Abstract

Efficient and long-lived interfaces between light and matter are crucial for the development of quantum information technologies. Integrated photonic solutions for quantum storage devices offer improved performances due to light confinement and enable more complex and scalable designs. We demonstrate a novel platform for quantum light storage based on laser written waveguides. The new adopted writing regime allows us to attain waveguides with improved confining capabilities compared to previous demonstrations. We report the first demonstration of single-photon storage in laser written waveguides. While we achieve storage efficiencies comparable to those observed in massive samples, the power involved for the memory preparation is strongly reduced, by a factor 100, due to an enhancement of the light–matter interaction of almost one order of magnitude. Moreover, we demonstrate excited-state storage times 100 times longer than previous realizations with single photons in integrated quantum memories. Our system promises to effectively fulfill the requirements for efficient and scalable integrated quantum storage devices.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. INTRODUCTION

Implementing solid-state quantum storage devices with guided wave optics has several advantages, such as compactness, scalability, efficiency due to enhanced light–matter interaction, and improved mechanical stability [1]. The possibility of connection with other integrated quantum devices, i.e., single-photon sources [2], photonic circuits [3], and detectors [4], enables the implementation of complex integrated quantum architectures. In addition, the compatibility of waveguide-based devices with fiber optics opens the way to the interconnection between quantum memories and the current fiber networks [5] with proven extraordinary telecommunication capabilities.

Rare-earth-ion-doped crystals, already widely known as long-lived and multiplexed optical memories with quantum storage capabilities [6], promise to be excellent systems for the development of on-chip quantum storage devices. The different approaches pursued in this respect include quantum light storage in Tm3+ or Er3+ in LiNbO3 waveguides [7,8] and storage with optically controlled retrieval of weak coherent states in a nanophotonic crystal cavity in Nd3+:YVO4 [9]. In these remarkable realizations, the ions used do not offer the possibility of storage at the ground state, thus, the storage times are inherently limited by the lifetime of the excited state (further shortened in the case of the nanophotonic crystals by the Purcell enhancement in the cavity). Photon echoes and long spin-state lifetimes have been observed in a hybrid integrated system [10] composed by a TeO2 slab waveguide deposited onto a Pr3+:Y2SiO5 substrate that potentially enables efficient [11] and long-lived storage with on-demand read-out [12]. This solution, nonetheless, sacrifices the enhancement of the light–matter interaction, as the coupling between the guided wave optics, the TeO2 waveguide, and the optically active Pr3+ ions happens through the evanescent field.

Optical channel waveguides can be also fabricated in the bulk of crystalline substrates by femtosecond (fs) laser micromachining (FLM). This powerful technique is rapid, cost-effective, and features unique three-dimensional fabrication capabilities. Different writing regimes can be adopted [13]. One possibility consists of irradiating the sample at high energy fluence for producing a pair of damaged material tracks that form the waveguide cladding, and light is guided between them due to a stress-induced positive refractive index change. This kind of structure is called a type II waveguide. Another possibility, substantially different from the previous one, consists of directly writing the waveguide core and fabricating a so-called type I waveguide. In this case, a much lower energy fluence is required for a positive refractive index change at the irradiated material volume. Identifying a processing window for operating in the first regime is relatively simple; in fact, type II waveguides have been demonstrated in numerous different materials, including rare-earth-doped crystals, mainly for integrated laser source applications [1416]. Type II waveguides have also been recently fabricated in Pr3+:Y2SiO5 and successfully employed for the coherent storage of classical pulses [17]. On the contrary, the fabrication of type I waveguides in crystalline substrates is a very challenging task, since it requires finding a very narrow processing parameter window, if any. So far, this fabrication regime has been demonstrated only in a very limited number of cases [1820], including lithium niobate [2123], potassium dihydrogen phosphate (KDP) [24], and polycrystalline matrices [25,26].

In the present paper, we report on the realization of type I waveguides in a Pr3+:Y2SiO5 crystal, and we characterize the light guiding properties at 606 nm, where Pr3+ features an optical transition of interest for photonic storage purposes. The small dimensions of the waveguide modes guarantee the compatibility with optical fibers and a significant enhancement of the light–matter interaction. Moreover, spectroscopic investigations reveal that the fabrication does not affect the optical properties of Pr3+ ions in the light guiding region. Finally, to assess the potential of our new platform as quantum memory, we implement quantum storage of heralded single photons. The achieved storage times are 100 times longer than in previous experiments with single photons in waveguides [7,8].

2. NEW WAVEGUIDE WRITING REGIME IN Pr3+:Y2SiO5

A. Fabrication of Type I Waveguides

Type I waveguides have been directly written by FLM in the volume of a Pr3+:Y2SiO5 crystal with a dopants concentration of 0.05%. We employed the second harmonic at 520 nm of an Yb-based commercial fs laser source (SPIRIT-1040, Spectra-Physics), which emits a train of ultrashort laser pulses with a duration of 350 fs. The waveguides are written in a single transverse scan geometry with the laser beam propagating along the crystal D1 axis and the sample translating along the b crystallographic direction (3.7 mm length). The laser pulses are focused 100 μm below the sample top surface by means of a 0.6 NA microscope objective. We experimentally found the optimal irradiation conditions for obtaining high quality optical waveguides as 40 nJ pulse energy, 100 μm/s translation speed, and 20 kHz repetition rate, giving a total fabrication time of 40s for a single waveguide. These waveguides support a single optical mode at the wavelength of 606 nm, polarized along the crystal D2 axis. An important parameter in the waveguide fabrication is the polarization of the writing laser, which must be linear and perpendicular to the sample translation direction, otherwise, no guiding effect is observed at the irradiated lines. The refractive index change of the core with respect to the substrate was estimated numerically from the guided mode profile (according to the method described in Ref. [27]) as Δn1.6×103. As type I waveguides in crystals are often thermally unstable [23], it is worth mentioning that no visible degradation of our type I waveguides in Pr3+:Y2SiO5 was observed after several months of exposure to normal ambient and cryogenic conditions.

The physical mechanisms that contribute to inducing a type I waveguide in crystals strongly depend on the specific material considered, encompassing the formation of lattice defects and a local change in the material polarizability. Understanding their relative weight is a difficult task. Detailed studies performed on type I waveguides written in LiNbO3 and Nd:YCa4O(BO3)3 (Nd:YCOB) crystals showed that the positive refractive index change in these materials results mainly from a weak lattice distortion and partial ion migration effects taking place at the irradiated area [19,21]. Such modifications, typically observed in a regime close to the material processing threshold, essentially preserve the bulk properties of the crystal, as demonstrated for type I waveguides in LiNbO3 [22]. Regarding type I waveguides in Pr3+:Y2SiO5, a fundamental explanation of the origin of the positive index change induced by ultrafast laser irradiation is still under investigation.

B. Benchmarking Type I versus Type II Waveguides

An experimental characterization was performed for comparing the guiding properties of type I and type II waveguides fabricated in Pr3+:Y2SiO5. To this purpose, five different type II waveguides were inscribed in the sample employing the same setup described before, but using the laser fundamental harmonic at 1040 nm, a pulse energy of 575 nJ, and a scan speed of 60 μm/s. The five waveguides differ in the separation d between the tracks forming the cladding, ranging from 10 to 20 μm. It should be noted that the chosen irradiation parameters provide the best waveguiding properties for all values of d. Values of d25μm led to multimode behavior.

We coupled all waveguides with light at 633 nm from a He–Ne source polarized along the D2 crystal axis by means of a plano-convex lens (75 mm focal length, 1 in. diameter). The 1/e2 diameter of the laser beam before the lens was 6 mm. We collected the light at the waveguide output by means of a microscope objective (40 times, 0.65 NA). In this way, we were able to acquire with a CCD camera the normalized mode intensity profile of all waveguides, reported in Fig. 1(a), together with a microscope picture of their transverse cross sections. In addition, we measured for every waveguide the total insertion losses (IL), defined as the ratio between the light power measured before and after the waveguide. Note that the value of IL includes the waveguide propagation losses due to scattering and absorption, the coupling losses due to mode mismatch at the waveguide input, and the Fresnel losses caused at the crystal/air interface at the waveguide output, where no anti-reflection coating was present. The results of this measurement are shown in Fig. 1(b), where we plot the IL values as a function of the horizontal full width at half-maximum (FWHM) mode diameter for all waveguides. It is clearly visible that the reduction of d in type II waveguides (black squares) reduces the mode size, but, at the same time, leads to a dramatic increase of IL. On the other hand, the type I waveguide (blue circle) supports the smallest mode (3.1μm×5.9μm FWHM diameters) and, simultaneously, exhibits the lowest value of IL among all waveguides analyzed. This fact is readily explained by looking at the waveguides’ longitudinal profiles shown in Fig. 1(c). The type I waveguide features a smooth and very uniform profile along the propagation direction. The side tracks of type II waveguides, instead, present a rough and less uniform profile that increases light scattering, especially for small values of d. A more detailed analysis of the different contributions to the waveguide IL can be found in Supplement 1.

 figure: Fig. 1.

Fig. 1. (a) Optical microscope image of the waveguide transverse cross section (top row) and guided mode intensity profile (bottom row) of all fabricated waveguides. The numbers above the figures specify the separation between the laser tracks in type II waveguides. Scale bar is 10 μm. (b) Insertion losses versus FWHM mode diameter in the horizontal direction for type I (blue circle) and type II (black squares) waveguides. (c) Microscope picture of the longitudinal profile of a type I (top) and a type II waveguide with d=10μm (bottom). Scale bar is 20 μm. (d) Bending losses as a function of the radius of curvature for type I (blue circles) and type II (black squares) waveguides. Dashed lines are best exponential fits of experimental data [28]. Error bars in plots (b) and (d) are smaller than the data markers.

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As a further comparison, we measured for the two types of waveguide the additional bending losses (BL) caused by the coupling of light to radiative modes during the propagation in a curved guided path. We, thus, compared the IL of a straight waveguide with that of a curved one, containing an S band of known length and fabricated with constant radius of curvature RC. We performed this measurement for values of RC of 30, 50, and 90 mm, both on the type I and type II waveguides with d=20μm, fabricated in a dedicated sample (S-band length=7mm, total sample length=9mm). The results obtained are shown in Fig. 1(d). As expected, the values of BL increase for shorter RC for both types. However, in type I, the measured BL reach particularly low values and become almost negligible for RC>90mm. This outcome agrees with the well known fact that BL decrease in more confining waveguides [28].

