Two-mode squeezing over deployed fiber coexisting with conventional communications

TL;DR

This paper demonstrates coexistent two-mode squeezing over deployed fiber with classical signals, enabling quantum networks and sensing without dedicated dark fiber, by distributing and measuring multi-mode squeezed states in real-world fiber networks.

Contribution

It presents the first measurement of multi-mode squeezing coexisting with classical signals over deployed fiber, advancing practical quantum network implementations.

Findings

Achieved -0.9 dB squeezing over 5 km fiber with classical signals present

Achieved -0.5 dB squeezing over campus fibers (~250 m and 1.2 km) with classical signals

Demonstrated compatibility of squeezed light with existing classical communication infrastructure

Abstract

Squeezed light is a crucial resource for continuous-variable (CV) quantum information science. Distributed multi-mode squeezing is critical for enabling CV quantum networks and distributed quantum sensing. To date, multi-mode squeezing measured by homodyne detection has been limited to single-room experiments without coexisting classical signals, i.e., on ``dark'' fiber. Here, after distribution through separate fiber spools (5~km), $-0.9\pm0.1$-dB coexistent two-mode squeezing is measured. Moreover, after distribution through separate deployed campus fibers (about 250~m and 1.2~km), $-0.5\pm0.1$-dB coexistent two-mode squeezing is measured. Prior to distribution, the squeezed modes are each frequency multiplexed with several classical signals -- including the local oscillator and conventional network signals -- demonstrating that the squeezed modes do not need dedicated dark fiber. After distribution, joint two-mode squeezing is measured and recorded for post-processing using triggered homodyne detection in separate locations. This demonstration enables future applications in quantum networks and quantum sensing that rely on distributed multi-mode squeezing.