Quantum Enhancement of Phase Sensitivity for the Bright-Seeded SU(1,1) Interferometer with Direct Intensity Detection

An SU(1,1) interferometer can be realized by replacement of the beam splitters in the Mach-Zehnder interferometer with parametric amplifiers. This interferometer scheme was proposed more than 30 years ago [Yurke et al., Phys. Rev. A 33, 4033 (1986)] and the enhancement of its signal-to-noise ratio was experimentally demonstrated in our recent work [Hudelist et al., Nat. Commun. 5, 3049 (2014)]. The sensitivity of any interferometer increases with increasing internal photon number. Therefore, the bright-seeded SU(1,1) interferometer has an advantage of boosted sensitivity. However, the phase sensitivity used to characterize the bright-seeded SU(1,1) interferometer has not been shown yet. In this article, with direct intensity detection, we experimentally demonstrate the phase-sensitivity enhancement of such a bright-seeded SU(1,1) interferometer compared with the phase-sensitivity scaling given by 1/Ns, known as the "shot-noise limit," where Ns is the average photon number inside the interferometer. It is this direct intensity detection that brings about the major advance of quantum enhancement for the bright-seeded SU(1,1) interferometer in real time, which is substantially different from our previous work. Our results may find applications in quantum metrology.