Joshua C. Bienfang

Strong Loophole-Free Test of Local Realism

We performed an loophole-free test of Bells inequalities. The probability that local realism is compatible with our results is less than 5.9×10^-9.

Quantum key distribution with 1.25 Gbps clock synchronization

Clock recovery techniques at 1.25 Gbps enable continuous quantum key distribution at demonstrated sifted-key rates up to 1.0 Mbps. This rate is two orders of magnitude faster than has been reported previously

Single-photon detection efficiency up to 50% at 1310 nm with an InGaAs/InP avalanche diode gated at 1.25 GHz

We describe a gated Geiger-mode single-photon avalanche diode (SPAD) detection system in which both gating and avalanche discrimination are implemented by coherent addition of discrete harmonics of the fundamental gate frequency. With amplitude and phase control for each harmonic at the cathode, we form 65 dB suppression, allowing avalanche-discrimination thresholds at the anode below 2 mV or <8 fC. The low threshold not only accurately discriminates diminutive avalanches but also achieves usable detection efficiencies with lower total charge, reducing the afterpulse probability and allowing the use of gate pulses that exceed the SPAD breakdown voltage by more than 10 V, both of which increase detection efficiency. With detection efficiency of 0.19 ± 0.01, we measure per-gate afterpulse probability below 6.5 × 10−4 after 3.2 ns, and with detection efficiency of 0.51 ± 0.02 we measure per-gate afterpulse probabilit...

Experimental study of high speed polarization-coding quantum key distribution with sifted-key rates over Mbit/s

We present a quantitative study of various limitations on quantum cryptographic systems operating with sifted-key rates over Mbit/s. The dead time of silicon APDs not only limits the sifted-key rate but also causes correlation between the neighboring key bits. In addition to the well-known count-rate dependent timing jitter in avalanche photo-diode (APD), the faint laser sources, the vertical cavity surface emission lasers (VCSELs) in our system, also induce a significant amount of data-dependent timing jitter. Both the dead time and the data-dependent timing jitter are major limiting factors in designing QKD systems with sifted-key rates beyond Mbit/s.

High-speed quantum key distribution system supports one-time pad encryption of real-time video

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. NISTs 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.

Expeditious reconciliation for practical quantum key distribution

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.

Time-domain measurements of afterpulsing in InGaAs/InP SPAD gated with sub-nanosecond pulses

Afterpulsing was investigated experimentally in an InGaAs single-photon avalanche diode (SPAD) operating in the biasing and sensing regime of periodic-gating techniques. These techniques support single-photon counting at rates in the 100 MHz range with low afterpulse probability and are characterized by sub-nanosecond active gates that limit total avalanche-charge flows to the 100 fC range or less. We achieved comparable gating and sensing performance with a system using non-periodic gates and were able to make traditional double-pulse afterpulse measurements from 4.8 ns to 2 µs in this new low-avalanche-current regime. With 0.50 ns gate duration and a detection efficiency of 0.15 at 1310 nm the per-gate afterpulse probability at 4.8 ns is 0.008, while with a 1.5 ns gate it is almost two orders of magnitude higher. We provide a quantitative connection between afterpulse probability and total avalanche charge, and between the performance observed in traditional gating techniques for InGaAs SPADs and those observed with periodic gating techniques.

Up-conversion single-photon detector using multi-wavelength sampling techniques

The maximum achievable data-rate of a quantum communication system can be critically limited by the efficiency and temporal resolution of the systems single-photon detectors. Frequency up-conversion technology can be used to increase detection efficiency for IR photons. In this paper we describe a scheme to improve the temporal resolution of an up-conversion single-photon detector using multi-wavelength optical-sampling techniques, allowing for increased transmission rates in single-photon communications systems. We experimentally demonstrate our approach with an up-conversion detector using two spectrally and temporally distinct pump pulses, and show that it allows for high-fidelity single-photon detection at twice the rate supported by a conventional single-pump up-conversion detector. We also discuss the limiting factors of this approach and identify important performance-limiting trade offs.

Calibrating photon-counting detectors to high accuracy: background and deadtime issues

When photon-counting detectors are calibrated in the presence of a background signal, deadtime effects can be significant and must be carefully accounted for to achieve high accuracy. We present a method for separating the correlated signal from the background signal that appropriately handles deadtime effects. This method includes consideration of pulse timing and afterpulsing issues that arise in typical avalanche photodiode (APD) detectors. We illustrate how these effects should be accounted for in the calibration process. We also discuss detector timing issues that should be considered in detector calibration.

Stabilizing unstable steady states using extended time-delay autosynchronization

We describe a method for stabilizing unstable steady states in nonlinear dynamical systems using a form of extended time-delay autosynchronization. Specifically, stabilization is achieved by applying a feedback signal generated by high-pass-filtering in real time the dynamical state of the system to an accessible system parameter or variables. Our technique is easy to implement, does not require knowledge of the unstable steady state coordinates in phase space, automatically tracks changes in the system parameters, and is more robust to broadband noise than previous schemes. We demonstrate the controllers efficacy by stabilizing unstable steady states in an electronic circuit exhibiting low-dimensional temporal chaos. The simplicity and robustness of the scheme suggests that it is ideally suited for stabilizing unstable steady states in ultra-high-speed systems. (c) 1998 American Institute of Physics.