  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 a dual-frequency probe laser
interacts with atoms in one arm of a Mach-Zehnder interferometer, then beats
against a bright local oscillator 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 \emph{active subtraction} in which
anti-noise is actively written onto the local oscillator 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 ultra-low technical noise limited sensitivity of 51
nrad$/\sqrt{\R{Hz}}$ over a measurement bandwidth of 60 Hz to 8 kHz using a 230
$\mu$W probe, and stay within $\sim$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.
