Environmental radiation impact on lifetimes and quasiparticle tunneling rates of fixed-frequency transmon qubits

Abstract

Quantum computing relies on the operation of qubits in an environment as free of noise as possible. Assessing the quality of this environment is a key aspect of ensuring high-fidelity implementations based on superconducting qubits. Relaxation, decoherence, dephasing, and quasiparticle tunneling rates have been measured for various shielding configurations used in the measurement environment for state-of-the-art transmon qubits. An ensemble of approximately 120 control devices was used for this study, with five different capacitor pad designs. The shielding elements varied in the configuration included an indium gasket at the qubit can's lid, Cryoperm magnetic shielding, the mixing chamber shield of the dilution refrigerator, the inclusion of a vacuum pump-out port, and capping unused subminiature version A connectors at the top of the measurement can's lid. It was found that the qubit lifetimes T1, T2, and Tϕ are robust to the all of configuration changes tried until the mixing chamber shield was removed, significantly increasing blackbody radiation levels in the qubit measurement space, where in that limit it was found that tapering the qubit pads reduced the amount of loss. In contrast, the quasiparticle tunneling rates were found to be extremely sensitive to all configuration changes tested. Consistent with earlier reports [McEwen et al., arXiv:2104.05219 (2021); Cardani et al., Nat. Commun. 12, 2733 (2021); Wilen et al., Nature 594, 369–373 (2021); Ristè et al. Nat. Commun. 4, 1913 (2013)], the findings from this study indicate that non-equilibrium quasiparticles do not currently limit the lifetimes of well-shielded transmon qubits.

Material matters in superconducting qubits

“…Enhanced RF filtering along the microwave lines with Eccosorb and additional Al shielding surrounding Al 3D offset-charge transmon qubits led to a near doubling of quality factor and an increase in average quasiparticle tunneling time by two orders of magnitude [71], [287]. Measurements performed on Nb transmons showed that, while quasiparticle tunneling rates were highly sensitive to both the degree of shielding as well as the shunting capacitor size and shape, their relaxation times were not significantly impacted until the mixing chamber shield had been removed [69]. Modeling of spurious antenna modes that can be generated within transmon qubit designs at frequencies much larger than that associated with their |0⟩-|1⟩ transition have been conducted, suggesting that quasiparticle creation from radiation with these energies can produce an effective blackbody temperature of approximately 300 mK [164].…”

“…FIG. 4: Comparison of quality factors of transmon qubits fabricated using different deposition methods and superconducting metallization: sputtered [68], [69], evaporated [70], [71] and MBE [72].…”

Combating fluctuations in relaxation times of fixed-frequency transmon qubits with microwave-dressed states

“…With the advantages of long coherence times [1][2][3][4] and simplified demands on the electronic control circuits, fixedfrequency transmon qubit [5] has been demonstrated as a leading qubit modality for quantum computing [1]. This can be partially manifested by the progress that diverse qubit architectures, which utilize fixed-frequency transmon qubits, have been demonstrated with high-fidelity gate performance, such as all-microwave controlled qubit architecture with fixed inter-qubit coupling [6][7][8][9][10][11][12][13], qubit architecture with tunable bus [14] or tunable coupler [15,16], and qubit architecture combining both fixed-frequency qubits and frequency-tunable qubits [17].…”

“…Meanwhile, in conditions where qubit relaxations contain non-negligible contributions from quasiparticles, the variations of qubit relaxation times could also be attributed to the fluctuation in the quasiparticle density near the qubit junctions [27][28][29] or the fluctuations in quasiparticle dissipation channels [30,31]. Nevertheless, for well-shielded transmon qubit devices, transmon qubits have been shown not yet limited by losses related to quasiparticles [4]. Additionally, a recent result also suggests that quasiparticles trapped in shallow subgap states can also behave similarly to TLS [32].…”

Scalable error mitigation for noisy quantum circuits produces competitive expectation values

“…However, the hardware-platform has witnessed tremendous improvements since this initial demonstration. The size of the largest quantum processors based on this architecture now lies at 65 qubits [9], reported transmon coherence times are exceeding several hundred microseconds [27], cross-resonance gate fidelities are approaching 99.9 % with sub-100 ns gate times [28], with steady progress in holistic metrics such as quantum volume [29] as well. This raises a tantalizing question of how much these hardware improvements could extend the reach of zero-noise extrapolation to perform accurate quantum computation at a larger scale, into the realm of quantum advantage.…”