Detection-Loss Tolerant Real-Time Amplitude Measurement for Ultra-Fast Optical Quantum Processors: 5-dB Squeezing from DC to 43 GHz
Asuka Inoue, Takahiro Kashiwazaki, Taichi Yamashima, Naoto Takanashi, Takushi Kazama, Koji Enbutsu, Kei Watanabe, Takeshi Umeki, Mamoru Endo, Akira Furusawa · 2023
Measurement-based optical quantum computers (MBOQCs) using continuous-variables (CVs) of light are a candidate for large-scale fault-tolerant quantum computing [1]. This form of processing has a high carrier frequency of light, so it can also perform ultra-fast quantum computing. To achieve ultra-fast MBOQCs, broadband squeezed light generation and ultra-fast realtime amplitude measurement are required. So far, over-4.5-dB squeezed light generation, which is required for preparing a two-dimensional cluster state, has been demonstrated in THz bandwidth by using noise intensity measurement with a narrow bandwidth [2], [3]. However, the realtime amplitude measurement frequency is limited to several GHz. Up to now, the highest frequency is still limited to 9 GHz with 0.1-dB squeezing even though specific detectors have been designed [4]. This is because there is a tradeoff relationship between the bandwidth and the quantum efficiency of homodyne detectors. As a result, we cannot use a fast balanced homodyne detector such as a detector for high-speed optical communications with a large optical loss, which collapses the quantum states of light. To achieve ultra-fast realtime amplitude measurement, we need a new measurement method to overcome the tradeoff limitations.