  We have designed, fabricated and tested an XY-addressable readout system that
is specifically tailored for the reading of superconducting flux qubits in an
integrated circuit that could enable adiabatic quantum optimization. In such a
system, the flux qubits only need to be read at the end of an adiabatic
evolution when quantum mechanical tunneling has been suppressed, thus
simplifying many aspects of the readout process. The readout architecture for
an $N$-qubit adiabatic quantum optimization system comprises $N$ hysteretic dc
SQUIDs and $N$ rf SQUID latches controlled by $2\sqrt{N} + 2$ bias lines. The
latching elements are coupled to the qubits and the dc SQUIDs are then coupled
to the latching elements. This readout scheme provides two key advantages:
First, the latching elements provide exceptional flux sensitivity that
significantly exceeds what may be achieved by directly coupling the flux qubits
to the dc SQUIDs using a practical mutual inductance. Second, the states of the
latching elements are robust against the influence of ac currents generated by
the switching of the hysteretic dc SQUIDs, thus allowing one to interrogate the
latching elements repeatedly so as to mitigate the effects of stochastic
switching of the dc SQUIDs. We demonstrate that it is possible to achieve
single qubit read error rates of $<10^{-6}$ with this readout scheme. We have
characterized the system-level performance of a 128-qubit readout system and
have measured a readout error probability of $8\times10^{-5}$ in the presence
of optimal latching element bias conditions.
