Low Density Parity Check (LDPC) error correction is a one-way algorithm that has become popular for quantum key
distribution (QKD) post-processing. Graphic processing units (GPUs) provide an interesting attached platform that may
deliver high rates of error correction performance for QKD. We present the details of our various LDPC GPU
implementations and both error correction and execution throughput performance that each achieves. We also discuss
the potential for implementation on a GPU platform to achieve Gbit/s throughput.
Polar coding is the most recent encoding scheme in the quest for error correction codes that approaches the Shannon
limit, has a simple structure, and admits fast decoders. As such, it is an interesting candidate for the quantum key
distribution (QKD) protocol that normally operates at high bit error rates and requires codes that operate near the
Shannon limit. This paper describes approaches that integrate Polar codes into the QKD environment and provides
performance results of Polar code designs within the QKD protocol.
We present the quantum key distribution (QKD) secure key ratio expression in a form that exposes the parameters that
affect the Reconciliation (error correction) stage. Reconciliation is the least well understood in practical terms and is
typically described through a model that provides little guidance when it comes to efficient implementation, although it
requires significant resources and is required to achieve a performance level commensurate with the other stages of the
QKD protocol implementation. We addresses the issue of practical QKD error correction, questions of performance
based on our data and data that have been published, and addresses the issue of platforms capable of handling rates of
Secret keys can be established through the use of a Quantum channel monitored through classical channel which can be
thought of as being error free. The quantum channel is subject to massive erasures and discards of erroneously measured
bit values and as a result the error correction mechanism to be used must be accordingly modified. This paper addresses
the impact of error correction (known as Reconciliation) on the secrecy of the retained bits and issues concerning the
efficient software implementation of the Low Density Parity Check algorithm in the Quantum Key Distribution
environment. The performance of three algorithmic variants are measured through implementations and the collected
sample data suggest that the implementation details are particularly important
Complete high-speed quantum key distribution (QKD) systems over fiber networks for campus and metro areas have
been developed at NIST. The systems include an 850-nm QKD system for a campus network, a 1310-nm QKD system
for metro networks, and a 3-user QKD network and network manager. In this paper we describe the key techniques
used to implement these systems, including polarization recovery, noise reduction, frequency up-conversion detection
based on PPLN waveguide, custom high-speed data handling and network management. A QKD-secured video
surveillance system has been used to experimentally demonstrate these systems.
Quantum Cryptography has demonstrated the potential for ultra-secure communications. However, with quantumchannel
transmission rates in the MHz range, typical link losses and signal-to-noise ratios have resulted in keyproduction
rates that are impractical for continuous one-time-pad encryption of high-bandwidth communications. We have developed high-speed data handling electronics that support quantum-channel transmission rates up to 1.25 GHz.
This system has demonstrated error-corrected and privacy-amplified key rates above 1 Mbps over a free-space link.
While the transmission rate is ultimately limited by timing jitter in the single-photon avalanche photodiodes (SPADs),
we find the timing resolution of silicon SPADs sufficient to operate efficiently with temporal gates as short as 100 ps.
We have developed systems to implement such high-resolution gating in our system, and anticipate the attendant
reduction in noise to produce significantly higher secret-key bitrates.
Free-space Quantum key distribution (QKD) has shown the potential for the practical production of cryptographic key for ultra-secure communications. The performance of any QKD system is ultimately limited by the signal to noise ratio on the single-photon channel, and over most useful communications links the resulting key rates are impractical for performing continuous one-time-pad encryption of today's broadband communications. We have adapted clock and data recovery techniques from modern telecommunications practice, combined with a synchronous classical free-space optical communications link operating in parallel, to increase the repetition rate of a free-space QKD system by roughly 2 orders of magnitude over previous demonstrations. We have also designed the system to operate in the H-alpha Fraunhofer window at 656.28 nm, where the solar background is reduced by roughly 7 dB. This system takes advantage of high efficiency silicon single-photon avalanche photodiodes with <50ps timing resolution that are expected to enable operation at a repetition rate of 2.5 GHz. We have identified scalable solutions for delivering sustained one-time-pad encryption at 10 Mbps, thus making it possible to integrate quantum cryptography into first-generation Ethernet protocols.
