Asuka Inoue, Takahiro Kashiwazaki, Taichi Yamashima, Naoto Takanashi, Takushi Kazama, Koji Enbutsu, Kei Watanabe, Takeshi Umeki, Mamoru Endo, and Akira Furusawa
Continuous-variable optical quantum information processing, where quantum information is encoded in a traveling wave of light called a f lying qubit, is a candidate for a practical quantum computer with high clock frequencies. Homodyne detectors for quadrature-phase amplitude measurements have been the major factor limiting the clock frequency. Here, we developed a real-time amplitude measurement method using a modular optical parametric amplifier (OPA) and a broadband balanced photodiode that is commercially used for coherent wavelength-division multiplexing telecommunication of the fifth-generation mobile communication systems (5G). The OPA amplifies one quadrature-phase component of the quantum-level signal to a loss-tolerant macroscopic level and suppresses the loss after the OPA from 92.4% to only 0.4%. This method was applied to a broadband squeezed vacuum measurement with a center wavelength of 1545.32nm. In the time-domain measurement, the squeezing level of 5.160.1dB without loss correction was obtained by a real-time oscilloscope with a sampling rate of 160GHz and an analog bandwidth of 63GHz. The frequency-domain analysis also shows that a squeezing level of 5.260.5dB is obtained from DC to 43GHz, which is limited by the balanced detector. This indicates that the proposed method can be easily broadened by using a broader bandwidth measurement instrument. By applying this method, not only can optical quantum computers with high clock frequencies be realized but also multi-core systems can be realized.