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We report the generation of 1.5-μm-band energy-time entangled photon pairs using a periodically poled lithium niobate (PPLN) waveguide. We performed a two-photon interference experiment and obtained coincidence fringes with 77.3% visibilities without subtracting accidental coincidences.
©2007 Optical Society of America
1. Introduction
The generation of entangled photon pairs is a very important technology for quantum communication systems, such as quantum cryptography, quantum teleportation, and quantum repeaters. Furthermore, entangled photon pairs in the 1.5-μm telecom band, where silica fiber has its minimum loss, are clearly advantageous for scalable quantum communication networks operating over optical fiber. Of the several types of entanglement, polarization entanglement is well known and widely studied. However, it is unsuitable for transmission use due to the polarization mode dispersion that occurs in optical fiber, which limits the transmission distance. On the other hand, energy-time or time-bin entanglement is robust as regards polarization mode dispersion, giving this type of entanglement the potential for transmission use over long distances. Although several experiments have already been successfully performed [1, 2, 3], only one group has reported the generation and direct observation of a photon pair where both photons are in the 1.5-μm band [3, 4, 5]. They successfully observed time-bin entanglement using spontaneous four-wave mixing in dispersion-shifted fiber. However, their method was adversely affected by noise photons caused by the spontaneous Raman scattering process[3, 4, 5]. In this paper, we report the generation of 1.5-μm-band energy-time entanglement using a periodically poled lithium niobate (PPLN) waveguide, and a two-photon interference experiment using planar lightwave circuit (PLC) Mach-Zehnder interferometers. This method is unaffected by the noise photons induced by the Raman scattering process and so is expected to provide an entanglement photon source that is free from noise photons. This paper is organized as follows. Section 2 briefly explains energy-time entanglement. Section 3 describes the experimental setup. Section 4 reports the results of a coincidence measurement experiment and a two-photon interference experiment. Section 5 concludes this paper.
2. Energy-time entanglement
First, we briefly describe energy-time entanglement to provide some background to our experiment. Let us assume a situation where two photon pairs are produced simultaneously but at an uncertain time from a pump photon, and the well-determined pump photon energy is shared between these two photons. Here, the energy and the time of creation of each particle are uncertain, but the sum of their energies and the difference between their emission times, which is nearly zero, are well-determined. This type of entanglement is known as energy-time entanglement. To observe energy-time entanglement, Franson proposed the non-local interference experimental setup shown in Fig. 1 [6]. Each photon is transmitted through an unbalanced Mach-Zehnder interferometer, both with the same length difference between the long and the short arm, ∆L. There will be no single photon interference when ∆L is longer than the coherence length of each single photon. Alternatively, when both photons pass through the long arm (LL) or the short arm (SS), they are indistinguishable and exhibit interference fringes. Whereas when one photon passes through the long arm and the other through the short arm (SL, LS), they can be distinguished because one photon clearly arrives before the other at the detector. Thus, when both photons are detected simultaneously, an interference pattern is observed that depends on the phase difference between the two arms.
3. Experimental setup
Figure 2 shows the experimental setup. A continuous wave light from a laser diode with a wavelength of 780 nm was used as a pump light. The coherence length of the pump light is ∼1 μsec, which is much longer than the path length difference of our Mach-Zehnder interferometer. This light was passed through a 780-nm band pass filter to suppress residual spectral components. The light was polarization controlled and then launched into a PPLN waveguide. Frequency degenerated photon pairs were generated in the PPLN at around 1560 nm by the spontaneous parametric down conversion process. The excess and out-coupling loss of the PPLN waveguide was estimated to be 2.0 dB. After passing through the PPLN waveguide, the output light was filtered to suppress the 780-nm pump light. The output light was also filtered with a 1560-nm band pass filter whose spectral width was 1 nm to reduce the influence of the dispersion in the planar lightwave circuit (PLC) interferometer used in our measurement. Note that the spectral width of each photon was sufficiently broad for there to be no single photon interference. The excess losses of these filters were 2.5 and 0.5 dB, respectively. The entangled photon pairs were launched into a 3-dB fiber coupler, which separated the signal and idler with 50% probability. They were then launched into a PLC Mach-Zehnder interferometer whose path length difference was 20 cm [7]. The excess loss of the interferometer was about 2.0 dB. The phase difference between the two paths was precisely adjusted by controlling the temperature of the interferometer, and stable operation was possible. Photon detectors based on an InGaAs APD operated in a gated mode with a 5-MHz frequency were installed at one output of the Mach-Zehnder interferometer. The gate width of the detector was about 1 nsec. The quantum efficiency of the detectors for the signal and idler were 5.0 % and 5.2 %, respectively. The dark count rate per gate was about 3.5×10-5. The output signals of the photon detectors were input into a time interval analyzer (TIA) to measure the coincidence.
