We report a field trial of differential phase shift quantum key distribution (QKD) using polarization independent frequency up-conversion detectors. A frequency up-conversion detector is a promising device for achieving a high key generation rate when combined with a high clock rate QKD system. However, its polarization dependence prevents it from being applied to practical QKD systems. In this paper, we employ a modified polarization diversity configuration to eliminate the polarization dependence. Applying this method, we performed a long-term stability test using a 17.6-km installed fiber. We successfully demonstrated stable operation for 6 hours and achieved a sifted key generation rate of 120 kbps and an average quantum bit error rate of 3.14 %. The sifted key generation rate was not the estimated value but the effective value, which means that the sifted key was continuously generated at a rate of 120 kbps for 6 hours.
© 2007 Optical Society of America
Quantum key distribution (QKD) is being studied as a way of providing unconditionally secure communication . Many QKD experiments have been performed both inside the laboratory and in the field, and the feasibility of QKD schemes has been demonstrated . Since optical fiber networks are already widely deployed, the demonstration of a QKD system over installed optical fibers is an important step toward its practical use. To the best of our knowledge, Gisin’s group was the first to perform a QKD field experiment over an installed fiber . Since then, several groups have completed successful field QKD experiments [4, 5, 6, 7]. In these QKD field experiments, InGaAs avalanche photodiodes (APDs) have been used for single photon detection. However, an InGaAs APD usually requires gated mode operation with a gate frequency of <10 MHz owing to a large afterpulse probability. This drawback limited the key generation rate in those experiments. Recently, several fascinating single photon detectors have been developed for high-speed operation in the 1.5 µm telecom band. These include frequency up-conversion detectors [8, 9, 10], sinusoidally gated InGaAs/InP APDs , and superconducting single-photon detectors (SSPD)[12, 13]. Especially, the potential of the frequency up-conversion detector applying to several QKD protocols, such as BB84 with decoy state, has thoroughly investigated in . The feasibility of these detectors for use in QKD systems has been demonstrated in laboratory experiments. Although each detector has certain advantages and disadvantages, they are promising candidates for high performance QKD systems [15, 16, 17, 18, 19].
In this paper, we report a field experiment undertaken with a differential-phase-shift QKD (DPS-QKD) that had frequency up-conversion detectors. The frequency up-conversion detector has a serious drawback in terms of practical system applications in that it is polarization dependent. To overcome this problem, a polarization diversity configuration has been proposed and its feasibility demonstrated . In the present experiment, we employed a polarization diversity configuration modified from the previous scheme to eliminate the polarization dependence. We then used the polarization independent frequency up-conversion detectors to perform a QKD field experiment over a 17.6-km installed fiber. A sifted key generation rate of 120 kbps and a quantum bit error rate (QBER) of 3.14 % was stably obtained for 6 hours.
2. Differential-phase-shift quantum key distribution (DPS-QKD)
First, we briefly explain our QKD scheme. Differential-phase-shift QKD (DPS-QKD) is a new quantum key distribution scheme that was proposed by one of the present authors . Figure 1 shows the setup of the DPS-QKD scheme. Alice randomly phase-modulates a pulse train of weak coherent states by 0, π for each pulse and sends it to Bob with an average photon number of less than one per pulse. Bob measures the phase difference between two sequential pulses using a 1-bit delay Mach-Zehnder interferometer and photon detectors, and records the photon arrival time and which detector clicked. After raw level transmission, Bob tells Alice the time instances at which a photon was counted. From this time information and her modulation data, Alice knows which detector clicked at Bob’s site. Under an agreement that a click by detector 1 denotes “0” and a click by detector 2 denotes “1”, for example, Alice and Bob obtain an identical bit string. The DPS-QKD scheme has certain features including a simple configuration, efficient time domain use, and robustness against photon number splitting attack. In particular, a high repetition frequency is possible through the use of one-way transmission and a pulse train. The longest transmission distance and the highest key generation rate in QKD systems were demonstrated experimentally with this scheme using a fiber spool .
