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MIT Physicists Harness Quantum “Time Reversal” for Detecting Gravitational Waves and Dark Matter

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A new technique to measure vibrating atoms could improve the precision of atomic clocks and of quantum sensors for detecting dark matter or

Noise from the classical world, which can quickly overpower small atomic vibrations and make any changes to those oscillations devilishly hard to detect, is a significant barrier in the way of improved quantum measurements.

However,

Before you go out and buy a DeLorean, let me assure you that they haven’t discovered a means to reverse time itself. Instead, the scientists forced atoms that were quantumly entangled to act as if they were evolving backward in time. Any alterations to the atomic oscillations were magnified and made easy to monitor as the researchers essentially rewound the tape of atomic oscillations.

In research published on July 14, 2022, in the journal U06P

MIT physicists have shown they can significantly amplify quantum changes in atomic vibrations, by putting the particles through two key processes: quantum entanglement and time reversal. Credit: Jose-Luis Olivares, MIT, with figures from iStockphoto

The technique could improve the

The study’s MIT co-authors include first author Simone Colombo, Edwin Pedrozo-Peñafiel, Albert Adiyatullin, Zeyang Li, Enrique Mendez, and Chi Shu.

Entangled timekeepers

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MIT researchers used a system of lasers to first entangle, then reverse the evolution of a cloud of ultracold atoms. Credit: Simone Colombo

In 2020, Vuletic’s group showed that the precision of current atomic clocks could be improved by entangling the atoms — a quantum phenomenon by which particles are coerced to behave in a collective, highly correlated state. In this entangled state, the oscillations of individual atoms should shift toward a common frequency that would take far fewer attempts to accurately measure.

“At the time, we were still limited by how well we could read out the clock phase,” Vuletic says.

That is, the tools used to measure atomic oscillations were not sensitive enough to read out, or measure any subtle change in the atoms’ collective oscillations.

Reverse the sign

In their new study, instead of attempting to improve the resolution of existing readout tools, the team looked to boost the signal from any change in oscillations, such that they could be read by current tools. They did so by harnessing another curious phenomenon in quantum mechanics: time reversal.

It’s thought that a purely quantum system, such as a group of atoms that is completely isolated from everyday classical noise, should evolve forward in time in a predictable manner, and the atoms’ interactions (such as their oscillations) should be described precisely by the system’s “Hamiltonian” — essentially, a mathematical description of the system’s total energy. 

In the 1980s, theorists predicted that if a system’s Hamiltonian were reversed, and the same quantum system was made to de-evolve, it would be as if the system was going back in time.

“In quantum mechanics, if you know the Hamiltonian, then you can track what the system is doing through time, like a quantum trajectory,” Pedrozo-Peñafiel explains. “If this evolution is completely quantum, quantum mechanics tells you that you can de-evolve, or go back and go to the initial state.”

“And the idea is, if you could reverse the sign of the Hamiltonian, every small perturbation that occurred after the system evolved forward would get amplified if you go back in time,” Colombo adds.

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Shown here is the chamber in which researchers trapped and entangled a cloud of 400 ultracold ytterbium atoms. Credit: Simone Colombo

For their new study, the team studied 400 ultracold atoms of ytterbium, one of two atom types used in today’s atomic clocks. They cooled the atoms to just a hair above

They then measured the particles’ oscillations as they settled back into their unentangled states, and found that their final phase was markedly different from their initial phase — clear evidence that a quantum change had occurred somewhere in their forward evolution.

The team repeated this experiment thousands of times, with clouds ranging from 50 to 400 atoms, each time observing the expected amplification of the quantum signal. They found their entangled system was up to 15 times more sensitive than similar unentangled atomic systems. If their system is applied to current state-of-the-art atomic clocks, it would reduce the number of measurements these clocks require, by a factor of 15.

Going forward, the researchers hope to test their method on atomic clocks, as well as in quantum sensors, for instance for dark matter.

“A cloud of dark matter floating by Earth could change time locally, and what some people do is compare clocks, say, in Australia with others in Europe and the U.S. to see if they can spot sudden changes in how time passes,” Vuletic says. “Our technique is exactly suited to that, because you have to measure quickly changing time variations as the cloud flies by.”

Reference: “Time-reversal-based quantum metrology with many-body entangled states” by Simone Colombo, Edwin Pedrozo-Peñafiel, Albert F. Adiyatullin, Zeyang Li, Enrique Mendez, Chi Shu and Vladan Vuletić, 14 July 2022, Nature Physics.
DOI: 10.1038/s41567-022-01653-5

This research was supported, in part, by the National Science Foundation and the Office of Naval Research.



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