  Non-destructive detection of photonic qubits will enable important
applications in photonic quantum information processing and quantum
communications. Here, we present an approach based on a solid-state cavity
containing an ensemble of rare-earth ions. First a probe pulse containing many
photons is stored in the ensemble. Then a single signal photon, which
represents a time-bin qubit, imprints a phase on the ensemble that is due to
the AC Stark effect. This phase does not depend on the exact timing of the
signal photon, which makes the detection insensitive to the time-bin qubit
state. Then the probe pulse is retrieved and its phase is detected via homodyne
detection. We show that the cavity leads to a dependence of the imprinted phase
on the {\it probe} photon number, which leads to a spreading of the probe
phase, in contrast to the simple shift that occurs in the absence of a cavity.
However, we show that this scenario still allows non-destructive detection of
the signal. We discuss potential implementations of the scheme, showing that
high success probability and low loss should be simultaneously achievable.
