Guang Wu

Laser ranging at 1550 nm with 1-GHz sine-wave gated InGaAs/InP APD single-photon detector

We demonstrated a laser ranging system with single photon detection at 1550 nm. The single-photon detector was a 1-GHz sine-wave gated InGaAs/InP avalanche photodiode. In daylight, 8-cm depth resolution was achieved directly by using a time-of-flight approach based on time-correlated single photon counting measurement. This system presented a potential for low energy level and eye-safe laser ranging system in long-range measurement.

Stable quantum key distribution with active polarization control based on time-division multiplexing

Polarization-encoding provides a promising approach for the practical quantum key distribution (QKD) system due to the simple encoding and decoding method. Here we present a long-term stable polarization-encoding QKD with a real-time polarization control. The polarization of the signal photons in the optical fiber was maintained stable by using time-division-multiplexed reference pulses for feedback control. We implemented this technique in the QKD experiment in 50 km fiber to show that it could facilitate polarization-encoded quantum communication. The system operated stably for ~460 min, and the quantum bit error rate was ~5.27%. The raw key generating rate in this system was kept stable at ~500 bits s-1. The advantages and disadvantages of the previous polarization-control schemes were also rigorously analyzed.

Low-Timing-Jitter Single-Photon Detection Using 1-GHz Sinusoidally Gated InGaAs/InP Avalanche Photodiode

We demonstrate an efficient way to improve the timing jitter of sine-wave gated InGaAs/InP avalanche photo diode (APD) by combining sinusoidally balanced differencing and low-pass filtering techniques to minimize detrimental waveform distortion of the avalanche signal, and realize a 1-GHz gated InGaAs/InP single-photon detector with the timing jitter as low as 60 ps. The detection efficiency could reach 10.4% at 1550 nm with the maximum count rate ~100 MHz, after pulse probability ~3.0%, and dark count rate ~6.1 × 10-6 per gate. Such low timing jitter single-photon detectors offer facility for quantum key distribution at gigahertz clock rate.

High-speed InGaAs/InP-based single-photon detector with high efficiency

An efficient single-photon detector at telecom wavelength of 1.55 μm was realized with an InGaAs/InP avalanche photodiode at −30 °C. By implementing a short gating pulse and optimizing the self-differencing circuit, a detection efficiency of 29.3% was achieved with an error count probability of 6% at the gating frequency of 200 MHz, paving the way for the high-efficiency and low-noise fast detection of the infrared single photons.

Single-photon routing by time-division phase modulation in a Sagnac interferometer

In this letter, we report the experimental demonstration of a single-photon router based on a time-division Sagnac interferometer, wherein differential phase shifts are applied on either the clockwise or counterclockwise quasi-single-photon pulses to determine the single photon interference and consequently output photon routing. High fidelity (>85%) of single-photon routing was demonstrated over a long-distance Sagnac loop. Stable performance was guaranteed by passive compensation of stress and temperature dependent drifts of the fiber-optic path. Experimental data show that time-division single-photon routing can be realized by controlling the applied electric pulses on the integrated phase modulators in the Sagnac loop, which makes this setup suitable for a practical quantum cryptography system.

Photon-number-resolving detection based on InGaAs/InP avalanche photodiode in the sub-saturated mode

We demonstrated a robust spike cancellation by virtue of optical balancing technique for the near-infrared single-photon detection based on InGaAs/InP avalanche photodiode. A 31 dB suppression of the spike noise provided an efficient technique to read out weak avalanche currents at the early built-up, allowing the study on the photon number resolving dynamics of the InGaAs/InP avalanche photodiode. With the detection efficiency varied from 1% to 36%, the avalanche gain was shown to vary from the linear mode to the saturated mode and evidenced as a sub-saturated avalanche state. Multi-photon avalanche saturation was observed at different photon numbers as the avalanche gain varied. A photon-number-resolving detection was achieved with the detection efficiency as high as 36%.

Experimental demonstration of counterfactual quantum key distribution

Counterfactual quantum key distribution provides natural advantage against the eavesdropping on the actual signal particles. It can prevent the photon-number-splitting attack when a weak coherent light source is used for the practical implementation. We experimentally realized the counterfactual quantum key distribution in an unbalanced Mach-Zehnder interferometer of 12.5-km-long quantum channel with a high-fringe visibility of 97.4%. According to the security analysis, the system was robust against the photon-number-splitting attack. The article is published in the original.

1550-nm time-of-flight ranging system employing laser with multiple repetition rates for reducing the range ambiguity.

We demonstrated a time-of-flight (TOF) ranging system employing laser pulses at 1550 nm with multiple repetition rates to decrease the range ambiguity, which was usually found in high-repetition TOF systems. The time-correlated single-photon counting technique with an InGaAs/InP avalanche photodiode based single-photon detector, was applied to record different arrival time of the scattered return photons from the non-cooperative target at different repetition rates to determine the measured distance, providing an effective and convenient method to increase the absolute range capacity of the whole system. We attained hundreds of meters range with millimeter accuracy by using laser pulses of approximately 10-MHz repetition rates.

Two-bit quantum random number generator based on photon-number-resolving detection

Here we present a new fast two-bit quantum random number generator based on the intrinsic randomness of the quantum physical phenomenon of photon statistics of coherent light source. Two-bit random numbers were generated according to the number of detected photons in each light pulse by a photon-number-resolving detector. Poissonian photon statistics of the coherent light source guaranteed the complete randomness of the bit sequences. Multi-bit true random numbers were generated for the first time based on the multi-photon events from a coherent light source.

Quantum random-number generator based on a photon-number-resolving detector

We demonstrated a high-efficiency quantum random number generator which takes inherent advantage of the photon number distribution randomness of a coherent light source. This scheme was realized by comparing the photon flux of consecutive pulses with a photon number resolving detector. The random bit generation rate could reach 2.4 MHz with a system clock of 6.0 MHz, corresponding to a random bit generation efficiency as high as 40%. The random number files passed all the stringent statistical tests.