3. Polarization-independent frequency up-conversion detectors
In the present experiment, we used single photon detectors based on frequency up-conversion. Since frequency up-conversion detectors can be operated in a non-gated mode, a high key generation rate will be achieved when they are combined with a high clock rate QKD system. However, the efficiency of a nonlinear process in a periodically poled lithium niobate (PPLN) waveguide utilized for frequency conversion is polarization dependent so the polarization of a single incoming photon must be adjusted for proper operation. This polarization dependence prevents it from being applied to practical QKD systems, because it is difficult to control the polarization of a photon transmitted through installed fiber due to the fluctuations in fiber birefringence. Furthermore, if we leave the detectors polarization dependent, an imbalance of the count rate of the detectors will be appear depending on the condition of the fiber, which induces the imbalance of the number of “0” bit and “1” bit in the sifted key. The reason of this imbalance is that receiver’s side also consists of fiber components which has birefringence. In such a situation, Eve can utilize this imbalance to perform the eavesdropping. QKD system assumes “0” bit and “1” bit randomly appear in the sifted key so that this imbalance must induce a serious security hole. To overcome this problem, we employed a modified polarization diversity configuration. This method is one kind of polarization scrambler.
Figure 2 shows the setup of our polarization-independent frequency-up-conversion detector. A 1.5-µmsignal pulse (photon) was input into a fiber-coupled polarization beam splitter (PBS), which split the polarization of the incoming photon into horizontally and vertically polarized pulses. All the components after the PBS, described below, were connected with polarization maintaining fibers. The horizontally polarized pulse was directly input into a polarization maintaining 50:50 coupler. The vertically polarized pulse was input into a fiber delay line as a horizontally polarized pulse by twisting the axis at the connection between the PBS output fiber and the delay fiber. The pulse passed through the delay line, and was input into the polarization maintaining 50:50 coupler. The delay line was used to avoid interference at the 50:50 coupler. Since the pulse width in our experiment was 66 ps, we chose a delay time of 200 ps. One output of the 50:50 coupler was connected to a wavelength division multiplexer (WDM) coupler, which meant this setup had an intrinsic loss of 3 dB. The excess loss from the PBS to the 50:50 coupler was 2 dB. The 1.5-µmsignal pulse output from the 50:50 coupler was combined with a strong pump light whose wavelength was 980 nm at 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 by using a combination consisting of a short-pass filter, prisms and a spatial filter. The SFG 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 a 1-GHz signal [16, 23]. When the input pomp power was 20 mW, the quantum efficiency and dark count rate were 0.66 % and 2.8 kcps, respectively.
Before performing the QKD experiment, we confirmed the polarization independence of our detector. A 1-GHz pulse stream whose wavelength was 1551 nm was used as a signal. The pulse stream was strongly attenuated to 0.01 photons per pulse. The pulse stream passed through a fiber coupled polarization controller and was input into the polarization independent frequency up-conversion detector. We measured the count rate while rotating the half wave plate of the polarization controller. Figure 3 shows the results. The count rate remained almost stable irrespective of the polarization state of the input light. The slight fluctuation of the count rate was due to the slight difference of the excess loss between the PBS to the 50:50 coupler. Thus, we confirmed the polarization independence of this setup.
4. Experimental setup
Using the above polarization independent frequency up-conversion detectors, we performed a DPS-QKD experiment over installed fibers. In this experiment, we used two round-trip dispersion shifted fibers installed between our research center and an NTT (Nippon Telegraph and Telephone corporation) telephone exchange office. The total length of these fibers was 17.6 km. Figure 4 shows the experimental setup. These experimental instruments were set in the air-conditioned room. At Alice’s site a continuous light from an external cavity semiconductor laser was modulated into a pulse stream with a 1-GHz clock frequency using a LiNbO 3 intensity modulator. The pulse width was 66 ps. Each pulse was randomly phase-modulated by 0, π with a LiNbO 3 phase modulator. We used a pseudo-random bit sequence with a length of 11 k bits as the phase modulation signal. The pulse was attenuated to 0.2 photons per pulse and then transmitted to Bob’s site over the installed fiber. The start pulse, which indicated the head of a pseudo-random bit sequence, was generated by a distributed feedback laser with an electro-absorption modulator (EA-DFB), and transmitted over the other installed fiber. The interval of the start pulse was 11 µsec. The excess losses of these installed fibers were 7.0 and 7.2 dB, respectively. After the transmission, the 1-GHz pulse stream was input into a Mach-Zehnder interferometer based on planar lightwave circuit technology. The path length difference and the excess loss were 20 cm and 2.0 dB, respectively. The extinction ratio was greater than 20 dB and the polarization dependence was negligible . The phase difference between the two paths in the Mach-Zehnder interferometer could be stably adjusted by controlling the temperature of the waveguide chip. In this experiment, no feed-back mechanism that adjusted the the temperature of the Mach-Zehnder interferometer depending on the fluctuation of the center frequency of the laser source was implemented. The output ports of the Mach-Zehnder interferometer were connected to the polarization independent up-conversion detectors. The detected signals were input into a time interval analyzer (TIA) by way of a logic gate to record the photon detection events. The start pulses were received by a photo diode (PD) and converted into an electrical signal, which was used as a reference time in the TIA. A personal computer (PC3) continuously retrieved the detection events from the TIA, and transmitted them to another computer (PC2). Bob’s server and monitor server were installed on PC2. Bob’s server was driven by a detection event packet sent from PC2. From these packets, Bob’s server generated his sifted key and sent the time information to Alice’s server. Bob’s server also sent his key to the monitor server. On the other hand, Alice’s server generated her key from the phase modulation information and the time information received from Bob’s server, and then sent her key to the monitor server. The monitor server received the keys from Alice’s and Bob’s servers, and estimated the key generation and quantum bit error rates.
