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Professor Ed Daw, The Quantum Sensors for the Hidden Sector talks axions, dilution refrigerators and new training opportunities at the University of Sheffield
In the quest to unravel one of the universe's greatest mysteries, Professor Ed Daw of the University of Sheffield is leading a project that could increase our understanding of dark matter. The Quantum Sensors for the Hidden Sector (QSHS) collaboration, funded by the Science and Technologies Facilities Council (STFC), is venturing beyond traditional dark matter theories to explore 'hidden-sector' particles.
In March, the STFC funded the university’s purchase of an Oxford Instruments ProteoxMX dilution refrigerator to perform experiments at a temperature a hundredth of a degree centigrade above absolute zero. Here we talk to Ed about what this allows - from searching for axions, to making precise measurements of the performance and properties of quantum electronics devices.
Together with our collaborators in University College London, University of Oxford, University of Lancaster, Royal Holloway University London and the National Physical Laboratory, we aim to increase major new UK activity in the field of searches for hidden sector particles and hidden sector dark matter.
Identifying the nature of the dark matter that dominates the mass distribution of galaxies and that plays a key role in our understanding of cosmology is a central unsolved problem of modern physics. Attention over the past 30+ years has focused on weakly interacting dark matter (WIMPs), however, a smaller but active community has been searching instead for ‘hidden-sector’ particles, using some of the world’s most sensitive electronics.
We work collaboratively with the Axion Dark Matter Experiment (ADMX), which brings together scientists from 10 institutions from the United States and the United Kingdom to hunt for a theoretical particle called an axion, which may solve the mystery of dark matter.
Essentially, we're searching for axions, which might be the dark matter that permeates the universe. Dark matter tends to concentrate in galaxies due to its mass, and if axions are the answer to dark matter, they would have played a role in galaxy formation. Since axions are everywhere, we can build our detectors in a lab, rather than needing to go deep underground.
However, axions are incredibly elusive—they were produced in a phase transition long ago, and because they rarely interact with other particles, they have extremely long lifetimes, longer than the age of the universe. This makes them good candidates for dark matter since they haven't decayed yet.
Our detector is based on a concept developed by Pierre Sikivie, which uses a resonant electromagnetic structure—a metal cavity. This cavity is essentially an empty box that we cool down to extremely low temperatures. Inside the cavity, there's a movable metal element that adjusts the resonant frequency of the box. Resonance here works similarly to how a stretched string vibrates at certain frequencies based on its length, tension, and density.
When the cavity is at the right frequency, it can resonate with the axions, leading to their conversion into electromagnetic waves. This resonance only occurs in a strong magnetic field, which induces the axion-photon conversion. However, since we don't know the exact mass of the axion, we have to "tune" the experiment, adjusting the cavity's frequency to match the unknown axion mass.
The signal we're looking for is extremely weak, on the order of 10 yoctowatts (10^-23 watts), which is an incredibly tiny amount of power. To detect such a faint signal, we need to minimise noise. That's why we use a dilution refrigerator to create an ultra-cold environment, reducing the motion of charged particles in the detector's walls and lowering the background noise to manageable levels.
So, in summary, our experiment requires a powerful magnet and a very cold environment to search for axions. The unique selling point of Oxford instruments, and why we were attracted to the Proteox system for this, is that they have magnet experts and fridge experts in house in the same building.
The Sheffield Ultra Low Temperature facility opening earlier this year
One of the big challenges in detecting axions is the tuning process. It has to be done very slowly, like trying to tune into a faint radio signal. When I was a kid, I used to try to find obscure stations on my dad's shortwave radio, and it’s a similar experience—moving the dial very slowly and listening for the faintest signal. This makes the experiment take a long time, though there’s ongoing R&D to try and speed it up. Ultimately I would like to use novel and interesting resonators that are tuned using external electronics rather than by moving a physical tuning rod element inside the fridge, but that's my future research.
Another challenge is physically tuning the cavity at very low temperatures. Moving equipment inside a cryogenic refrigerator is tricky. There are predictable failures that still happen anyway, despite our best efforts. It's both frustrating and amusing—highly intellectual discussions about theoretical physics often come down to mundane problems, like things getting stuck.
We also face issues with the electronics. Detecting such a tiny signal requires amplifying it, but the electronics themselves can add noise. So, we need the lowest-noise electronics possible, which are often developed by national labs, big companies, and universities. A key goal of the program funding our experiment is to encourage UK engineers and physicists to develop usable, high-tech devices.
Yes, the connection between our work on quantum electronics devices, like amplifiers, and the search for dark matter lies in reducing noise. When searching for axions, it's like trying to hear a whisper in a crowded bar—there’s a lot of background noise. Part of that noise comes from the physical temperature of the cavity we use in the experiment. The lower the temperature, the better. The other part comes from the noise produced by the amplifier itself, which is described in terms of its "noise temperature."
To improve our chances of detecting axions, we need amplifiers with the lowest possible noise temperature. The quantum electronics devices we're working with, such as travelling wave parametric amplifiers and microwave SQUID amplifiers (called SLUGs), are designed to minimise this noise. We also collaborate with the Oxford group led by Peter Leek, whose team is developing qubit arrays that we hope to embed in waveguides or cavities as potential axion detectors.
We are part of several other collaborations to generate devices. For example, with Yuri Pashkin and Ed Laird at Lancaster, Boon Kok Tan and Stafford Withington, both at Oxford, Ling Hao and Ed Romans at the National Physical Laboratory and University College London, and Phil Meeson who operates the SuperFab facility at Royal Holloway. Each of these groups is developing a different candidate low noise readout design for the axion search, and their designs will be field-tested on the Proteox fridge and magnet at Sheffield.
There’s a clear connection between our axion research and quantum computing because both fields deal with sensitive detectors coupled to resonators, making their noise background challenges similar. Although it's still early days for this research.
Our lab, which houses the only closed-cycle dilution refrigerator in the northeast, offers unique opportunities for both undergraduate and PhD students. We have a very diverse group, including students from underrepresented backgrounds. For example, one of our PhD students is the first in his family to attend university. Another student, here on a special scholarship for underrepresented groups, already has outstanding hardware skills and is quickly developing expertise in other areas too.
We've also had several undergraduates working on related projects, particularly in electronics. For instance, we're teaching students to program field-programmable gate arrays (FPGAs), which are cutting-edge in electronics. This hands-on experience not only excites students but also provides them with valuable skills that are highly relevant to employers. This aligns with the government’s goals of fostering national competitiveness and equipping young people with high-tech skills.
It's exciting to bring this level of high-tech research to Sheffield. While Sheffield isn't traditionally known as a high-tech hub like Oxford, Cambridge, or London, we're working to change that by building up expertise in areas like electronics and cryogenics. And while Sheffield already has strengths in other quantum technologies, we're expanding the scope of high-tech research here with our lab.
As an equipment manufacturer, the overlapping dual track of qubit and quantum sensor development is really exciting, as our Proteox dilution fridges show their strengths in both quantum technologies and dark matter search.
One of the reasons that Proteox is ideal for dark matter detection experiments is Oxford Instruments combined expertise in cryogenics and magnets. We develop and manage both technologies in-house and our teams work together side-by-side to support successful integrations.
Find out more about the Proteox family here and contact us for more information.
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