A scalable readout system for a superconducting adiabatic quantum optimizationsystem

Abstract

We have designed, fabricated and tested an XY-addressable readout system thatis specifically tailored for the reading of superconducting flux qubits in anintegrated circuit that could enable adiabatic quantum optimization. In sucha system, the flux qubits only need to be read at the end of an adiabaticevolution when quantum mechanical tunneling has been suppressed, thussimplifying many aspects of the readout process. The readout architecture for anN-qubit adiabatic quantum optimization system comprisesN hysteretic dcSQUIDs and N rf SQUID latches controlled by bias lines. The latching elements are coupled to the qubits and the dc SQUIDs are thencoupled to the latching elements. This readout scheme provides two key advantages: first,the latching elements provide exceptional flux sensitivity that significantly exceedswhat may be achieved by directly coupling the flux qubits to the dc SQUIDsusing a practical mutual inductance. Second, the states of the latching elementsare robust against the influence of ac currents generated by the switching of thehysteretic dc SQUIDs, thus allowing one to interrogate the latching elementsrepeatedly 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 ofa 128-qubit readout system and have measured a readout error probability of8 × 10 − 5 in the presence of optimal latching element bias conditions.