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IEEE International Symposium on Information Theory (ISIT)

Adaptive pulse-position modulation for high-dimensional quantum key distribution

Hongchao Zhou, Gregory W. Wornell

Published Jul 1, 2013• 12 citations
Publisher

Abstract

High-dimensional quantum key distribution (QKD) systems that exploit temporal correlation among entangled photons are of growing practical interest. In such systems, the observation time is typically partitioned into frames of fixed duration, with pulse-position modulation (PPM) coding used within each frame, via which a secret key is established between the parties. Such schemes can be very inefficient in their use of photons, since only a fraction of the frames can be used. As an alternative, we describe an efficient class of schemes with adaptive frame size whose performance can converge to the fundamental limit much more quickly. We analyze and compare the performances of both fixed and adaptive PPM schemes, taking into account photon transmission and detection losses. Further numerical results reveal the significant performance gain of adaptive PPM relative to fixed PPM.

DOI
10.1109/isit.2013.6620248

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High-dimensional quantum key distribution (QKD) systems that exploit temporal correlation among entangled photons are of growing practical interest. In such systems, the observation time is typically partitioned into frames of fixed duration, with pulse-position modulation (PPM) coding used within each frame, via which a secret key is established between the parties. Such schemes can be very inefficient in their use of photons, since only a fraction of the frames can be used. As an alternative, we describe an efficient class of schemes with adaptive frame size whose performance can converge to the fundamental limit much more quickly. We analyze and compare the performances of both fixed and adaptive PPM schemes, taking into account photon transmission and detection losses. Further numerical results reveal the significant performance gain of adaptive PPM relative to fixed PPM.
Pulse-position modulation (PPM) is a promising technique that can be used to improve the efficiency of quantum key distribution (QKD) based on a Poisson photon source. In this paper, we first investigate a simple entanglement-and- PPM-based QKD protocol and demonstrate the improvement in secret key rate. However, such a PPM-based QKD protocol that utilizes only frames with a single click is still inefficient because it ignores frames with two and more clicks. For this reason we propose to use such multi-click frames to further improve the efficiency of PPM-based QKD by employing a better sifting strategy. Specifically, we focus on using the frames with two clicks in addition to those with a single click. Finally, we analyze the secret key rate under various noise levels in the scenario of high channel loss, which has been faced by most QKD applications. With the analytical results, we show the advantage of the proposed PPM-based QKD.
The binary (one-bit-per-photon) encoding that most existing quantum key distribution (QKD) protocols employ puts a fundamental limit on their achievable key rates, especially under high channel loss conditions associated with long-distance fiber-optic or satellite-to-ground links. Inspired by the pulse-position-modulation (PPM) approach to photon-starved classical communications, we design and demonstrate the first PPM-QKD, whose security against collective attacks is established through continuous-variable entanglement measurements that also enable a novel decoy-state protocol performed conveniently in post processing. We achieve a throughput of 8.0 Mbit/s (2.5 Mbit/s for loss equivalent to 25 km of fiber) and secret-key capacity up to 4.0 bits per detected photon, thus demonstrating the significant enhancement afforded by high-dimensional encoding. These results point to a new avenue for realizing high-throughput satellite-based or long-haul fiber-optic quantum communications beyond their photon-reception-rate limits.
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We investigate modulation and coding techniques that approach the fundamental limits of communication and key distribution over optical channels, in the regime of simultaneously high photon and bandwidth efficiencies. First, we develop a simple and robust system design for free-space optical communication that incorporates pulse-position modulation (PPM) over multiple spatial degrees of freedom in order to achieve high photon and spectral efficiency. Further, in the context of key distribution, we determine the optimal rate using a Poisson source of entangled photon pairs and photon detectors, and show how to approach it using PPM parsing of the detected photon stream.
Quantum key distribution (QKD) exploits the quantum nature of light to share provably secure keys, allowing secure communication in the presence of an eavesdropper. The first QKD schemes used photons encoded in two states, such as polarization. Recently, much effort has turned to large-alphabet QKD schemes, which encode photons in high-dimensional basis states. Compared to binary-encoded QKD, large-alphabet schemes can encode more secure information per detected photon, boosting secure communication rates, and also provide increased resilience to noise and loss. High-dimensional encoding may also improve the efficiency of other quantum information processing tasks, such as performing Bell tests and implementing quantum gates. Here, we demonstrate a large-alphabet QKD protocol based on high-dimensional temporal encoding. We achieve record secret-key rates and perform the first field demonstration of large-alphabet QKD. This demonstrates a new, practical way to optimize secret-key rates and marks an important step towards transmission of high-dimensional quantum states in deployed networks.
Conventional quantum key distribution (QKD) typically uses binary encoding based on photon polarization or time-bin degrees of freedom and achieves a key capacity of at most one bit per photon. Under photon-starved conditions the rate of detection events is much lower than the photon generation rate, because of losses in long distance propagation and the relatively long recovery times of available single-photon detectors. Multi-bit encoding in the photon arrival times can be beneficial in such photon-starved situations. Recent security proofs indicate high-dimensional encoding in the photon arrival times is robust and can be implemented to yield high secure throughput. In this work we demonstrate entanglement-based QKD with high-dimensional encoding whose security against collective Gaussian attacks is provided by a high-visibility Franson interferometer. We achieve unprecedented key capacity and throughput for an entanglement-based QKD system because of four principal factors: Franson interferometry that does not degrade with loss; error correction coding that can tolerate high error rates; optimized time–energy entanglement generation; and highly efficient WSi superconducting nanowire single-photon detectors. The secure key capacity yields as much as 8.7 bits per coincidence. When optimized for throughput we observe a secure key rate of 2.7 Mbit s −1 after 20 km fiber transmission with a key capacity of 6.9 bits per photon coincidence. Our results demonstrate a viable approach to high-rate QKD using practical photonic entanglement and single-photon detection technologies.