  We demonstrate the suppression of photon shot noise dephasing in a
superconducting qubit by eliminating its dispersive coupling to the readout
cavity. This is achieved in a tunable coupling qubit, where the qubit frequency
and coupling rate can be controlled independently. We observe that the
coherence time approaches twice the relaxation time and becomes less sensitive
to thermal photon noise when the dispersive coupling rate is tuned from several
MHz to 22 kHz. This work provides a promising building block in circuit quantum
electrodynamics that can hold high coherence and be integrated into larger
systems.
