However, in many architectures, the qubits cannot be operated at the sweet spot indefinitely while performing logic operations 18, 19. In order to protect the qubits from flux noise, these qubits are typically operated at bias points where their frequency is first-order insensitive to small changes in flux, colloquially referred to as a sweet spot, see Fig. Due to their design, transmons are mostly insensitive to charge noise 15, but suffer from noise in magnetic flux, which is used to tune their frequency. In our experiment, we employ flux-tunable transmon qubits 15, which are widely used in contemporary superconducting quantum processors 16, 17. These noise sources are primarily intrinsic and local to the device 10, 11-as opposed to noise in the control electronics-though recently there has been some evidence of correlated noise between the qubits 12, 13, 14. The dominant source of decoherence in superconducting qubits is typically either charge noise or flux noise but depending on the qubit design also photon shot noise 8 or Bogoliubov quasiparticles may contribute 9. 7 employs a slightly different approach in a trapped-ion system, where spectator qubits are used to probe spatially correlated errors in the control laser amplitude and targeting. 6, it was further shown that this method can be used to decouple an electron spin from low-frequency magnetic field fluctuations, resulting in improved coherence times. 5, a method based on interleaving the probing sequences with separate periods of time used for the computation was introduced and demonstrated to mitigate the effect of slow magnetic field fluctuations. These experiments are based on continuous monitoring of the system, which inevitably decoheres to the quantum state. There is an increasing number of experiments where feedback is used to modify the evolution of quantum systems, such as stabilizing the motion of the atoms in optical cavities 2, cooling quantum mechanical resonators 3, or stabilizing Rabi oscillations in superconducting qubits 4. For many qubit modalities, including superconducting qubits, the qubit frequencies and their controls are subject to temporally correlated noise-most notably 1/ f-type noise 1-resulting in correlated errors and slow drifts in frequency.Ĭlosed-loop feedback control is ubiquitously used in a wide variety of engineering applications. High-fidelity single- and two-qubit gates are a prerequisite for high-depth circuits and quantum error correction. This approach is helpful in large qubit grids, where frequency crowding and parasitic interactions between the qubits limit their performance. Importantly, the resulting high-fidelity operation remains effective even away from the qubit flux-noise insensitive point, significantly increasing the frequency bandwidth over which the qubit can be operated with high fidelity. Here we experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26% and reducing the single-qubit error rate from (8.5 ± 2.1) × 10 −4 to (5.9 ± 0.7) × 10 −4. With recent advances in qubit control, both single- and two-qubit gate fidelities are now in many cases limited by the coherence times of the qubits. In order for the processor to reach practical viability, the gate errors need to be further suppressed and remain stable for extended periods of time. Superconducting qubits are a promising platform for building a larger-scale quantum processor capable of solving otherwise intractable problems.
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