Investigation and discovering of new physics, is always connected to a significant improvement of measurements precision, as demonstrated recently with gravitational waves detection by means of large scale interferometers. Even if the sensitivity of such instruments is already impressive, a joint measurement with a pair of such detectors could allow, in some cases, identifying signals orders of magnitude below the sensitivity of the single one. For example, a faint correlated phase noise source acting in both the interferometers can emerge by evaluating their cross-correlation(-spectrum). This is the idea at the base of the Fermilab experiment [2], devoted to the search of specific quantum gravity effects at the Plank scale.

Based on previous theoretical investigation [3,4], we proved experimentally the advantage of using quantum light (squeezing and bipartite correlations) in a measurement apparatus composed by two interferometers whose outputs are jointly measured. Each interferometer exploits a 2-D power recycling cavity and is operated close to the dark fringe, emulating the configuration used in large scale experiments such as the gravitational wave detectors [1] or the Fermilab "Holometer”.

First, we have shown that the injection of the squeezed light in both the detector reduces the integration time for the correlation measurement of QE^4, where QE is the linear quantum enhancement in the single one. For testing, we have measured an injected white noise signal with amplitude 1/5 of the shot noise of the individual interferometer, reaching a sensitivity of 1/10 of the shot noise (10^{-17 }m/√Hz in absolute terms) in 1s of integration time.

^{}In the second place, we have injected a bipartite quantum correlated state (TWB-like quadrature correlation) demonstrating a noise reduction in the photocurrent subtraction, making this configuration suitable for detecting a small difference between the signals in the two interferometers (with sub shot noise sensitivity) as well as the presence of faint uncorrelated signals.

Summarizing, this work represents the first feasibility test of a quantum enhanced correlated interferometry system which can have immediate applications for example in the search of quantum gravity signature in large scale experiment. At the same time, it explores the potential of using quantum correlated states in new interferometric schemes.

[1] J. Aasi, et al., Nat. Phot. 7, 613–619 (2013).

[2] Aaron S. Chou, et al.,Phys. Rev. Lett. 117, 111102 (2016).

[3] I. Ruo Berchera, et al.,Phys. Rev. Lett. 110, 213601 (2013).

[4] I. Ruo-Berchera, et al., Phys. Rev. A 92,053821 (2015).

Quantum sensors and quantum metrology , Quantum optics and non-classical light sources