Abstract
One of the main limitations in state-of-the art solid-state quantum processors is qubit decoherence and relaxation due to noise from adsorbates on surfaces, impurities at interfaces, and material defects. For the field to advance towards full fault-tolerant quantum computing, a better understanding of these microscopic noise sources is therefore needed. Here, we use an ultra-high vacuum package to study the impact of vacuum loading, UV-light exposure, and ion irradiation treatments on relaxation and coherence times, as well as slow parameter fluctuations of flux tunable superconducting transmon qubits. The treatments studied do not significantly impact the relaxation rate Γ1 and the echo decay rate {{{Gamma }}}_{2,{{{rm{SS}}}}}^{{{{rm{e}}}}} at the sweet spot, except for Ne ion bombardment which reduces Γ1. In contrast, flux noise parameters are improved by removing magnetic adsorbates from the chip surfaces with UV-light and NH3 treatments. Additionally, we demonstrate that SF6 ion bombardment can be used to adjust qubit frequencies in situ and post-fabrication without affecting qubit relaxation and coherence times at the sweet spot.
Highlights
Current solid-state quantum processors are still limited by intrinsic error mechanisms that lead to the relaxation of the qubit state, the loss of phase coherence over time, i.e. dephasing, as well as readout and unitary errors when performing gate operations[1]
One distinguishes two basic effects: energy exchange with the environment which is characterized by the relaxation time T1, as well as pure dephasing, where the qubit level splitting f01 is affected by environmental fluctuations that lead to the decay of the phase of superposition states on a timescale Tφ in the ensemble average over many repeated experiments
We have studied a variety of surface treatments and their effect on coherence and noise parameters of flux-tunable transmon qubits using a UHV package
Summary
Current solid-state quantum processors are still limited by intrinsic error mechanisms that lead to the relaxation of the qubit state, the loss of phase coherence over time, i.e. dephasing, as well as readout and unitary errors when performing gate operations[1]. Frequency tunable qubits that either include a magnetic field tunable SQUID5–8 or an electric field tunable semiconductor–superconductor hybrid junction[9,10,11,12], are typically more sensitive to environmental fluctuations when tuned away from sweet-spots which can impact coherence times. Our results show the different effects of the treatments on the outlined coherence parameters and indicate that ion milling can be used to trim the qubit frequency of fixed frequency qubits after fabrication without a significant reduction in coherence
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