Fang-Xiang Wang

Robust Quantum Random Number Generator Based on Avalanche Photodiodes

We propose and demonstrate a scheme to realize a high-efficiency truly quantum random number generator (RNG) at room temperature. Using an effective extractor with simple time bin encoding method, the avalanche pulses of avalanche photodiode (APD) are converted into high-quality random numbers that are robust to slow varying noise such as fluctuations of pulse intensity and temperature. A light source is compatible but not necessary in this scheme. Therefor the robustness of the system is effective enhanced. The random bits generation rate of this proof-of-principle system is 0.69 Mb/s with double APDs and 0.34 Mb/s with single APD. The results indicate that a high-speed RNG chip based on the scheme is potentially available with an integrable APD array.

Non-Markovian Property of Afterpulsing Effect in Single-Photon Avalanche Detector

The single-photon avalanche photodiode (SPAD) has been widely used in research on quantum optics. The afterpulsing effect, which is an intrinsic character of SPAD, affects the system performance in most experiments and needs to be carefully handled. For a long time, afterpulsing has been presumed to be determined by the pre-ignition avalanche. We studied the afterpulsing effect of a commercial InGaAs/InP SPAD (The avalanche photodiode model is: Princeton Lightwave PGA-300) and demonstrated that its afterpulsing is non-Markovian, with a memory effect in the avalanching history. Theoretical analysis and experimental results clearly indicate that the embodiment of this memory effect is the afterpulsing probability, which increases as the number of ignition-avalanche pulses increase. This conclusion makes the principle of the afterpulsing effect clearer and is instructive to the manufacturing processes and afterpulsing evaluation of high-count-rate SPADs. It can also be regarded as a fundamental premise to handle the afterpulsing signals in many applications, such as quantum communication and quantum random number generation.

Scalable orbital-angular-momentum sorting without destroying photon states

Single photons with orbital angular momentum (OAM) have attracted substantial attention from researchers. A single photon can carry infinite OAM values theoretically. Thus, OAM photon states have been widely used in quantum information and fundamental quantum mechanics. Although there have been many methods for sorting quantum states with different OAM values, the nondestructive and efficient sorter of high-dimensional OAM remains a fundamental challenge. Here, we propose a scalable OAM sorter which can categorize different OAM states simultaneously, meanwhile, preserving both OAM and spin angular momentum. Fundamental elements of the sorter are composed of symmetric multiport beam splitters (BSs) and Dove prisms with cascading structure, which in principle can be flexibly and effectively combined to sort arbitrarily high-dimensional OAM photons. The scalable structures proposed here greatly reduce the number of BSs required for sorting high-dimensional OAM states. In view of the nondestructive and extensible features, the sorters can be used as fundamental devices not only for high-dimensional quantum information processing, but also for traditional optics.

Controlled-phase manipulation module for orbital-angular-momentum photon states

Phase manipulation is essential to quantum information processing, for which the orbital angular momentum (OAM) of photon is a promising high-dimensional resource. Dove prism (DP) is one of the most important elements to realize the nondestructive phase manipulation of OAM photons. DP usually changes the polarization of light and thus increases the manipulation error for a spin-OAM hybrid state. DP in a Sagnac interferometer also introduces a mode-dependent global phase to the OAM mode. In this work, we implemented a high-dimensional controlled-phase manipulation module (PMM), which can compensate the mode-dependent global phase and thus preserve the phase in the spin-OAM hybrid superposition state. The PMM is stable for free running and is suitable to realize the high-dimensional controlled-phase gate for spin-OAM hybrid states. Considering the Sagnac-based structure, the PMM is also suitable for classical communication with the spin-OAM hybrid light field.

Single-path Sagnac interferometer with Dove prism for orbital-angular-momentum photon manipulation.

Orbital angular momentum (OAM) is an important resource in high-dimensional quantum information processing, as its quantum number can be infinite. Dove prism (DP) is a most common tool to manipulate OAM light. However, the Dove prism changes the polarization of the photon states and decreases the sorting fidelity of the interferometer. In this work, we analyze the polarization-dependent effect of the DP on OAM light manipulation in the normal single-path Sagnac interferometers (SPSIs) with beam splitter (BS) and polarizing beam splitter (PBS). The results demonstrate that the BS SPSI is more sensitive to the input polarization and the specific parameters of the DP. We have also proposed and realized a modified BS SPSI, of which the sorting fidelity can be 100% in principle and is independent on the input polarization and the transmission matrix of the DP. The experiments demonstrate that the fidelity of the modified BS SPSI is about 5%~10% higher than that of the normal one. The modified BS SPSI is easy to implement (only two more half-wave plates are required) and is stable for free running at the scale of several hours. These merits make the structure suitable for applications in critical quantum information processing tasks, such as quantum cryptography.Fang-Xiang Wang, Wei Chen1,2,3,∗ Ya-Ping Li, Guo-Wei Zhang, Zhen-Qiang Yin, Shuang Wang, Guang-Can Guo, and Zheng-Fu Han CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026, People’s Republic of China Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China State Key Laboratory of Cryptology, P. O. Box 5159, Beijing 100878, People’s Republic of China

Realistic Device Imperfections Affect the Performance of Hong-Ou-Mandel Interference With Weak Coherent States

Measurement-device-independent quantum key distribution (MDI QKD) is a promising and practical protocol for remote secret sharing, where the Hong-Ou-Mandel (HOM) type interference in the measurement side provides a simple and economic way to resist malicious attacks on the measurement devices. Because the HOM interference greatly indicates the performance of MDI QKD system, its interferential behavior in practical situations deserve to be studied. In this work, we analyzed HOM visibility with several practical imperfections, such as the deviation of the beam splitting ratio, detection efficiency mismatch of detectors, inconsistent intensities of two incident pulses. In particular, we discussed the afterpulse effect of InGaAs avalanche photodiode detectors with a non-Markovian model, where we take the avalanche history into consideration and provide a new and more accurate perspective for afterpulse analysis. Simulation results indicate that the afterpulse effect in practical situation has a greater impact on the HOM performance than other imperfections. The results exhibit its value in realistic high-speed MDI QKD systems as well as other fiber-based HOM applications.