Abstract: While there hasbeen tremendous recent progress in the realization of small-scale quantumcircuits comprising of order 10 qubits, research indicates that afault-tolerant quantum computer that exceeds what is possible on existingclassical machines will require a network of thousands or millions of qubits,far beyond current capabilities. Robust approaches to the measurement andcontrol of large-scale next-generation quantum processors have yet to bedeveloped.
In this talk I describean experimental program to develop high-fidelity qubit measurement and controlcircuitry based on the superconducting Single Flux Quantum (SFQ) digital logicfamily. Qubit measurement is performed by mapping the state to the microwavephoton occupation of a linear resonator followed by subsequent photodetectionwith a microwave photon counter. This scheme provides access to the binarydigital output of qubit measurement at the millikelvin experimental stage,without the need for room-temperature heterodyne measurement and thresholding. Rawsingle-shot measurement fidelity up to 92% has been achieved, limited by qubitrelaxation. Moreover, we exploit the intrinsic damping of the counter to efficientlyextract the energy released by the measurement process, allowing fast, repeatedhigh-fidelity measurements.
Coherent controlof the qubit is performed using trains of quantized SFQ flux pulses. Each ofthese pulses provides a delta function-like kick to the qubit, inducing atrajectory on the Bloch sphere that can be tailored to minimize leakage errors.We describe the fabrication and characterization of a transmon qubit chip incorporatingan on-chip SFQ pulse driver. We achieve gate fidelity up to around 98%, limitedby quasiparticle generation from the dissipative SFQ pulse driver. I discussnext-generation designs that are expected to significantly mitigatequasiparticle poisoning.
These two effortspoint a direction toward the integration of a large-scale superconductingquantum processor with proximal classical superconducting logic for thepurposes of reducing latency, wiring heat load, and overall system footprint.
Host: Vladimir Manucharyan