This experimental analysis allows us to conclude that type I waveguides in Pr3+:Y2SiO5 show better guiding performances than their type II counterparts, both in terms of waveguide losses and in terms of light confinement. Interestingly, the measured values of BL for type I waveguides are compatible with the fabrication of complex integrated devices, i.e., directional couplers or waveguide arrays, as bending radii in the order of tens of millimeters (mm) allow for a flexible engineering of evanescent waveguide coupling, even in samples with a limited length.

3. EXPERIMENTAL SETUP

Figure 2 represents the experimental setup for the spectroscopy and single-photon storage measurements, presented in the next paragraphs, and the hyperfine splitting of the ground and excited crystal field levels of Pr3+ involved in the optical transition relevant for the storage (inset). Note that we address only ions in one of the two possible crystallographic sites (site 1).

 figure: Fig. 2.

Fig. 2. Setup: a pump laser at 426 nm (blue beam) shines a periodically poled lithium niobate (PPLN) crystal in a bow-tie cavity. Photon pairs (gray beam) are generated by spontaneous parametric down-conversion and divided with a dichroic mirror (DM). The idler photon at 1436 nm (purple beam) is sent to a filter cavity (FC). The signal photon at 606 nm (orange beam) passes through an etalon and enters input 2 of a fiber beam-splitter (BS). Two chopper wheels are used to alternate between locking and the measurement period. The signal photon and the preparation light (from input 1 of the BS) emerging from one output of the BS are coupled into the Pr3+:Y2SiO5 waveguide. The hyperfine splitting of the first sublevels of the ground H43 and the excited D21 manifolds of Pr ions in Y2SiO5 is shown in the inset along with the optical absorption spectrum (the data points are experimental values and the solid curve the Gaussian fit). The orange arrows highlight the specific transition chosen for the storage protocol.

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A CW laser at 606 nm (linewidth 20 kHz) is modulated with acousto-optic modulators (AOMs) in double-pass configuration to produce the preparation pulse sequences. The preparation light is coupled into one input of a fiber beam splitter (BS, input 1 in Fig. 2). In input 2, we send the input for the storage, either classical pulses or heralded single photons (see below). One output is sent to an independent optical table, where the Pr3+:Y2SiO5 crystal is maintained at 3 K, while the second output is used as reference. The light is coupled into the waveguide using a 75 mm lens, which focuses the beam to a waist <10μm at the input facet of the crystal. The outcoming light from the waveguide is collected with a 50 mm lens and sent to a detection stage after a path of about 2 m. The detection is implemented with a CCD camera for imaging and alignment, with a photo detector for protocols with classical light, or with a single-photon detector (SPD) for experiments with single photons. For auto-correlation measurements, a Hanbury Brown–Twiss setup is assembled with fiber BSs and additional SPDs. All of the experiments are synchronized with the cycle of the cryostat (1.4 Hz). Because of vibrations, the light is efficiently coupled in the waveguide for less than 300 ms in each cycle. Further details on the setup can be found in Supplement 1 [29].

Our heralded single photons are created with a new generation [30] of the photon pair source described in Refs. [31,32]. It is based on cavity-enhanced type I spontaneous parametric down-conversion (SPDC) in a 2-cm-long periodically poled lithium niobate (PPLN) crystal (Fig. 2). This is placed in a bow-tie cavity (BTC) and pumped with a 426 nm laser. The BTC is in resonance both with the signal photon at 606 nm and its heralding photon at 1436 nm (idler). The cavity lock exploits the Pound–Drever–Hall technique [32], and two mechanical choppers are used to alternate between the locking period and the single-photon measurement. The two collinearly generated photons are separated after the BTC using a dichroic mirror (DM, Fig. 2). The signal and idler photons are distributed in eight spectral modes. The idler photons pass through a home-made Fabry–Perot filter cavity [FC, Fig. 2, linewidth 80 MHz, free spectral range (FSR)=17GHz] to guarantee single-spectral-mode heralding. They are then coupled into a single-mode fiber to an SPD. The 606 nm photons after passing through an etalon filter are coupled to a single-mode polarization-maintaining (PM) fiber, and then to input 2 of the fiber BS. The heralding efficiency of the SPDC source is ηHSPDC25% after the PM fiber and ηHWG7% in front of the waveguide. The loss is due to the BS and mode–diameter mismatch between the PM fiber and the fiber BS and could be readily reduced by using a fiber switch.

4. SPECTROSCOPIC AND COHERENCE MEASUREMENTS

The inhomogeneous broadening of the transition of Pr3+ at 606 nm in the waveguide has been estimated from the transmission of an optical pulse through the crystal while tuning the laser frequency. The FWHM is 9GHz in optical depth (OD, see inset in Fig. 2), which is in good agreement with bulk samples with similar ion concentrations. Moreover, the central frequency did not shift with respect to the bulk. The hyperfine splitting of the levels and the oscillator strength of the transitions, measured following the spectral hole-burning experiments described in Ref. [33], are also consistent with those measured in the bulk. The unaltered spectroscopic properties confirm that the fabrication process does not substantially alter the crystalline structure of the modified zone.

To use the waveguide for quantum memory application, the coherence of the ions in the guiding zone should also be maintained. Coherence and lifetime of the optical transition are measured by means of, respectively, two-pulse [34] and three-pulse stimulated photon echo [35] experiments (TPE and SPE). A typical pulse sequence for the heterodyne detection of the TPE is shown in the inset of Fig. 3(a). The echo is detected in the form of oscillations on a probe pulse 10 MHz-detuned with respect to the excitation and rephasing pulses. From the exponential decay of the echo intensity (proportional to the square of the oscillation amplitude) while increasing the time τ2 [Fig. 3(a)], we extract the coherence time of the Pr3+ ions in the waveguide. Figure 3(b) represents examples of heterodyne detected TPE at different storage times. In our case, probing a single-class absorption feature at the 1/2g1/2e transition, we measure a maximum coherence time T2=57±10μs. This is only slightly lower than the maximal T2 measured in bulk samples or type II waveguides [17]. We note, however, that T2 is affected by instantaneous spectral diffusion and that higher values could in principle be achieved by decreasing the average number of atoms excited [17].

 figure: Fig. 3.

Fig. 3. (a) Decay in the echo intensity for a two-photon echo pulse measurement, from which we extract the optical transition coherence time T2 in the type I waveguide. The black solid line is the fit to an exponential decay, and the dotted lines indicate the error. The inset shows the temporal sequence used. The time interval between the first and second pulses is called τ2 (solid arrow). (b) Examples of heterodyne detected TPE at different times τ2. The solid orange line is the experimental signal, and the dotted black line the sinusoidal fit. The echo intensity is proportional to the square of the oscillation amplitude. (c) Area of the stimulated photon echo (SPE) in arbitrary units, varying the times τ1 and τ2; (d) temporal trace of the SPE process. The SPE signal is highlighted by the orange area. The time interval between the second and third pulses is called τ1 (dotted arrow); (e) values of the SPE areas at τ2=0 (extracted from the exponential decays of the SPE areas over τ2) plotted as a function of τ1. From the decay of the SPE versus τ1 (for τ2=0), we extract the excited-state lifetime of the ions, T1; (f) homogeneous broadening of the optical transition for increasing τ1. The values are extracted from the decays of the SPE versus τ2 for different τ1 values. The black solid line is the value extracted from the T2 measurement for τ1=0 with the dashed lines indicating the error.

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To study the lifetime of the transition and slow dephasing mechanisms, e.g., spectral diffusion, we analyze the SPE measured with direct detection. A typical output of a SPE measurement is shown in Fig. 3(c), while Fig. 3(d) sketches the temporal sequence used. To reduce the errorbar, we perform these measurements with a higher excitation power than that used in the TPE measurement; thus, we expect the absolute value of T2 to be lower due to instantaneous spectral diffusion. We measure the decay of the SPE with τ2 for different values of τ1. By fitting these decays with exponentials, we extract the expected areas of the SPE(τ1) for τ2=0 [Fig. 3(e)]. Their decay with τ1 directly depends on the lifetime of the excited state |e, namely, T1=118±35μs. This value is lower than that obtained from fluorescence measurements (about 160 μs) but in good agreement with that measured with SPE in the same bulk sample, T1=103±11μs. In the presence of spectral diffusion, the ions could experience frequency shifts induced by flips of the surrounding nuclear spins. This would cause a slow broadening of the ions linewidth Γhom for increasing τ1 [36]. As we can clearly see from Fig. 3(f), Γhom(τ1)=1/[πT2(τ1)] remains consistent over time with the value at τ1=0 (solid line, about 10 kHz), proving the absence of spectral diffusion in the analyzed timescale and excitation power. The same trend is observed in SPE measurements in the bulk crystal.

One of the advantages of optical waveguides is the enhanced light–ions interaction due to the strong light confinement. To quantify the strength of this interaction, we measure the Rabi frequency ΩR of the optical transition by means of optical nutation [37]. We prepare by optical pumping a single-class absorption feature on the 1/2g3/2e transition and measuring the population inversion time tπ induced by a long resonant probe pulse. For an optically dense and inhomogeneously broadened ensemble, the Rabi frequency ΩR is calculated as ΩRtπ=5.1 [from Eq. (1) in Ref. [37]]. We repeat the measurement for several probe powers P. From the linear fit of ΩR versus P, we extract ΩR=2π×1.75MHz/mW, i.e., an increase of 1.6 with respect to the type II waveguide [17] and almost one order of magnitude with respect to the bulk crystal (maintaining the same optics), fully matching with the expected increase due to the stronger light confinement (further details can be found in Supplement 1 [29]).

5. STORAGE OF HERALDED SINGLE PHOTONS

A. Characterization of the Heralded Single Photons

Before storing the signal photons, we measure the properties of the generated heralded single photons (more details can be found in Supplement 1 [29]). We build a coincidence histogram using the idler detection as the start and the signal photon as the stop [inset in Fig. 4(a)]. The correlation time of our biphoton is estimated by fitting the histogram [32] (black dashed lines) as 121 ns, which corresponds to a biphoton linewidth of Γ=1.8MHz. This is smaller than the hyperfine splitting of the excited state, thus narrow enough to address a single transition of Pr3+ ions. We calculate the normalized second-order cross-correlation function as gs,i(2)(Δt)=ps,i/(ps·pi), where ps,i is the probability to detect a coincidence in a temporal window Δt, while ps (pi) is the probability to detect a signal (idler) count in a temporal window of the same size. The measured gs,i(2) values for a window Δt=400ns for different pump powers are plotted in Fig. 4(a) (empty orange circles). The highest value gs,i(2)(400ns)=209±9 is achieved at the lowest measured pump power (0.1 mW) and decreases while increasing the pump power, as expected for a two-mode squeezed state [38].

 figure: Fig. 4.

