Squeezed light states are quantum states with reduced noise in one quadrature compared to vacuum states, making them valuable for various quantum applications including computing, metrology, sensing, and particularly quantum communication and key distribution (QKD). These states can enhance noise tolerance, provide security benefits, and prevent information leakage to adversaries. However, measuring squeezed light presents significant challenges due to requirements for precise phase noise control and mode matching, which has limited most implementations to free-space or simulated loss channels.
Traditionally, squeezed light detection uses coherent detection techniques where the squeezed light combines with a strong laser beam (local oscillator) on a balanced beam splitter. Effective coherent detection requires precise matching of polarization, spatial, and temporal modes between the local oscillator and squeezed light, while maintaining stable phase reference.
For practical applications like QKD across distances, significant challenges arise as the generation and detection typically happen at separate locations connected by optical fiber or free-space channels. This separation requires phase stabilization through complex optical phase lock loops or by multiplexing the local oscillator with squeezed light—creating security vulnerabilities. Additional complications include fiber-induced polarization drift requiring real-time alignment and clock synchronization between source and receiver.
The authors propose and experimentally demonstrate a novel method for reconstructing squeezed light post-measurement using a series of unitary transformations to digitally compensate for channel effects. Their approach uses radio-frequency heterodyne detection with a locally generated local oscillator, enabling simultaneous measurement of both field quadratures.
The method’s effectiveness was validated through two practical experiments: first, distributing squeezed states over 10 km of single-mode fiber without requiring phase locking or polarization alignment before measurement; second, implementing a passive continuous-variable quantum key distribution system using squeezed vacuum states transmitted through deployed fiber between remote locations.
This digital reconstruction approach significantly simplifies squeezed light distribution in fiber channels, creating new possibilities for practical quantum communication applications and distributed quantum sensing networks by eliminating the need for complex active stabilization systems.
npj Quantum Information, Published online: 03 May 2025; doi:10.1038/s41534-025-01018-9