Abstract
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 
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 × 10 − 5
in the presence of optimal latching element bias conditions.