Zhengjun Wei

An integral gated mode single photon detector at telecom wavelengths

We demonstrate an integral gated mode single photon detector at telecom wavelengths. The charge number of an avalanche pulse rather than the peak current is monitored for single photon detection. The transient spikes in conventional gated mode operation are cancelled completely by integrating, which enables one to effectively improve the performance of single photon detector with the same avalanche photodiode. This method achieved a detection efficiency of 29.9% at the dark count probability per gate equal to 5.57 × 10−6/gate (1.11 × 10−6 ns−1) at 1550 nm.

Efficient quantum key distribution via single-photon two-qubit states

We propose a scheme for quantum key distribution, in which single photons are encoded with polarization states and then divided and delayed to encode phase differences between two sequential pulses. In an example, the BB84 and DPS (differential phase shift) protocols are combined to achieve single-photon two-qubit quantum key distribution. This scheme can increase the key generation efficiency up to 7/6, higher than that of the conventional single-photon one-qubit quantum key distribution scheme.

A gain-flatness optimization solution for feedback technology of wideband low noise amplifiers *

The S parameter expression of high-frequency models of the high electron mobility transistors (HEMTs) with basic feedback structure, especially the transmission gain S21, is presented and analyzed. In addition, an improved feedback structure and its theory are proposed and demonstrated, in order to obtain a better gain-flatness through the mutual interaction between the series inductor and the parallel capacitor in the feedback loop. The optimization solution for the feedback amplifier can eliminate the negative impacts on transmission gain S21 caused by things such as resonance peaks. Furthermore, our theory covers the shortage of conventional feedback amplifiers, to some extent. A wideband low-noise amplifier (LNA) with the improved feedback technology is designed based on HEMT. The transmission gain is about 20 dB with the gain variation of 1.2 dB from 100 MHz to 6 GHz. The noise figure is lower than 2.8 dB in the whole band and the amplifier is unconditionally stable.

A rigorous theoretical analysis for an In0.53Ga0.47As/InP single photon avalanche photodiode under Geiger mode operation

A rigorous theoretical model for In0.53Ga0.47As/InP single photon avalanche photodiodes operated in the Geiger mode is developed to calculate dark count probabilities over a wide range of temperatures and widths of multiplication layer. Both nonlocal ionization and low field impact ionization in the absorption layer are considered in this model. The calculated results confirm that impact ionization in the absorption and charge layers increases the dark count probability. The primary mechanism of dark counts depends on both device structure and operating conditions. For a 1 µm multiplication width, the dominant mechanism of the dark counts at a temperature above 233 K is the generation–recombination in the absorber while at a temperature below 233 K the tunnelling effect in the multiplication layer begins to dominate.

The study of single photon detector for quantum key distribution

This paper introduces the design of a single photon detector based on the AT89C51 single chip microcontroller for quantum key distribution at telecommunication wavelengths. An avalanche photodiode (APD) is operated in Geiger mode and stabilized at temperature of 228K for translating the single photon signals into electrical pulses. The photon induced avalanche pulses are amplified and converted to digital data. The microcontroller discriminates the data, counting the number of photons, and then displays the photon number and the detection efficiency on a LCD display. The instrument can effectively support the study of quantum key distribution.

Experimental demonstration of polarization encoding quantum key distribution system based on intrinsically stable polarization-modulated units.

A proof-of-principle demonstration of a one-way polarization encoding quantum key distribution (QKD) system is demonstrated. This approach can automatically compensate for birefringence and phase drift. This is achieved by constructing intrinsically stable polarization-modulated units (PMUs) to perform the encoding and decoding, which can be used with four-state protocol, six-state protocol, and the measurement-device-independent (MDI) scheme. A polarization extinction ratio of about 30 dB was maintained for several hours over a 50 km optical fiber without any adjustments to our setup, which evidences its potential for use in practical applications.

Determination of breakdown voltage of In0.53Ga0.47As/InP single photon avalanche diodes

We examine the saturation of relative current gain of In0.53Ga0.47As/InP single photon avalanche diodes (SPADs) operated in Geiger mode. The punch-through voltage and breakdown voltage of the SPADs can be measured using a simple and accurate method. The analysis method is temperature-independent and can be applied to most SPADs.

PUNCH-THROUGH CHARACTERISTICS OF AVALANCHE PHOTODIODES UNDER THE GEIGER MODE

A passive quench circuit is used to study the punch-through characters of avalanche photodiodes under the Geiger mode. The photocurrent–voltage curve indicated clearly the punch-through voltage while the dark current–voltage curve is insensitive to the punch-through. The experiments demonstrate different distributions of the carries. The dark carriers counts increase much faster than the photo-carriers counts due to the different collection efficiency. A proper selection of the bias can increase the signal-to-noise ratio (SNR) of the single photon detector.

Photon Emitting, Absorption and Reconstruction of Photons

Photon cannot keep itself unchanged from emission to absorption. The information encoded on the photon is also changed due to interaction with environment. There has no definitely demonstration that the photon being absorbed is the original one from ideal light source since the quantum mechanics itself is an indeterminate theory that the physical measurement is only the probability. We divide the change of the photon state into two parts that one can be compensated and the other cannot be compensated. A concept of photon reconstruction is introduced to explain every optical phenomena including Raman scattering, multi-photon absorption, nonlinear phenomena, free electron lasing, quantum entanglement, high order coherence, ghost imaging and the de-phase which result in error bits or information loss in the quantum information process. An experimental result is explained to show that the signal photon can modify the background even the energy of the photon is not enough for absorption in the wide-band gap semiconductor material. The photon-current-voltage curve and dark-current-voltage curve of an absorption, grating, and multiplication InGaAs/InP avalanche photodiode is analyzed to show that 1550nm input light modified the dark background even the applied reverse bias voltage is far below the punch-through voltage. This increase of the dark count directly relates to the input photons at the 1550nm wavelength but is not due to absorption in the absorption layer and insensitive to the applied voltage.

An infrared single photon detector based on avalanche photodiodes with transmission lines

A new method to detect infrared single photon using avalanche photodiodes is proposed, which combine the requirements of a single photon detector control circuit and features of the electric pulses generator by transient process of transmission lines. When the terminated boundary conditions of transmission lines are changed quickly, voltage across the switch devices will drop suddenly in the electric pulses generator model and at the same time a signal that can reflect the drive information of the switch device will export from load resistance. So we can replace the switch with avalanche photodiodes who can act as an excellent optoelectronic switch. Then when faint laser pulses enter the avalanche photodiode, resistance of the avalanche photodiode will decrease quickly and with that voltage across avalanche photodiodes will drop simultaneously and a signal that can reflect the single-photon laser pulses will export from the load resistance. The simulation results are given for the new quenching circuit model of avalanche photodiodes and the analyses emphasis on the feasibility of the quenching model and the influence of each electrical component on the performance of the quenching circuit. Finally optimal parameter of the electrical components in the model is demonstrated.