Dark matter research is accelerated by quantum technology

Dark matter can be deduced from an assortment of physical clues in the universe. NASA

Almost a century after dark matter was first proposed to explain the movement of clusters of galaxies, physicists still have no idea what it is made of.

Researchers around the world have built dozens of detectors in hopes of discovering dark matter. As a graduate student, I helped design and operate one of these detectors, aptly named HAYSTAC. But despite decades of experimental efforts, scientists have yet to identify the dark matter particle.

Now, dark matter research has received unlikely help from the technology used in quantum computing research. In a new article published in the journal Nature, my colleagues on the HAYSTAC team and I describe how we used a little quantum trickery to double the speed at which our detector can search for dark matter. Our result adds an essential speed increase in the search for this mysterious particle.

Finding a dark matter signal

There is compelling evidence from astrophysics and cosmology that an unknown substance called dark matter constitutes over 80% of the matter in the universe. Theoretical physicists have proposed dozens of new fundamental particles that could explain dark matter. But to determine which of these theories, if any, is correct, researchers must build different detectors to test each one.

An important theory proposes that dark matter is made up of still hypothetical particles called axions that collectively behave like an invisible wave oscillating at a very specific frequency across the cosmos. Axion detectors – including HAYSTAC – work much like radio receivers, but instead of converting radio waves into sound waves, they aim to convert axion waves into electromagnetic waves. Specifically, axis detectors measure two quantities called electromagnetic field quadratures. These quadratures are two distinct types of oscillations in the electromagnetic wave that would be produced if axions existed.

The main challenge in finding axions is that no one knows the frequency of the hypothetical axion wave. Imagine that you are in an unfamiliar city looking for a particular radio station while browsing the FM band one frequency at a time. Axion hunters do much the same thing: they tune their detectors over a wide range of frequencies in discrete steps. Each step can only cover a very small range of possible axis frequencies. This small range is the detector bandwidth.

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Tuning to a radio usually involves pausing for a few seconds at each step to see if you’ve found the station you’re looking for. It is more difficult if the signal is weak and there is a lot of static. An axion signal – even in the most sensitive detectors – would be extremely weak compared to the statics of random electromagnetic fluctuations, which physicists call noise. The more noise there is, the longer the detector must stay at each tuning step to listen for an axion signal.

Unfortunately, researchers cannot count on the emission of the axion after a few dozen turns of the radio dial. An FM radio transmits only 88 to 108 MHz (one megahertz equals one million hertz). The axial frequency, on the other hand, can be between 300 hertz and 300 billion hertz. At the current rate of detectors, finding the axion or proving it doesn’t exist could take more than 10,000 years.

Squeeze quantum noise

In the HAYSTAC team, we don’t have that kind of patience. So in 2012 we decided to speed up the axial search by doing everything possible to reduce noise. But in 2017 we ran into a fundamental minimum noise limit due to a law of quantum physics known as the Uncertainty Principle.

The uncertainty principle states that it is impossible to know the exact values ​​of certain physical quantities simultaneously – for example, you cannot know both the position and momentum of a particle at the same time. Remember that axis detectors look for the axion by measuring two quadratures – these specific types of electromagnetic field oscillations. The uncertainty principle prohibits the precise knowledge of the two quadratures by adding a minimum of noise to the quadrature oscillations.

In conventional axis detectors, the quantum noise of the uncertainty principle obscures the two quadratures equally. This noise cannot be eliminated, but with the right tools it can be controlled. Our team has developed a way to mix quantum noise in the HAYSTAC detector, reducing its effect on one quadrature while increasing its effect on the other. This noise manipulation technique is called quantum compression.

In an effort led by graduate students Kelly Backes and Dan Palken, the HAYSTAC team took on the challenge of implementing compression in our detector, using superconducting circuit technology borrowed from quantum computing research. General-purpose quantum computers are still a long way off, but our new article shows that this compression technology can immediately speed up the search for dark matter.

Greater bandwidth, faster search

Our team succeeded in reducing noise in the HAYSTAC detector. But how did we use this to speed up axion search?

Quantum compression does not reduce noise uniformly over the bandwidth of the axion detector. Instead, it has the greatest effect on the edges. Imagine you set your radio to 88.3 megahertz, but the station you want is actually 88.1. With quantum compression, you will be able to hear your favorite song playing at a station.

In the world of radio, that would be the recipe for disaster, as different stations would interfere with each other. But with just one dark matter signal to search for, a wider bandwidth allows physicists to search faster by covering more frequencies at once. In our last result, we used compression to double the bandwidth of HAYSTAC, which allowed us to find axions twice as fast as before.

Quantum compression alone is not enough to cover all the possible axis frequencies in a reasonable time. But doubling the scan rate is a big step in the right direction, and we believe further improvements to our quantum compression system could allow us to digitize 10 times faster.

No one knows if axions exist or if they will solve the mystery of dark matter; but thanks to this unexpected application of quantum technology, we are one step closer to answering these questions.

This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Benjamin Brubaker, University of Colorado Boulder.

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Benjamin Brubaker is a collaborator of the HAYSTAC experiment, which has received funding from the National Science Foundation, the Department of Energy and the Heising-Simons Foundation.