Automated Tuning of Tunnel Couplings and Gate Operations for Semiconductor Spin Qubits

Quantum computers require large numbers of qubits with access to high-fidelity initialization, control and readout. While spin qubits in gate-defined quantum dots have demonstrated many of these properties, they need to be individually tuned by adjusting the voltages applied to electrostatic gates. This is a manual and time-consuming process [1] which will become unfeasible for large arrays of qubits. To address this challenge we present an iterative machine-learning algorithm [2] for automated fine-tuning of quantum dots. We estimate the system response by Bayesian updates and thus reduce the number of required measurements. Benchmarks show that the algorithm is able to adjust the tunnel and lead couplings in a singlet-triplet (ST) qubit in GaAs within 5 iterations.

In addition to the calibration of static dot parameters, high-fidelity quantum operations require precise tuning of time-dependent voltage pulses with respect to incoherent and coherent errors. While decoherence can be reduced by numerical pulse engineering, inaccuracies in the qubit model used for pulse engineering can cause coherent errors in the experimental implementation. In order to remove these errors, we present a self-consistent calibration routine for single- and two-qubit gates [2], which can be extended to a larger number of qubits and different sets of quantum gates. We apply this approach to a ST qubit in GaAs, and show experimentally that single qubit gate fidelities of 99.5% can be reached [3]. We use an accurate qubit model to simulate the performance of the calibration routine, and show that two-qubit gate fidelities of 99.9% (99.99%) should be achievable in GaAs (Si).

[1] Botzem et al., Physical Review Applied 10, 054026 (2018)
[2] All our software packages maintaining high software-engineering standards are available open source, see www.quantuminfo.physik.rwth-aachen.de/code
[3] Unpublished results relating to Cerfontaine et al., arXiv:1606.01897