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arXiv:2202.01435 · Nature Communications

Engineering superconducting qubits to reduce quasiparticles and charge noise

Xianchuang Pan, Yuxuan Zhou, Haolan Yuan, Lifu Nie, Weiwei Wei, Libo Zhang, Jian Li, Song Liu, Zhi Hao Jiang, Gianluigi Catelani, Ling Hu, Fei Yan, Dapeng Yu

Published Nov 23, 2022• 70 citations
arXivHTMLPDF

Abstract

Identifying, quantifying, and suppressing decoherence mechanisms in qubits are important steps towards the goal of engineering a quantum computer or simulator. Superconducting circuits offer flexibility in qubit design; however, their performance is adversely affected by quasiparticles (broken Cooper pairs). Developing a quasiparticle mitigation strategy compatible with scalable, high-coherence devices is therefore highly desirable. Here we experimentally demonstrate how to control quasiparticle generation by downsizing the qubit, capping it with a metallic cover, and equipping it with suitable quasiparticle traps. Using a flip-chip design, we shape the electromagnetic environment of the qubit above the superconducting gap, inhibiting quasiparticle poisoning. Our findings support the hypothesis that quasiparticle generation is dominated by the breaking of Cooper pairs at the junction, as a result of photon absorption by the antenna-like qubit structure. We achieve record low charge-parity switching rate (<1 Hz). Our aluminium devices also display improved stability with respect to discrete charging events.

DOI
10.1038/s41467-022-34727-2

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Identifying, quantifying, and suppressing decoherence mechanisms in qubits are important steps towards the goal of engineering a quantum computer or simulator. Superconducting circuits offer flexibility in qubit design; however, their performance is adversely affected by quasiparticles (broken Cooper pairs). Developing a quasiparticle mitigation strategy compatible with scalable, high-coherence devices is therefore highly desirable. Here we experimentally demonstrate how to control quasiparticle generation by downsizing the qubit, capping it with a metallic cover, and equipping it with suitable quasiparticle traps. Using a flip-chip design, we shape the electromagnetic environment of the qubit above the superconducting gap, inhibiting quasiparticle poisoning. Our findings support the hypothesis that quasiparticle generation is dominated by the breaking of Cooper pairs at the junction, as a result of photon absorption by the antenna-like qubit structure. We achieve record low charge-parity switching rate (<1 Hz). Our aluminium devices also display improved stability with respect to discrete charging events.
Suppression of Quasiparticle Poisoning to 10−1110^{-11}10−11 Levels in Superconducting Qubits via Infrared Shielding
Quasiparticle poisoning bottlenecks superconducting qubits, limiting coherence and the scalability of quantum processors. In this work, we systematically investigate quasiparticle poisoning in superconducting qubits under three infrared (IR) shielding configurations, ranging from a dedicated multi-layer design to a simplified implementation. By measuring quasiparticle-induced parity switching, we demonstrate a suppression of the switching rate by over four orders of magnitude via the implementation of improved shielding. In the best configuration, the rate decreases over time following cooldown and reaches 0.069 \,Hz on day 34, corresponding to an anticipated quasiparticle density per Cooper pair of 1.88×10−111.88\times10^{-11}1.88×10−11. To our knowledge, this represents the lowest quasiparticle density reported in the literature to date. The remaining quasiparticle population is likely dominated by sporadic phonon bursts stemming from mechanical stress release in the on-chip films, as well as from the surrounding environment. The effective qubit temperature follows the phonon bath down to 17 \,mK, enabling initialization errors of ∼0.01%\sim 0.01\%∼0.01% for 3 \,GHz qubits. These results demonstrate that proper IR shielding and thermalization are essential for suppressing quasiparticle poisoning and enabling high-coherence, scalable superconducting qubit systems.
The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1 μm^{-3}, tens of orders of magnitude greater than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and can induce dephasing. Here we show that a dominant mechanism for quasiparticle poisoning is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity of the qubit. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticles. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning.
The performance of various superconducting devices operating at ultra-low temperatures is impaired by the presence of non-equilibrium quasiparticles. Inelastic quasiparticle (QP) tunneling across Josephson junctions in superconducting qubits results in decoherence and spurious excitations and, notably, can trigger correlated errors that severely impede quantum error correction. In this work, we use "gap engineering" to suppress the tunneling of low-energy quasiparticles in Al-based transmon qubits, a leading building block for superconducting quantum processors. By implementing potential barriers for QP, we strongly suppress QP tunneling across the junction and preserve charge parity for over 10310^3103 seconds. The suppression of QP tunneling also results in a reduction in the qubit energy relaxation rates. The demonstrated approach to gap engineering can be easily implemented in all Al-based circuits with Josephson junctions.
We study single-Cooper-pair-transistors in which the island is made from an ultra-thin (10nm) aluminum film. The value of the superconducting gap (2Delta) changes dramatically with film thickness therefore allowing different superconducting gaps for the islands and leads simply by using different film thicknesses. Devices engineered in this way can be made less susceptible to having a single quasiparticle trapped on the island (often known as the quasiparticle poisoning problem). This is an advantage for charge-based superconducting qubits where quasiparticle poisoning is a source of decoherence. We perform radio-frequency measurements and observe 2e-periodic supercurrent peaks as a function of gate charge. With the device biased on a supercurrent oscillation, a fast random-telegraph signal due to individual quasiparticle poisoning events is observed. By analyzing the statistics of the poisoning and un-poisoning events we determine the energetics and thermal activation of these processes
A known source of decoherence in superconducting qubits is the presence of broken Cooper pairs, or quasiparticles. These can be generated by high-energy radiation, either present in the environment or purposefully introduced, as in the case of some hybrid quantum devices. Here, we systematically study the properties of a transmon qubit under illumination by focused infrared radiation with various powers, durations, and spatial locations. Despite the high energy of incident photons, our observations agree well with a model of low-energy quasiparticle dynamics dominated by trapping. This technique can be used for understanding and potentially mitigating the effects of high-energy radiation on superconducting circuits with a variety of geometries and materials.
The field of quantum computing is growing at a rapid rate with the promise of dramatic improvements in the ability to solve critical computational problems. A leading approach for implementing fault-tolerant quantum computers is based on superconducting circuits containing Josephson junctions to form qubits. These qubits have many attractive qualities, but predicting the device performance requires detailed knowledge of the junction properties. Thus, precise characterization is crucial for realizing different qubit designs. The coherence of superconducting qubits is limited by several sources in the circuit en- vironment. Building a quantum computer requires effort to reduce gate errors caused by decoherence. This is typically achieved with quantum error correction. An alternative ap- proach involves designing qubits that are intrinsically protected from noise sources. One such design is the charge-parity qubit, which combines Josephson junctions in a novel regime. The capacitance of the junctions is a critical parameter for this qubit design that directly affects the engineered protection against decoherence. Beyond the conventional geometric capaci- tance, there is an excess contribution from virtual quasiparticle tunneling; we refer to this component as the electronic capacitance. This thesis focuses on the characterization of the capacitance of Josephson junctions, with the goal of isolating and measuring the electronic capacitance contribution, particularly for use in the charge-parity qubit. The thesis first reviews the initial implementation of the charge-parity qubit and the challenges that were encountered. Next, the theoretical origin is described and a framework for measuring the junction capacitance using superconducting resonant circuits is described. This technique is then applied to arrays of junctions fabricated with different parameters and the scaling of the electronic capacitance is presented. Finally, based on this knowledge of the junction capacitance, second-generation charge-parity qubits are implemented and the experimental results are presented.