We demonstrate “simultaneous unbalanced shared local oscillator heterodyne interferometry” (SUSHI) a new method for minimally destructive, high signal-to-noise-ratio (SNR) dispersive detection of atomic spins. In SUSHI a dual-frequency probe laser interacts with atoms in one arm of a Mach–Zehnder interferometer, and then beats against a bright local oscillator (LO) beam traversing the other arm, resulting in two simultaneous, independent heterodyne measurements of the atom-induced phase shift. Measurement noise due to mechanical disturbances of beam paths is strongly rejected by the technique of active subtraction, in which antinoise is actively written onto the LO beam via an optical phase-locked loop. In SUSHI, technical noise due to phase, amplitude, and frequency fluctuations of the various laser fields 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 ultralow technical-noise-limited sensitivity of over a measurement bandwidth of 60 Hz to 8 kHz using a 230 μW probe, and stay within 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 quantum nondemolition measurements for preparing spin-squeezed states and for high-SNR, truly continuous observations of ground-state Rabi flopping in cold atom ensembles.
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