A post-processing-free single-photon random number generator with ultra-low latency.

The low-latency requirements of a practical loophole-free Bell test preclude time-consuming post-processing steps that are often used to improve the statistical quality of a physical random number generator (RNG). Here we demonstrate a post-processing-free RNG that produces a random bit within 2.4(2) ns of an input trigger. We use weak feedback to eliminate long-term drift, resulting in 24 hour operation with output that is statistically indistinguishable from a Bernoulli process. We quantify the impact of the feedback on the predictability of the output as less than 6.4×10-7 and demonstrate the utility of the Allan variance as a tool for characterizing non-idealities in RNGs.

Simple autocorrelation method for thoroughly characterizing single-photon detectors.

We introduce and demonstrate a simple and highly sensitive method for characterizing single-photon detectors. This method is based on analyzing multi-order correlations among time-tagged detection events from a device under calibrated continuous-wave illumination. First- and second-order properties such as detection efficiency, dark count rate, afterpulse probability, dead time, and reset behavior are measured with high accuracy from a single data set, as well as higher-order properties such as higher-order afterpulse effects. While the technique is applicable to any type of click/no-click detector, we apply it to two different single-photon avalanche diodes, and we find that it reveals a heretofore unreported afterpulse effect due to detection events that occur during the device reset.

Strong Loophole-Free Test of Local Realism.

We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p values as small as 5.9×10^{-9} for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p value is 2.3×10^{-7}. We therefore reject the hypothesis that local realism governs our experiment.

NIR Fluorescence lifetime macroscopic imaging with a novel time-gated SPAD camera.

SwissSPAD3 is the latest of a family of widefield time-gated SPAD imagers developed for fluorescence lifetime imaging (FLI) applications. Its distinctive features are (i) the ability to define shorter gates than its predecessors (width W < 1 ns), (ii) support for laser repetition rates up to at least 80 MHz and (iii) a dual-gate architecture providing an effective duty cycle of 100%. We present widefield macroscopic FLI measurements of short lifetime NIR dyes, analyzed using the phasor approach. The results are compared with those previously obtained with SwissSPAD2 and to theoretical predictions.

Massively parallel, real-time multispeckle diffuse correlation spectroscopy using a 500 × 500 SPAD camera.

Diffuse correlation spectroscopy (DCS) is a promising noninvasive technique for monitoring cerebral blood flow and measuring cortex functional activation tasks. Taking multiple parallel measurements has been shown to increase sensitivity, but is not easily scalable with discrete optical detectors. Here we show that with a large 500 × 500 SPAD array and an advanced FPGA design, we achieve an SNR gain of almost 500 over single-pixel mDCS performance. The system can also be reconfigured to sacrifice SNR to decrease correlation bin width, with 400 ns resolution being demonstrated over 8000 pixels.

Low-noise photon counting above 100 million counts per second with a high-efficiency reach-through single-photon avalanche diode system.

We demonstrate a method that allows a high-efficiency single-photon-avalanche diode (SPAD) with a thick absorption region (> 10 µm) to count single photons at rates significantly higher than previously demonstrated. We apply large (> 30 V) AC bias gates to the SPAD at 1 GHz and detect minute avalanches with a discrimination threshold of 5(1) mV by means of radio-frequency (RF) interferometry. We measure a reduction by a factor of ≈ 500 in the average charge per avalanche when compared to operation in its traditional active-quenching module, and a relative increase of >19 % in detection efficiency at 850 nm. The reduction in charge strongly suppresses self-heating effects in the diode that can degrade performance at high avalanche rates. We show that the single-photon detection system maintains high efficiency at count rates exceeding 108s-1.

Low-bias high-speed quantum random number generator via shaped optical pulses.

We present an optical quantum random number generator (QRNG) based on the digitized time interval between random photon arrivals. By tailoring the photon flux of the laser diode, the statistics of the waiting-time distribution are altered to approximate the ideal, uniform case. This greatly reduces the need for post-processing, and enables fast, secure quantum random number generation at rates exceeding 110 Mbit/s.