Generation and control of frequency-dependent squeezing via Einstein–Podolsky–Rosen entanglement

Quantum noise-limited displacement sensors such as gravitational wave detectors can be improved by using non-classical light¹. This has been achieved in limited bands and in a single quadrature (that is, only one of a pair of conjugate variables) by injecting single-mode squeezed vacuum states^2,3. Quantum noise in gravitational wave detectors, however, results from input noise in both quadratures, with the dominant quadrature being a function of Fourier frequency. Broadband reduction of this noise via squeezed light injection then requires a method of rotating this quadrature. This can be accomplished with a low-loss, all-pass optical filter with bandwidth in the low audio frequencies^4,5, a substantial technical challenge. We present a proof-of-principle demonstration of a recent proposal⁶ to use two-mode squeezed vacuum states with Einstein–Podolsky–Rosen (EPR) entanglement, which allows the gravitational detector to simultaneously serve as the optical filter, eliminating the need for a separate apparatus.