Nearly a century after dark matter was first proposed to explain the motion of galaxies, 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, named HAYSTAC. But despite decades of experimental efforts, scientists have yet to identify the particle of dark matter.
The search for dark matter has received an unlikely help from technology used in quantum computer research. In a new article published in the journal Nature, my colleagues in the HAYSTAC team and I describe how we used some quantum cheats to double the rate at which our detector can search for dark matter. Our result adds a much-needed speed boost to the hunt for this mysterious particle.
Search 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 can explain dark matter. But to determine which – if any – of these theories are correct, researchers need to build different detectors to test each one.
One prominent theory suggests that dark matter consists of still hypothetical particles called actions that act collectively like an invisible wave that oscillates through the cosmos at a very specific frequency. Axis detectors – including HAYSTAC – work like radio receivers, but instead of converting radio waves to sound waves, it aims to convert action waves into electromagnetic waves. Specifically, action detectors measure two quantities called squares of electromagnetic field. These squares are two different types of oscillations in the electromagnetic wave that will be produced if actions exist.
The biggest challenge in the search for action is that no one knows the frequency of the hypothetical action wave. Imagine that in an unknown city you are searching for a specific radio station by working one frequency at a time through the FM band. Axion hunters do much the same: they tune their detectors over a wide range of frequencies in discrete steps. Each step can only cover a very small range of possible action frequencies. This small range is the bandwidth of the detector.
To set up a radio, usually pause for a few seconds at each step to see if you have found the station you are looking for. It is more difficult when the signal is weak and there is a lot of static. An action sense – even in the most sensitive detectors – would be extremely faint compared to static from random electromagnetic fluctuations, which physicists call noise. The more noise there is, the longer the detector has to sit at each setting step to listen to an action signal.
Unfortunately, researchers can not count on picking up the action broadcast after a few dozen turns of the radio switch. An FM radio voice ranges from 88 to 108 megahertz (one megahertz is one million hertz). The action frequency, on the other hand, can be between 300 hertz and 300 billion hertz. At the rate at which today’s detectors are going, it can take more than 10,000 years to find or prove the action does not exist.
Press the quantum sound
In the HAYSTAC team, we do not have that kind of patience. Therefore, in 2012, we decided to speed up the action search by doing everything possible to reduce noise. But by 2017, we have found that we are facing 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 the momentum of a particle at the same time. Remember that action detectors search for the action by measuring two squares – those specific types of electromagnetic field oscillations. The uncertainty principle prohibits precise knowledge of both squares by adding a minimum amount of noise to the quadrature oscillations.
In conventional action detectors, the quantum noise of the uncertainty principle obscures both squares equally. This noise can not be eliminated, but with the right tools it can be controlled. Our team worked out a way to shift the quantum noise in the HAYSTAC detector, reducing the effect on one quadrature while increasing the other on the other. This sound 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 printing in our detector using superconducting circuit technology borrowed from quantum computer research. Quantum computers for general purposes are still a long way off, but our new article shows that this purple technology can speed up the search for dark matter immediately.
Greater bandwidth, faster search
Our team managed to capture the noise in the HAYSTAC detector. But how did we use it to speed up the action search?
Quantum pumps do not reduce the noise uniformly across the bandwidth of the action detector. Instead, it has the greatest effect on the edge. Imagine tuning your radio to 88.3 megahertz, but the station you want is actually 88.1. With quantum pressure, you can hear your favorite song play one station further.
In the world of radio broadcasting, it would be a recipe for disaster, because different stations would interfere with each other. But with only one dark matter signal to look at, a wider bandwidth allows physicists to search faster by covering more frequencies simultaneously. In our latest result, we doubled the bandwidth of HAYSTAC so that we could search for action twice as fast as possible.
Quantum pressing alone is not enough to scan through every possible action frequency within a reasonable amount of time. But doubling the scan rate is a big step in the right direction, and we believe that further improvements to our quantum printing system can enable us to scan 10 times faster.
No one knows whether actions exist and whether it 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 published from The Conversation, a non-profit 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 contributor to the HAYSTAC experiment, which received funding from the National Science Foundation, the Department of Energy and the Heising-Simons Foundation.