Immerse a superconducting qubit in liquid helium-three to drown out the noise that destroys its quantum state. Qubits are extraordinarily sensitive to stray heat and atomic-scale defects in the materials around them, which introduce errors and limit how long the quantum information survives. After decades of incremental improvements through better device design, coherence times have stalled. This project tests a radical alternative: instead of trying to eliminate defects, bathe the circuit in a quantum fluid that simultaneously cools it and couples to the noise sources, suppressing their harmful effects. If it works, the approach could deliver a step-change in qubit performance without requiring complex error correction that currently demands many physical qubits to make one reliable logical qubit. That would accelerate progress toward fault-tolerant quantum computers capable of solving problems in drug design, materials science, and cryptography that classical computers cannot touch. The work is fundamental—much of the physics of how a quantum fluid interacts with a quantum circuit remains unknown—but the payoff is a scalable cooling and noise-mitigation strategy that industry could adopt directly.
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Quantum computers will have a transformative impact by solving hitherto intractable problems in science and enable multiple innovations across society and the economy, derived from the exponential increase in computing power. Ultimately performance and ease of implementation are primarily limited by the quality of the basic building block; the qubit. Furthermore quantum sensors will lead to a new era of discovery in fundamental science, due to step changes in detector sensitivity. Superconducting qubits provide a scalable technology, strongly favoured by industry. However, quantum states are very fragile, making the qubit highly sensitive to its environment. This includes spurious sources of energy (heat) and the quantum bath of material defects. These notoriously ubiquitous defects can broadly be categorised as two types: surface spins and two-level system defects (TLS). Both are a source of noise and decoherence in circuits and constitute a significant roadblock to technological applications. The ability to both adequately cool circuits and eliminate the deleterious effects of defects, the nature of which remains poorly understood after decades of study, are the two major obstacles towards improved coherence and large-scale quantum computing. The central research hypothesis behind this proposal is that immersion of the quantum circuit in a quantum fluid bath (for example liquid helium-three) presents an elegant, scalable, solution to both these problems. It is motivated by striking results we obtained on a simple quantum circuit (superconducting resonator) immersed in liquid helium-three. The overarching objective of this project is a systematic investigation of the suppression of decoherence in superconducting quantum circuits (qubits and resonators) cooled through immersion in a quantum fluid bath, and to achieve a fundamental understanding of its origin via the interaction of the circuit and its environment to the quantum fluids. This will be combined with strain and electric field tuning to pin-point TLS, enabling new circuit designs to optimally draw upon immersion cooling for enhanced coherence. The coupling between qubit and the quantum fluid bath provides a new pathway to mitigate decoherence. In contrast over the last decades, mitigation of decoherence by TLS through device design has yielded most of the improvements in coherence times. The materials science of eliminating TLS is a major future challenge, now receiving much attention, here with a completely new tool at our disposal. Thus far the relative stagnation in coherence times has driven an approach to quantum computing with error correction in which many physical qubits are required to realise a single logical qubit. Furthermore, we aim to identify the optimal quantum bath conditions at which to operate circuits for enhanced performance. Through engagement with the theoretical physics community, we aim to develop testable hypotheses for the quantum fluid-quantum circuit interaction, to help guide the experimental programme towards most efficiently achieving the main objective: a step-change in qubit performance. We aim to significantly advance the understanding of properties of amorphous dielectrics in quantum circuits (nature of TLS and their interactions), in particular on surfaces. Finally, we will investigate the feasibility of quantum fluid immersion scalability for quantum computers, with a view to accelerate the impact of our fundamental research. In this work quantum circuits meet quantum fluids, and much fundamental work remains to unpick the underlying mechanisms at play. The promise of performance optimisation lies in the tunability of the quantum fluid and its interface with the quantum circuit. We therefore believe that the success of this project will trigger a step-change in the progress towards fault-tolerant quantum computing.
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