Nearly a century after dark matter was first proposed to explain the motion of galaxy clusters, physicists still have no idea what it consists 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 effort, scientists have yet to identify the dark matter particle.
Now, the search for dark matter has received unlikely help from technology used in quantum computer research. In a new paper published in the journal Nature, my colleagues on the HAYSTAC team and I describe how we used a little quantum trick to double the speed at which our detector can search for dark matter. Our result adds a much-needed speed boost to the hunt for this mysterious particle.
Scan for a dark matter signal
There is compelling evidence from astrophysics and cosmology that an unknown substance called dark matter makes up more than 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, if any, of these theories is correct, researchers need to build several detectors to test each one.
A prominent theory holds that dark matter is made of hitherto hypothetical particles called axions that act together like an invisible wave oscillating through the cosmos at a very specific frequency. Axion detectors – including HAYSTAC – work something like radio receivers, but instead of converting radio waves into sound waves, they try to convert axion waves into electromagnetic waves. Specifically, axion detectors measure two quantities called electromagnetic field squares. These squares are two different types of oscillations in the electromagnetic wave that would be produced if axes exist.
The main challenge in searching for axions is that no one knows the frequency of the hypothetical axion wave. Imagine looking for a particular radio station in an unfamiliar city by running through the FM band one frequency at a time. Axion hunters do much the same: they tune their detectors in discrete steps to a wide frequency range. Each step can only cover a very small range of possible axion frequencies. This small range is the bandwidth of the detector.
When tuning a radio, you should pause for a few seconds at each step to see if you have found the station you are looking for. This is more difficult when the signal is weak and there is a lot of noise. An axion signal – even in the most sensitive detectors – would be extremely weak compared to static electricity due to random electromagnetic fluctuations, which physicists call noise. The more noise there is, the longer the detector has to sit at each tuning step to listen for an axion signal.
Unfortunately, researchers cannot count on picking up the axion broadcast after a few dozen turns of the radio dial. An FM radio tunes from only 88 to 108 megahertz (one megahertz is one million hertz). The axion frequency, on the other hand, can be anywhere from 300 hertz to 300 billion hertz. At the pace of current detectors, it could take more than 10,000 years to find the axion or prove it doesn’t exist.
Squeezing the quantum noise
We don’t have that kind of patience with the HAYSTAC team. So in 2012, we wanted to speed up the search for axions by doing everything possible to reduce noise. But by 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 at the same time – for example, you cannot know both the position and the momentum of a particle at the same time. Remember, axion detectors look for the axion by measuring two squares – those specific types of electromagnetic field oscillations. The uncertainty principle prohibits accurate knowledge of both squares by adding a minimal amount of noise to the quadrature oscillations.
In conventional axion detectors, the uncertainty principle quantum noise obscures both squares equally. This noise cannot be eliminated, but with the right tools it can be controlled. Our team has come up with a way to shuffle around the quantum noise in the HAYSTAC detector, reducing the effect on one quadrature and increasing the effect on the other. This noise manipulation technique is called quantum squeezing.
In an effort led by graduate students Kelly Backes and Dan Palken, the HAYSTAC team took up the challenge of implementing squeezing in our detector, using superconducting circuit technology derived from quantum computer research. General purpose quantum computers are still a long way off, but our new paper shows that this press technology can immediately accelerate the search for dark matter.
Greater bandwidth, faster search
Our team managed to suppress the noise in the HAYSTAC detector. But how did we use this to speed up the search for axions?
Quantum squeezing does not reduce noise evenly across the bandwidth of the axion detector. Instead, it has the greatest effect on the edges. Imagine tuning your radio to 88.3 megahertz, but the station you want is actually 88.1. With quantum pinching, you could hear your favorite number one station play further.
In the world of radio broadcasting, this would be a recipe for disaster as different stations would interfere with each other. But with only one dark matter signal to search for, a wider bandwidth allows physicists to search faster by covering more frequencies at once. In our latest result, we used squeezing to double the bandwidth of HAYSTAC, allowing us to search for axions at twice the speed of before.
Quantum pinching alone is not enough to scan through every possible axion frequency in a reasonable amount of time. But doubling the scan speed is a big step in the right direction, and we believe that further improvements to our quantum squeezing system could allow us to scan 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 was republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It is written by: Benjamin Brubaker, University of Colorado Boulder.
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Benjamin Brubaker is an associate of the HAYSTAC experiment, which has received funding from the National Science Foundation, the Department of Energy and the Heising-Simons Foundation.