Hybrid quantum network for sensing in the acoustic frequency range.

Ultimate limits for the sensing of fields and forces are set by the quantum noise of a sensor. Entanglement allows for suppression of such noise and for achieving sensitivity beyond standard quantum limits. Applicability of quantum optical sensing is often restricted by fixed wavelengths of available photonic quantum sources. Another ubiquitous limitation is associated with challenges of achieving quantum-noise-limited sensitivity in the acoustic noise frequency range relevant for several applications. Here we demonstrate a tool for broadband quantum sensing by performing quantum state processing that can be applied to a wide range of the optical spectrum and by suppressing quantum noise over an octave in the acoustic frequency range. An atomic spin ensemble is strongly coupled to one of the frequency-tunable beams of an Einstein-Podolsky-Rosen (EPR) source of light. The other EPR beam of light, entangled with the first one, is tuned to a disparate wavelength. Engineering the spin ensemble to act as a negative-mass or positive-mass oscillator, we demonstrate frequency-dependent quantum noise reduction for measurements at the disparate wavelength. The tunability of the spin ensemble enables targeting quantum noise in a variety of systems with dynamics ranging from kHz to MHz. As an example of broadband quantum noise reduction in the acoustic frequency range, we analyse the applicability of our approach to gravitational-wave detectors (GWDs). Other possible applications include continuous-variable quantum repeaters and distributed quantum sensing.