The existence of dark matter was first proposed in 1933 to explain anomalies in the movement of galaxies. As described in Nature, the observed movements couldn’t be fully explained by the gravitational pull of visible objects. Rather, objects seemed to be reacting to gravity from some invisible unknown source, which scientists named “dark matter.” Dark matter is now thought to make up about 80% of the matter in the universe, Space.com notes — but what it is remains a mystery.
The search for dark matter recently received a boost from quantum technology. The two leading candidates are currently weakly interacting massive particles (WIMPs) and axions. Both these subatomic particles were proposed in the 1970s, but neither one has been proven to exist. But a new quantum breakthrough may provide an opening to unpack the mystery.
Axion Detectors
Recently, the search for axions gained speed by incorporating technology that was developed for quantum computers. This new approach was adopted for the HAYSTAC experiment, which stands for the Haloscope At Yale Sensitive to Axion CDM (cold dark matter). HAYSTAC was launched in 2010 to advance the search for axions and is a collaboration between Yale, the University of California, Berkeley and the University of Colorado Boulder. The first results from HAYSTAC were published in 2017. The dark matter detector was then rebuilt in 2018 to take advantage of new quantum-enhanced detection technologies, which led to a Nature publication in 2021.
Axions are predicted to have no charge, no spin and a minuscule amount of mass, with a single axion perhaps billions to trillions of times smaller than an electron. Therefore, for axions to account for the perceived dark matter in the universe, there must be 10 trillion to 100 trillion axions per cubic centimeter. However, axions almost never interact with ordinary matter, so extremely sensitive techniques are required to detect them. The process is analogous to looking for a needle in a haystack.
As explained in Berkeley News, if axions in a microwave cavity pass through a strong magnetic field, a small number of them should theoretically transform into microwave-frequency photons, which can be detected as particles of light. The photon frequency is determined by axion mass, which is unknown. The predicted frequency is anywhere between 300 hertz and 300 billion hertz, so the goal of HAYSTAC and similar projects is to carefully and systematically scan through that range, to eliminate possibilities until axions are either found or disproved.
The Quantum Limit
Unless new technologies are adopted and developed, it could take thousands of years to scan through all the possible axion frequencies, as a HAYSTAC collaborator discusses in The Conversation. When the first HAYSTAC paper was published in 2017, the existing technology had essentially reached the limits imposed by a fundamental law of quantum mechanics. The Heisenberg uncertainty principle indicates that it’s impossible to know the exact value of two different properties of a quantum system — such as a photon — at the same time.
In axion searches, the quantum limit introduces excessive noise into the measurement process. Even at temperatures near absolute zero, photons are ubiquitous and produce random electromagnetic fluctuations. The more noise there is, the longer a dark matter detector must sit at each frequency to listen for an extremely faint axion signal. The HAYSTAC experiment started by exploring the lower end of the possible axion frequency range and was able to rule out a subset of predicted axion models. The noise would only increase as higher frequencies were explored.
Researchers had essentially exhausted options to amplify the signal and sort it from noise, so the HAYSTAC team decided to develop a new axion detector that could circumvent the quantum limit.
Quantum Squeezing
Axion detectors measure two quadratures (two properties of incoming light waves). The Heisenberg uncertainty principle stipulates that measuring both quadratures at the same time would result in some uncertainty in both measurements. The HAYSTAC team reasoned that it would be better to measure one quadrature at a time, if they could increase the accuracy of that measurement while decreasing accuracy for the other quadrature.
A noise manipulation technique called quantum squeezing was applied, which was originally developed for quantum computing. While ordinary computer chips hold bits of data in an “on” or “off” position, quantum computer chips (“qubits”) can hold data in an intermediate position, which will make them ideal for situations involving uncertainty. Quantum computers are still in the early stages of development, but tools designed to handle the challenges of quantum computing are finding applications elsewhere.
One such tool is the Josephson Parametric Amplifier (JPA), which was developed to improve the fast readout fidelity of quantum computers. The HAYSTAC team used the JPA to “squeeze” the light they were getting from their axion detection experiment, to reduce uncertainty in one dimension while increasing it in another.
Reduced Noise, Increased Bandwidth
The new HAYSTAC detector uses a microwave cavity held at a temperature near absolute zero (to decrease noise) and placed in a strong magnetic field. The detector takes advantage of the uncertainty principle by reducing the uncertainty of the X component of the cavity electromagnetic (EM) field by squeezing with a JPA. The squeezed X component is then amplified with a second JPA. If an axion field is present, it should displace the amplified squeezed state in a random direction.
“The detection process is based on detecting the weakly coupled oscillating axion field, rather than individual particles, since the axion is assumed to be many orders of magnitude lighter than the known standard-model particles and would be unlikely to be directly detectable as a particle,” explains Dr. Mike Fitelson, a consulting engineer for Northrop Grumman Mission Systems who was not involved in the study.
By decreasing background noise and increasing bandwidth, this new dark matter detector was able to scan two times faster than the old detector. While no axions were detected in the 100-day trial run, doubling the speed of the dark matter detector is a major step in the right direction. The HAYSTAC team believes that additional improvements to the quantum squeezing system could allow scanning to occur 10 times faster, and advances in other areas could add even more speed.
Additional Implications of Quantum Technology
Verifying the existence of axions would also solve the “strong charge-parity (CP) problem” in particle physics. If a neutron’s positive and negative charges are inverted, it exhibits mirror-image behavior in terms of electrical charge and other properties. The strong force — which explains why atomic nuclei don’t fly apart from the repulsive force between positively charged protons — obeys CP symmetry. This is problematic because symmetry isn’t required by the Standard Model of particle physics. The axion was first proposed to explain this symmetry.
Dr. Jonathan Green, director of Disruptive Concepts & Technologies for Northrop Grumman Mission Systems who was not involved in the HAYSTAC study, describes the work of the HAYSTAC team as “fundamental research focused on establishing our basic understanding of the mechanisms of nature.”
“This can be contrasted with applied research where our understanding of nature is applied to creating a useful technology,” he continues.
“The main benefit from this work could be the design of new detectors that are more sensitive,” adds Dr. Fitelson. “For example, instead of exotic particles, this technology might be adaptable to single radio-frequency (RF) photon detection and the detection of very weak electric fields.”
Dr. Greene also points out that the Laser Interferometer Gravitational-Wave Observatory (LIGO) is also using quantum squeezing to detect gravitational waves, as MIT News describes.
“We currently live in exciting times for scientific discovery,” he enthuses. “In the past decade, we’ve observed the Higgs boson, detected gravitational waves and directly imaged a black hole. There still remain many fundamental questions about the nature of matter and the universe that our technology may allow us to answer in the near future.”
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