Physicists harness quantum ‘time reversal’ to measure vibrating atoms

Physicists harness quantum ‘time reversal’ to measure vibrating atoms

Physicists harness quantum 'time reversal' to measure vibrating atoms

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

Quantum vibrations in atoms contain a miniature world of information. If scientists can accurately measure these atomic oscillations and their evolution over time, they can refine the accuracy of atomic clocks as well as quantum sensors, which are systems of atoms whose fluctuations can indicate the presence of dark matter, a passing gravitational wave, or even new and unexpected phenomena.

A major obstacle in the way of better quantum measurements is the noise of the classical world, which can easily overwhelm the subtle atomic vibrations, making any changes in those vibrations fiendishly difficult to detect.

Now, MIT physicists have shown they can significantly amplify quantum changes in atomic vibrations, subjecting particles to two key processes: quantum entanglement and time inversion.

Before they started shopping for DeLoreans, no, they didn’t find a way to reverse time itself. Instead, physicists manipulated quantum-entangled atoms so that the particles behaved as if moving backwards in time. As the researchers effectively rewound the tape of the atomic oscillations, any changes in those oscillations were amplified, in a way that could be easily measured.

In an article published today in Natural Physicsthe team demonstrates that the technique, which they dubbed SATIN (for time-inversion signal amplification), is the most sensitive method for measuring quantum fluctuations developed to date.

The technique could improve the accuracy of today’s state-of-the-art atomic clocks by a factor of 15, making their timing so precise that over the entire age of the universe, the clocks would be off by less than 20 milliseconds. The method could also be used to further focus quantum sensors designed to detect gravitational waves, dark matter and other physical phenomena.

“We think this is the paradigm of the future,” says lead author Vladan Vuletic, Lester Wolfe professor of physics at MIT. “Any quantum interference that works with many atoms can benefit from this technique.”

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

Tangled Timekeepers

A given type of atom vibrates at a particular, constant frequency which, if correctly measured, can serve as a very precise pendulum, keeping time at intervals much shorter than the second of a kitchen clock. But on the scale of a single atom, the laws of quantum mechanics take over and the oscillation of the atom changes like the face of a coin each time it is flipped. Only by taking many measurements of an atom can scientists get an estimate of its true oscillation – a limitation known as the standard quantum limit.

In state-of-the-art atomic clocks, physicists measure the oscillation of thousands of ultracold atoms, multiple times, to increase their chances of getting an accurate measurement. However, these systems have some uncertainty and their timing could be more accurate.

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

“At the time, we were still limited by the quality of our reading of the clock phase,” says Vuletic.

In other words, the tools used to measure atomic oscillations were not sensitive enough to read or measure any subtle changes in the collective oscillations of atoms.

reverse the sign

In their new study, instead of trying to improve the resolution of existing reading tools, the team sought to amplify the signal of any changes in the oscillations, so that they could be read by current tools. They did this by exploiting another curious phenomenon of quantum mechanics: time inversion.

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

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

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

“And the idea is that if you could reverse the sign of the Hamiltonian, every little disturbance that happened after the system evolved would be amplified if you went back in time,” Colombo adds.

For their new study, the team studied 400 ultracold ytterbium atoms, one of two types of atoms used in today’s atomic clocks. They cooled the atoms to just a hair’s breadth above absolute zero, to temperatures where most classical effects such as heat fade and the behavior of atoms is governed solely by quantum effects.

The team used a system of lasers to trap the atoms, then sent blue-tinted “entangled” light, which forced the atoms to oscillate in a correlated state. They let the entangled atoms evolve over time, then exposed them to a small magnetic field, which introduced a tiny quantum shift, slightly shifting the collective oscillations of the atoms.

Such a displacement would be impossible to detect with existing measurement tools. Instead, the team applied time inversion to amplify this quantum signal. To do this, they sent another red-tinted laser that stimulated the atoms to unravel, as if moving backwards in time.

They then measured the oscillations of the particles 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 shift had occurred somewhere in their evolution. forward.

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

In the future, the researchers hope to test their method on atomic clocks, as well as on quantum sensors, for example for dark matter.

“A cloud of dark matter floating near Earth could change the time locally, and what some people do is compare clocks, for example, in Australia with others in Europe and the United States to see if they can detect sudden changes in the way time passes,” says Vuletic. . “Our technique is exactly suited for this, because you need to measure variations in weather that change rapidly as the cloud passes.”

A new type of atomic clock could help scientists detect dark matter and study the effect of gravity on time

More information:
Simone Colombo et al, Time inversion-based quantum metrology with entangled many-body states, Natural Physics (2022). DOI: 10.1038/s41567-022-01653-5

Provided by Massachusetts Institute of Technology

Quote: Physicists Harness Quantum ‘Time Reversal’ to Measure Vibrating Atoms (2022, July 14) Retrieved July 15, 2022 from .html

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