NTT develops quantum light source operating over optical fiber

NTT, in collaboration with the University of Tokyo, and RIKEN, unveiled an optical fiber-coupled quantum light source (squeezed light source) with the potential to serve as a building block for fault-tolerant, rack-sized, universal optical quantum computers.

Squeezed light is described as a non-classical light that has an even number of photons and squeezed quantum noise. It is used to generate quantum entanglement. NTT said squeezed light also plays an extremely important role in quantum error correction, since quantum error correction is made possible by utilizing the parity of the number of photons. 

In this project, the researchers sought a fiber-coupled squeezed light source with highly squeezed quantum noise and photon number parity that is maintained even in high-photon-number components (a squeezing level of over 65% is required to generate time-domain multiple quantum entanglement (two-dimensional clustered states) that can be used for large-scale quantum computation.) 

The researchers developed a new optical fiber-coupled quantum light source that operates at optical communication wavelengths. By combining it with optical fiber components, the researchers ached continuous-wave squeezed light with more than 75% squeezed quantum noise with more than 6 THz sideband frequency even in an optical fiber closed system for the first time. This means that the key device in optical quantum computers has been realized in a form that is compatible with optical fibers while maintaining the broadband nature of light. This will enable the development of an optical quantum computer in a stable and maintenance-free system using optical fibers and optical communication devices. 

NTT claims that by using a low-loss optical fiber as a propagation medium for flying optical qubits, large-scale quantum entangled states will be able to be generated freely and stably in combination with optical communication devices. Specifically, with only four squeezed light sources, two optical fibers of different lengths (optical delay lines), and five beam splitters, large-scale two-dimensional clustered states can be generated that are necessary for universal quantum computations. 

https://group.ntt/en/newsrelease/2021/12/22/211222a.html