How a world-record squeeze could bring comfort to dark matter hunters

Quantum engineers at UNSW have developed a new amplifier that could help other scientists search for elusive dark matter particles.

Imagine throwing a ball. You would expect science to be able to determine its exact speed and position at any given moment, right? Well, the theory of quantum mechanics says that you can’t know both simultaneously with infinite precision.

It turns out that the determination of the ball’s speed becomes increasingly inaccurate the more precisely the location of the ball is measured.

This puzzle is commonly called the Heisenberg uncertainty principle, named after the famous physicist Werner Heisenberg who first described it.

This effect is not noticeable in the sphere, but in the quantum world of small electrons and photons the measurement uncertainty suddenly becomes very large.

A team of engineers at UNSW Sydney is working on this problem. They have developed an amplifier device that can precisely measure very weak microwave signals using a process called squeezing.

Squeeze in the microwave

Squeezing reduces the certainty of one property of a signal in order to obtain high-precision measurements of another property.

The UNSW research team led by Associate Professor Jarryd Pla has improved the accuracy of measuring signals in the microwave frequency range, such as those emitted by your mobile phone, so significantly that a new world record has been set.

The accuracy of signal measurement is generally limited by noise. Noise is the fuzziness that creeps in and masks signals. You may have noticed this if you have ever been out of range of the receiver while listening to AM or FM radio.

However, due to the uncertainty in the quantum world, there is a limit to how low the noise in a measurement can be.

“Even in a vacuum, a space where everything is empty, the uncertainty principle tells us that there must still be noise. We call this ‘vacuum noise’. In many quantum experiments, vacuum noise is the dominant effect that prevents us from making more precise measurements,” says A/Prof. Pla from the Department of Electrical Engineering and Telecommunications at UNSW and co-author of a paper published in Nature communication.

The squeezer developed by the UNSW team can go below this quantum limit.

“The device amplifies the noise in one direction so that the noise in another direction is significantly reduced or ‘squashed’. Imagine the noise like a tennis ball: if we stretch it vertically, it has to shrink horizontally to maintain its volume. We can then use the reduced part of the noise for more precise measurements,” says A/Prof. Pla.

“Crucially, we have shown that the squeezer is capable of reducing noise to a record low.”

The device is the result of painstaking work. PhD student Arjen Vaartjes, lead author of the paper along with UNSW colleagues Dr Anders Kringhøj and Dr Wyatt Vine, adds: “Squeezing is very difficult at microwave frequencies because the materials used tend to destroy the fragile squeezed noise quite easily.”

“We’ve done a lot of engineering to eliminate sources of loss. This means we use very high quality superconducting materials to build the amplifier.”

And the team believes the new device could help speed up the search for the notoriously elusive particles known as axions. Axions are only known theoretically so far, but are considered by many to be the secret ingredient in the mysterious dark matter.

Axion measurements

Making precise measurements is the domain of scientists trying to figure out what dark matter is made of, which is thought to make up about 27 percent of the known universe but remains a cosmic mystery because scientists have failed to actually identify it.

As the name suggests, it neither emits nor absorbs light, making it “invisible.” However, physicists believe it must be there and exert a gravitational force, otherwise galaxies would fly apart.

There are many different theories about what dark matter might consist of – including the suspected existence of so-called axions.

Axions themselves have also never been discovered. The theory assumes that they are almost unimaginably small, have an extremely low mass as a single particle and therefore interact practically imperceptibly with other known matter.

However, one theory suggests that when axions are exposed to strong magnetic fields, they should produce very weak microwave signals. Scientists use highly sensitive equipment and make careful measurements to detect these tiny signals.

But as A/Prof. Pla says, “When you try to detect particles as ghostly as axions, even vacuum noise can be deafening.”

The work done at UNSW on squeezing means these measurements can now be made up to six times faster, increasing the chances of discovering an elusive axion.

“Axion detectors can use squeezers to reduce noise and speed up their measurements. Our results show that these experiments could now be performed even faster than before,” says A/Prof. Pla.

“Scientists can see the effects of dark matter on galaxies, but no one has ever discovered it. Until you can physically measure an axion, it will always be just a theory about how dark matter manifests itself.”

Wide use

Co-author Dr. Vine says there are other possible applications for the team’s new amplifier device.

“What we also showed in our study is that the device can operate at higher temperatures than previous squeeze devices and also in large magnetic fields,” says Dr. Vine.

“This opens the door to application in techniques such as spectroscopy, which is used to study the structure of new materials and biological systems such as proteins. The compressed noise means you can study smaller volumes or measure samples with greater precision.”

Dr. Kringhøj points out that the squeezed noise itself could even be used in future quantum computers.

“It turns out that compressed vacuum noise is an ingredient for building a certain type of quantum computer. Excitingly, the level of compression we have achieved is not far from the level required to build such a system,” he says.