Silencing the noise in quantum circuits by a Quantum fluid Bath - SQuBa

量子计算机将通过解决迄今为止难以解决的科学问题，并通过计算能力的指数级增长，在社会和经济领域实现多种创新，从而产生变革性的影响。最终，性能和实现的易用性主要受到基本构建块质量的限制；量子位。此外，由于探测器灵敏度的阶跃变化，量子传感器将引领基础科学发现的新时代。超导量子比特提供了一种可扩展的技术，受到工业界的强烈青睐。然而，量子态非常脆弱，这使得量子比特对环境非常敏感。这包括虚假的能量来源（热）和物质缺陷的量子浴。这些众所周知的普遍缺陷可以大致分为两种类型：表面自旋和两级系统缺陷（TLS）。两者都是电路中的噪声和退相干的来源，并构成技术应用的重大障碍。充分冷却电路和消除缺陷的有害影响的能力，经过几十年的研究，其性质仍然知之甚少，是改进相干性和大规模量子计算的两个主要障碍。这一提议背后的核心研究假设是，将量子电路浸入量子流体浴（例如液态氦- 3）中，为这两个问题提供了一种优雅的、可扩展的解决方案。它的动机是我们在一个简单的量子电路（超导谐振器）上获得的惊人结果，浸入液态氦- 3。该项目的总体目标是系统地研究通过浸泡在量子流体浴中冷却的超导量子电路（量子比特和谐振器）中的退相干抑制，并通过电路及其环境与量子流体的相互作用实现对其起源的基本理解。这将与应变和电场调谐相结合，以实现定点TLS，使新的电路设计能够最佳地利用浸入式冷却来增强相干性。量子比特与量子流体浴之间的耦合为减轻退相干提供了一条新的途径。相比之下，在过去的几十年里，通过器件设计缓解TLS的退相干已经产生了相干时间的大部分改进。消除TLS的材料科学是未来的主要挑战，现在受到了广泛关注，在这里我们有一个全新的工具可供使用。到目前为止，相干时间的相对停滞已经推动了一种带有纠错的量子计算方法，在这种方法中，需要许多物理量子位来实现单个逻辑量子位。此外，我们的目标是确定最佳的量子浴条件，以提高电路的性能。通过与理论物理界的合作，我们的目标是为量子流体-量子电路相互作用开发可测试的假设，以帮助指导实验计划最有效地实现主要目标：量子位性能的阶跃变化。我们的目标是显著推进对量子电路中非晶介电体性质的理解（TLS的性质及其相互作用），特别是在表面上。最后，我们将研究量子流体浸泡可扩展性在量子计算机上的可行性，以期加速我们基础研究的影响。在这项工作中，量子电路遇到了量子流体，许多基础工作仍然需要解开其中的潜在机制。性能优化的前景在于量子流体及其与量子电路接口的可调性。因此，我们相信这个项目的成功将引发容错量子计算进展的阶段性变化。

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.