High-speed periodic gating of InGaAs/InP single-photon avalanche diodes (SPADs) has allowed these detectors to operate at count rates above 108 per second with low afterpulsing. However, a drawback of high-speed periodic gating is that bias gates are applied continuously, regardless of whether an avalanche has occurred or not. This is disadvantageous because gates immediately following an avalanche have elevated afterpulse probabilities, and the additional charge from these secondary events contributes to the overall afterpulse probability. We investigate this phenomenon in a proof-of-principle experiment in which the series of bias gates is briefly interrupted after an avalanche, and we measure the resulting impact on the afterpulse probability. We observe a significant reduction in afterpulsing when such a bias-gate hold-o_ is applied to an InGaAs/InP SPAD gated at 1.25 GHz; when one bias gate is omitted after an avalanche the per-gate afterpulse probability is reduced by more than 40 %. These results indicate that afterpulsing noise at high count rates can be further reduced in high-speed-gated SPADs.
KEYWORDS: Avalanche photodiodes, Single photon detectors, Control systems design, Picosecond phenomena, Analog electronics, Signal detection, Diodes, Sensors, Interferometers, Destructive interference
We discuss the performance of a 1.25 GHz gated single-photon avalanche diode (SPAD) with bias gates of 150 ps FWHM and AC amplitude up to 25 V, a high-efficiency, high-speed SPAD system. This system uses an interferometric readout technique known as harmonic subtraction, and recent development efforts have enabled the use of up to the 4th harmonic of the gate to discriminate avalanche signals from the gate transient. With an improved design of the RF control system we have been able to demonstrate an ultra-low minimum detectable charge. We discuss the performance of this system, particularly its afterpulsing performance when counting at rates > 108 s-1. Systems of this type require unique characterization techniques, and we will discuss the methods we have developed for this purpose.
We gate a InGaAs/InP single-photon avalanche diode with a narrow periodic gate obtained by summing a 1.25 GHz sinusoid with its second and third harmonic. The temporal full-width at half maximum (FWHM) of the gate is kept below 200 ps by adjusting relative weight of the harmonic components. Measurements of detection efficiency and afterpulse probability as the gate pulse duration is reduced show that it is possible to reach the same detection efficiency obtainable with wider gates with the advantage of significantly reducing afterpulse probability.
We present an InGaAs/InP single-photon detection system operating at 1.25 GHz with detection efficiency above 50 % and per-gate afterpulse probability, measured 24.8 ns after an avalanche, below 0.2 %. The high efficiency and low afterpulse probabilities we observe are achieved with an avalanche discrimination system whose threshold for detection approaches the fundamental limit imposed by Johnson noise on a 50 Ohm load; we measure the threshold to be less than 8 fC. We discuss the design and performance of our approach, and tradeoffs between detection efficiency, afterpulse probability, and maximum count rate.
We discuss avalanche discrimination in a periodically-gated InGaAs/InP single-photon avalanche diode. We investigate
the interrelation between the minimum detectable avalanche charge and the detection efficiency, and we show that the
technical solutions we implement can improve performance. Gating the detector at 1.25 GHz, single-photon count rates
above 250x106 s-1 can be obtained while maintaining low afterpulse probability with detection efficiencies larger than 0.10.
We adapt a previously-demonstrated gating technique for InGaAs SPADs to enable double-bias-pulse measurements of
afterpulsing at nanosecond time scales with gate durations down to 500 ps. We present preliminary results for afterpulse
probabilities below 10 ns, time scales comparable to those in the self-differencing technique, and show that afterpulse
probabilities low enough to support reliable counting above 100 MHz can be observed.
We discuss high-speed electronics that support the use of single-photon avalanche diodes (SPADs) in gigahertz singlephoton
communications systems. For InGaAs/InP SPADs, recent work has demonstrated reduced afterpulsing and count
rates approaching 500 MHz can be achieved with gigahertz periodic-gating techniques designed to minimize the total
avalanche charge to less than 100 fC. We investigate afterpulsing in this regime and establish a connection to
observations using more conventional techniques. For Si SPADs, we report the benefits of improved timing electronics
that enhance the temporal resolution of Si SPADs used in a free-space quantum key distribution (QKD) system operating
in the GHz regime. We establish that the effects of count-rate fluctuations induced by daytime turbulent scintillation are
significantly reduced, benefitting the performance of the QKD system.
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.
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