We used the setup described above to perform a long-term stability test. Figure 5 shows the experimental results, which reveal that stable operation was obtained over 6 hours. We achieved a sifted key generation rate of 120 kbps and an average quantum bit error rate of 3.14 %. Note that the sifted key generation rate is not an estimated value but an actually obtained value including classical communications and other data processing, which means the sifted key was continuously generated at a rate of 120 kbps at Alice’s and Bob’s servers.
In the following, we describe the experimental conditions in more detail. First, we discuss the key generation rate. Throughout the experiment, the total detector count rate was 160 kcps. When Bob’s server generated a sifted key, a 500-ps time window was employed to reduce errors caused by the timing jitter and dark counts, through which some of the detection events were discarded. In addition, the time interval analyzer had a dead time of 0.5 µsec per start pulse. As a result of the effect of the time window and the dead time of the TIA, the sifted key generation rate was 30 % lower than the count rate.
Next, we discuss the origin of the quantum bit error rate (QBER). The dark count rate was 2.8 kbps. Taking the effect of the time window into account, the effective dark count rate was estimated to be 1.4 kbps and the QBER due to the dark count was estimated to be 1.2 %. The QBER as a result of the imperfection of the Mach-Zehnder interferometer was estimated to be 1 % because the extinction ratio was 20 dB. The remainder must be due to system operation errors including the timing jitter of the detectors. The QBER resulting from system error was estimated to be 1 %.
During the long stability test, we observed a slight degradation in the QBER. This must result from the imperfection of the interference. Although we were unable to determine the exact cause, the center frequency of the laser source or the temperature of PLC Mach-Zehnder must have drifted slightly. The implementation of a feedback control for the temperature of the PLC Mach-Zehnder interferometer could reduce this degradation.
Finally, we discuss the secure key generation rate. Although only sifted keys were generated in this experiment, the observed error rate was good enough to distill a secure key through error correction and the privacy amplification process. Based on an analysis of the security against a general individual attack , the secure key generation rate was estimated to be 13.6 kbps. Note that the unconditional security of DPS-QKD has not been proved yet. Several attacks which does not include in a general individual attack model have been proposed[25, 26]. However, these attacks are effective for the long distance transmisson experiment. For the short distance experiment as we described in this paper, the general individulal attack is currently the most tight secuirty model.
We described a field trial of differential phase shift QKD using polarization independent frequency up-conversion detectors. Although a frequency up-conversion detector is advantageous for high-speed operation, its polarization dependence prevents it from being applied to practical systems. We proposed a simple way to avoid this polarization dependence. Using polarization independent frequency up-conversion detectors, we performed a long-term QKD stability test over 17.6 km of installed fiber. We demonstrated stable operation for 6 hours and achieved a sifted key generation rate of 120 kbps and an average quantum bit error rate of 3.14 %. The sifted key generation rate that we describe is not an estimated value but was actually obtained, which means the sifted key was continuously generated at a rate of 120 kbps.
This work was supported in part by the National Institute of Information and Communications Technology (NICT) of Japan.
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