Measurement and Control of Quasiparticle Dynamics in a Superconducting Qubit (1406.7300v2)

Abstract: Superconducting circuits have attracted growing interest in recent years as a promising candidate for fault-tolerant quantum information processing. Extensive efforts have always been taken to completely shield these circuits from external magnetic field to protect the integrity of superconductivity. Surprisingly, here we show vortices can improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we quantitatively distinguish between recombination and trapping mechanisms in controlling the dynamics of residual quasiparticles, and show quantized changes in quasiparticle trapping rate due to individual vortices. These results highlight the prominent role of quasiparticle trapping in future development of superconducting qubits, and provide a powerful characterization tool along the way.

Measurement and Control of Quasiparticle Dynamics in a Superconducting Qubit: An Analytical Overview

This paper presents a detailed paper of the role of quasiparticles (QPs) in the performance characteristics of superconducting qubits, detailing a method to measure and control their dynamics for optimizing qubit coherence. The research primarily investigates how the presence and control of vortices can influence the QP dynamics and, consequently, the coherence properties of superconducting qubits.

Quasiparticle Dynamics and Superconducting Qubits

Superconducting qubits are advancing rapidly as viable elements for quantum computing architectures, primarily due to their potential for fault tolerance. However, the presence of quasiparticles—single electron-like excitations within the superconducting state—has been recognized as a significant limiting factor in achieving long qubit coherence times. Quasiparticles can give rise to energy relaxation and decoherence phenomena, with rates proportional to QP density.

Operating at temperatures as low as 20 mK, superconducting systems theoretically maintain negligible QP populations. Nonetheless, substantial quasiparticle backgrounds have been observed in diversified quantum circuits including Cooper-pair transistors and kinetic inductance detectors, illustrating the sustained challenge posed by non-equilibrium quasiparticles.

Methodology and Results

A core contribution of the paper is the implementation of a contactless QP injection technique within a 3D transmon qubit setup, permitting high dynamic range measurements of QP densities. Through this setup, the paper differentiates between recombination and trapping mechanisms governing QP dynamics. The research reveals that QP trapping by vortices, understood as regions with diminished superconducting gaps, significantly impacts the coherence properties of qubits.

The researchers demonstrate a strong correlation between vortex-induced quasiparticle trapping and reduced QP densities, leading to notably improved qubit coherence times. Specifically, the introduction of vortices is shown to enhance the relaxation and coherence times (T1 and T2E, respectively) by more than a factor of two under certain geometrical designs. The intrinsic trapping power of individual vortices is calculated as $P = (6.7\pm0.5) \times 10^{-2}$ cm²s⁻¹, a quantification pivotal to understanding QP dynamics.

Implications and Future Directions

The paper’s findings underscore the critical influence of vortex trapping in the dynamics of quasiparticles and provide a valuable framework for optimizing superconducting circuits' design and operation. The measured recombination constant and the QP generation rates quantified offer insights for the design of future devices aiming to mitigate the effects of non-equilibrium QP populations.

This research opens pathways for enhanced device engineering in quantum computing applications, emphasizing the practical merit of controlled vortex insertion to suppress unwanted QP dynamics. Future investigations might explore alternative trapping mechanisms, perhaps through engineered materials or geometric configurations, further enhancing the coherence properties of superconducting quantum systems.

Conclusions

By elucidating the dynamics of quasiparticles and their control via vortices, this paper contributes significantly to the refinement of superconducting qubits aimed at large-scale quantum computing implementations. The techniques and insights derived from this paper provide a robust base for future explorations into more efficient qubit architectures, ensuring lower QP densities and enhanced device stability.