Continuous-wave non-classical light with GHz squeezing bandwidth

TL;DR

This paper reports the generation of continuous-wave non-classical light at 1550 nm with a squeezing bandwidth exceeding 2 GHz, which is promising for high-rate quantum key distribution.

Contribution

It demonstrates GHz-bandwidth squeezing at telecom wavelength without resonant enhancement, expanding the potential for high-speed quantum communication.

Findings

Squeezing observed over 2 GHz bandwidth

Squeezing up to 0.3 dB below vacuum noise

Bandwidth limited by detector, not crystal properties

Abstract

Squeezed states can be employed for entanglement-based continuous-variable quantum key distribution, where the secure key rate is proportional to the bandwidth of the squeezing. We produced a non-classical continuous-wave laser field at the telecommunication wavelength of 1550 nm, which showed squeezing over a bandwidth of more than 2 GHz. The experimental setup used parametric down-conversion via a periodically poled potassium titanyl phosphate crystal (PPKTP). We did not use any resonant enhancement for the funda- mental wavelength, which should in principle allow a production of squeezed light over the full phase-matching bandwidth of several nanometers. We measured the squeezing to be up to 0.3 dB below the vacuum noise from 50 MHz to 2 GHz limited by the measuring bandwidth of the homodyne detector. The squeezing strength was possibly limited by thermal lensing inside the non-linear crystal.