Fig. 4. (a) Measured gs,i(2) for different pump powers with just the source (empty light dots) and after the pit (full dark points). The inset represents the time-resolved signal–idler coincidence histogram with just the source: the darker region is the window considered for the calculation of the gs,i(2) (400 ns), and the black dashed line is the temporal fit of the biphoton correlation; (b), (c) coincidences between signal photons split with a fiber BS, respectively, before and after passing through a pit in the waveguide, sorted by the number of heralding photons between pairs of contiguous signal counts.

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We demonstrate the quantum nature of the signal–idler correlations violating the Cauchy-Schwartz (CS) inequality by more than 20 standard deviations (see Ref. [29] for details). The classical bound is set by the parameter R=(gs,i(2))2gs,s(2)gi,i(2)1, where gs,s(2) and gi,i(2) are the unconditional auto-correlations of the signal and idler photons, respectively. To demonstrate the single-photon nature of our source, we measure the heralded auto-correlation of the signal photons gi:s,s(2)(Δt). This can be extracted from the histogram of Fig. 4(b), built like in Ref. [32,39]. We find gi:s,s(2)(400ns)=0.12±0.01 for a pump power of 1.7 mW. This value is considerably lower than the classical bound gi:s,s(2)1 and compatible with the single-photon behavior (gi:s,s(2)0.5) [29].

We then send the signal photons through the waveguide, where we hole-burn a transparency window (pit) of 16MHz in the inhomogeneously broadened absorption profile of the ions [orange line in the inset of Fig. 5(a)]. We measure the gs,i(2) versus pump power, sending the signal photons through the pit [full brown circles in Fig. 4(a)]. The correlations after the pit become remarkably higher because the crystal acts as a spectral filter: the pit selects a single frequency mode, while all of the others are absorbed by the Pr3+ ions spread over the whole inhomogeneously broadened absorption line [4042]. We find R=524±84 after the pit for the highest measured power, violating the CS inequality by more than 6 standard deviations (assuming gs,s(2)=2). Furthermore, we measure the gi:s,s(2)(Δt) of the signal photons after the pit: from the histogram of Fig. 4(c), we extract gi:s,s(2)(400ns)=0.06±0.04 (pump power 1.7mW).

 figure: Fig. 5.

Fig. 5. (a) Time-resolved histogram of the idler–signal coincidences for signal photons passing through the pit (gray histogram) or the AFC for τ=1.5μs (orange histogram). The counts in the AFC are multiplied by three. The darker regions of the peaks show the windows considered for ηAFC and gs,i(2) calculations. The black and brown dashed lines are the temporal decays of the correlations [inset of Fig. 4(a)] renormalized for losses and efficiencies after the pit and the AFC, respectively. The shaded rectangle is the region in which the accidental counts for the AFC echo are measured. The absorption profiles of the pit (orange) and the comb (brown) are plotted in the inset with the spectrum of the single photons (black points and line, see Ref. [29]); (b) internal storage efficiency ηAFC at different storage times τ for single photons (full orange points, error bars account for Poissonian statistics) and classical light (empty black circles); (c) cross-correlation values between idler photons and stored signal photons gAFC,i(2) for different τ. The orange dashed line is the classical upper bound gs,s(2)·gi,i(2)=1.58±0.02 (assuming gs,s(2)=2).

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B. Storage of Heralded Single Photons

The storage protocol that we use is called atomic frequency comb (AFC) [43]: the inhomogeneously broadened absorption of the ions is shaped in a spectral comb with periodicity Δ. When a photon is absorbed by the comb, its state is mapped onto a collective excitation of atoms, described by a collective Dicke state: kei2πδktck|g1ekgN, where the frequency detuning of the atom k is δk=mkΔ (being an integer number mk). After a first inhomogeneous dephasing, the atoms will collectively rephase at a predetermined time τ=1/Δ, giving rise to a photon re-emission in the forward direction, called AFC echo.

The sequence of optical pulses used to create the AFC is very similar to the one explained in Refs. [44,45]: after creating a 16-MHz-wide pit to empty the 1/2g and 3/2g states, we repopulate the 1/2g level with a single class of ions (duration 80ms). Then, we send a series of pulses resonant with the 1/2g3/2e transition, whose Fourier transform is the AFC that we want to create (duration 35ms). The generated feature for a storage time of 1.5 μs is plotted as a brown line in the inset of Fig. 5(a). Note that the spectrum of the input photons [black line in the inset of Fig. 5(a)] matches perfectly with the AFC (see Ref. [29] for details on the biphoton spectrum). Thanks to the enhanced light–ions interaction, the maximum power that we inject in the waveguide during the AFC creation is 100μW, i.e., two orders of magnitude lower than what is usually necessary in bulk, in agreement with the ΩR increase. The measurement is performed in the remaining time (150ms), resulting in a duty cycle for the AFC storage of 21% (accounting for the duty cycle due to the cryostat vibrations). When a herald is detected, an AOM in front of the pump laser is closed to reduce the noise coming from the source during the AFC echo retrieval [29]. The minimum response time of this gate is tPoff=1.2μs, which limits our minimum storage time to τ=1.5μs. For longer storage times, tPoff is delayed in order to maintain τtPoff=0.3μs. The AFC echo for a storage time τ=1.5μs is shown in Fig. 5(a) (orange trace). The exponential fit of the biphoton temporal decay, measured with only the source [black dashed line of the inset in Fig. 4(a)], is plotted on top of the input (black dashed line) and the AFC echo (brown dashed line), renormalized for the different count-rates. Note that the linewidth of the photons, both after the pit and emitted by the comb, remains unchanged, confirming that there is no mismatch between the width of the comb and the spectral profile of the photons. Before and after each storage experiment, we measure the coincidences between the idler and the signal photons after the pit. To account for fluctuations in power, we consider the average of the two as our reference input [gray peak in Fig. 5(a)]. We evaluate the storage efficiency by integrating the counts of the histogram in a 400 ns window centered at the AFC echo [dark orange region at about 1.5 μs, Fig. 5(a)] divided by the counts inside a 400 ns window centered at the input [dark gray region about 0 μs, Fig. 5(a)], the latter normalized by the transmission through the pit [85%, see inset in Fig. 5(a)]. The resulting efficiency is the internal efficiency of the process ηAFC. The total efficiency of our device, i.e., the ratio between the output signal and the input signal before entering the waveguide, is calculated multiplying ηAFC by the coupling into the waveguide (40%). We perform storage experiments with an average pump power of 1.7 mW. The internal efficiencies for different storage times are shown in Fig. 5(b) (full orange points). For comparison, we measure the internal efficiency of our memory with classical pulses [empty black circles in Fig. 5(b)] using the same comb preparation sequences, showing a good overlap between the quantum and the classical regimes. The efficiency decrease for increasing τ is fitted with an exponential decay e4/(T2*Δ) [45], from which we extract the effective coherence time of our storage protocol T2*=8μs, which is much smaller than T2 (see Section 4). This suggests that our storage time is at the moment limited by technical issues and not yet by the coherence time T2 of the Pr3+ in the waveguide. We estimate that a factor of 2 could be given by instantaneous spectral diffusion because, due to the time limit imposed by the cryostat cycle, we implement the optical pumping for the AFC preparation with relatively high power. The remaining mismatch between T2* and T2 is likely due to the finite laser linewidth (see Section 3) and power broadening.

The gAFC,i(2) of the AFC echo is measured similarly to the one of the input: we find ps,i integrating the counts in a window of 400 ns centered at the AFC echo (the same region considered for ηAFC); ps·pi is measured integrating the accidental coincidences after the AFC echo up to the last stored noise count tPoff+τ [light orange rectangle in Fig. 5(a)], renormalized to a 400 ns window. Figure 5(c) shows the gAFC,i(2) values for AFC echoes measured at different τ. The cross-correlation increases after the storage up to 61±12 for a storage time of 1.5 μs with respect to 36±3 after the pit. This could be explained by the presence of broadband noise from the SPDC source, which is not in resonance with the inhomogeneous absorption line of the Pr3+ ions. Such noise would not be present in the temporal mode of the AFC echo, where the pump is gated off [40]. The value of gAFC,i(2) should remain constant for different τ, as the storage efficiency is the same for the AFC echo and for the stored noise. But for longer storage times, as ηAFC decreases, our signal-to-noise ratio is limited by the background noise. Nevertheless, the gAFC,i(2) remains higher than the classical bound (orange dashed line) for all the measured storage times up to τ=5.5μs, for which we violate the CS inequality with R=34±18 (above the classical bound by almost 2 standard deviations), effectively demonstrating the longest quantum storage in an integrated solid-state optical memory (100 times longer than the previous demonstrations of single-photon storage in waveguides [7,8]). Moreover, thanks to the convenient energy level scheme, our system enables the full spin-wave AFC storage, thus giving access to both longer storage times and on-demand read-out [44,46,47].

6. CONCLUSION AND OUTLOOK

In the present work, we propose a new platform for the implementation of integrated quantum storage devices. We demonstrated the generation of type I waveguides in a Pr3+:Y2SiO5 crystal using FLM in a new writing regime. We showed that the fabrication of type I waveguides preserves the measured spectroscopic properties of Pr3+. We implemented a quantum storage protocol for heralded single photons, observing high non-classical correlations for storage times 100 times longer than in previous waveguide demonstrations. The use of type I waveguides in Pr3+:Y2SiO5 gives several advantages with respect to type II ones. In type I waveguides, the guided mode is in general sensibly smaller than what is obtainable in a type II waveguide with comparable losses. This fact yields an enhancement in the interaction of the guided light with the rare earth dopants. Moreover, the very good mode matching between type I waveguides and standard single-mode optical fibers would allow us to glue the Pr3+:Y2SiO5 samples directly to fiber patch cords with low coupling losses, sensibly simplifying the procedure of light coupling inside the cryostat and avoiding the temporal constraints on the photon storage given by the cryostat vibrations. High-quality type I waveguides will also allow us to produce linear cavities with high-quality factors by directly writing Bragg gratings superimposed to the waveguide [48]. In addition, type I waveguides in Pr3+:Y2SiO5 could also be easily interfaced with laser written optical circuits in glass, potentially opening the way to the realization of integrated hybrid glass/crystal platforms [49] embedding quantum memories. Remarkably, taking advantage of the intrinsic three-dimensional capabilities of FLM, one can envision high spatial multiplexing by an efficient exploitation of the substrate volume with matrices of quantum memories interconnected to linear fiber arrays by glass circuits. Finally, for this kind of waveguide, the absence of lateral damage tracks (present in the type II counterpart) enables greater freedom in engineering the evanescent coupling of light between different waveguides. This, together with the tighter bend radii achievable in type I waveguides, permits us to easily inscribe optical circuits in Pr3+:Y2SiO5 crystals embedding directional couplers and other integrated optics devices, for performing complex tasks besides quantum light storage, fully on-chip.