We previously demonstrated a high speed, point to point, quantum key distribution (QKD) system with polarization
coding over a fiber link, in which the resulting cryptographic keys were used for one-time pad encryption of real time
video signals. In this work, we extend the technology to a three-node active QKD network - one Alice and two Bobs. A
QKD network allows multiple users to generate and share secure quantum keys. In comparison with a passive QKD
network, nodes in an active network can actively select a destination as a communication partner and therefore, its
sifted-key rate can remain at a speed almost as high as that in the point-to-point QKD. We demonstrate our three-node
QKD network in the context of a QKD secured real-time video surveillance system. In principle, the technologies for the
three-node network are extendable to multi-node networks easily. In this paper, we report our experiments, including
the techniques for timing alignment and polarization recovery during switching, and discuss the network architecture and
its expandability to multi-node networks.
Quantum cryptography asserts that shared secrets can be established over public channels in such a way that the total information of an eavesdropper can be made arbitrarily small with probability arbitrarily close to 1. As we will show below, the current state of affairs, especially as it pertains to engineering issues, leaves something to be desired.
A complete fiber-based polarization encoding quantum key distribution (QKD) system based on the BB84 protocol has been developed at National Institute of Standard and Technology (NIST). The system can be operated at a sifted key rate of more than 4 Mbit/s over optical fiber of length 1 km and mean photon number 0.1. The quantum channel uses 850 nm photons from attenuated high speed VCSELs and the classical channel uses 1550 nm light from normal commercial coarse wavelength division multiplexing devices. Sifted-key rates and quantum error rates at different transmission rates are measured as a function of distance (fiber length). A polarization auto-compensation module has been developed and utilized to recover the polarization state and to compensate for temporal drift. An automatic timing alignment device has also been developed to quickly handle the initial configuration of quantum channels so that detection events fall into the correct timing window. These automated functions make the system more practical for integration into existing optical local area networks.
We have implemented a quantum key distribution (QKD) system with polarization encoding at 850 nm over 1 km of optical fiber. The high-speed management of the bit-stream, generation of random numbers and processing of the sifting algorithm are all handled by a pair of custom data handling circuit boards. As a complete system using a clock rate of 1.25 Gbit/s, it produces sifted keys at a rate of 1.1 Mb/s with an error rate lower than 1.3% while operating at a transmission rate of 312.5 Mbit/s and a mean photon number μ = 0.1. With a number of proposed improvements this system has a potential for a higher key rate without an elevated error rate.
The paper proposes algorithmic and environmental modifications to the extant reconciliation algorithms within the BB84 protocol so as to speed up reconciliation and privacy amplification. These algorithms have been known to be a performance bottleneck 1 and can process data at rates that are six times slower than the quantum channel they serve2. As improvements in single-photon sources and detectors are expected to improve the quantum channel throughput by two or three orders of magnitude, it becomes imperative to improve the performance of the classical software. We developed a Cascade-like algorithm that relies on a symmetric formulation of the problem, error estimation through the segmentation process, outright elimination of segments with many errors, Forward Error Correction, recognition of the distinct data subpopulations that emerge as the algorithm runs, ability to operate on massive amounts of data (of the order of 1 Mbit), and a few other minor improvements. The data from the experimental algorithm we developed show that by operating on massive arrays of data we can improve software performance by better than three orders of magnitude while retaining nearly as many bits (typically more than 90%) as the algorithms that were designed for optimal bit retention.
We describe the status of the NIST Quantum Communication Testbed (QCT) facility. QCT is a facility for exploring quantum communication in an environment similar to that projected for early commercial implementations: quantum cryptographic key exchange on a gigabit/second free-space optical (FSO) channel. Its purpose is to provide an open platform for testing and validating performance in the application, network, and physical layers of quantum communications systems. The channel uses modified commercial FSO equipment to link two buildings on the Gaithersburg, MD campus of the National Institute of Standards and Technology (NIST), separated by approximately 600 meters. At the time of writing, QCT is under construction; it will eventually be made available to the research community as a user facility. This paper presents the basic design considerations underlying QCT, and reports the status of the project.