  We show how to realize high-fidelity quantum non-demolition qubit readout
using longitudinal qubit-oscillator interaction. This is realized by modulating
the longitudinal coupling at the cavity frequency. The qubit-oscillator
interaction then acts as a qubit-state dependent drive on the cavity, a
situation that is fundamentally different from the standard dispersive case.
Single-mode squeezing can be exploited to exponentially increase the
signal-to-noise ratio of this readout protocol. We present an implementation of
this idea in circuit quantum electrodynamics and a possible multi-qubit
architecture.