Funding

H2020 European Research Council (ERC) (742745); Ministerio de Economía y Competitividad (MINECO) and European Regional Development Fund (ERDF) (FIS2015-69535-R); MINECO Severo Ochoa (SEV-2015-0522 and Ph.D. fellowship program for A. S.); Fundación Cellex; CERCA Programme/Generalitat de Catalunya.

 

Please see Supplement 1 for supporting content.

REFERENCES AND NOTES

1. A. Orieux and E. Diamanti, “Recent advances on integrated quantum communications,” J. Opt. 18, 083002 (2016). [CrossRef]  

2. P. Vergyris, F. Kaiser, E. Gouzien, G. Sauder, T. Lunghi, and S. Tanzilli, “Fully guided-wave photon pair source for quantum applications,” Quantum Sci. Technol. 2, 024007 (2017). [CrossRef]  

3. I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017). [CrossRef]  

4. F. Raffaelli, G. Ferranti, D. H. Mahler, P. Sibson, J. E. Kennard, A. Santamato, G. Sinclair, D. Bonneau, M. G. Thompson, and J. C. F. Matthews, “A homodyne detector integrated onto a photonic chip for measuring quantum states and generating random numbers,” Quantum Sci. Technol. 3, 025003 (2018). [CrossRef]  

5. F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten, C. Simon, and W. Tittel, “Prospective applications of optical quantum memories,” J. Mod. Opt. 60, 1519–1537 (2013). [CrossRef]  

6. H. Riedmatten and M. Afzelius, “Quantum light storage in solid state atomic ensembles,” in Engineering the Atom-Photon Interaction: Controlling Fundamental Processes with Photons, Atoms and Solids (Springer, 2015), pp. 241–273.

7. E. Saglamyurek, N. Sinclair, J. Jin, J. A. Slater, D. Oblak, F. Bussieres, M. George, R. Ricken, W. Sohler, and W. Tittel, “Broadband waveguide quantum memory for entangled photons,” Nature 469, 512–515 (2011). [CrossRef]  

8. M. F. Askarani, M. Grimau Puigbert, T. Lutz, V. B. Verma, M. D. Shaw, S. W. Nam, N. Sinclair, D. Oblak, and W. Tittel, “Storage and retrieval of heralded telecommunication-wavelength photons using a solid-state waveguide quantum memory,” arXiv: 1804.05699 (2018).

9. T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017). [CrossRef]  

10. S. Marzban, J. G. Bartholomew, S. Madden, K. Vu, and M. J. Sellars, “Observation of photon echoes from evanescently coupled rare-earth ions in a planar waveguide,” Phys. Rev. Lett. 115, 013601 (2015). [CrossRef]  

11. M. P. Hedges, J. J. Longdell, Y. Li, and M. J. Sellars, “Efficient quantum memory for light,” Nature 465, 1052–1056 (2010). [CrossRef]  

12. G. Heinze, C. Hubrich, and T. Halfmann, “Stopped light and image storage by electromagnetically induced transparency up to the regime of one minute,” Phys. Rev. Lett. 111, 033601 (2013). [CrossRef]  

13. F. Chen and J. Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photon. Rev. 8, 251–275 (2014). [CrossRef]  

14. Y. Tan, A. Rodenas, F. Chen, R. R. Thomson, A. K. Kar, D. Jaque, and Q. Lu, “70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser,” Opt. Express 18, 24994–24999 (2010). [CrossRef]  

15. T. Calmano, J. Siebenmorgen, F. Reichert, M. Fechner, A.-G. Paschke, N.-O. Hansen, K. Petermann, and G. Huber, “Crystalline Pr:SrAl12O19 waveguide laser in the visible spectral region,” Opt. Lett. 36, 4620–4622 (2011). [CrossRef]  

16. C. Grivas, C. Corbari, G. Brambilla, and P. G. Lagoudakis, “Tunable, continuous-wave Ti:sapphire channel waveguide lasers written by femtosecond and picosecond laser pulses,” Opt. Lett. 37, 4630–4632 (2012). [CrossRef]  

17. G. Corrielli, A. Seri, M. Mazzera, R. Osellame, and H. de Riedmatten, “Integrated optical memory based on laser-written waveguides,” Phys. Rev. Appl. 5, 054013 (2016). [CrossRef]  

18. T. Calmano and S. Müller, “Crystalline waveguide lasers in the visible and near-infrared spectral range,” IEEE J. Sel. Top. Quantum Electron. 21, 401–413 (2015). [CrossRef]  

19. A. Rodenas and A. K. Kar, “High-contrast step-index waveguides in borate nonlinear laser crystals by 3D laser writing,” Opt. Express 19, 17820–17833 (2011). [CrossRef]  

20. L. Li, W. Nie, Z. Li, C. Romero, R. I. Rodriguez, J. Aldana, and F. Chen, “Laser-writing of ring-shaped waveguides in BGO crystal for telecommunication band,” Opt. Express 25, 24236–24241 (2017). [CrossRef]  

21. J. Burghoff, S. Nolte, and A. Tünnermann, “Origins of waveguiding in femtosecond laser-structured LiNbO3,” Appl. Phys. A 89, 127–132 (2007). [CrossRef]  

22. R. Osellame, M. Lobino, N. Chiodo, M. Marangoni, G. Cerullo, R. Ramponi, H. T. Bookey, R. R. Thomson, N. D. Psaila, and A. K. Kar, “Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient,” Appl. Phys. Lett. 90, 241107 (2007). [CrossRef]  

23. R. Thomson, S. Campbell, I. Blewett, A. Kar, and D. Reid, “Optical waveguide fabrication in z-cut lithium niobate (LiNnO3) using femtosecond pulses in the low repetition rate regime,” Appl. Phys. Lett. 88, 111109 (2006). [CrossRef]  

24. L. Huang, P. Salter, M. Karpinski, B. Smith, F. Payne, and M. Booth, “Waveguide fabrication in KDP crystals with femtosecond laser pulses,” Appl. Phys. A 118, 831–836 (2015). [CrossRef]  

25. A. Rodenas, A. Benayas, J. Macdonald, J. Zhang, D. Tang, D. Jaque, and A. Kar, “Direct laser writing of near-IR step-index buried channel waveguides in rare earth doped YAG,” Opt. Lett. 36, 3395–3397 (2011). [CrossRef]  

26. J. Macdonald, R. Thomson, S. Beecher, N. Psaila, H. Bookey, and A. Kar, “Ultrafast laser inscription of near-infrared waveguides in polycrystalline ZnSe,” Opt. Lett. 35, 4036–4038 (2010). [CrossRef]  

27. I. Mansour and F. Caccavale, “An improved procedure to calculate the refractive index profile from the measured near-field intensity,” J. Lightwave Technol. 14, 423–428 (1996). [CrossRef]  

28. R. G. Hunsperger, Integrated Optics (Springer, 1995), vol. 4.

29. Supplement 1 contains details about the experimental setup and the characterization measurements of the heralded single photons from the SPDC source.

30. N. Maring, D. Lago-Rivera, A. Lenhard, G. Heinze, and H. de Riedmatten, “Quantum frequency conversion of storable single photons from 606 nm to the telecom c-band,” arXiv: 1801.03727 (2018).

31. J. Fekete, D. Rieländer, M. Cristiani, and H. de Riedmatten, “Ultranarrow-band photon-pair source compatible with solid state quantum memories and telecommunication networks,” Phys. Rev. Lett. 110, 220502 (2013). [CrossRef]  

32. D. Rieländer, A. Lenhard, M. Mazzera, and H. de Riedmatten, “Cavity enhanced telecom heralded single photons for spin-wave solid state quantum memories,” New J. Phys. 18, 123013 (2016). [CrossRef]  

33. M. Nilsson, L. Rippe, S. Kröll, R. Klieber, and D. Suter, “Hole-burning techniques for isolation and study of individual hyperfine transitions in inhomogeneously broadened solids demonstrated in Pr3+:Y2SiO5,” Phys. Rev. B 70, 214111 (2004). [CrossRef]  

34. A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: I. echoes excited by two optical pulses,” Eur. J. Phys. 10, 291–297 (1989). [CrossRef]  

35. A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: II. three pulse echoes and collision effects,” Eur. J. Phys. 12, 234–239 (1991). [CrossRef]  

36. R. Yano, M. Mitsunaga, and N. Uesugi, “Stimulated-photon-echo spectroscopy. I. Spectral diffusion in Eu3+:YAlO3,” Phys. Rev. B 45, 12752–12759 (1992). [CrossRef]  

37. Y. Sun, G. M. Wang, R. L. Cone, R. W. Equall, and M. J. M. Leask, “Symmetry considerations regarding light propagation and light polarization for coherent interactions with ions in crystals,” Phys. Rev. B 62, 15443–15451 (2000). [CrossRef]  

38. P. Sekatski, N. Sangouard, F. Bussières, C. Clausen, N. Gisin, and H. Zbinden, “Detector imperfections in photon-pair source characterization,” J. Phys. B 45, 124016 (2012). [CrossRef]  

39. S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin, and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New J. Phys. 6, 163 (2004). [CrossRef]  

40. D. Rieländer, K. Kutluer, P. M. Ledingham, M. Gündoğan, J. Fekete, M. Mazzera, and H. de Riedmatten, “Quantum storage of heralded single photons in a praseodymium-doped crystal,” Phys. Rev. Lett. 112, 040504 (2014). [CrossRef]  

41. H. Zhang, M. Sabooni, L. Rippe, C. Kim, S. Kröll, L. V. Wang, and P. R. Hemmer, “Using quantum memory techniques for optical detection of ultrasound,” Appl. Phys. Lett. 100, 131102 (2012). [CrossRef]  

42. S. Beavan, E. A. Goldschmidt, and M. J. Sellars, “Demonstration of a dynamic bandpass frequency filter in a rare-earth ion-doped crystal,” J. Opt. Soc. Am. B 30, 1173–1177 (2013). [CrossRef]  

43. M. Afzelius, C. Simon, H. de Riedmatten, and N. Gisin, “Multimode quantum memory based on atomic frequency combs,” Phys. Rev. A 79, 052329 (2009). [CrossRef]  

