Beating the thermal limit of qubit initialization with a Bayesian Maxwell's demon

Abstract

Fault-tolerant quantum computing requires initializing the quantum register in a well-defined fiducial state. In solid-state systems, this is typically achieved through thermalization to a cold reservoir, such that the initialization fidelity is fundamentally limited by temperature. Here, we present a method of preparing a fiducial quantum state with a confidence beyond the thermal limit. It is based on real time monitoring of the qubit through a negative-result measurement -- the equivalent of a `Maxwell's demon' that triggers the experiment only upon the appearance of a qubit in the lowest-energy state. We experimentally apply it to initialize an electron spin qubit in silicon, achieving a ground-state initialization fidelity of 98.9(4)%, corresponding to a 20$\times$ reduction in initialization error compared to the unmonitored system. A fidelity approaching 99.9% could be achieved with realistic improvements in the bandwidth of the amplifier chain or by slowing down the rate of electron tunneling from the reservoir. We use a nuclear spin ancilla, measured in quantum nondemolition mode, to prove the value of the electron initialization fidelity far beyond the intrinsic fidelity of the electron readout. However, the method itself does not require an ancilla for its execution, saving the need for additional resources. The quantitative analysis of the initialization fidelity reveals that a simple picture of spin-dependent electron tunneling does not correctly describe the data. Our digital `Maxwell's demon' can be applied to a wide range of quantum systems, with minimal demands on control and detection hardware.

15 pages, 7 figures

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Cited by in corpus (13)

• Review of performance metrics of spin qubits in gated semiconducting nanostructures
• High-fidelity operation and algorithmic initialisation of spin qubits above one kelvin
• Measurement-based quantum thermal machines with feedback control
• Real-time two-axis control of a spin qubit
• Feedback-based active reset of a spin qubit in silicon
• Scalable entanglement of nuclear spins mediated by electron exchange
• Error channels in quantum nondemolition measurements on spin systems
• Efficient adaptive Bayesian estimation of a slowly fluctuating Overhauser field gradient
• A SWAP Gate for Spin Qubits in Silicon
• Optimally Fast Qubit Reset
• Spin-qubit readout analysis based on a hidden Markov model
• Electron readout contrast enhancement in the parallel nuclear regime of an exchange-coupled donor spin qubit system
• Maximizing the nondemolition nature of a quantum measurement via an adaptive readout protocol