Phase sensing beyond the standard quantum limit with a variation on the SU(1,1) interferometer

An SU(1,1) interferometer, which replaces the beam splitters in a Mach–Zehnder interferometer with nonlinear interactions, offers the potential of achieving improved phase sensitivity in applications with low optical powers. We present a novel variation on the SU(1,1) interferometer in which the second nonlinear interaction is replaced with balanced homodyne detection. We show theoretically that this “truncated SU(1,1) interferometer” can achieve the same potential phase sensitivity as the conventional SU(1,1) interferometer. We build an experimental realization of this device using seeded four-wave mixing in Rb85 vapor as the nonlinear interaction, thus employing a bright two-mode squeezed state as the phase-sensing quantum state inside the interferometer. Measurements as a function of operating point show that even with ≈35% loss, this device can surpass the standard quantum limit by 4 dB. This device is simpler to build and operate than the conventional SU(1,1) interferometer, and also eliminates some sources of loss, thus making it useful for applications in precision metrology.