44. A. Seri, A. Lenhard, D. Rieländer, M. Gündoğan, P. M. Ledingham, M. Mazzera, and H. de Riedmatten, “Quantum correlations between single telecom photons and a multimode on-demand solid-state quantum memory,” Phys. Rev. X 7, 021028 (2017). [CrossRef]  

45. P. Jobez, N. Timoney, C. Laplane, J. Etesse, A. Ferrier, P. Goldner, N. Gisin, and M. Afzelius, “Towards highly multimode optical quantum memory for quantum repeaters,” Phys. Rev. A 93, 032327 (2016). [CrossRef]  

46. M. Gündoğan, P. M. Ledingham, K. Kutluer, M. Mazzera, and H. de Riedmatten, “Solid state spin-wave quantum memory for time-bin qubits,” Phys. Rev. Lett. 114, 230501 (2015). [CrossRef]  

47. P. Jobez, C. Laplane, N. Timoney, N. Gisin, A. Ferrier, P. Goldner, and M. Afzelius, “Coherent spin control at the quantum level in an ensemble-based optical memory,” Phys. Rev. Lett. 114, 230502 (2015). [CrossRef]  

48. G. D. Marshall, P. Dekker, M. Ams, J. A. Piper, and M. J. Withford, “Directly written monolithic waveguide laser incorporating a distributed feedback waveguide-Bragg grating,” Opt. Lett. 33, 956 (2008). [CrossRef]  

49. S. Atzeni, A. S. Rab, G. Corrielli, E. Polino, M. Valeri, P. Mataloni, N. Spagnolo, A. Crespi, F. Sciarrino, and R. Osellame, “Integrated sources of entangled photons at the telecom wavelength in femtosecond-laser-written circuits,” Optica 5, 311–314 (2018). [CrossRef]  

References

  • View by:

  1. A. Orieux and E. Diamanti, “Recent advances on integrated quantum communications,” J. Opt. 18, 083002 (2016).
    [Crossref]
  2. P. Vergyris, F. Kaiser, E. Gouzien, G. Sauder, T. Lunghi, and S. Tanzilli, “Fully guided-wave photon pair source for quantum applications,” Quantum Sci. Technol. 2, 024007 (2017).
    [Crossref]
  3. I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017).
    [Crossref]
  4. F. Raffaelli, G. Ferranti, D. H. Mahler, P. Sibson, J. E. Kennard, A. Santamato, G. Sinclair, D. Bonneau, M. G. Thompson, and J. C. F. Matthews, “A homodyne detector integrated onto a photonic chip for measuring quantum states and generating random numbers,” Quantum Sci. Technol. 3, 025003 (2018).
    [Crossref]
  5. F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten, C. Simon, and W. Tittel, “Prospective applications of optical quantum memories,” J. Mod. Opt. 60, 1519–1537 (2013).
    [Crossref]
  6. H. Riedmatten and M. Afzelius, “Quantum light storage in solid state atomic ensembles,” in Engineering the Atom-Photon Interaction: Controlling Fundamental Processes with Photons, Atoms and Solids (Springer, 2015), pp. 241–273.
  7. E. Saglamyurek, N. Sinclair, J. Jin, J. A. Slater, D. Oblak, F. Bussieres, M. George, R. Ricken, W. Sohler, and W. Tittel, “Broadband waveguide quantum memory for entangled photons,” Nature 469, 512–515 (2011).
    [Crossref]
  8. M. F. Askarani, M. Grimau Puigbert, T. Lutz, V. B. Verma, M. D. Shaw, S. W. Nam, N. Sinclair, D. Oblak, and W. Tittel, “Storage and retrieval of heralded telecommunication-wavelength photons using a solid-state waveguide quantum memory,” arXiv: 1804.05699 (2018).
  9. T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
    [Crossref]
  10. S. Marzban, J. G. Bartholomew, S. Madden, K. Vu, and M. J. Sellars, “Observation of photon echoes from evanescently coupled rare-earth ions in a planar waveguide,” Phys. Rev. Lett. 115, 013601 (2015).
    [Crossref]
  11. M. P. Hedges, J. J. Longdell, Y. Li, and M. J. Sellars, “Efficient quantum memory for light,” Nature 465, 1052–1056 (2010).
    [Crossref]
  12. G. Heinze, C. Hubrich, and T. Halfmann, “Stopped light and image storage by electromagnetically induced transparency up to the regime of one minute,” Phys. Rev. Lett. 111, 033601 (2013).
    [Crossref]
  13. F. Chen and J. Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photon. Rev. 8, 251–275 (2014).
    [Crossref]
  14. Y. Tan, A. Rodenas, F. Chen, R. R. Thomson, A. K. Kar, D. Jaque, and Q. Lu, “70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser,” Opt. Express 18, 24994–24999 (2010).
    [Crossref]
  15. T. Calmano, J. Siebenmorgen, F. Reichert, M. Fechner, A.-G. Paschke, N.-O. Hansen, K. Petermann, and G. Huber, “Crystalline Pr:SrAl12O19 waveguide laser in the visible spectral region,” Opt. Lett. 36, 4620–4622 (2011).
    [Crossref]
  16. C. Grivas, C. Corbari, G. Brambilla, and P. G. Lagoudakis, “Tunable, continuous-wave Ti:sapphire channel waveguide lasers written by femtosecond and picosecond laser pulses,” Opt. Lett. 37, 4630–4632 (2012).
    [Crossref]
  17. G. Corrielli, A. Seri, M. Mazzera, R. Osellame, and H. de Riedmatten, “Integrated optical memory based on laser-written waveguides,” Phys. Rev. Appl. 5, 054013 (2016).
    [Crossref]
  18. T. Calmano and S. Müller, “Crystalline waveguide lasers in the visible and near-infrared spectral range,” IEEE J. Sel. Top. Quantum Electron. 21, 401–413 (2015).
    [Crossref]
  19. A. Rodenas and A. K. Kar, “High-contrast step-index waveguides in borate nonlinear laser crystals by 3D laser writing,” Opt. Express 19, 17820–17833 (2011).
    [Crossref]
  20. L. Li, W. Nie, Z. Li, C. Romero, R. I. Rodriguez, J. Aldana, and F. Chen, “Laser-writing of ring-shaped waveguides in BGO crystal for telecommunication band,” Opt. Express 25, 24236–24241 (2017).
    [Crossref]
  21. J. Burghoff, S. Nolte, and A. Tünnermann, “Origins of waveguiding in femtosecond laser-structured LiNbO3,” Appl. Phys. A 89, 127–132 (2007).
    [Crossref]
  22. R. Osellame, M. Lobino, N. Chiodo, M. Marangoni, G. Cerullo, R. Ramponi, H. T. Bookey, R. R. Thomson, N. D. Psaila, and A. K. Kar, “Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient,” Appl. Phys. Lett. 90, 241107 (2007).
    [Crossref]
  23. R. Thomson, S. Campbell, I. Blewett, A. Kar, and D. Reid, “Optical waveguide fabrication in z-cut lithium niobate (LiNnO3) using femtosecond pulses in the low repetition rate regime,” Appl. Phys. Lett. 88, 111109 (2006).
    [Crossref]
  24. L. Huang, P. Salter, M. Karpinski, B. Smith, F. Payne, and M. Booth, “Waveguide fabrication in KDP crystals with femtosecond laser pulses,” Appl. Phys. A 118, 831–836 (2015).
    [Crossref]
  25. A. Rodenas, A. Benayas, J. Macdonald, J. Zhang, D. Tang, D. Jaque, and A. Kar, “Direct laser writing of near-IR step-index buried channel waveguides in rare earth doped YAG,” Opt. Lett. 36, 3395–3397 (2011).
    [Crossref]
  26. J. Macdonald, R. Thomson, S. Beecher, N. Psaila, H. Bookey, and A. Kar, “Ultrafast laser inscription of near-infrared waveguides in polycrystalline ZnSe,” Opt. Lett. 35, 4036–4038 (2010).
    [Crossref]
  27. I. Mansour and F. Caccavale, “An improved procedure to calculate the refractive index profile from the measured near-field intensity,” J. Lightwave Technol. 14, 423–428 (1996).
    [Crossref]
  28. R. G. Hunsperger, Integrated Optics (Springer, 1995), vol. 4.
  29. Supplement 1 contains details about the experimental setup and the characterization measurements of the heralded single photons from the SPDC source.
  30. N. Maring, D. Lago-Rivera, A. Lenhard, G. Heinze, and H. de Riedmatten, “Quantum frequency conversion of storable single photons from 606  nm to the telecom c-band,” arXiv: 1801.03727 (2018).
  31. J. Fekete, D. Rieländer, M. Cristiani, and H. de Riedmatten, “Ultranarrow-band photon-pair source compatible with solid state quantum memories and telecommunication networks,” Phys. Rev. Lett. 110, 220502 (2013).
    [Crossref]
  32. D. Rieländer, A. Lenhard, M. Mazzera, and H. de Riedmatten, “Cavity enhanced telecom heralded single photons for spin-wave solid state quantum memories,” New J. Phys. 18, 123013 (2016).
    [Crossref]
  33. M. Nilsson, L. Rippe, S. Kröll, R. Klieber, and D. Suter, “Hole-burning techniques for isolation and study of individual hyperfine transitions in inhomogeneously broadened solids demonstrated in Pr3+:Y2SiO5,” Phys. Rev. B 70, 214111 (2004).
    [Crossref]
  34. A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: I. echoes excited by two optical pulses,” Eur. J. Phys. 10, 291–297 (1989).
    [Crossref]
  35. A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: II. three pulse echoes and collision effects,” Eur. J. Phys. 12, 234–239 (1991).
    [Crossref]
  36. R. Yano, M. Mitsunaga, and N. Uesugi, “Stimulated-photon-echo spectroscopy. I. Spectral diffusion in Eu3+:YAlO3,” Phys. Rev. B 45, 12752–12759 (1992).
    [Crossref]
  37. Y. Sun, G. M. Wang, R. L. Cone, R. W. Equall, and M. J. M. Leask, “Symmetry considerations regarding light propagation and light polarization for coherent interactions with ions in crystals,” Phys. Rev. B 62, 15443–15451 (2000).
    [Crossref]
  38. P. Sekatski, N. Sangouard, F. Bussières, C. Clausen, N. Gisin, and H. Zbinden, “Detector imperfections in photon-pair source characterization,” J. Phys. B 45, 124016 (2012).
    [Crossref]
  39. S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin, and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New J. Phys. 6, 163 (2004).
    [Crossref]
  40. D. Rieländer, K. Kutluer, P. M. Ledingham, M. Gündoğan, J. Fekete, M. Mazzera, and H. de Riedmatten, “Quantum storage of heralded single photons in a praseodymium-doped crystal,” Phys. Rev. Lett. 112, 040504 (2014).
    [Crossref]
  41. H. Zhang, M. Sabooni, L. Rippe, C. Kim, S. Kröll, L. V. Wang, and P. R. Hemmer, “Using quantum memory techniques for optical detection of ultrasound,” Appl. Phys. Lett. 100, 131102 (2012).
    [Crossref]
  42. S. Beavan, E. A. Goldschmidt, and M. J. Sellars, “Demonstration of a dynamic bandpass frequency filter in a rare-earth ion-doped crystal,” J. Opt. Soc. Am. B 30, 1173–1177 (2013).
    [Crossref]
  43. M. Afzelius, C. Simon, H. de Riedmatten, and N. Gisin, “Multimode quantum memory based on atomic frequency combs,” Phys. Rev. A 79, 052329 (2009).
    [Crossref]
  44. A. Seri, A. Lenhard, D. Rieländer, M. Gündoğan, P. M. Ledingham, M. Mazzera, and H. de Riedmatten, “Quantum correlations between single telecom photons and a multimode on-demand solid-state quantum memory,” Phys. Rev. X 7, 021028 (2017).
    [Crossref]
  45. P. Jobez, N. Timoney, C. Laplane, J. Etesse, A. Ferrier, P. Goldner, N. Gisin, and M. Afzelius, “Towards highly multimode optical quantum memory for quantum repeaters,” Phys. Rev. A 93, 032327 (2016).
    [Crossref]
  46. M. Gündoğan, P. M. Ledingham, K. Kutluer, M. Mazzera, and H. de Riedmatten, “Solid state spin-wave quantum memory for time-bin qubits,” Phys. Rev. Lett. 114, 230501 (2015).
    [Crossref]
  47. P. Jobez, C. Laplane, N. Timoney, N. Gisin, A. Ferrier, P. Goldner, and M. Afzelius, “Coherent spin control at the quantum level in an ensemble-based optical memory,” Phys. Rev. Lett. 114, 230502 (2015).
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  49. S. Atzeni, A. S. Rab, G. Corrielli, E. Polino, M. Valeri, P. Mataloni, N. Spagnolo, A. Crespi, F. Sciarrino, and R. Osellame, “Integrated sources of entangled photons at the telecom wavelength in femtosecond-laser-written circuits,” Optica 5, 311–314 (2018).
    [Crossref]

