Fig. 1: Parallel approach to broadband quantum noise suppression.
From: Hybrid quantum network for sensing in the acoustic frequency range
a, General parallel scheme for the sensitivity enhancement of a quantum-noise-limited sensor. b, Layout of the experimental set-up. The spin ensemble is probed by light at 852 nm (idler), which is entangled with a 1,064 nm (signal) optical field. A suitably engineered atomic spin ensemble provides the required quadrature-rotation dynamics, described by Φ(Ω). The two fields are analysed by the corresponding homodyne detectors, with θs defining the quadrature phase of the signal field and the phases ϕi and δθi defining the detection phase θi for the idler field. Conditioning the signal photocurrent on the idler that has interacted with the spin ensemble results in the noise-reduction spectrum required for a particular application. c, The noise in the phase space for the idler field as a function of the frequency Ω. Far away from Ωa, the idler noise (blue-shaded area) corresponds to the EPR state. Closer to Ωa, the idler experiences a single-axis-twisting transformation as a result of interaction with the atomic spin ensemble. d, Conditioning the signal on the idler by the post-processing of the photocurrents, as shown in panel b, results in conditional squeezing in the signal observable Qs|i(θs), for which the squeezing phase θs = Φ(Ω) changes with Ω, as illustrated by the rotated red-shaded ellipses. This process counteracts the quantum noise induced by optical probing of the force sensor by reducing the amplitude noise at Ωa and reducing the phase noise far away from Ωa, thus enabling broadband quantum-enhanced sensing. Dashed circles indicate vacuum noise.
