ИСТИНА

We perform the phase measurement in an unseeded SU(1,1) interferometer composed of two cascaded optical parametric amplifiers (OPAs) with direct detection. Theoretically, such a device can provide phase sensitivity overcoming the shot-noise limit and ideally achieving the Heisenberg limit, the only obstacle being the losses inside and outside of the interferometer. While the internal losses are difficult to overcome, the external (detection) losses can be overcome by making the interferometer unbalanced, with the parametric gain of the second OPA considerably exceeding the first one [1]. In our recent experiment, we demonstrate a phase sensitivity overcoming the shot noise limit by up to 3dB, with the number of photons in the interferometer being between 1 and 10. We show that the phase sensitivity is considerably improved by increasing the parametric gain of the second OPA. In particular, we show that with the gain of the second OPA as high as 7, the phase sensitivity of the interferometer is preserved even with the detection efficiency reduced by an order of magnitude, by means of additional loss introduced in the detection scheme. [1] M. Manceau, F. Khalili, and M. Chekhova, New Journal of Physics 19, 013014 (2017).