Quantum sensor overcomes major obstacle in search for dark matter and gravitational waves

To build a quantum sensor, light must be prepared in a carefully controlled state where its frequency, polarization and intensity are all well controlled. Here, the polarization of blue light is altered before it is used to cool the atoms to absolute zero. Credit: Dr. Thomas Walker, Imperial College London

A prototype quantum sensor developed by researchers at Imperial has demonstrated for the first time that a key principle behind next-generation quantum detectors can work under realistic conditions.

The study shows how comparing two long-baseline atom interferometers, instruments that use lasers to precisely measure the behavior of atoms, allows experimental noise to be effectively canceled.

This enables signals to be recovered even when individual measurements are overwhelmed and opens the door to searches for gravitational waves from the early universe and signatures of exotic forms of dark matter.

The work forms part of the Atom Interferometer Observatory and Network (AION) collaboration. Led by Imperial, AION brings together researchers from institutions across the U.K. to develop next-generation quantum sensing technologies.

This research is published in Nature .

Canceling noise in quantum measurements

Understanding what the universe is made of and identifying new sources of gravitational waves remain major challenges in modern physics.

Both problems require measuring extremely small signals that can easily be lost in background noise. Finding reliable ways to detect them is essential for exploring parts of the universe that current experiments cannot access.

The small glowing ball in the center of this chamber is a cloud of atoms at close to absolute zero, levitating on blue laser light. These atoms will be cooled even further before becoming tiny sensors, turned to listen for gravitational waves and dark matter. Credit: Dr. Thomas Walker, Imperial College London

Long-baseline atom interferometers are emerging as one of the most promising tools for this. They work by using lasers to split clouds of atoms and then bring them back together, allowing tiny changes in their motion to be measured with extreme precision.

These experiments rely on comparing the behavior of two clouds of atoms held at different locations and interrogated by the same laser. Any difference between the two could point to previously hidden signals, for example, the presence of a dark matter field .

However, the technique faces a major challenge. The laser used to control the experiment produces phase noise that is far greater than the signals researchers are trying to measure. Left uncorrected, this noise completely obscures these effects.

To overcome this, scientists have proposed a differential approach, comparing two interferometers so that shared noise cancels out. This method underpins plans for next-generation detectors but had not previously been demonstrated under realistic conditions.

Speaking about the significance of the advance, Dr. Charles Baynham, co-lead of the Ultracold Strontium Laboratory at Imperial College London, said, "We've known for a long time that quantum sensors can help us understand the universe, but it's only recently that it's become possible to build them with the resolution needed.

"We're immensely proud of our team's efforts to make these sensors a reality—I can't wait for the day when signals from an atom are telling us about a black hole that merged millions of years ago."

Simulated black hole mergers in the observable universe, with projected sensitivities of existing and proposed gravitational wave detectors. The new class of atom-based sensors pioneered in this work (AION/ AEDGE) may help us see Intermediate Mass Black Holes (IMBHs) that played a pivotal role in our galaxy's formation. Credit: Dr. Thomas Walker, Dr. Elizabeth Pasatembou, Dr. Charles Baynham, Imperial College London

Testing the approach

In the new study, researchers set out to test this principle experimentally.

In the Imperial Ultracold Strontium Laboratory, they built a tabletop prototype with two macroscopically separated clouds of ultracold strontium-87, interrogated by a single ultrastable clock laser.

The setup was designed to mimic the conditions expected in much larger future experiments, where controlling noise becomes increasingly difficult.

To push the method to its limits, the team deliberately introduced large amounts of additional phase noise into the system—far more than clock lasers naturally produce—to simulate the conditions expected in long-baseline detectors.

Individually, each interferometer became unusable, with its signal obscured by noise. The interference patterns that normally allow measurements to be made were effectively erased.

However, when the two interferometers were compared, a clear signal could still be recovered. Even though each individual measurement appeared random, the correlation between them revealed the underlying behavior of the system. The combined measureme…