2018 (2)

F. Raffaelli, G. Ferranti, D. H. Mahler, P. Sibson, J. E. Kennard, A. Santamato, G. Sinclair, D. Bonneau, M. G. Thompson, and J. C. F. Matthews, “A homodyne detector integrated onto a photonic chip for measuring quantum states and generating random numbers,” Quantum Sci. Technol. 3, 025003 (2018).
[Crossref]

S. Atzeni, A. S. Rab, G. Corrielli, E. Polino, M. Valeri, P. Mataloni, N. Spagnolo, A. Crespi, F. Sciarrino, and R. Osellame, “Integrated sources of entangled photons at the telecom wavelength in femtosecond-laser-written circuits,” Optica 5, 311–314 (2018).
[Crossref]

2017 (5)

A. Seri, A. Lenhard, D. Rieländer, M. Gündoğan, P. M. Ledingham, M. Mazzera, and H. de Riedmatten, “Quantum correlations between single telecom photons and a multimode on-demand solid-state quantum memory,” Phys. Rev. X 7, 021028 (2017).
[Crossref]

P. Vergyris, F. Kaiser, E. Gouzien, G. Sauder, T. Lunghi, and S. Tanzilli, “Fully guided-wave photon pair source for quantum applications,” Quantum Sci. Technol. 2, 024007 (2017).
[Crossref]

I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017).
[Crossref]

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
[Crossref]

L. Li, W. Nie, Z. Li, C. Romero, R. I. Rodriguez, J. Aldana, and F. Chen, “Laser-writing of ring-shaped waveguides in BGO crystal for telecommunication band,” Opt. Express 25, 24236–24241 (2017).
[Crossref]

2016 (4)

A. Orieux and E. Diamanti, “Recent advances on integrated quantum communications,” J. Opt. 18, 083002 (2016).
[Crossref]

G. Corrielli, A. Seri, M. Mazzera, R. Osellame, and H. de Riedmatten, “Integrated optical memory based on laser-written waveguides,” Phys. Rev. Appl. 5, 054013 (2016).
[Crossref]

D. Rieländer, A. Lenhard, M. Mazzera, and H. de Riedmatten, “Cavity enhanced telecom heralded single photons for spin-wave solid state quantum memories,” New J. Phys. 18, 123013 (2016).
[Crossref]

P. Jobez, N. Timoney, C. Laplane, J. Etesse, A. Ferrier, P. Goldner, N. Gisin, and M. Afzelius, “Towards highly multimode optical quantum memory for quantum repeaters,” Phys. Rev. A 93, 032327 (2016).
[Crossref]

2015 (5)

M. Gündoğan, P. M. Ledingham, K. Kutluer, M. Mazzera, and H. de Riedmatten, “Solid state spin-wave quantum memory for time-bin qubits,” Phys. Rev. Lett. 114, 230501 (2015).
[Crossref]

P. Jobez, C. Laplane, N. Timoney, N. Gisin, A. Ferrier, P. Goldner, and M. Afzelius, “Coherent spin control at the quantum level in an ensemble-based optical memory,” Phys. Rev. Lett. 114, 230502 (2015).
[Crossref]

T. Calmano and S. Müller, “Crystalline waveguide lasers in the visible and near-infrared spectral range,” IEEE J. Sel. Top. Quantum Electron. 21, 401–413 (2015).
[Crossref]

L. Huang, P. Salter, M. Karpinski, B. Smith, F. Payne, and M. Booth, “Waveguide fabrication in KDP crystals with femtosecond laser pulses,” Appl. Phys. A 118, 831–836 (2015).
[Crossref]

S. Marzban, J. G. Bartholomew, S. Madden, K. Vu, and M. J. Sellars, “Observation of photon echoes from evanescently coupled rare-earth ions in a planar waveguide,” Phys. Rev. Lett. 115, 013601 (2015).
[Crossref]

2014 (2)

F. Chen and J. Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photon. Rev. 8, 251–275 (2014).
[Crossref]

D. Rieländer, K. Kutluer, P. M. Ledingham, M. Gündoğan, J. Fekete, M. Mazzera, and H. de Riedmatten, “Quantum storage of heralded single photons in a praseodymium-doped crystal,” Phys. Rev. Lett. 112, 040504 (2014).
[Crossref]

2013 (4)

J. Fekete, D. Rieländer, M. Cristiani, and H. de Riedmatten, “Ultranarrow-band photon-pair source compatible with solid state quantum memories and telecommunication networks,” Phys. Rev. Lett. 110, 220502 (2013).
[Crossref]

G. Heinze, C. Hubrich, and T. Halfmann, “Stopped light and image storage by electromagnetically induced transparency up to the regime of one minute,” Phys. Rev. Lett. 111, 033601 (2013).
[Crossref]

F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten, C. Simon, and W. Tittel, “Prospective applications of optical quantum memories,” J. Mod. Opt. 60, 1519–1537 (2013).
[Crossref]

S. Beavan, E. A. Goldschmidt, and M. J. Sellars, “Demonstration of a dynamic bandpass frequency filter in a rare-earth ion-doped crystal,” J. Opt. Soc. Am. B 30, 1173–1177 (2013).
[Crossref]

2012 (3)

C. Grivas, C. Corbari, G. Brambilla, and P. G. Lagoudakis, “Tunable, continuous-wave Ti:sapphire channel waveguide lasers written by femtosecond and picosecond laser pulses,” Opt. Lett. 37, 4630–4632 (2012).
[Crossref]

H. Zhang, M. Sabooni, L. Rippe, C. Kim, S. Kröll, L. V. Wang, and P. R. Hemmer, “Using quantum memory techniques for optical detection of ultrasound,” Appl. Phys. Lett. 100, 131102 (2012).
[Crossref]

P. Sekatski, N. Sangouard, F. Bussières, C. Clausen, N. Gisin, and H. Zbinden, “Detector imperfections in photon-pair source characterization,” J. Phys. B 45, 124016 (2012).
[Crossref]

2011 (4)

2010 (3)

2009 (1)

M. Afzelius, C. Simon, H. de Riedmatten, and N. Gisin, “Multimode quantum memory based on atomic frequency combs,” Phys. Rev. A 79, 052329 (2009).
[Crossref]

2008 (1)

2007 (2)

J. Burghoff, S. Nolte, and A. Tünnermann, “Origins of waveguiding in femtosecond laser-structured LiNbO3,” Appl. Phys. A 89, 127–132 (2007).
[Crossref]

R. Osellame, M. Lobino, N. Chiodo, M. Marangoni, G. Cerullo, R. Ramponi, H. T. Bookey, R. R. Thomson, N. D. Psaila, and A. K. Kar, “Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient,” Appl. Phys. Lett. 90, 241107 (2007).
[Crossref]

2006 (1)

R. Thomson, S. Campbell, I. Blewett, A. Kar, and D. Reid, “Optical waveguide fabrication in z-cut lithium niobate (LiNnO3) using femtosecond pulses in the low repetition rate regime,” Appl. Phys. Lett. 88, 111109 (2006).
[Crossref]

2004 (2)

M. Nilsson, L. Rippe, S. Kröll, R. Klieber, and D. Suter, “Hole-burning techniques for isolation and study of individual hyperfine transitions in inhomogeneously broadened solids demonstrated in Pr3+:Y2SiO5,” Phys. Rev. B 70, 214111 (2004).
[Crossref]

S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin, and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New J. Phys. 6, 163 (2004).
[Crossref]

2000 (1)

Y. Sun, G. M. Wang, R. L. Cone, R. W. Equall, and M. J. M. Leask, “Symmetry considerations regarding light propagation and light polarization for coherent interactions with ions in crystals,” Phys. Rev. B 62, 15443–15451 (2000).
[Crossref]

1996 (1)

I. Mansour and F. Caccavale, “An improved procedure to calculate the refractive index profile from the measured near-field intensity,” J. Lightwave Technol. 14, 423–428 (1996).
[Crossref]

1992 (1)

R. Yano, M. Mitsunaga, and N. Uesugi, “Stimulated-photon-echo spectroscopy. I. Spectral diffusion in Eu3+:YAlO3,” Phys. Rev. B 45, 12752–12759 (1992).
[Crossref]

1991 (1)

A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: II. three pulse echoes and collision effects,” Eur. J. Phys. 12, 234–239 (1991).
[Crossref]

1989 (1)

A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: I. echoes excited by two optical pulses,” Eur. J. Phys. 10, 291–297 (1989).
[Crossref]

Afzelius, M.

