Quantum Technology Accelerates the Search for Dark Matter

  

2 Oct 2021

by Adil Aftab

 Quantum Technology Accelerates the Search for Dark Matter

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 remains a mystery.

The search for dark matter recently received a boost from quantum technology. The two leading candidates are currently weakly interacting with 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 HAYSTACK 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 HAYSTACK collaborator discusses in The Conversation. When the first HAYSTACK 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 HAYSTACK 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 HAYSTACK 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 HAYSTACK 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 HAYSTACK 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 HAYSTACK 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.

“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.

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. 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 particle physics.