We report an experimental demonstration of the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-μm non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. Combining this approach with ultra-low-loss filters to eliminate the pump light and separate signal and idler photons, we obtained an efficient entangled photon pair source. To detect the photons, we used single-photon detectors based on frequency up-conversion. These detectors operated in a non-gated mode so that we could use a pulse stream of time correlated entangled photon pairs at a high repetition frequency (1 GHz). Using these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber and performed a two-photon interference experiment. We obtained a coincidence fringe of 81.6% visibility without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bell’s inequality. Thus, we successfully distributed time-bin entangled photon pairs over 100 km.
©2007 Optical Society of America
Entangled photon pairs are very important resources for quantum communication systems , such as quantum cryptography [2, 3], quantum teleportation , and quantum repeater  systems. Photons are robust against decoherence, which makes them suitable for long-distance quantum communications. Optical fiber is the most promising photon transmission line. Silica fiber has its minimum loss in the 1.5-μm wavelength band (telecom band), and so entangled photons in this band are clearly advantageous for scalable quantum communication networks operating over optical fiber.
Distributing entangled photon pairs over optical fiber is the first step toward sophisticated quantum communications. Recently, the generation and distribution of entangled photon pairs in the 1.5-μm telecom band has been intensively investigated [6, 7, 8, 9, 10, 11, 13]. To achieve the long-distance transmission of entangled photon pairs, we need an efficient entangled photon pair source and efficient single photon detectors. In addition, a stable experimental setup is indispensable for distributing and measuring entangled photon pairs.
As regards the long-distance transmission of entangled photon pairs over optical fiber, Marcikic et al. were the first to demonstrate the distribution of time-bin entangled photon pairs over 50 km of optical fiber . They generated time-bin entangled photon pairs at wavelengths of 1.3 and 1.55 μm by using the spontaneous parametric down conversion process in LBO (lithium triborate), and detected 1.3-μm photons with a passively quenched germanium avalanche photodiode (APD) cooled by liquid nitrogen. In 2006, Takesue successfully transmitted time-bin entangled photon pairs over 60 km of optical fiber [10, 11]. He developed an efficient entangled photon pair source based on spontaneous four wave mixing in a dispersion shifted fiber, which was cooled with liquid nitrogen to suppress noise photons. He also used planar lightwave circuit (PLC) Mach-Zehnder interferometers to achieve good stability. In 2006, Liang et al. succeeded in transmitting polarization entangled photon pairs over 100 km of optical fiber . They successfully observed two-photon interference with more than 90 % visibility. However, in their experiments, quantum correlation could only be observed after subtracting background counts. In 2007, Hubel et al. succeeded in transmitting polarization entangled photon pairs over 100 km of optical fiber . They generated polarization entangled photon pairs at wavelengths of 0.8 and 1.55 μm, and detected 0.8 and 1.55-μm photons with a silicon avalanche photo diode (Si-APD) and an InGaAs APD, respectively. To enable the 1.55-μm correlated photons to be detected efficiently, the InGaAs APD was triggered by an electrical signal from the Si-APD.
In this paper, we demonstrate the distribution of entangled photon pairs both in the 1.5-μm telecom band over 100 km of optical fiber, and without subtracting any background noise, such as accidental coincidence or dark count. In order to stably distribute entangled photon pairs over long distance optical fiber, time-bin coding scheme was used. However, in this experiment, we observed only the interference part (energy-basis), so that we did not use the double pulses but sequential pulse stream . Non-degenerated time-bin photon pairs were generated by the spontaneous parametric down conversion process in a periodically poled lithium niobate (PPLN) waveguide [15, 16, 17]. We developed an efficient entangled photon source by using ultra-low-loss filters to eliminate the pump light and separate the signal and idler photons. To detect the correlated photon pairs efficiently, we used single photon detectors based on frequency up-conversion. In these detectors, 1.5-μm photons were converted into short-wavelength photons, and then detected by a non-gated Si-APD. The quantum efficiency of our frequency up-conversion detectors were not so high. However, due to a non-gated operation of Si-APD, it was possible to distribute entangled photon pairs with a high repetition frequency (1 GHz). In this sense frequency up-conversion detectors are more efficient than conventional detectors based on InGaAs APDs, which requires gated operation.
