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.