P. Jobez, N. Timoney, C. Laplane, J. Etesse, A. Ferrier, P. Goldner, N. Gisin, and M. Afzelius, “Towards highly multimode optical quantum memory for quantum repeaters,” Phys. Rev. A 93, 032327 (2016).
[Crossref]

P. Jobez, C. Laplane, N. Timoney, N. Gisin, A. Ferrier, P. Goldner, and M. Afzelius, “Coherent spin control at the quantum level in an ensemble-based optical memory,” Phys. Rev. Lett. 114, 230502 (2015).
[Crossref]

F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten, C. Simon, and W. Tittel, “Prospective applications of optical quantum memories,” J. Mod. Opt. 60, 1519–1537 (2013).
[Crossref]

M. Afzelius, C. Simon, H. de Riedmatten, and N. Gisin, “Multimode quantum memory based on atomic frequency combs,” Phys. Rev. A 79, 052329 (2009).
[Crossref]

H. Riedmatten and M. Afzelius, “Quantum light storage in solid state atomic ensembles,” in Engineering the Atom-Photon Interaction: Controlling Fundamental Processes with Photons, Atoms and Solids (Springer, 2015), pp. 241–273.

Aldana, J.

L. Li, W. Nie, Z. Li, C. Romero, R. I. Rodriguez, J. Aldana, and F. Chen, “Laser-writing of ring-shaped waveguides in BGO crystal for telecommunication band,” Opt. Express 25, 24236–24241 (2017).
[Crossref]

F. Chen and J. Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photon. Rev. 8, 251–275 (2014).
[Crossref]

Alibart, O.

S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin, and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New J. Phys. 6, 163 (2004).
[Crossref]

Ams, M.

Askarani, M. F.

M. F. Askarani, M. Grimau Puigbert, T. Lutz, V. B. Verma, M. D. Shaw, S. W. Nam, N. Sinclair, D. Oblak, and W. Tittel, “Storage and retrieval of heralded telecommunication-wavelength photons using a solid-state waveguide quantum memory,” arXiv: 1804.05699 (2018).

Atzeni, S.

Baldi, P.

S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin, and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New J. Phys. 6, 163 (2004).
[Crossref]

Banchi, L.

I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017).
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Bartholomew, J. G.

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
[Crossref]

S. Marzban, J. G. Bartholomew, S. Madden, K. Vu, and M. J. Sellars, “Observation of photon echoes from evanescently coupled rare-earth ions in a planar waveguide,” Phys. Rev. Lett. 115, 013601 (2015).
[Crossref]

Beavan, S.

Beecher, S.

Benayas, A.

Bentivegna, M.

I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017).
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Bettinelli, M.

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
[Crossref]

Beveratos, A.

S. Fasel, O. Alibart, S. Tanzilli, P. Baldi, A. Beveratos, N. Gisin, and H. Zbinden, “High-quality asynchronous heralded single-photon source at telecom wavelength,” New J. Phys. 6, 163 (2004).
[Crossref]

Beyer, A. D.

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
[Crossref]

Blewett, I.

R. Thomson, S. Campbell, I. Blewett, A. Kar, and D. Reid, “Optical waveguide fabrication in z-cut lithium niobate (LiNnO3) using femtosecond pulses in the low repetition rate regime,” Appl. Phys. Lett. 88, 111109 (2006).
[Crossref]

Bonneau, D.

F. Raffaelli, G. Ferranti, D. H. Mahler, P. Sibson, J. E. Kennard, A. Santamato, G. Sinclair, D. Bonneau, M. G. Thompson, and J. C. F. Matthews, “A homodyne detector integrated onto a photonic chip for measuring quantum states and generating random numbers,” Quantum Sci. Technol. 3, 025003 (2018).
[Crossref]

Bookey, H.

Bookey, H. T.

R. Osellame, M. Lobino, N. Chiodo, M. Marangoni, G. Cerullo, R. Ramponi, H. T. Bookey, R. R. Thomson, N. D. Psaila, and A. K. Kar, “Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient,” Appl. Phys. Lett. 90, 241107 (2007).
[Crossref]

Booth, M.

L. Huang, P. Salter, M. Karpinski, B. Smith, F. Payne, and M. Booth, “Waveguide fabrication in KDP crystals with femtosecond laser pulses,” Appl. Phys. A 118, 831–836 (2015).
[Crossref]

Bose, S.

I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017).
[Crossref]

Brambilla, G.

Burghoff, J.

J. Burghoff, S. Nolte, and A. Tünnermann, “Origins of waveguiding in femtosecond laser-structured LiNbO3,” Appl. Phys. A 89, 127–132 (2007).
[Crossref]

Bussieres, F.

E. Saglamyurek, N. Sinclair, J. Jin, J. A. Slater, D. Oblak, F. Bussieres, M. George, R. Ricken, W. Sohler, and W. Tittel, “Broadband waveguide quantum memory for entangled photons,” Nature 469, 512–515 (2011).
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Bussières, F.

F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten, C. Simon, and W. Tittel, “Prospective applications of optical quantum memories,” J. Mod. Opt. 60, 1519–1537 (2013).
[Crossref]

P. Sekatski, N. Sangouard, F. Bussières, C. Clausen, N. Gisin, and H. Zbinden, “Detector imperfections in photon-pair source characterization,” J. Phys. B 45, 124016 (2012).
[Crossref]

Caccavale, F.

I. Mansour and F. Caccavale, “An improved procedure to calculate the refractive index profile from the measured near-field intensity,” J. Lightwave Technol. 14, 423–428 (1996).
[Crossref]

Calmano, T.

T. Calmano and S. Müller, “Crystalline waveguide lasers in the visible and near-infrared spectral range,” IEEE J. Sel. Top. Quantum Electron. 21, 401–413 (2015).
[Crossref]

T. Calmano, J. Siebenmorgen, F. Reichert, M. Fechner, A.-G. Paschke, N.-O. Hansen, K. Petermann, and G. Huber, “Crystalline Pr:SrAl12O19 waveguide laser in the visible spectral region,” Opt. Lett. 36, 4620–4622 (2011).
[Crossref]

Campbell, S.

R. Thomson, S. Campbell, I. Blewett, A. Kar, and D. Reid, “Optical waveguide fabrication in z-cut lithium niobate (LiNnO3) using femtosecond pulses in the low repetition rate regime,” Appl. Phys. Lett. 88, 111109 (2006).
[Crossref]

Caprara, D.

I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017).
[Crossref]

Cavalli, E.

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
[Crossref]

Cerullo, G.

R. Osellame, M. Lobino, N. Chiodo, M. Marangoni, G. Cerullo, R. Ramponi, H. T. Bookey, R. R. Thomson, N. D. Psaila, and A. K. Kar, “Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient,” Appl. Phys. Lett. 90, 241107 (2007).
[Crossref]

Chen, F.

Chiodo, N.

R. Osellame, M. Lobino, N. Chiodo, M. Marangoni, G. Cerullo, R. Ramponi, H. T. Bookey, R. R. Thomson, N. D. Psaila, and A. K. Kar, “Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient,” Appl. Phys. Lett. 90, 241107 (2007).
[Crossref]

Clark, P. M.

A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: II. three pulse echoes and collision effects,” Eur. J. Phys. 12, 234–239 (1991).
[Crossref]

A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: I. echoes excited by two optical pulses,” Eur. J. Phys. 10, 291–297 (1989).
[Crossref]

Clausen, C.

P. Sekatski, N. Sangouard, F. Bussières, C. Clausen, N. Gisin, and H. Zbinden, “Detector imperfections in photon-pair source characterization,” J. Phys. B 45, 124016 (2012).
[Crossref]

Cone, R. L.

Y. Sun, G. M. Wang, R. L. Cone, R. W. Equall, and M. J. M. Leask, “Symmetry considerations regarding light propagation and light polarization for coherent interactions with ions in crystals,” Phys. Rev. B 62, 15443–15451 (2000).
[Crossref]

Corbari, C.

Corrielli, G.

Craiciu, I.

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
[Crossref]

Crespi, A.

S. Atzeni, A. S. Rab, G. Corrielli, E. Polino, M. Valeri, P. Mataloni, N. Spagnolo, A. Crespi, F. Sciarrino, and R. Osellame, “Integrated sources of entangled photons at the telecom wavelength in femtosecond-laser-written circuits,” Optica 5, 311–314 (2018).
[Crossref]

I. Pitsios, L. Banchi, A. S. Rab, M. Bentivegna, D. Caprara, A. Crespi, N. Spagnolo, S. Bose, P. Mataloni, R. Osellame, and F. Sciarrino, “Photonic simulation of entanglement growth and engineering after a spin chain quench,” Nat. Commun. 8, 1569 (2017).
[Crossref]

Cristiani, M.

J. Fekete, D. Rieländer, M. Cristiani, and H. de Riedmatten, “Ultranarrow-band photon-pair source compatible with solid state quantum memories and telecommunication networks,” Phys. Rev. Lett. 110, 220502 (2013).
[Crossref]

de Riedmatten, H.