2. Frequency up-conversion detectors
To detect photons, we used single photon detectors based on frequency up-conversion [18, 19, 20]. Frequency up-conversion detectors can be operated in a non-gated mode to allow us to increase the signal repetition frequency to a limit below which the detector can discriminate adjacent pulses. In experiments that involve distributing entangled photon pairs, coincidence events are very rare because of the large transmission loss. Therefore, a high signal repetition rate is very advantageous in terms of shortening the measurement time. As described in Section 3, we used a 1-GHz pulse stream of time correlated entangled photon pairs.
Figure 1 shows the setup we used for our frequency up-conversion detector. We used a pump light whose wavelength was 980 nm, which was identical to . A 1.5-μm photon was combined with a strong pump light whose wavelength was 980 nm by a wavelength division multiplexer (WDM) coupler, and injected into a PPLN waveguide. In the PPLN waveguide, a 600 nm photon was generated via the sum frequency generation (SFG) process. The converted signal, pump, and spurious light after the PPLN waveguide were separated with a combination consisting of a short-pass filter, prisms and a spatial filter. In order to sufficiently supress the pump, two prisms were used. The converted photon was detected with a single photon counting module (SPCM) based on a Si-APD (MPD). The jitter of this Si-APD was low enough to discriminate 1-GHz signal pulses [21, 22].
Before describing the performance of our detectors, we should mention a peculiar feature as regards detecting entangled photons with frequency up-conversion detectors. While an entangled photon pair generated in a bulk medium generally has a broad spectrum, the phase-matching bandwidth of the PPLN waveguide is very narrow. In our device, the full width at half maximum of the phase-matching bandwidth was approximately 25 GHz. Therefore, the PPLN waveguide effectively worked as a narrow band-pass filter. When the signal photon wavelength did not perfectly satisfy the phase-matching condition, the conversion efficiency was low, resulting in a low detection efficiency. Figure 2 shows quantum efficiency as a function of signal wavelength. When the coupled pump power was 30 mW, the overall quantum efficiency was around 3.3% for a photon whose wavelength was 1547 nm. When we chose a photon bandwidth of 100 GHz (0.8 nm), the average quantum efficiency became 1.4% as a result of phase mismatching. That is, frequency up-conversion detectors can detect photons whose wavelength are satisfied the phase-matching condition, so that frequency up-conversion detectors are very selective. The dark count rate in free running mode was 4 kcps.
3. Experimental setup
Figure 3 shows our experimental setup for entangled-photon distribution. A continuous lightwave emitted at a wavelength of 1551 nm from an external-cavity semiconductor laser was converted into a pulse stream by a LiNbO 3 intensity modulator. The pulse width and repetition frequency were 100 ps and 1 GHz, respectively. The pulse stream was amplified by an erbium-doped fiber amplifier (EDFA) and filtered by a fiber Bragg grating (FBG) filter to suppress the amplified spontaneous emission noise from the EDFA. After passing through a polarization controller, the pulse stream was launched into PPLN(1), where a 775.5-nm pulse stream was generated by the second harmonic generation process. The output light from PPLN(1) was input into filters that transmitted the 775.5-nm pulse stream while eliminating the remaining 1551-nm light. The 775.5-nm pulse stream was polarization-controlled and then input into PPLN(2). A series of time-correlated entangled photon pairs was generated in PPLN(2) by the parametric down conversion process. The pump, signal and idler frequencies were fp, fs and fi, respectively, with a relationship of fp = fs + fi. The output from PPLN(2) was input into a filter that transmitted the 1551-nm pulse stream while eliminating the remaining 775.5-nm light. The excess loss of this filter was 0.05 dB for 1.5-μm photons. The photons were input into a dielectric band-pass filter that separated the 1547-nm signal light and the 1555-nm idler light. The spectral width of this filter was 100 GHz, and its excess loss was 0.35 dB for 1547 nm and 0.53 dB for 1555 nm. After this separation, the signal and idler were transmitted over a 50-km dispersion shifted fiber (DSF) with a loss of 0.21 dB/km. They were then launched into 1-bit delayed PLC Mach-Zehnder interferometers whose path length difference was 20 cm. The excess loss of the interferometer was about 2.0 dB. The phase difference between the two paths was precisely and stably adjusted by controlling the interferometer temperature. Photon detectors based on frequency up-conversion and described in Section 2 were installed at one output port of the Mach-Zehnder interferometer. The output signals of the photon detectors were input into a time interval analyzer (TIA) to measure the coincidence.
