Physicists still don't know what dark matter is made of, over a century after it was initially hypothesized to explain the motion of galaxy clusters. Hundreds of detectors have been created by researchers all around the world in the hopes of discovering dark matter. As a graduate student, I assisted in the design and operation of one of these detectors, appropriately designated HAYSTAC (Haloscope At Yale Sensitive To Axion CDM). However, despite decades of research, scientists have yet to discover the dark matter particle.
The search for dark matter has now gained an unexpected boost from quantum computing technology. My colleagues on the HAYSTAC team and I describe how we employed quantum trickery to increase the rate at which our detector can hunt for dark matter in a new paper published in the journal Nature. Our discovery provides a much-needed boost to the search for this enigmatic particle.
Searching for Dark Matter Signals
Astrophysics and cosmology provide solid evidence that an unknown element known as dark matter accounts for more than 80% of the matter in the universe. Hundreds of new elementary particles postulated by theoretical physicists could explain dark matter. However, in order to discover which — if any — of these ideas is accurate, researchers must construct separate detectors to test each one. According to one popular idea, dark matter is composed of as-yet hypothetical particles known as axions, which collectively behave like an unseen wave pulsing at a very particular frequency throughout the universe. Axion detectors, such as HAYSTAC, function similarly to radio receivers, except that instead of converting radio waves to sound waves, they try to convert axion waves to electromagnetic waves. Axion detectors, in particular, measure two values known as electromagnetic field quadratures. These quadratures are two unique types of electromagnetic wave oscillations that would be formed if axions existed. The primary difficulty in looking for axions is that no one knows the frequency of the hypothetical axion wave. Assume you're in a foreign city looking for a specific radio station by scanning the FM spectrum one frequency at a time. Axion hunters do the same thing: they adjust their detectors in discrete steps across a wide range of frequencies. Each step can only cover a small portion of the possible axion frequencies. This narrow range is the detector's bandwidth.
Tuning a radio usually entails pausing for a few seconds at each step to see if you've discovered the desired channel. This is more difficult if the signal is weak and there is a lot of static. An axion signal would be extremely low in even the most sensitive detectors when compared to static from random electromagnetic oscillations, which physicists refer to as noise. The longer the detector must wait at each tuning step to listen for an axion signal, the more noise there is. Unfortunately, researchers cannot rely on picking up the axion transmission after a few dozen radio dial rotations. An FM radio has a frequency range of 88 to 108 megahertz. The axion frequency, on the other hand, can range between 300 hertz and 300 billion hertz. At the current rate of progress, finding the axion or establishing its absence could take more than 10,000 years.
Quantum Noise Squeezing
We don't have that type of patience on the HAYSTAC crew. So, in 2012, we set out to accelerate the axion search by reducing noise as much as possible. However, by 2017, we have reached a fundamental minimum noise limit due to a quantum physics law known as the uncertainty principle. The uncertainty principle asserts that it is impossible to know the exact values of certain physical quantities at the same time — for example, you can't know a particle's position and momentum at the same time. Axion detectors look for the axion by measuring two quadratures, which are certain types of electromagnetic field oscillations. By adding a little quantity of noise to the quadrature oscillations, the uncertainty principle prevents precise knowledge of both quadrature.
The quantum noise from the uncertainty principle obscures both quadratures equally in ordinary axion detectors. This noise cannot be eradicated, but it can be regulated with the correct tools. Our group devised a method to shuffle the quantum noise in the HAYSTAC detector, minimizing its effect on one quadrature while increasing its effect on the other. This method of noise manipulation is known as quantum squeezing. The HAYSTAC team took on the problem of implementing squeezing in our detector, employing superconducting circuit technology drawn from quantum computing research, led by graduate students Kelly Backes and Dan Palken. Although general-purpose quantum computers are still a long way off, our new article reveals that this squeezing technology can significantly accelerate the search for dark matter.
Increased Bandwidth, Faster Search
Our team was successful in reducing noise in the HAYSTAC detector. But how did we use it to accelerate the axion search? Quantum squeezing does not equally reduce noise across the axion detector bandwidth. Instead, it has the greatest impact on the edges. Assume you set your radio to 88.3 megahertz, but the station you want is at 88.1. With quantum squeezing, you could hear your favorite tune playing on another channel. This would be a disaster in the world of radio transmission since different stations would interfere with one another. However, with only one dark matter signal to search for, a wider bandwidth allows physicists to examine more frequencies at once, allowing them to search faster. In our most recent finding, we used squeezing to double HAYSTAC's bandwidth, allowing us to search for axions twice as fast as previously.
Quantum squeezing is insufficient to scan over every possible axion frequency in an acceptable amount of time. However, doubling the scan rate is a significant step forward, and we expect that subsequent enhancements to our quantum squeezing technology will allow us to scan 10 times quicker. Nobody knows whether axions exist or whether they will solve the puzzle of dark matter, but we're one step closer to finding out owing to this unexpected application of quantum technology.