Quantum Compression | symmetry magazine

In the 1960s, researchers at the Ford Motor Company Science Laboratory developed the Superconducting Quantum Interference Device, also known as “SQUID”. It was the first usable sensor to take advantage of a quantum mechanical property – in this case, superconductivity.

This made the SQUID one of the first generation quantum sensors: devices that use a quantum system, quantum properties, or quantum phenomena to make a physical measurement. Physicists took the idea and continued it, coming up with new types of sensors that they continue to use and improve today.

SQUIDs played a key role in the development of ultra-sensitive electrical and magnetic measurement systems and are still in use. For example, they amplify detector signals for super cryogenic search for dark matter. “As particle physicists, we’ve been using quantum sensing techniques for decades,” says Lauren Hsu, SuperCDMS physicist at the US Department of Energy’s Fermi National Accelerator Laboratory.

But SQUIDs are no longer the only quantum sensors. An important recent development in quantum sensing is known as quantum compression – a way to circumvent the quantum limitations that even quantum sensors have faced in the past.

The first quantum sensors

Ford’s SQUIDs, which had to be cooled to a few degrees above absolute zero, used superconducting loops to measure tiny magnetic fields.

SQUIDs have not proven very useful in an automobile. But not all Ford researchers expected their creations to end up in a car. “It shows you how different the world was in the 1960s,” says Kent Irwin, a physicist at Stanford University and SLAC National Accelerator Laboratory. “These days, Ford doesn’t do basic physics.”

A few decades later, while at university, Irwin used Ford’s SQUID idea to develop a new quantum sensor: the first practical transitional superconducting sensor.

Irwin took advantage of the fact that the superconducting material loses its superconductivity when it heats up, regaining its resistance at a precise temperature. By keeping a superconducting material as close to this temperature limit as possible, he could create a sensor that would undergo a significant change at the introduction of even a small amount of energy. A single photon hitting one of Irwin’s transition sensors would cause it to switch to a different state.

The transition edge sensor is well known and has been widely adopted in X-ray astronomy, dark matter detection, and cosmic microwave background radiation measurements. “It’s really old-school Quantum 1.0,” says Irwin.

Quantum sensing for gravitational waves

A new generation of quantum sensors goes beyond quantum 1.0. Some of today’s sensors use more than superconductivity: they have managed to use Heisenberg’s uncertainty principle, generally seen as a limitation on physicists’ ability to make measurements, to their advantage.

Heisenberg’s uncertainty principle limits how accurately you can measure a pair of related properties. For example, the more you know about a particle’s position, the less you can know about its momentum.

Quantum compression takes advantage of these relationships by deliberately tipping the scales: shifting all the uncertainty of a measurement one way or the other.

Gravitational wave detectors, such as LIGO in the US, and Virgo and GEO in Europe, have used quantum compression to great effect. In 2015, LIGO – the Laser Interferometer Gravitational Wave Observatory – detected the first gravitational waves, ripples in spacetime first predicted by Albert Einstein. Once it started, it detected new signs of gravitational wave events every month.

LIGO detects gravitational waves using an interferometer, an L-shaped device in which two beams of light are configured to bounce off identical mirrors and back. Under normal conditions, the beams will arrive at the same time and cancel each other out. No signal will reach the detector.

But if some subtle outside force gets them out of sync with each other, they won’t cancel out and the photons will reach the detector. If a gravitational wave passes through both beams, it will hit one and then the other, interrupting their pattern.

LIGO’s measurements are limited by the quantum properties of the photons that make up their light beams. At the quantum level, photons are affected by fluctuations, virtual particles appearing and disappearing in the vacuum. These fluctuations could cause a false signal in the detector. How could LIGO researchers make a difference?

“LIGO uses the strongest lasers they can build and the best mirrors they can build, and their backs are against the wall,” says Irwin. “The only way to do better is to start beating quantum mechanics.”

Scientists at LIGO and other gravitational wave detectors have turned to quantum compression to help solve their virtual photon problem.

To generate compressed light, the researchers used a technology called an optical parametric oscillator, in which an input wave of laser light is converted into two output waves with smaller frequencies. This process entangles pairs of photons, and the resulting correlations of their properties serve to reduce uncertainty in one aspect of incoming photons, allowing LIGO scientists to better measure another aspect, helping them sort signal from noise.

Since April 2019, when LIGO started working with quantum presses, the observatory has been able to detect new gravitational wave signals — signs of collisions between massive objects such as black holes and neutron stars — more frequently. , going from about one detection per month to about one per week.

Quantum sensing for detecting dark matter

Quantum compression has also recently found application in the search for dark matter.

Dark matter has never been observed directly, but clues in cosmology indicate that it makes up about 85% of the matter in the universe. There are several different theories that describe what a dark matter particle might be.

“Mass can range from a billionth the size of an electron to a supermassive black hole,” Hsu says. “There are over 100 orders of magnitude it can cover.”

The most promising small dark matter candidates are axions. In the presence of a strong magnetic field, axions sometimes transform into photons, which can then be detected by the sensors of an experiment.

Like someone trying to find a radio station on a car trip in the middle of nowhere, they scan for a while on a frequency, to see if they pick up a signal. Otherwise, they turn the dial a bit and try the next size up.

It takes time to listen to each “station” once the detector is tuned to a particular possible axion signal; the more noise, the longer it takes to determine if there might be a signal.

The HAYSTAC experiment – for Haloscope at Yale Sensitive to Axion Cold Dark Matter – searches for axions by measuring two different components of electromagnetic field oscillations. Like LIGO, it is limited by the uncertainty principle; HAYSTAC researchers are unable to accurately measure both oscillations at once.

But they didn’t need it. Like the LIGO scientists, the HAYSTAC scientists realized that if they could squeeze all the precision into just one side of the equation, it would improve the speed of their research. In early 2021, the researchers announced that at HAYSTAC they had also succeeded in using quantum compression to reduce noise levels in their experiment.

Several groups demonstrated promising new applications of superconducting circuit technology for axion detection.

The “RF Quantum Upconverter” uses devices similar to Ford’s SQUIDs to evade Heisenberg’s uncertainty principle in searches for dark matter at lower frequencies than HAYSTAC searches. Another uses a technology borrowed from quantum computing – qubits – as a sensor to escape Heisenberg limits at frequencies above HAYSTAC. Although none of these technologies have yet been used in dark matter searches, scientists believe they could speed up searches by orders of magnitude.

At the current rate, it will take thousands of years for axion experiments to sweep through all possible axion “stations”. They may be lucky and find what they are looking for early in the search, but it is more likely that they will still have to find other ways to accelerate their progress, perhaps with advances in quantum sensing, explains Daniel Bowring, a Fermilab physicist involved in another axion research, the Axion Dark Matter experiment.

“It’s going to take a lot of people with really good imaginations,” Bowring says.

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