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Frontiers in Optics

3 Pulse Differential Phase Shift Quantum Key Distribution with Spatial, or Time, Multiplexed

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

Published Jan 1, 2019• 0 citations
Publisher

Abstract

We demonstrated 3 pulse differential phase shift quantum key distribution with 30 km quantum channel with two different approaches, namely path superposition and time bin superposition.
DOI
10.1364/fio.2019.jw3a.54

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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.
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.
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.