4. Coincidence measurement
Before undertaking the two-photon interference experiment, we performed a coincidence measurement. The purpose of the measurement was to estimate the average number of photon pairs per pulse generated by our entanglement source . The setup was the same as that shown in Fig. 3 except for the 50-km DSFs and PLC Mach-Zehnder interferometers. We measured the coincidence rate at matched and unmatched time slots, denoted as Rm and Rum, respectively, as described below. In our experiment, the pump pulse duration was longer than the coherence time of the down-converted photons so that the probability of n pairs in a given pulse was described according to a Poisson distribution [23, 24]. Assuming that only correlated photon pairs (no noise photons) are generated, the average count rates per time slot for the signal and idler channels are expressed as
where μc,αx and ds are average numbers 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), respectively. Using these expressions, we express the ratio of true coincidence to accidental coincidence C as
We can calculate μc from experimentally obtained C,αs and αi. In our experimental setup, the signal and idler arm losses were 3.4 and 4.0 dB, respectively. We performed coincidence measurements for several pump powers, and estimated μc for each pump power. Figure 4 shows the results. Based on these results, we set μc = 0.015 for a 0-km transmission and μc = 0.07 for a 100-km transmission in the following two-photon interference experiment.
5. Two-photon interference experiments
Finally, we performed two-photon interference experiments. First, we undertook an experiment without 50-km fiber spools to confirm the generation of entangled photon pairs. In this experiment, the coincidence was measured with the temperature of the PLC Mach-Zehnder interferometer fixed for the signal and changed for the idler. Changing the temperature corresponded to changing the phase difference in the interferometer. Figure 5 shows the results, where the closed circles indicate the coincidence rate per signal photon and the crosses indicate the idler photon count rate. Throughout the measurement, the count rates of the signal and idler detectors were 31 and 28 kcps, respectively. With the count rates unchanged, we observed a deep modulation in the coincidence rate, as when changing the temperature. A coincidence fringe of 93.1% visibility was obtained without subtracting any background noise, such as accidental coincidence or dark count. We also estimated theoretically the visibility in our experimental setup. When we assume that the Mach-Zehnder interferometer splits a photon contained in two sequential pulses into three time slots and only the central time slot is used, which means that the Mach-Zehnder interferometer has an intrinsic 3 dB loss, the visibility is given by 
Substituting the estimated average number of photon pairs per pulse of 0.015 and the loss of each arm, we theoretically estimated the visibility to be 96.1 %, which is close to the experimental value. We then inserted a 50-km DSF spool between the entanglement source and the interferometer in both the signal and idler arms, and undertook a two-photon interference experiment. The total loss of 50-km DSF spool, including connector and splices losses, was 10.5 dB. Therefore, the total loss between PPLN(2) and the photon detector was 19.0 and 19.5 dB. The results are shown Fig. 6. μc is around 0.07. The closed circles indicate the coincidence rate per signal photon detection and the crosses indicate the idler photon count rate. Throughout the measurement, the count rates of the signal and idler detectors were 16 and 14 kcps, respectively. A coincidence fringe of 81.6% visibility was obtained. Again, no background noise, for example accidental coincidence or dark count, was subtracted. This is good enough to violate Bell’s inequality. The theoretically estimated visibility was 80.4%, which is very close to the experimental value. Thus we successfully distributed time-bin entangled photon pairs over 100 km.
We demonstrated the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-μm non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. By combining this approach with ultra-low-loss filters to suppress the pump light and separate the signal and idler, we obtained an efficient entangled photon pair source. To detect these photons, we used single-photon detectors based on frequency up-conversion detectors. These detectors operated in a non-gated mode so that we could increase the repetition frequency of time-bin entangled photon pairs, which shortened the measurement time. With these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber, and performed a two-photon interference experiment. A coincidence fringe of 81.6% visibility was obtained without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bell’s inequality. This result confirmed the distribution of time-bin entangled photon pairs over 100 km.
This work was supported in part by the National Institute of Information and Communications Technology (NICT) of Japan.
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