A. Seri, A. Lenhard, D. Rieländer, M. Gündoğan, P. M. Ledingham, M. Mazzera, and H. de Riedmatten, “Quantum correlations between single telecom photons and a multimode on-demand solid-state quantum memory,” Phys. Rev. X 7, 021028 (2017).
[Crossref]

D. Rieländer, A. Lenhard, M. Mazzera, and H. de Riedmatten, “Cavity enhanced telecom heralded single photons for spin-wave solid state quantum memories,” New J. Phys. 18, 123013 (2016).
[Crossref]

G. Corrielli, A. Seri, M. Mazzera, R. Osellame, and H. de Riedmatten, “Integrated optical memory based on laser-written waveguides,” Phys. Rev. Appl. 5, 054013 (2016).
[Crossref]

M. Gündoğan, P. M. Ledingham, K. Kutluer, M. Mazzera, and H. de Riedmatten, “Solid state spin-wave quantum memory for time-bin qubits,” Phys. Rev. Lett. 114, 230501 (2015).
[Crossref]

D. Rieländer, K. Kutluer, P. M. Ledingham, M. Gündoğan, J. Fekete, M. Mazzera, and H. de Riedmatten, “Quantum storage of heralded single photons in a praseodymium-doped crystal,” Phys. Rev. Lett. 112, 040504 (2014).
[Crossref]

J. Fekete, D. Rieländer, M. Cristiani, and H. de Riedmatten, “Ultranarrow-band photon-pair source compatible with solid state quantum memories and telecommunication networks,” Phys. Rev. Lett. 110, 220502 (2013).
[Crossref]

F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten, C. Simon, and W. Tittel, “Prospective applications of optical quantum memories,” J. Mod. Opt. 60, 1519–1537 (2013).
[Crossref]

M. Afzelius, C. Simon, H. de Riedmatten, and N. Gisin, “Multimode quantum memory based on atomic frequency combs,” Phys. Rev. A 79, 052329 (2009).
[Crossref]

N. Maring, D. Lago-Rivera, A. Lenhard, G. Heinze, and H. de Riedmatten, “Quantum frequency conversion of storable single photons from 606  nm to the telecom c-band,” arXiv: 1801.03727 (2018).

Dekker, P.

Diamanti, E.

A. Orieux and E. Diamanti, “Recent advances on integrated quantum communications,” J. Opt. 18, 083002 (2016).
[Crossref]

Durrant, A. V.

A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: II. three pulse echoes and collision effects,” Eur. J. Phys. 12, 234–239 (1991).
[Crossref]

A. V. Durrant, J. Manners, and P. M. Clark, “Understanding optical echoes using Schrodinger’s equation: I. echoes excited by two optical pulses,” Eur. J. Phys. 10, 291–297 (1989).
[Crossref]

Equall, R. W.

Y. Sun, G. M. Wang, R. L. Cone, R. W. Equall, and M. J. M. Leask, “Symmetry considerations regarding light propagation and light polarization for coherent interactions with ions in crystals,” Phys. Rev. B 62, 15443–15451 (2000).
[Crossref]

Etesse, J.

P. Jobez, N. Timoney, C. Laplane, J. Etesse, A. Ferrier, P. Goldner, N. Gisin, and M. Afzelius, “Towards highly multimode optical quantum memory for quantum repeaters,” Phys. Rev. A 93, 032327 (2016).
[Crossref]

Faraon, A.

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
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S. Marzban, J. G. Bartholomew, S. Madden, K. Vu, and M. J. Sellars, “Observation of photon echoes from evanescently coupled rare-earth ions in a planar waveguide,” Phys. Rev. Lett. 115, 013601 (2015).
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M. Gündoğan, P. M. Ledingham, K. Kutluer, M. Mazzera, and H. de Riedmatten, “Solid state spin-wave quantum memory for time-bin qubits,” Phys. Rev. Lett. 114, 230501 (2015).
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P. Jobez, C. Laplane, N. Timoney, N. Gisin, A. Ferrier, P. Goldner, and M. Afzelius, “Coherent spin control at the quantum level in an ensemble-based optical memory,” Phys. Rev. Lett. 114, 230502 (2015).
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D. Rieländer, K. Kutluer, P. M. Ledingham, M. Gündoğan, J. Fekete, M. Mazzera, and H. de Riedmatten, “Quantum storage of heralded single photons in a praseodymium-doped crystal,” Phys. Rev. Lett. 112, 040504 (2014).
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J. Fekete, D. Rieländer, M. Cristiani, and H. de Riedmatten, “Ultranarrow-band photon-pair source compatible with solid state quantum memories and telecommunication networks,” Phys. Rev. Lett. 110, 220502 (2013).
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Phys. Rev. X (1)

A. Seri, A. Lenhard, D. Rieländer, M. Gündoğan, P. M. Ledingham, M. Mazzera, and H. de Riedmatten, “Quantum correlations between single telecom photons and a multimode on-demand solid-state quantum memory,” Phys. Rev. X 7, 021028 (2017).
[Crossref]

Quantum Sci. Technol. (2)

F. Raffaelli, G. Ferranti, D. H. Mahler, P. Sibson, J. E. Kennard, A. Santamato, G. Sinclair, D. Bonneau, M. G. Thompson, and J. C. F. Matthews, “A homodyne detector integrated onto a photonic chip for measuring quantum states and generating random numbers,” Quantum Sci. Technol. 3, 025003 (2018).
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P. Vergyris, F. Kaiser, E. Gouzien, G. Sauder, T. Lunghi, and S. Tanzilli, “Fully guided-wave photon pair source for quantum applications,” Quantum Sci. Technol. 2, 024007 (2017).
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Science (1)

T. Zhong, J. M. Kindem, J. G. Bartholomew, J. Rochman, I. Craiciu, E. Miyazono, M. Bettinelli, E. Cavalli, V. Verma, S. W. Nam, F. Marsili, M. D. Shaw, A. D. Beyer, and A. Faraon, “Nanophotonic rare-earth quantum memory with optically controlled retrieval,” Science 357, 1392–1395 (2017).
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Other (5)

M. F. Askarani, M. Grimau Puigbert, T. Lutz, V. B. Verma, M. D. Shaw, S. W. Nam, N. Sinclair, D. Oblak, and W. Tittel, “Storage and retrieval of heralded telecommunication-wavelength photons using a solid-state waveguide quantum memory,” arXiv: 1804.05699 (2018).

H. Riedmatten and M. Afzelius, “Quantum light storage in solid state atomic ensembles,” in Engineering the Atom-Photon Interaction: Controlling Fundamental Processes with Photons, Atoms and Solids (Springer, 2015), pp. 241–273.

R. G. Hunsperger, Integrated Optics (Springer, 1995), vol. 4.

Supplement 1 contains details about the experimental setup and the characterization measurements of the heralded single photons from the SPDC source.

N. Maring, D. Lago-Rivera, A. Lenhard, G. Heinze, and H. de Riedmatten, “Quantum frequency conversion of storable single photons from 606  nm to the telecom c-band,” arXiv: 1801.03727 (2018).

Supplementary Material (1)

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Supplement 1       Supplementary information

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Figures (5)

Fig. 1.
Fig. 1. (a) Optical microscope image of the waveguide transverse cross section (top row) and guided mode intensity profile (bottom row) of all fabricated waveguides. The numbers above the figures specify the separation between the laser tracks in type II waveguides. Scale bar is 10 μm. (b) Insertion losses versus FWHM mode diameter in the horizontal direction for type I (blue circle) and type II (black squares) waveguides. (c) Microscope picture of the longitudinal profile of a type I (top) and a type II waveguide with d=10μm (bottom). Scale bar is 20 μm. (d) Bending losses as a function of the radius of curvature for type I (blue circles) and type II (black squares) waveguides. Dashed lines are best exponential fits of experimental data [28]. Error bars in plots (b) and (d) are smaller than the data markers.
Fig. 2.
Fig. 2. Setup: a pump laser at 426 nm (blue beam) shines a periodically poled lithium niobate (PPLN) crystal in a bow-tie cavity. Photon pairs (gray beam) are generated by spontaneous parametric down-conversion and divided with a dichroic mirror (DM). The idler photon at 1436 nm (purple beam) is sent to a filter cavity (FC). The signal photon at 606 nm (orange beam) passes through an etalon and enters input 2 of a fiber beam-splitter (BS). Two chopper wheels are used to alternate between locking and the measurement period. The signal photon and the preparation light (from input 1 of the BS) emerging from one output of the BS are coupled into the Pr3+:Y2SiO5 waveguide. The hyperfine splitting of the first sublevels of the ground H43 and the excited D21 manifolds of Pr ions in Y2SiO5 is shown in the inset along with the optical absorption spectrum (the data points are experimental values and the solid curve the Gaussian fit). The orange arrows highlight the specific transition chosen for the storage protocol.
Fig. 3.
Fig. 3. (a) Decay in the echo intensity for a two-photon echo pulse measurement, from which we extract the optical transition coherence time T2 in the type I waveguide. The black solid line is the fit to an exponential decay, and the dotted lines indicate the error. The inset shows the temporal sequence used. The time interval between the first and second pulses is called τ2 (solid arrow). (b) Examples of heterodyne detected TPE at different times τ2. The solid orange line is the experimental signal, and the dotted black line the sinusoidal fit. The echo intensity is proportional to the square of the oscillation amplitude. (c) Area of the stimulated photon echo (SPE) in arbitrary units, varying the times τ1 and τ2; (d) temporal trace of the SPE process. The SPE signal is highlighted by the orange area. The time interval between the second and third pulses is called τ1 (dotted arrow); (e) values of the SPE areas at τ2=0 (extracted from the exponential decays of the SPE areas over τ2) plotted as a function of τ1. From the decay of the SPE versus τ1 (for τ2=0), we extract the excited-state lifetime of the ions, T1; (f) homogeneous broadening of the optical transition for increasing τ1. The values are extracted from the decays of the SPE versus τ2 for different τ1 values. The black solid line is the value extracted from the T2 measurement for τ1=0 with the dashed lines indicating the error.
Fig. 4.
Fig. 4. (a) Measured gs,i(2) for different pump powers with just the source (empty light dots) and after the pit (full dark points). The inset represents the time-resolved signal–idler coincidence histogram with just the source: the darker region is the window considered for the calculation of the gs,i(2) (400 ns), and the black dashed line is the temporal fit of the biphoton correlation; (b), (c) coincidences between signal photons split with a fiber BS, respectively, before and after passing through a pit in the waveguide, sorted by the number of heralding photons between pairs of contiguous signal counts.
Fig. 5.
Fig. 5. (a) Time-resolved histogram of the idler–signal coincidences for signal photons passing through the pit (gray histogram) or the AFC for τ=1.5μs (orange histogram). The counts in the AFC are multiplied by three. The darker regions of the peaks show the windows considered for ηAFC and gs,i(2) calculations. The black and brown dashed lines are the temporal decays of the correlations [inset of Fig. 4(a)] renormalized for losses and efficiencies after the pit and the AFC, respectively. The shaded rectangle is the region in which the accidental counts for the AFC echo are measured. The absorption profiles of the pit (orange) and the comb (brown) are plotted in the inset with the spectrum of the single photons (black points and line, see Ref. [29]); (b) internal storage efficiency ηAFC at different storage times τ for single photons (full orange points, error bars account for Poissonian statistics) and classical light (empty black circles); (c) cross-correlation values between idler photons and stored signal photons gAFC,i(2) for different τ. The orange dashed line is the classical upper bound gs,s(2)·gi,i(2)=1.58±0.02 (assuming gs,s(2)=2).

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