Thermal neutron detection is of vital importance to many disciplines, including neutron scattering, workplace monitoring, and homeland protection. We survey recent results from our collaboration which couple low-pressure noble gas scintillation with novel approaches to neutron absorbing materials and geometries to achieve potentially advantageous detector concepts. Noble gas scintillators were used for neutron detection as early as the late 1950's. Modern use of noble gas scintillation includes liquid and solid forms of argon and xenon in the dark matter and neutron physics experiments and commercially available high pressure applications have achieved high resolution gamma ray spectroscopy. Little attention has been paid to the overlap between low pressure noble gas scintillation and thermal neutron detection, for which there are many potential benefits.
Quantum key distribution (QKD) channels are typically realized by transmitting and detecting single photons, and
therefore suffer from dramatic reductions in throughput due to both channel loss and noise. These shortcomings can be
mitigated by applying telecommunications clock-recovery techniques to maximize the bandwidth of the single-photon
channel and minimize the system's exposure to noise. We demonstrate a QKD system operating continuously at a
quantum-channel transmission rate of 1.25 GHz, with dedicated data-handling hardware and error-correction/privacy
amplification. We discuss the design and performance of our system and highlight issues which limit our maximum
transmission and key production rates.
The desire for quantum-generated cryptographic key for broadband encryption services has motivated the development
of high-transmission-rate single-photon quantum key distribution (QKD) systems. The maximum operational
transmission rate of a QKD system is ultimately limited by the timing resolution of the single-photon detectors and
recent advances have enabled the demonstration of QKD systems operating at transmission rates well in to the GHz
regime. We have demonstrated quantum generated one-time-pad encryption of a streaming video signal with high
transmission rate QKD systems in both free-space and fiber. We present an overview of our high-speed QKD
architecture that allows continuous operation of the QKD link, including error correction and privacy amplification, and
increases the key-production rate by maximizing the transmission rate and minimizing the temporal gating on the
single-photon channel. We also address count-rate concerns that arise at transmission rates that are orders of magnitude
higher than the maximum count rate of the single-photon detectors.
Quantum key distribution (QKD) can produce secure cryptographic key for use in symmetric cryptosystems. By adopting clock-recovery techniques from modern telecommunications practice we have demonstrated a free-space quantum key distribution system operating at a transmission rate of 625 MHz at 850 nm. The transmission rate of this system is ultimately limited by the timing resolution of the single-photon avalanche photodiodes (SPADs), and we present a solution to take advantage of SPADs with higher timing resolution that can enable repetition rates up to 2.5 GHz. We also show that with high-repetition-rate sub-clock gating these higher-resolution SPADs can reduce the system's exposure to solar background photons, thus reducing the quantum-bit error rate (QBER) and improving system performance.
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.
Proc. SPIE. 6244, Quantum Information and Computation IV
KEYWORDS: Avalanche photodetectors, Coarse wavelength division multiplexing, Clocks, Polarization, Single mode fibers, Computer programming, Vertical cavity surface emitting lasers, Local area networks, Quantum key distribution, Picture Archiving and Communication System
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.
NIST has developed a high-speed quantum key distribution (QKD) test bed incorporating both free-space and fiber systems. These systems demonstrate a major increase in the attainable rate of QKD systems: over two orders of magnitude faster than other systems. NIST's approach to high-speed QKD is based on a synchronous model with hardware support. Practical one-time pad encryption requires high key generation rates since one bit of key is needed for each bit of data to be encrypted. A one-time pad encrypted surveillance video application was developed and serves as a demonstration of the speed, robustness and sustainability of the NIST QKD systems. We discuss our infrastructure, both hardware and software, its operation and performance along with our migration to quantum 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.
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.