The ability to generate broadband, continuous-wave squeezed light can improve quantum information techniques by speeding up and downsizing quantum processors. Using a single-mode waveguide, Kashiwazaki et al. exceeded previous limitations in wide-bandwidth, single-pass light squeezing.

“4.5 dB squeezing is a threshold for generation of the 2D-cluster state, which is the most important state in optical cluster quantum computing,” said author Takahiro Kashiwazaki.

Although this threshold has been surpassed before, it has not been done using a single-pass method, which is necessary for realizing wide-bandwidth quantum computing. Using a single-mode waveguide, the group detected squeezing up to 6.3 dB without degradation or vacuum noise contamination.

They did so by first separating the continuous-wave light into two optical fiber beams – the first beam undergoes frequency doubling and acts as a pump for the waveguide, while the second is used as a local oscillator. Then, squeezed light is generated by recombining the two beams, where the first beam interferes with the oscillator from the second beam and creates high spatial mode-matching. In this way, the mode-matching problem of multi-mode waveguides, which causes the squeezing to degrade, is eliminated.

The authors believe these results can be applied to building fault-tolerant, high-speed quantum information processors. However, two challenges remain to be overcome.

“One is realizing higher squeezing levels. 6.3 dB is sufficient for the generation of 2D cluster states, but is insufficient for quantum error correction,” Kashiwazaki said. “Second is the integration of optical functions in optical circuits.”

Source: “Continuous-wave 6-dB-squeezed light with 2.5-THz-bandwidth from single-mode PPLN waveguide,” by Takahiro Kashiwazaki, Naoto Takanashi, Taichi Yamashima, Takushi Kazama, Koji Enbutsu, Ryoichi Kasahara, Takeshi Umeki, and Akira Furusawa, APL Photonics (2020). The article can be accessed at