In order to realize a practical optical quantum computer that can be accessed on the cloud, the ability to keep the entire system stable over a long period of time is essential. However, manual alignment in conventional free-space-based systems [1] is troublesome. By introducing a fiber-based system [2], spatial alignment becomes unnecessary. Instead, we find many unstable properties in a fiber-based system such as phase, polarization, power of light, and coupling ratio of fiber directional couplers. Moreover, squeezed light, a fundamental resource of optical quantum information processing, rapidly degrades by optical losses. Here, we develop a variety of techniques to overcome the difficulties. Also, we demonstrate continuous measurement of squeezed light generated from a waveguide optical parametric amplifier [3] keeping a squeezing level of -4.420±0.006 dB for 24 hours by automatically aligning the system once every 30 minutes.

We introduce and develop many devices to solve various instabilities of fiber systems. A variable optical attenuator is used to stabilize the optical power, a fiber stretcher is constructed to lock the phase. Furthermore, we stabilize the coupling ratio of the fiber directional coupler by controlling temperature, resulting in suppressing the drift of the coupling ratio from 2% to less than 0.02%. For polarization fluctuations, we invent a new low-loss polarization control method. In general fiber-based polarization controllers, single-mode fiber is pressurized by a piezoelectric actuator to rotate its polarization [4], resulting in a large optical loss of about 5%. In the new method, the polarization is rotated by stretching the fiber, and polarization control with an optical loss of 0.5% is realized. Besides, polarization modulation is applied to improve accuracy. These individual stabilization devices are driven by a Field Programmable Gate Array. We connect these devices to a single network, enabling integrated control of the entire system from a single computer.

[1] S. Yokoyama et al., “Ultra-large-scale continuous-variable cluster states multiplexed in the time domain”, Nature Photon 7, 982–986 (2013).[2] M. V. Larsen et al., “Deterministic generation of a two-dimensional cluster state”. Science 366, 369–372 (2019).[3] T. Kashiwazaki et al., "Fabrication of low-loss quasi-single-mode PPLN waveguide and its application to a modularized broadband high-level squeezer", Appl. Phys. Lett. 119, 251104 (2021). [4] S. C. Huang, “Automatic Polarization Compensation Tracking Method for Maximum Visibility of Fiber Interferometric Sensors”, J. Light. Technol., 27, 18, 15(2009).