  The future development of quantum information using superconducting circuits
requires Josephson qubits with long coherence times combined to a high-delity
readout. Major progress in the control of coherence has recently been achieved
using circuit quantum electrodynamics (cQED) architectures, where the qubit is
embedded in a coplanar waveguide resonator (CPWR) which both provides a well
controlled electromagnetic environment and serves as qubit readout. In
particular a new qubit design, the transmon, yields reproducibly long coherence
times. However, a high-delity single-shot readout of the transmon, highly
desirable for running simple quantum algorithms or measuring quantum
correlations in multi-qubit experiments, is still lacking. In this work, we
demonstrate a new transmon circuit where the CPWR is turned into a
sample-and-hold detector, namely a Josephson Bifurcation Amplifer (JBA), which
allows both fast measurement and single-shot discrimination of the qubit
states. We report Rabi oscillations with a high visibility of 94% together with
dephasing and relaxation times longer than 0.5 $\mu$s. By performing two
subsequent measurements, we also demonstrate that this new readout does not
induce extra qubit relaxation.
