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Time-bin multiplexed differential phase shift quantum key distribution (Conference Presentation)

Anil Prabhakar, Gautam Kumar Shaw, Shyam Sridharan, Foram Shingala, Prabha Mandyam, Shashank Kumar Ranu

Published Mar 30, 2020• 0 citations
Publisher

Abstract

We present an experimental demonstration of the 4-state differential-phase-shift quantum key distribution (4S-DPS-QKD) over a 30 km quantum channel with two different approaches, namely path superposition and time-bin superposition. 4S-DPS-QKD offers enhanced security compared to the pulse-train DPS-QKD against individual attacks. We show that the key generation efficiency, and security, improve with an increase in the number of path delays or time bin superpositions. Our implementation establishes an ease in implementation of the time-bin superposition approach, over the path superposition approach. The sender (Alice) uses a photon in a superposition state, corresponding to either 3 spatial paths or temporal bins. In the temporal case, Alice uses a weak coherent source (WCS) with a pulse width of 3 ns, that we interpret as a single wave packet comprising of three time-bins of 1 ns each.. Alice encodes her random key bit [0,1] as a random phase [0,pi] between |a> and |b>, and |b> and |c>, i.e. successive paths or time bins of the WCS, with mean photon number < n>=0.1, and pulse widths of 0.5 ns. The phase encoded bits are then transmitted over a single mode optical fibre. Both path and timb-bin implementations follow similar setups beyond the transmitter. At the receiver, a delay line interferometer (DLI) introduces a delay of 1 ns, measures < a|b> and< b|c>, and recovers the phase introduced by Alice as a detection at one of the 2 output ports of the DLI with a time resolution of 50 ps using a time-to-digital convertor (TDC). Our experiments have yielded a QBER of 21% for path superposition and 17% for time-bin superposition. We thus establish the equivalence of the two approaches, and note that the time-bin approach is easily extended to more than 3 time-bins, and an increased secure key rate.

DOI
10.1117/12.2556051

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Key generation efficiency, and security, in differential phase-shift quantum key distribution (DPS-QKD) improve with an increase in the number of optical delays or time-bin superpositions. We demonstrate the implementation of superposition states using time-bins, with two different approaches. In Type-A, we use an optical pulse and create superposition states with optical splitters and path delays. Similar superposition states are created, in Type-B, by applying direct phase modulation within a single weak coherent pulse. We establish the equivalence between both the approaches, and note that higher-order superposition states of Type-B are easier to generate for DPS-QKD. We set up DPS-QKD, over 105 km of single mode optical fiber, with a quantum bit error rate of less than 15% at a secure key rate of 2 kbps. With temporal guard bands, the QBER reduced to less than 10%, but with a 20% reduction in the key rate.
This paper presents novel quantum key distribution (QKD) schemes that use differential phases of sequential pulses as an information carrier. Alice sends a photon in a linear superposition state of three temporal slots, in which each amplitude is phase-modulated by {0,π}. Bob measures the phase difference of an asymmetric interferometer, and tells Alice time-instance at which the photon was counted. From this time information and her modulation data, Alice knows which detector counted the photon in Bob s site. Then, the two parties create a secret key. The scheme is suitable for fiber transmission systems and offers key creation efficiency higher than conventional phase-encoding QKD. The above scheme utilizes fully non-orthogonal four states. Differential phase shift QKD utilizing two non-orthogonal states is also presented. Alice sends a coherent pulse train with less than one photon per pulse, which is phase modulated by {0,π} for each pulse, and Bob measures the pulse train by a one-bit delay circuit. The system has a simple configuration such that Alice has no interferometer and sends no intense reference pulse, unlike to conventional QKD scheme using two non-orthogonal states, and also has an advantage of high key creation efficiency.
A novel type of quantum key distribution (QKD) protocol, called DPS (differential-phase-shift) QKD, was proposed several years ago. A sender transmits a highly-attenuated coherent pulse train with {0, π} phase, and a receiver receives it with a one-bit delay Mach-Zehnder interferometer followed by photon detectors. A secret key is created from photon detection events, whose security is based on the fact that an eavesdropper cannot perfectly measure the phase information of a highly-attenuated coherent pulse train. This protocol has some features of simple setup, potential for a high key creation rate, and robustness against photon-number-splitting attack. This paper overviews DPS-QKD. The operation mechanism is described, and then some experimental efforts are introduced, featuring use of a glass waveguide Mach-Zehnder interferometer and advanced single-photon detectors. The highest key rate and the longest distance have been achieved with the DPS-QKD protocol. Some modified schemes are also presented, including that utilizing quantum entanglement, that using decoy pulses, and that using macroscopic coherent light.
Quantum key distribution allows for a provably secure transmission of cryptographic keys over an optical channel. Encoded polarization states or time-bin degree of freedom have been used for successful demonstrations. However, photon losses in long fibers, slow single photon detectors, and detector dark counts significantly limit the overall bit rate. Improving key throughput and reducing the overhead of key reconciliation remain as major challenges. Methods which utilize multiple time bins allow for multiple key bits to be encoded in a single photon, thus increasing the fidelity of transmitted keys and decreasing the overhead of key reconciliation in real-world conditions. Previous implementations of these methods required that Alice and Bob share a time reference by sharing a dedicated classical channel used for synchronization. This work presents a technique that allows two parties to exchange time-bin encoded photons without the need for synchronized time references. Our technique uses a framing protocol which allows Alice to encode a time reference along with a key which is determined by Alice before transmission. Security can be achieved by monitoring the visibility of a pair of Franson interferometers, using decoy pulses and measuring the round trip time between Alice and Bob. The bit rate of this technique is limited only by the recovery time of the detector and the speed of the modulation electronics. We experimentally demonstrate a raw bit rate of 5Mb/s over an optical channel with 55dB of loss, which is competitive with current research. We also demonstrate absolute timing synchronization with an accuracy of 20ps.
Differential phase-shift (DPS) quantum key distribution (QKD) is a unique QKD protocol that is different from traditional ones, featuring simplicity and practicality. In this work, we simulated the DPS-QKD experiment conducted by (Liu et al., 2013), using OptiSystem 7. To the best of our knowledge, this is the first simulation work on DPS-QKD using a single photon source.We used a random number generator to get the phase modulation pattern of N=5, 7,9,11 and 13, while for the 3 and 15 pulse cases, the pattern adopted in the experiment was used. When the number of pulse (N) was 3, a quantum bit error rate (QBER) of 3.0%, which is lower than the minimum QBER of 4.12% required for unconditional security, was obtained. The key creation efficiency increases with the increase in the number of pulse up to 15, as it reaches 93.4% but at the expense of the increment in QBER. The result of our simulation is, on some aspect, in agreement with the experimental result. However, we were able to extend the transmission distance from 3 meter, as in the experiment, to 10 meter. The coincidence count obtained was also in total agreement with the one obtained from the experiment. The result of the average QBER indicated that increase in the pulse number N causes the QBER to raise up due to longer rise and fall time of phase modulation step which affect the MZ inference. Therefore, we suggest using a faster waveform generator with shorter rise and fall times will remarkably lower the QBER. Extending the transmission coverage to a longer distance while, at the same time reducing the QBER with full unconditional security will part of the future research.