How a world record ‘squeeze’ could provide comfort to dark matter hunters

UNSW quantum engineers 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 the exact speed and location at any time, right? Well, the theory of quantum mechanics says that you can’t know both at the same time with infinite precision.

It turns out that the more accurately you measure where the ball is, its speed becomes less and less accurate.

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

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

That’s the problem being tackled by a team of engineers from UNSW Sydney, who have developed an amplifier device that makes precise measurements of very weak microwave signals, and this is done through a process known as squeezing.

Microwave squeeze

Squeezing reduces the certainty of one property of a signal to obtain ultra-precise measurements of another property.

The team of researchers from UNSW, led by Associate Professor Jarryd Pla, have significantly increased the accuracy of measuring signals at microwave frequencies, such as those emitted by your mobile phone, to the point of setting a new world record.

The accuracy of measuring any signal is fundamentally limited by noise. Noise is the fuzziness that creeps in and masks signals, something you may have experienced if you’ve ever been out of range when listening to AM or FM radio.

However, uncertainty in the quantum world means that there is a limit to the extent to which a measurement can produce little noise.

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

The squeezer produced by the UNSW team can beat this quantum limit.

“The device amplifies sound in one direction so that sound in another direction is significantly reduced, or ‘squeezed’. Think of the sound as a tennis ball: if we stretch it vertically, it must decrease along the horizontal line to maintain its volume We can then use the reduced part of the noise to make more accurate measurements,” says A/Prof. Pla says.

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

The device was the result of painstaking work. Ph.D. candidate Arjen Vaartjes, joint lead author of the article together 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 compressed sound quite easily.

“What we’ve done is a lot of engineering to eliminate sources of loss, which means we have to use very high quality superconducting materials to build the amplifier.”

And the team believes the new device could help speed up the search for notoriously elusive particles known as axions. These particles are only theoretical so far, but are proposed by many as the secret ingredient of mysterious dark matter.

Axion measurements

Making accurate measurements is the domain of scientists trying to discover what dark matter is, which is believed to make up about 27 percent of the known universe but remains a cosmic mystery because scientists have not been able to actually identify it.

As the name suggests, it does not emit or absorb light, making it ‘invisible’. But physicists believe it must be there, exerting a gravitational pull, otherwise galaxies would fly apart.

There are many different theories about what dark matter could be made of, including the proposed existence of so-called axions.

Axions themselves have also never been discovered; the theory is that they are almost unfathomably small, with extremely low mass as an individual particle, and therefore interact almost imperceptibly with other known matter.

However, one idea predicts that ashes should produce very weak microwave signals when exposed to large magnetic fields. Scientists use highly sensitive equipment and take meticulous measurements in an attempt to detect those tiny signals.

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

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

“Axion detectors can use pinchers to reduce noise and speed up their measurements. Our results indicate that these experiments can 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 physically measure an axion, it will only be a theory of how dark matter manifests.”

Wide usage

Joint lead author Dr. Vine says there are other uses for the team’s new amplification device.

“What we also showed in our research is that the device can be used at higher temperatures than previous squeezers and also at large magnetic fields,” says Dr. Vine.

“This opens the door for 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 allows you to study smaller volumes or measure samples with greater precision.”

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

“It turns out that squeezed vacuum noise is an ingredient to building a certain type of quantum computer. Excitingly, the level of squeezing we’ve achieved is not far off the amount needed to build such a system,” he says .