Simultaneous Unbalanced Shared Local Oscillator Heterodyne Interferometry (SUSHI) for high SNR, minimally destructive dispersive detection of time-dependent atomic spins

We demonstrate "Simultaneous Unbalanced Shared Local Oscillator Heterodyne Interferometry (SUSHI),“ a new method for minimally destructive, high SNR dispersive detection of atomic spins. In SUSHI, technical noise due to phase, amplitude, and frequency fluctuations of the probe laser is strongly rejected (i) for any mean phase bias between the interferometer arms, (ii) without the use of piezo actuated mirrors, and (iii) without signal balancing. We experimentally demonstrate an technical noise limited phase sensitivity of 37 nrad/rtHz for a measurement bandwidth of 60 Hz to 8 kHz using a 230 uW probe, and stay within 3 dB of the standard quantum limit as probe power is reduced by more than 5 orders of magnitude to as low as 650 pW. SUSHI is therefore well suited to performing QND measurements for preparing spin squeezed states and for high SNR, truly continuous observations of ground-state Rabi flopping in cold atom ensembles.