  Single-shot readout experiments were performed on the two lowest-energy
states of a superconducting qubit with three Josephson junctions embedded in a
superconducting loop. We measured the qubit state via switching current Isw of
a current-biased dc-SQUID, a quantum detector surrounding the qubit loop. The
qubit signals were measured in a small Isw regime of the SQUID, typically less
than 100 nA, where the Isw distribution is particularly narrow. The obtained
single-shot data indicate that the qubit state is readout, through the flux
generated by the qubit persistent-current, as energy eigenstates rather than
current eigenstates.
