  SU(1,1) interferometry, proposed in a classic 1986 paper by Yurke, McCall,
and Klauder [Phys. Rev. A 33, 4033 (1986)], involves squeezing, displacing, and
then unsqueezing two bosonic modes. It has, over the past decade, been
implemented in a variety of experiments. Here I take SU(1,1) interferometry
apart, to see how and why it ticks. SU(1,1) interferometry arises naturally as
the two-mode version of active-squeezing-enhanced, back-action-evading
measurements aimed at detecting the phase-space displacement of a harmonic
oscillator subjected to a classical force. Truncating an SU(1,1)
interferometer, by omitting the second two-mode squeezer, leaves a prototype
that uses the entanglement of two-mode squeezing to detect and characterize a
disturbance on one of the two modes from measurement statistics gathered from
both modes.
