A quantum parametric amplifier using quantum paraelectricity

轴子是一种粒子，被假设为回答当今物理学中两个最重要的悬而未决的问题：暗物质的组成是什么，其引力将星系聚集在一起？为什么强核力如此精确地服从电荷宇称对称性？轴子暗物质假说认为，在大爆炸后不久，大量的轴子被创造出来，并在每个星系中聚集成暗物质晕，包括我们自己的星系。在这个假设中，每秒有数万亿个轴子穿过我们的实验室（和我们的身体），但它们从未被测量过，因为它们与普通物质的相互作用非常微弱。直接探测银河系轴子将是粒子物理学和宇宙学的重大突破。量子搜索隐藏扇区（QSHS）合作是英国量子技术基础物理学计划的一部分，旨在通过测量轴子在磁场中衰变时发出的射频信号来探测轴子。因为这个信号很小（大约比移动的手机检测到的信号小10亿倍），所以只能使用灵敏度极高的电子放大器来测量。事实上，除非放大器以量子力学所允许的最高精度工作，否则需要许多人一生的平均时间来搜索轴子可能发射的可能频率。开发具有必要灵敏度的射频量子放大器，并在我们的测试设备中对其进行表征，是这项合作的主要任务之一。尽管量子放大器与传统的经典放大器相比，有望大大加快轴子搜索的速度，但它们存在一个重要的限制。需要磁场来刺激轴子衰变，但这样的场对所有现有的量子放大器都是致命的。在QSHS项目中，就像在其他轴子搜索中一样，这个问题将通过使用磁屏蔽来部分缓解，但它使实验工程比我们想象的要困难得多，也意味着我们无法充分利用量子放大器提供的功能。该项目将开发一种新的量子放大器，它对磁场具有鲁棒性，因此非常适合搜索轴子信号。我们的新设计采用了量子顺电晶体，这是一种非线性介电材料。非线性意味着我们可以将能量从泵浦电压转移到信号，从而放大它。为了实现这种新的放大器，我们将首先在低温下测量合适的量子顺电材料的特性，并使用结果来实现和测试原理验证设备。使用优化的材料，我们将制造一种先进的设备（所谓的行波放大器），能够放大宽范围的频率。最后，我们将在QSHS测试设备内操作放大器，并发现它是否确实可以在这个探测器和未来更大的实验中加速对轴子的搜索。如果这个新的放大器可以在磁场中进行量子限制测量，它将不仅是轴子搜索的突破，但在其他快速发展的量子技术领域，需要在磁场中进行极其精确的电学测量。例子包括使用半导体的量子计算，以及使用磁共振研究新材料。

The axion is a particle that has been hypothesised to answer two of the most important outstanding questions of present day physics: What is the composition of the dark matter whose gravity holds galaxies together? And why does the strong nuclear force so exactly obey charge-parity symmetry? The axion dark matter hypothesis is that enormous numbers of axions were created shortly after the Big Bang and have since congregated as a dark matter halo in every galaxy, including our own. In this hypothesis, trillions of axions pass through our laboratories (and our bodies) every second, but they have never been measured because their interaction with ordinary matter is so weak. A direct detection of galactic axions would be a major breakthrough in both particle physics and cosmology.The Quantum Search for the Hidden Sector (QSHS) collaboration, part of the UK's Quantum Technology for FundamentalPhysics programme, aims to detect axions by measuring a radio-frequency signal that they give off when they decay in a magnetic field. Because this signal is tiny (approximately billion billion times smaller than the signal detected by a mobile phone), it can only be measured using an exquisitely sensitive electronic amplifier. Indeed, unless the amplifier works at the highest precision allowed by quantum mechanics, it would take many human lifetimes' worth of averaging to search through the likely frequencies at which the axions might emit. Developing radio-frequency quantum amplifiers which have the necessary sensitivity, and characterising them in our test facility, is one of the main tasks of this collaboration.Although quantum amplifiers promise an enormous speed-up of axion searches compared to conventional classical amplifiers, they suffer an important limitation. A magnetic field is needed to stimulate axions to decay, but such a field is fatal to all existing quantum amplifiers. In the QSHS project, as in other axion searches, this problem will be partly mitigated using magnetic shields, but it makes the experimental engineering much harder than we would like and means that we cannot fully exploit the capabilities that quantum amplifiers offer.This project will develop a new class of quantum amplifier that is robust against magnetic fields and therefore perfectly suited to search for an axion signal. Our new design incorporates quantum paraelectric crystals, which are non-linear dielectric materials. The non-linearity means that we can transfer energy from a pump voltage to the signal, thus amplifyingit. This process of parametric amplification allows in principle for extremely low noise.To realise this new amplifier, we will first measure the properties of a suitable quantum paraelectric material at low temperature, and use the results to implement and test a proof-of-principle device. Using optimised materials, we will then fabricate an advanced device (a so-called travelling wave amplifier) capable of amplifying a wide range of frequencies. Finally we will operate the amplifier inside the QSHS test facility and find out whether it can indeed speed up the search for axions, both in this detector and in future larger experiments.If this new amplifier can perform quantum-limited measurements in a magnetic field, it will be a breakthrough not only for axion searches, but in other rapidly developing areas of quantum technology that require extremely precise electrical measurements in a magnetic field. Examples include quantum computing using semiconductors, and studying new materials using magnetic resonance.