Physicists use quantum "time reversal" to measure vibrating atoms

The quantum vibrations in atoms hold a miniature world of information. If scientists can accurately measure these atomic oscillations, and how they evolve over time, they can hone the precision of atomic clocks and quantum sensors, which are atomic systems whose fluctuations can indicate the presence of dark matter, passing gravitational waves, or even phenomena. unexpected new.

The main obstacle on the road to better quantum measurements is noise from the classical world, which can easily overwhelm the subtle vibrations of atoms, making any changes to those vibrations extremely difficult to detect.

Now, MIT physicists have shown that they can significantly amplify quantum changes in atomic vibrations, by placing particles through two key processes: quantum entanglement and time reversal.

Before you start shopping for DeLoreans, no, they haven’t figured out how to turn back time itself. Instead, physicists have manipulated quantum entangled atoms in such a way that the particles behave as if they were evolving backwards in time. When the researchers effectively played back recordings of atomic oscillations, any changes to those oscillations were amplified, in a way that could be easily measured.

In a paper appearing today in Nature Physics, the team demonstrates that the technique, which they have dubbed SATIN (for amplification of signals through time reversal), is the most sensitive method for measuring quantum fluctuations developed to date.

This technique can increase the accuracy of today’s advanced atomic clocks by a factor of 15, making their timing so precise that over the entire lifetime of the universe the clock would be less than 20 milliseconds. The method could also be used to better focus quantum sensors designed to detect gravitational waves, dark matter, and other physical phenomena.

“We think this is the paradigm of the future,” said lead author Vladan Vuletic, Lester Wolfe Professor of Physics at MIT. “Any quantum interference working with multiple atoms can take advantage of this technique.”

Co-authors of the MIT study include first author Simone Colombo, Edwin Pedrozo-Peñafiel, Albert Adiyatullin, Zeyang Li, Enrique Mendez, and Chi Shu.

The entangled timer

Certain types of atoms vibrate at a certain, constant frequency which, if measured correctly, can function as very precise pendulums, keeping time in much shorter intervals than the seconds of a kitchen clock. But on the scale of a single atom, the laws of quantum mechanics take over, and atomic oscillations change like the face of a coin each time it is tossed. Only by taking many measurements of an atom can scientists estimate its actual oscillations – a limit known as the Standard Quantum Limit.

In state-of-the-art atomic clocks, physicists measure the oscillations of thousands of ultracold atoms, many times over, to increase their chances of getting an accurate measurement. However, this system has some uncertainties, and the timing could be more precise.

In 2020, the Vuletic group demonstrated that the precision of today’s atomic clocks could be improved by involving atoms – a quantum phenomenon in which particles are forced to behave in highly correlated collective states. In this entangled state, the individual atomic oscillations would have to shift towards a common frequency which would require much less effort to measure accurately.

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

That is, the tools used to measure atomic oscillations are not sensitive enough to read, or measure subtle changes in atomic collective oscillations.

Reverse sign

In their new study, instead of trying to increase the resolution of an existing reading device, the team sought to increase the signal of any change in oscillation, so that it can be read by the current tool. They did so by exploiting another strange phenomenon in quantum mechanics: the reversal of time.

It is thought that a pure quantum system, such as a group of atoms completely isolated from everyday classical noise, should progress in time in a predictable way, and atomic interactions (such as their oscillations) should be precisely described by the “Hamilton” system – essentially , a mathematical description of the total energy of the system.

In the 1980s, theorists predicted that if the Hamiltonian system were reversed, and the same quantum system made for de-evolution, it would be as if the system went back in time.

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

“And the idea is, if you can reverse the Hamiltonian sign, any small perturbations that occur after the system evolves forward will be amplified if you go back in time,” Colombo added.

For their new study, the team studied 400 atoms of supercooled ytterbium, one of two types of atoms used in atomic clocks today. They cool atoms just a hair above absolute zero, at a temperature where most classical effects such as heat fade and atomic behavior is governed purely by quantum effects.

The team used a laser system to trap atoms, then sent a blue “wrapped” light, which forced the atoms to oscillate in a correlated state. They allowed the entangled atoms to evolve forward in time, then exposed them to tiny magnetic fields, which introduced tiny quantum changes, slightly shifting the collective oscillations of the atoms.

Such a shift is impossible to detect with existing measurement tools. Instead, the team applied time reversal to enhance this quantum signal. To do this, they sent another red laser that stimulated the atoms to disentangle, as if they had evolved backwards in time.

They then measured the oscillations of the particles as they settled back into their non-entangled state, and found that their late phase was very different from the initial phase — clear evidence that a quantum change had occurred somewhere in their advanced 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 entangled atomic systems. If their system were applied to today’s most advanced 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 quantum sensors, for example for dark matter.

“Clouds of dark matter floating on Earth can change time locally, and what some people do is compare the clock, say, in Australia with others in Europe and the US to see if they can see sudden changes in how time passes,” he said. Vuletic. . “Our technique is perfect for that, because you have to measure variations in time that change rapidly as the cloud passes.”

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

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