  We have fabricated and measured a high-Q Josephson junction resonator with a
tunable resonance frequency. A dc magnetic flux allows the resonance frequency
to be changed by over 10 %. Weak coupling to the environment allows a quality
factor of $\thicksim$7000 when on average less than one photon is stored in the
resonator. At large photon numbers, the nonlinearity of the Josephson junction
creates two stable oscillation states. This resonator can be used as a tool for
investigating the quality of Josephson junctions in qubits below the single
photon limit, and can be used as a microwave qubit readout at high photon
numbers.
