Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier

High-fidelity and rapid readout of a qubit state is key to quantum computing and communication, and it is a prerequisite for quantum error correction. We present a readout scheme for superconducting qubits that combines two microwave techniques: applying a shelving technique to the qubit that reduces the contribution of decay error during readout, and a two-tone excitation of the readout resonator to distinguish among qubit populations in higher energy levels. Using a machine-learning algorithm to post-process the two-tone measurement results further improves the qubit-state assignment fidelity. We perform single-shot frequency-multiplexed qubit readout, with a 140 ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout-without using a quantum-limited amplifier.

Automated summary

High-fidelity and rapid readout of qubit state is crucial for quantum computing and communication, and a prerequisite for quantum error correction. We propose a readout scheme for superconducting qubits that combines a shelving technique and two-tone excitation of the readout resonator. Using a machine-learning algorithm to post-process the measurement results further improves the fidelity of qubit-state assignment. We demonstrate single-shot frequency-multiplexed qubit readout with a 140 ns readout time and achieve high assignment fidelity without using a quantum-limited amplifier.