Scientists Created a Quantum Crystal That Will Search for Dark Matter

Scientists harnessed quantum entanglement and time reversal in a crystal that could solve one of the universe's biggest mysteries.
August 5, 2021, 6:01pm
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NIST physicists John Bollinger (left) and Matt Affolter work on the quantum crystal. Image: Jacobson/NIST
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Scientists have created a quantum crystal with unparalleled sensitivity that could potentially be used to detect dark matter, a mysterious substance that makes up the vast majority of matter in our universe, reports a new study.

The mind-boggling sensor was built at the US National Institute of Standards and Technology (NIST) and its performance relies on two very trippy processes: quantum entanglement and “time reversal,” according to the study, which was published on Thursday in Science

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Measuring just 200 micrometers wide, the tiny blue crystal is ten times more sensitive than previous atomic sensors. It can tune in to frequencies that scientists think are emitted by hypothetical particles called axions, a leading candidate for dark matter—including a range of signals that are not detectable to any other existing sensors.

Capturing a bonafide axion could be the key to unlocking one of the biggest unsolved mysteries in science: the nature of dark matter. Scientists know that dark matter exists because we can see its immense gravitational effects on the “normal” baryonic matter that makes up our bodies, planets, and stars. But because it doesn’t emit any conventional signals, dark matter has proved to be difficult to observe and explain, even though it is five times as abundant as normal matter.

Despite this tantalizing dark matter application, the team’s ability to achieve quantum entanglement between the crystal’s mechanical and electromagnetic properties is the “most important” takeaway of the research, noted Ana Maria Rey, a theoretical physicist at JILA, a joint institute of NIST and the University of Colorado Boulder, who co-authored the study.

“Using entanglement to actually enhance the sensitivity that we can’t afford with a classical system—that is the new part that is very exciting,” Rey said in a call. “We're using quantum tools and we're using resources offered only by quantum systems to do sensing that is not possible with classical resources.”

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Quantum entanglement occurs when the quantum states of particles become linked and cannot behave independently of each other, even across vast distances in some cases. In the NIST experiment, the team entangled two important physical properties governing the 150 beryllium ions that made up their quantum crystal: mechanical oscillation and spin.

The mechanical oscillation refers to the collective motion of the ions along a flat plane, while the spin refers to the individual orientations of the ions. The NIST researchers were able to use laser light to establish quantum entanglement of these two properties. 

While in this entangled state, the crystal is, in theory, able to sense the subtle electromagnetic wave produced by an axion as it hits the strong magnetic field inside the detector. Because axions only exist in theoretical models of dark matter, the team introduced a voltage to simulate the electric field an axion might make, to test out their method.

“The axion generates an electromagnetic wave that is in the form of an electric field with some specific frequency,” Rey explained. “If our ions resonate with this frequency, the electric field can induce motion, and this motion is what we can detect very precisely.”

This motion is known as “displacement.” Quantum entanglement allows for displacement to be captured without all the interference and noise that normally exists in quantum systems. 

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However, the team had to overcome a second problem in order to measure the simulated displacement in their experiment: the Heisenberg uncertainty principle. Basically, any attempt on the part of the researchers to observe displacement in the entangled system would re-introduce all the distortion that they had just worked so hard to tune out. 

“Entanglement is fantastic; it reduces the noise,” Rey said. “But if you want to take advantage of this entanglement, it is very complicated because when you measure it, you collapse the wave function,” which “adds more noise” and “destroys the gain that we got from entanglement.” 

That’s where the “time reversal” trick comes in. After the motion and spins of the ions were entangled and the displacement signal was applied, the NIST researchers untangled the properties again in a process that is like “going to the past,” Rey said. This is not time travel in the popular sense, but rather a form of reversing time in a quantum mechanical framework known as a Hamiltonian system.  

With the quantum entanglement turned off, noise emerged in the crystal system again. But crucially, information about the displacement introduced during the entanglement phase became mapped into the spin of the ions, like a quantum passport stamp, preserving the measurement data that the team could not obtain while the motion and spin were entangled.

The team aims to improve the stability and sensitivity of their crystal concept in the coming years, but this initial experiment opens the door to a new kind of dark matter sensor that could tune into axion frequencies that no other detector can pick up. 

The NIST crystal now joins a wide range of experiments that are hunting for dark matter particles, which theoretically exist all around us though they are very difficult to detect. If one of these efforts snagged the ultimate prize—an unambiguous observation of these elusive particles—it could finally expose the weird material that makes up most of the universe. 

“It would open up new directions in the field that require new theories to explain them,” Rey said. “I mean, it could be very exciting, obviously” because “we don't have a fundamental understanding of the composition of the universe.”