Changjun Liao

Photonic technology in quantum information challenges quantum limitation

Basic experiments for quantum information have demonstrated the important role that photons play in future practical quantum information system. Technical prospects of processing quantum information in quantum computing, quantum communication and quantum cryptography encourage researchers to develop advanced photonic devices with high precision. We recognized that the most essential is to develop component parts of every kind that can constitute a qualified quantum information system. By using these component parts with high precision, it is possible to decrease errors in a practical quantum information system. The errors come from interaction of quantum information with environment that appear as decoherence. In terms of the technical terminology, this kind of interaction is appeared in conventional characterization as loss of the devices that are responsible for the permutation between quantum information system and environment noise. Therefore we analyze several possible structures of the components and devices that should be considered in the design of the components and the devices for quantum information usage. The emitters should generate quantum eigen state that the quantum dot with specified shape is embedded in mode matched waveguide. The quantum bits should be encoded only by phase modulation without loss. The receiver should also be sensitive to the quantum state. Every step of the efforts challenges the quantum limitation and the technologies available.

Polarization coding and decoding by phase modulation in polarizing sagnac interferometers

We present implementation of phase modulated polarization coding by polarizing sagnac interferometers. At four different low input voltages of the phase modulator, coder encodes pulse into four different polarization states, 45°, 135° linearly polarized or right, left circle polarized, while the decoder serves as the complementary polarizers. The mean visibility of the system is tested at 94.31%.

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.

The cross-phase modulation in erbium-doped fiber amplifiers

The nonlinear Schroedinger equation (NLSE) in erbium-doped fiber (EDF) is obtained. Based on this NLSE, a more generalized form of the propagation equation in the erbium-doped fiber amplifiers (EDFA) is given which has included the phase shift that the erbium ions induce. The cross-phase modulation (XPM) in the EDFA is studied according to that erbium ions induced phase shift. An analytical expression is given to the frequency chirp that the doped-erbium ions induce. It was found that the frequency chirp dose not change much with the wavelength except at the neighbouring wavelengths (around 1.531 μm) where absorption and the emission cross-sections of the erbium ions reach their maximum, and the frequency chirp has opposite sign on the two sides around 1.531 μm. Furthermore, it was found that the frequency chirp will increase with increasing length of the EDF.

Two-evolution equations for nonlinear subpicosecond pulses in optical fibers and their equivalent solitary solutions

Abstract By means of the multiscale singular perturbation method and directly from the Maxwell vector wave equation containing the nonlinear polarization term, we have obtained two kinds of equations describing the propagation of nonlinear sub-picosecond optical pulse in optical fibers. These two equations are the nonlinear Schrodinger equations with different linear combinations of the higher order terms (the higher order dispersion term, the self-steepening term, and the delayed nonlinear response term). The approximate coefficients of the equations are derived under the weakly guiding condition. Using a perturbation method, we show that these two equations share a solitary wave solution.

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.

Phase shift that erbium-doped fiber amplifiers induce

The nonlinear Schroedinger equation in erbium-doped fiber and the more generalized form of the propagation equation in the erbium-doped fiber amplifiers are obtained which have included the phase shift that the erbium ions induce. An analytical expression is given to the erbium ions induced phase shift and frequency chirp. It was found that the frequency chirp dose not change much with the wavelength except at the neighboring wavelengths (around 1.531 micrometer) where the absorption and the emission cross-sections of the erbium ions reach their maximum, and the frequency chirp has opposite sign on the two sides around 1.531 micrometer.

10Gb/s transmission over 100 km of standard single-mode fiber with a dispersion tunable chirped fiber grating

A simple strain method is used to change uniform gratings into chirped gratings. The correspondent theory is presented. It is deduced that the product of the maximum bandwidth and the corresponding dispersion is nearly equal to a constant. The dispersion compensation distance for standard single-mode fiber G.652,10 Gb/s systems over 100 km is realized by using this method.