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arXiv:2003.04366 · SciPost Physics Lecture Notes

Bogoliubov quasiparticles in superconducting qubits

L. I. Glazman, Gianluigi Catelani

Published Jun 18, 2021• 68 citations
arXivHTMLPDF

Abstract

Extending the qubit coherence times is a crucial task in building quantum information processing devices. In the three-dimensional cavity implementations of circuit QED, the coherence of superconducting qubits was improved dramatically due to cutting the losses associated with the photon emission. Next frontier in improving the coherence includes the mitigation of the adverse effects of superconducting quasiparticles. In these lectures, we review the basics of the quasiparticles dynamics, their interaction with the qubit degree of freedom, their contribution to the qubit relaxation rates, and approaches to control their effect.

DOI
10.21468/scipostphyslectnotes.31

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Extending the qubit coherence times is a crucial task in building quantum information processing devices. In the three-dimensional cavity implementations of circuit QED, the coherence of superconducting qubits was improved dramatically due to cutting the losses associated with the photon emission. Next frontier in improving the coherence includes the mitigation of the adverse effects of superconducting quasiparticles. In these lectures, we review the basics of the quasiparticles dynamics, their interaction with the qubit degree of freedom, their contribution to the qubit relaxation rates, and approaches to control their effect.
A quantum computer will potentially solve far-reaching problems which are currently intractable on any classical computer. Many technological obstacles have prevented the realization of a quantum computer, the main obstacle being decoherence, which is the loss of quantum information. Decoherence arises from the undesired interaction between qubits and their environment. Isolated qubits have better coherence but are more difficult to control. Superconducting qubits are a promising platform since their macroscopic size allows for easy control and coupling to other qubits. While the coherence of superconducting qubits has substantially improved over the past two decades, further improvements in coherence are required.We have repeatedly and reliably increased the coherence times of superconducting qubits. Currently decoherence in these devices is dominated by coupling to material defects. These defects are present in the dielectrics used to fabricate these devices or introduced during fabrication. Using simpler resonators as a testbed, we individually isolate, characterize, and then improve each step of the more complicated fabrication of superconducting qubits. We increased the quality factor of resonators by a factor of four by first identifying the surfaces and interfaces as a major source of loss and then by optimizing the substrate preparation. Furthermore, we measure and subsequently mitigate additional defect loss, which is dependent on the position of ground plane holes used to limit the loss from magnetic vortices. Implementing these improvements led to an increase of our qubit coherence times by more than an order of magnitude.The progress made in coherence while maintaining a high degree of connectivity and controllability has been directly used in more complex circuits. One such device is a fully connected three qubit ring with both tunable qubit frequencies and adjustable qubit-qubit couplings. The considerable level of control allows us to generate the quantum dynamics resulting from a collision between Na and He atoms by encoding the problem into a time-dependent multi-qubit interaction. We compute a scattering cross section by measuring the probability of exciting the Na atom over a wide range of collision parameters. The scattering cross-section shows that there is a velocity where the excitation probability is maximized, in agreement with scattering experiments.
Quasiparticle-photon interactions in superconductors within a quantum electrodynamics (QED) cavity are crucial for quantum technologies and superconducting circuits. These transitions, driven by confined electromagnetic fields, influence quasiparticle dynamics and superconducting properties, enabling novel quantum states and hybrid quantum systems. This study investigates the transition rates for photon-quasiparticle emission from a cavity, using mathematical and numerical analysis methods. The transition rate function F(E) was evaluated for different values of normalized superconductor quasiparticle density states (n), specifically n = 1 and n = 10, across an energy gap range from −2 eV to 2 eV. The analysis reveals that for n = 1, the transition rate exhibits a pronounced peak near the Fermi energy, indicating higher transition probabilities at lower values of n due to fewer available states. Conversely, for n = 10, the transition rate profile becomes broader and the peak height diminishes, reflecting an increased