4. Results
4.1. Coincidence measurement
Before undertaking the two-photon interference experiment, we performed a coincidence measurement experiment. The purpose of this experiment was to estimate the average number of photon pairs per time slot. The experimental setup is the same as that shown in Fig. 2 except for the PLC Mach-Zehnder interferometers. We measured the coincidence rate at matched and unmatched time slots, which we refer to as Rm and Rum, respectively. We used a continuous wave pump in our experimental setup so that all the time slots except for the matched time slot corresponded to unmatched time slots. However, we operated our photon detectors in a gated mode so that we could easily define a discrete time slot [3, 4, 5]. That is, we defined a time slot in terms of the time during which the photon detectors were active. In addition we also employed a time window for the TIA output data in order to define a sharper time slot. In our experiment, we used a time window of 0.5 nsec. Assuming only correlated photon pairs (no noise photons), we expressed the average count rates per time slot for the signal and idler channels as
where μc,αx and ds are the average number of correlated photon pairs per time slot, the transmittance for channel x, and the dark count rate for channel x, with x = s (signal) or i (idler). Using these expressions, the ratio of true coincidence to accidental coincidence C is expressed as
From experimentally obtained C,αs and αi, we can calculate μc. In our experimental setup, the signal and idler arm losses were 8.0 dB. We performed measurements at several pump powers, and estimated μc for each pump power. Figure 3 shows the results. In the following two-photon interference experiment, we used μc = 0.12.
4.2. Two-photon interference experiment
Finally, we performed a two-photon interference experiment to confirm the generation of energy-time entangled photon pairs. For this experiment, we fixed the temperature of the PLC interferometer for the signal, changed that for the idler, and measured the coincidence. Figure 4 shows the experimental results. The circles indicate the experimentally obtained coincidence rate per signal photon detected for each temperature. The count rate of each detector was ∼1700 cps throughout the measurement. Although the count rates remained unchanged, we observed a deep modulation of the coincidence rate as we changed the temperature. The obtained coincidence fringes had 77.3% visibilities without subtracting the accidental coincidences. We also estimated the visibilities of our experimental setup theoretically. We took account of the fact that the Mach-Zehnder interferometer splits a photon into three time slots and only the central time slot is used, which means that the Mach-Zehnder interferometer has an intrinsic loss of 3 dB , and so the visibility is given by [3, 4, 5].
Substituting the estimated average number of photon pairs per time slot of 0.12 and the loss of each arm, we estimated the theoretical visibility to be 77.4%. The theoretical value was almost in agreement with the experimental results, which means that energy-time entanglement was confirmed, and very few noise photons were generated in our experiment.
5. Summary
In summary, we successfully generated a 1.5-μm band energy-time entangled photon pair using a periodically poled lithium niobate (PPLN) waveguide. We performed a two-photon interference experiment and obtained coincidence fringes with 77.3% visibilities without subtracting accidental coincidence. This experimental result was almost in agreement with the theoretically estimated value, thus confirming that energy-time entangled photon pairs were generated with very few noise photons in our experiment.
This work was supported in part by the National Institute of Information and Communication Technology (NICT) of Japan.
References and links
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