Alessandro Restelli

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...

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.

Monolithic silicon matrix detector with 50 μm photon counting pixels

A 48-element monolithic matrix detector, designed for parallel fluorescence detection, has been fabricated in silicon and tested. The pixels are single photon avalanche diodes (SPADs) with 50 µm diameter, ordered in a 6× 8 array with 240 µm pitch. The photon detection efficiency is remarkably uniform over the array: it has a peak of 48% at 530 nm and it is higher than 30% over all the visible range. Low dark counting rate (DCR) is obtained in operation at moderately low temperature (−15°C with thermoelectric cooling): the individual pixel DCR is 60 c s−1 for about 40% of the elements and it is below 5700 c s−1 in the rest of the array. It was verified that the afterpulsing probability is below 1% and that the optical crosstalk probability between elements is lower than 0.2%. Based on this matrix detector, we have developed a versatile and compact (20 cm× 8 cm × 4 cm) photon counting module that can be easily interfaced to a PC via USB link.

Efficient single-spatial-mode periodically-poled KTiOPO 4 waveguide source for high-dimensional entanglement-based quantum key distribution

We demonstrate generation of high-purity photon pairs at 1560 nm in a single spatial mode from a periodically-poled KTiOPO4 (PPKTP) waveguide. With nearly lossless spectral filtering, the PPKTP waveguide source shows approximately 80 % single-mode fiber coupling efficiency and is well suited for high-dimensional time-energy entanglement-based quantum key distribution. Using high-count-rate self-differencing InGaAs single-photon avalanche photodiodes configured with either square or sinusoidal gating, we achieve > 1 Mbit/s raw key generation with 3 bits-per-photon encoding, and, to the best of our knowledge, the highest reported Franson quantum-interference visibility of 98.2 % without subtraction of accidental coincidences.

Programmable instrumentation and gigahertz signaling for single-photon quantum communication systems

We discuss custom time-tagging instrumentation for high-speed single-photon metrology, focusing particularly on implementations that can tag and process detection events from multiple single-photon detectors with sub-nanosecond timing resolution and at detection rates above 100 MHz. The systems we present view the detector signal as if it were a serial data stream, tagging events according to the bit period in which a rising edge from the detector occurs. We achieve sub-nanosecond resolution with serial data receivers operating up to 10 Gb s−1. Data processing bottlenecks are avoided with pipelined algorithms and controlled data flow implemented in field-programmable gate arrays.

Avalanche discrimination and high-speed counting in periodically gated single-photon avalanche diodes

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.

Improved Timing Resolution Single-Photon Detectors in Daytime Free-Space Quantum Key Distribution With 1.25 GHz Transmission Rate

In free-space single-photon quantum key distribution (QKD), the error rate due to daytime background photons can be reduced with strong temporal filtering. In this case, the improvement in performance is determined by the receivers ability to resolve signal-photon arrival times. We use fast clock recovery and commercially available single-photon detectors with timing resolution enhanced by additional electronic circuitry to implement temporal gating down to 50 ps in a free-space QKD system. The single-photon channel operates at 850 nm, and the improved timing resolution enables transmission rates of 1.25 GHz. We observe daytime quantum bit error rates of 0.04, which is less than one-third of the ungated error rate. We present the design and performance of the system and demonstrate its benefit to free-space QKD.

Semiconductor-Based Detectors

Abstract This chapter presents a broad review of the principle, design, fabrication, operation, and performance of single-photon avalanche diodes (SPADs), and provides a comparative basis to assess the relative merits of the wide variety of devices and techniques covered. Particular attention is paid to the design of and operation of both silicon and high-speed-gated InGaAs/InP detection systems, as well as array-based systems.

Afterpulse Reduction Through Prompt Quenching in Silicon Reach-Through Single-Photon Avalanche Diodes

Reducing afterpulsing in single-photon avalanche diodes (SPADs) allows operation with shorter recovery times and higher detection rates. Afterpulsing in SPADs can be reduced by reducing the total avalanche charge. We use a periodic quenching system to arbitrarily vary the latency between the onset of an avalanche and the application of the quench, allowing us to characterize the afterpulsing behavior when the current flow is halted at time scales that are significantly shorter than can be achieved by standard active-quenching systems. Three different reach-through SPADs are characterized, and with prompt quenching we observe reductions in afterpulse probability of as much as a factor of 12. Beyond improving detection rates, reducing the total avalanche charge can also allow operation with higher excess bias voltages, which enables higher detection efficiency and more precise timing resolution.

A Green Laser Pointer Hazard

An inexpensive green laser pointer was found to emit 20 mW of infrared radiation during normal use. This is potentially a serious hazard that would not be noticed by most users of such pointers. We find that this infrared emission derives from the design of the pointer, and describe a simple method of testing for infrared emissions using common household items.