Design And Application Of Low-noise, Broadband Photo-detector In Quantum Optics

In order to accommodate the high-speed communication, high-density storage and precise detection in the future, scientists continually research on obtaining light source with lower noise in order to improve Signal Noise Ratio (SNR). However, even if we have got an ideal coherent light source, there will still be quantum noise (Quantum Noise Limit, QNL, or Shot Noise Limit, SNL) caused by the fluctuation of photon numbers. The appearance of squeezed states brings us lower-noise light. One of the quadratured-component of squeezed states is lower than the quantum noise of coherent state and exceeds the SNL. Squeezed states widely used in precision measurement of Break shot noise limit, Quantum cryptography, Quantum Computing.If we want to detect squeezed states in the quantum optics experiments, we must measure the SNL first. The noise fluctuation of squeezed states is lower than SNL. The photo-detector must satisfy the follow basic conditions:1 The quantum efficiency of photo-detector should be high enough to reduce the influence of quantum noise;2 The bandwidth of photo-detector should be wide enough to realize the measurement of squeezed states in wide bandwidth (the measurements of quantum noise are usually in a frequency range of a few MHz, tens of MHz, or even hundreds of MHz);3 The SNL detected must be at least lOdB higher than the electr-noise of photo-detector so that the degree of squeezing can be read according to the records of spectrum analyzer;4 The measurements of the SNL and squeezed states need two balanced photo-detectors, and the common-mode rejection ratio (CMRR) of the balanced photo-detector must be higher than 30dB in order to subtract the classical noise in the experiments.We presented the special requirements of photo-detectors in quantum optics experiments. And then we analyzed the influence factors of the performance of photo-detectors, the selection of the optical detection devices and the design of the amplifier circuit. We selected proper optical detection devices and operational amplifier, designed two balanced broad-band and low-noise photo-detectors. For a general photo-detector, its electr-noise is-91.5dB (the noise of Spectrum Analyzer is-98dBm), its bandwidth is 50MHz, and its saturation power is 16mW. For two balanced photo-detectors, the CMRR is above 30dB in the bandwidth of 2MHz-31MHz. In the experiment we measured the SNL by using three kinds of light, and the noise of all-solid-state continuous-wave single-frequency laser and the fiber laser.