number of states and a more dispersed transition rate. Additionally, the inverse temperature β shows an inverse relationship with the transition rate F(E), where higher temperatures (lower β) lead to increased transition rates due to enhanced thermal excitation. The study also examines the effective temperature TQCR of the quantum cavity resonator (QCR) environment as a function of the energy gap and photon frequency, showing that TQCR generally increases with the magnitude of the energy gap and decreases with increasing photon frequency. The scientific significance of these studies lies in their contribution to understanding the complex dynamics of photon-quasiparticle transitions, particularly how transition rates vary with energy gaps, photon frequencies, and temperature. This insight is crucial for improving coherence times and minimizing decoherence in superconducting quantum circuits. The potential application value is significant for advancing quantum computing and communication, as optimizing transition rates can lead to more efficient qubit initialization, readout, and control, ultimately enhancing the performance and scalability of quantum technologies. Received: 5 September 2024 | Revised: 28 October 2024 | Accepted: 24 February 2025 Conflicts of Interest The authors declare that they have no conflicts of interest to this work. Data Availability Statement Data are available from the corresponding author upon reasonable request. Author Contribution Statement Jayanti Chand: Conceptualization, Methodology, Software, Formal analysis, Resources, Data curation, Writing – original draft, Writing – review & editing, Visualization. Kishori Yadav: Software, Validation, Investigation, Supervision, Project administration. Saddam Husain Dhobi: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft, Writing – review & editing, Visualization, Supervision, Project administration.
This article reviews recent progress in superconducting quantum bits, including major improvements in design and coherence times. It first provides an overview of the basics of modern superconducting qubit devices and their architectures before turning to single-qubit Hamiltonians and reference frames. It then examines how decoherence originates with noise and shows how to characterize and mitigate this noise using magnetic-resonance-type pulse sequences. It also describes the first-generation superconducting qubits and the now-dominant circuit-quantum electrodynamics architecture in which qubits are coupled to microwave resonators. Finally, it considers several improved designs of superconducting qubits in which coherence times have been significantly improved by minimizing the sensitivity to fluctuating impurities and the coupling to external modes.
Abstract The fundamental excitations in superconductors—Bogoliubov quasiparticles—can be either a resource or a liability in superconducting devices: they are what enables photon detection in microwave kinetic inductance detectors, but they are a source of errors in qubits and electron pumps. To improve operation of the latter devices, ways to mitigate quasiparticle effects have been devised; in particular, combining different materials quasiparticles can be trapped where they do no harm and their generation can be impeded. We review recent developments in these mitigation efforts and discuss open questions.
Superconducting circuits exhibit quantum properties on a macroscopic scale, and are natural candidates for solid state quantum computing. Their low-energy physics can be described in terms of the phase of the order parameter, a single collective degree of freedom associated with billions of coherently paired electrons. In practice, however, on top of the superfluid condensate there are single-particle excitations (quasiparticles) with a continuous energy spectrum; the quasiparticles are coupled to the phase degree of freedom. The presence of quasiparticles in the system sets serious constraints on the performance of superconducting charge qubits. In this thesis, we study the kinetics of superconducting quantum circuits, and discuss the fundamental limitations on the energy and phase relaxation times in the presence of quasiparticles.
As low-loss nonlinear elements, Josephson junctions are the building blocks of superconducting qubits. The interaction of the qubit degree of freedom with the quasiparticles tunneling through the junction represents an intrinsic relaxation mechanism. We develop a general theory for the qubit decay rate induced by quasiparticles, and we study its dependence on the magnetic flux used to tune the qubit properties in devices such as the phase and flux qubits, the split transmon, and the fluxonium. Our estimates for the decay rate apply to both thermal equilibrium and nonequilibrium quasiparticles. We propose measuring the rate in a split transmon to obtain information on the possible nonequilibrium quasiparticle distribution. We also derive expressions for the shift in qubit frequency in the presence of quasiparticles.