Extended Data Fig. 1: Experimental set-up main components.
From: Hybrid quantum network for sensing in the acoustic frequency range
The experimental set-up includes two lasers: a 1,064-nm continuous-wave laser and a tunable continuous-wave Ti:sapphire laser at 852 nm. The two-colour EPR state set-up involves two nonlinear optical interactions: sum-frequency generation (SFG) and a NOPO. The 1,064-nm and 852-nm lasers produce the SFG light in a nonlinear crystal, which serves as the pump for the NOPO with pump phase θp. The NOPO generates the EPR state at 1,064 nm (signal) and 852 nm (idler), represented by dashed lines. The signal and idler beams are separated by a dichroic mirror. The signal is directed to balanced homodyne detection (HDs) with local oscillator phase θs, whereas the idler is mixed with the probe beam at a polarizing beam splitter (PBS) with relative phase ϕi and propagates in free space (approximately 10 m) to the atomic oscillator set-up. The probe–idler spatial profile is modified to a square top-hat beam by a top-hat shaper (THS) before being sent to the caesium (Cs) vapour cell. The cell is placed inside a magnetic shield with a set of coils controlling the bias magnetic field. The optical pump system prepares the spin ensemble in a highly polarized state. After interaction, the idler quantum state is sent to polarization homodyne detection (HDi), for which a quarter-wave plate (QWP) and a half-wave plate (HWP) determine the phase shift δθi. A part of the 1,064-nm laser is frequency-shifted by 3 MHz to produce the coherent-lock field (CLF) beam injected to the NOPO with phase ϕc, which provides the phase reference for the detection system and feedback control of the detected quadratures. The fields for the local oscillators LOs and LOi are produced by the 1,064-nm and 852-nm lasers, respectively. Each LO is filtered by a mode-cleaner cavity (not shown). Both photocurrents from the homodyne detectors are sent to a data acquisition (DAQ) system for recording and post-processing. The quadrature correlations at specific sideband frequencies for different set phases (ϕi + δθi, θs) are used to demonstrate the frequency-dependent conditional squeezing. The phases ϕi and θs are actively stabilized to fix the quadrature detection in relation to the pump phase θp.
