Josephson Parametric Amplifiers using CVD graphene junctions

这个合作项目将使用石墨烯制造超导结器件的国际合作者的能力与英国在超低温下执行低噪声电子测量的专业知识结合在一起，以开发将提高超导量子电路性能的超导放大器。量子系统通常对噪声非常敏感。例如，在量子计算中，量子比特的特征是相干，这是一种描述量子比特可以保持在给定量子状态多长时间的属性。噪声会导致消相干，从而限制这些微妙系统的寿命。噪声的一个来源是我们用来放大用于控制这些量子系统的极低功率信号的放大器。在超导电路中，我们可以使用使用约瑟夫森结的参数放大器来创建能够在仅受量子力学限制的最低噪声水平下工作的放大器。最常见的是，这些放大器使用约瑟夫森结，这种结是由两个超导体与绝缘体连接而成的，即所谓的SIS结。使用SIS结的参数放大器的工作频率可以使用磁通来调节，但是由于磁场的长程效应，这种磁通可能会干扰我们精密的量子设备，或者在多个放大器之间产生串扰。因此，筛选这种流动带来了额外的挑战。超导结也可以通过用石墨烯连接两个超导体来形成，这是一种SGS结。与使用SIS结的参数放大器不同，使用SGS结的参数放大器可以通过静电调谐。这是通过在石墨烯附近的栅电极上施加电压来实现的。这样，我们就可以避免由杂散磁通引起的干扰的危险。然而，SGS结的开发主要集中在使用从高质量石墨晶体中剥离出来的石墨烯。虽然这可以产生最高质量的石墨烯，但薄片的尺寸只有几微米，这限制了可以用一片石墨烯制造的结点的数量。这使得使用这些结的设备的实际实现非常具有挑战性。石墨烯也可以大面积生产，足以覆盖一个6英寸直径的硅片，而且很容易买到。这种形式的石墨烯通常质量较低，但是已经证明了使用大面积石墨烯的连接。不幸的是，几乎还没有开发出利用这些连接的设备。这个项目寻求开发一种制造超导结的可靠方法，然后利用这种结来开发参数放大器。通过这项概念验证工作，我们的目标是证明可扩展的、静电调谐的参数放大器是一种可能性。这些放大器可以用于读出超导量子比特，并且通过在非常低的噪声下工作，可以帮助减少这些系统的消相干。由于它们对磁场不那么敏感，这些放大器对这种干扰也会更健壮。更广泛地说，超导微波放大器被用于寻找暗物质候选轴子的实验，也可以用于射电天文学。石墨烯的低热容量也将使具有低噪声参数放大器的非常灵敏的测辐射热计得以制造。该项目的成功将把一个世界领先研究小组的专业知识带到英国，并提供关键的使能技术，这将加强英国作为量子技术世界领先者的地位。

This collaborative project brings together the capabilities of international collaborators who are experts in fabricating superconducting junctions devices using graphene, with UK based expertise in performing low-noise electronic measurements at ultra-low temperatures to develop superconducting amplifiers that will improve the performance of superconducting quantum circuits. Quantum systems are generally very sensitive to noise. In quantum computing, for example, qubits are characterised by coherence, a property which describes how long a qubit can remain in a given quantum state. Noise causes decoherence which limits the lifetime of these delicate systems. One source of noise is the amplifiers that we use to amplify the very low power signals used to control these quantum systems. In superconducting circuits we can use parametric amplifiers that employ Josephson junctions to create amplifiers that can operate at the very lowest possible noise levels, limited only by quantum mechanics. Most commonly, these amplifiers use Josephson junctions that are formed by connecting two superconductors with an insulator, a so-called SIS junction. The operating frequency of parametric amplifiers employing SIS junctions can be tuned using magnetic flux, however due to the long range effects of magnetic fields, this magnetic flux can interfere with our delicate quantum devices, or create cross-talk between multiple amplifiers. Screening this flux therefore creates additional challenges. Superconducting junctions can also be formed by connecting two superconductors with graphene, an SgS junctions. Unlike parametric amplifiers using SIS junctions, parametric amplifiers using SgS junctions can instead be tuned electrostatically. This is achieved by applying a voltage to a gate electrode near the graphene. In this way, we can avoid the perils of the interference caused by stray magnetic flux. However, the development of SgS junctions is overwhelmingly focused on using graphene that is exfoliated from high quality graphite crystals. Although this produces the highest quality graphene, the flakes are only a few micrometers in size, which limits the number of junctions that can be fabricated from a single flake. This makes a practical realisation of devices using these junctions very challenging. Graphene can also be produced in large areas, enough to cover a 6-inch diameter silicon wafer and can be readily purchased. This form of graphene is generally of a lower quality, however Junctions using large-area graphene have been demonstrated. Unfortunately, there has been almost no development of devices which exploit these junctions. This project seeks to develop a robust method for fabricating superconducting junctions and then to develop parametric amplifiers using the junctions. Through this proof-of-concept work we aim to demonstrate that scalable, electrostatically tuned parametric amplifiers is a possibility. These amplifiers could be used for the readout of superconducting qubits and by operating with very low noise, could help in reducing decoherence of these systems. As they are less sensitive to magnetic fields, these amplifiers would also be more robust against such interference. More widely, superconducting microwave amplifiers are used in experiments searching for Axions, a dark matter candidate and can also be used for radio astronomy. The low heat capacity of graphene will also allow the fabrication of very sensitive bolometers with low noise parametric amplifiers. Success in this project would bring the expertise of a world leading research group to the UK and deliver a key enabling technology that would strengthen the UK's position as a world leader in quantum technologies.