Active Resonator Reset in the Nonlinear Dispersive Regime of Circuit QED

Abstract

We present two pulse schemes to actively deplete measurement photons from a readout resonator in the nonlinear dispersive regime of circuit QED. One method uses digital feedback conditioned on the measurement outcome, while the other is unconditional. In the absence of analytic forms and symmetries to exploit in this nonlinear regime, the depletion pulses are numerically optimized using the Powell method. We speed up photon depletion by more than six inverse resonator linewidths, saving approximately 1650 ns compared to depletion by waiting. We quantify the benefit by emulating an ancilla qubit performing repeated quantum-parity checks in a repetition code. Fast depletion increases the mean number of cycles to a spurious error detection event from order 1 to 75 at a 1-μs cycle time.

Efficient protocol for qubit initialization with a tunable environment

“…Qubit initialization can also be achieved with feedback-based schemes, 11,[19][20][21][22] which rely on a single-shot measurement of the qubit state. In recent experiments, 19,20 the discrimination of the qubit state is predominantly limited by T 1 , with the current benchmark for the average assignment fidelity being 99.8% in 700-ns read-out time.…”

Quantum-circuit refrigerator

“…If demonstrated in a superconducting qubit, these numbers would represent a clear improvement in the present state-of-the-art initialization schemes [14][15][16][17][18][19][20] . Interestingly, Fig.…”

“…This device builds on the decade-long development of circuit quantum electrodynamics [7][8][9][10] , that is, the study of superconducting quantum bits, qubits, coupled to on-chip microwave resonators [11][12][13] . Although several methods have been demonstrated to initialize superconducting qubits [14][15][16][17][18][19][20] and resonators 21 , they are typically suited only for a very specific type of a system and the achieved fidelities fall below the demanding requirements of efficient fault-tolerant quantum computing. Thus, circuit quantum electrodynamics provides an ideal context for the demonstration of a quantum refrigerator.…”

Restless Tuneup of High-Fidelity Qubit Gates

“…With the exception of a few systems known with metrological precision [7], pulsing requires meticulous calibration by closed-loop tuning, i.e., pulse adjustment based on experimental observations. Numerical optimization algorithms have been implemented to solve a wide range of tuning problems with a cost-effective number of iterations [8][9][10][11][12][13]. However, relatively little attention has been given to quantitatively exploring the speed and robustness of the algorithms used.…”