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. 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. Building on this result, we first evaluate to what extent the readout technique is quantum-non-demolition (QND), an essential requirement for quantum error correction protocols. We find that the method is QND even if we excite the qubits to higher states before readout. Next, we investigate how to exploit the uncertainty in qubit-state discrimination. A feedforward neural network (FNN) classifier used to post-process the measurement result readily offers confidence information in the qubit-state assignment. We explore how to improve minimum weight matching decoders by incorporating this information in the weights of graph edges that correspond to stabilizer measurement errors.
Diskussionsledare: Assistant Professor Simone Gasparinetti
Examinator: Professor Per Delsing
Handledare: Dr Giovanna Tancredi and Associate Professor Jonas Bylander
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