Resisting high-energy impact events through gap engineering in superconducting qubit arrays (2402.15644v2)

Abstract: Quantum error correction (QEC) provides a practical path to fault-tolerant quantum computing through scaling to large qubit numbers, assuming that physical errors are sufficiently uncorrelated in time and space. In superconducting qubit arrays, high-energy impact events produce correlated errors, violating this key assumption. Following such an event, phonons with energy above the superconducting gap propagate throughout the device substrate, which in turn generate a temporary surge in quasiparticle (QP) density throughout the array. When these QPs tunnel across the qubits' Josephson junctions, they induce correlated errors. Engineering different superconducting gaps across the qubit's Josephson junctions provides a method to resist this form of QP tunneling. By fabricating all-aluminum transmon qubits with both strong and weak gap engineering on the same substrate, we observe starkly different responses during high-energy impact events. Strongly gap engineered qubits do not show any degradation in T1 during impact events, while weakly gap engineered qubits show events of correlated degradation in T1. We also show that strongly gap engineered qubits are robust to QP poisoning from increasing optical illumination intensity, whereas weakly gap engineered qubits display rapid degradation in coherence. Based on these results, gap engineering removes the threat of high-energy impacts to QEC in superconducting qubit arrays.

• The paper presents gap engineering as a technique to mitigate quasiparticle-induced errors in superconducting qubit arrays during high-energy impact events.
• Strongly gap-engineered qubits maintain T1 coherence under high-energy impacts and optical-induced quasiparticle poisoning, unlike weakly engineered ones.
• The research offers a scalable approach to quantum error correction by integrating gap engineering into existing transmon fabrication methods.

Overview of Gap Engineering in Superconducting Qubit Arrays for Resilience Against High-Energy Impact Events

The paper "Resisting high-energy impact events through gap engineering in superconducting qubit arrays," presents a novel approach to mitigating correlated errors in superconducting qubit arrays caused by high-energy impact events. The researchers focus on gap engineering—a technique that involves varying the superconducting gap across the Josephson junctions of qubits—to suppress the primary mechanisms of quasiparticle (QP) tunneling that induce error bursts.

Key Findings

The paper illustrates a significant contrast in performance between strongly and weakly gap-engineered transmon qubits when subjected to high-energy impact events. Strongly gap-engineered qubits exhibit no degradation in the energy relaxation time ($T_1$) during high-energy impacts, demonstrating robust immunity to QP-induced correlated errors. Conversely, weakly gap-engineered qubits show clear signs of correlated $T_1$ decay.

The paper reports that strongly gap-engineered qubits also remain unaffected by QP poisoning caused by increased optical illumination, showing resilience where the weakly gap-engineered counterparts fail. This resilience is attributed to the higher superconducting gap in the junction region, which creates an energy barrier that QPs cannot surmount, thereby preventing $T_1$ degradation.

Experimental Methods

The authors fabricated an array of 12 transmon qubits using all-aluminum substrates, with both layers of varying thickness to achieve differing levels of gap engineering. Measurements were conducted using rapid repetitive correlated sampling to identify and assess the effects of high-energy impacts on qubit error rates.

In addition, they employed optical illumination to artificially elevate QP densities, thereby assessing the qubits' coherence under controlled conditions. Notably, strongly gap-engineered qubits maintained coherent function under optical powers capable of severely affecting weakly engineered qubits, highlighting the effectiveness of this approach.

Implications and Future Directions

Gap engineering as demonstrated in this research offers a straightforward method to mitigate high-energy impact-related errors without the complexity associated with phonon or QP trapping techniques. This approach effectively addresses a major potential roadblock in scaling quantum error correction (QEC) techniques to achieve fault-tolerant quantum computing.

Despite the advancements shown, further exploration into reducing QP population through alternative methods remains necessary. Techniques like phonon trapping will be crucial to address other sources of loss, including QP scattering in the capacitor material. Additionally, since the gap engineering approach does not resolve TLS scrambling due to high-energy events, it remains vital to improve TLS density or implement dynamic frequency tuning techniques for real-time recalibration.

The paper's findings indicate that future QEC experiments can proceed without being limited by high-energy impact-induced error floors, thus enhancing the scalability and reliability of superconducting qubit systems. This development promises a significant step toward implementing large-scale quantum computations in practical settings. The ease of incorporating gap engineering into current superconducting qubit fabrication processes further accentuates its potential widespread adoption in the field of quantum technology.