Physicists use quantum ‘time reversal’ to measure vibrating atoms


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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 vibrations and how they evolve over time, they can hone in on the accuracy of atomic clocks, as well as quantum sensors, which are systems of atoms whose vibrations can indicate the presence of dark matter, a passing gravitational wave, or even new and unexpected phenomena.

The main obstacle to better quantum measurements is noise from the classical world, which can easily overwhelm subtle atomic vibrations, making any changes in those vibrations damn hard to detect.

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

Before you start buying DeLoreans, no, they haven’t found a way to turn back the clock. Rather, physicists have manipulated quantum entangled atoms in such a way that the particles behave as if they were evolving backwards in time. As the researchers effectively rewound the tape of atomic vibrations, any changes in those vibrations were amplified in such a way that they could be easily measured.

In an article published today in Physics of natureThe team demonstrated that the method, which they called SATIN (for signal amplification by time reversal), is the most sensitive method for measuring quantum fluctuations developed to date.

This method could improve the accuracy of today’s atomic clocks by a factor of 15, making them so accurate that, over the entire age of the universe, clocks would be less than 20 milliseconds behind. The technique 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 Vuletić, professor of physics at the Massachusetts Institute of Technology Lester Wolfe. “Any quantum interference that works with many atoms can benefit from this technique.”

The co-authors of the study at MIT are first author Simone Colombo, Edwin Pedroso-Penafiel, Albert Adiyatullin, Zeyan Li, Enrique Mendez, and Chi Shu.

Confused Timekeepers

A certain type of atom vibrates at a certain and constant frequency, which, if correctly measured, can serve as a very accurate pendulum, measuring time at much shorter intervals than the seconds of a kitchen clock. But on the scale of a single atom, the laws of quantum mechanics come into play, and the vibrations of an atom change like the face of a coin every time it is tossed. Only by taking many measurements of an atom can scientists get an estimate of its actual vibrations, a limit known as the standard quantum limit.

In modern atomic clocks, physicists repeatedly measure the vibrations of thousands of ultracold atoms to increase their chances of getting accurate results. However, these systems have some uncertainty and their timing could be more accurate.

In 2020, Vuletik’s group showed that the accuracy of today’s atomic clocks could be improved by atomic entanglement, a quantum phenomenon in which particles are forced to behave in a collective, highly correlated state. In this entangled state, the vibrations of the individual atoms should shift towards a common frequency, which would require far fewer attempts to accurately measure.

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

That is, the instruments used to measure atomic vibrations were not sensitive enough to read or measure any subtle changes in the collective vibrations of atoms.

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In their new study, rather than trying to improve the resolution of existing reading tools, the team sought to amplify the signal from any change in oscillation so that they could be read with existing reading tools. They did this by exploiting another curious phenomenon in quantum mechanics: time reversal.

It is believed that a purely quantum system, such as a group of atoms completely isolated from everyday classical noise, should evolve forward in time in a predictable way, and the interactions of atoms (for example, their vibrations) should be accurately described by the “Hamiltonian” of the system – in essence, a mathematical description of the complete system energy.

In the 1980s, theorists predicted that if the Hamiltonian of a system were reversed and the same quantum system deevolved, it would be like the system going back in time.

“In quantum mechanics, if you know the Hamiltonian, you can track what the system is doing over time, like a quantum trajectory,” explains Pedroso-Peñafiel. “If this evolution is entirely quantum, quantum mechanics tells you that you can deevolve, or go back and return to the original state.”

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

For their new study, the team studied 400 ultracold atoms of ytterbium, one of two types of atoms used in modern atomic clocks. They cooled atoms just above absolute zero, at temperatures at which most classical effects such as heat disappear and the behavior of atoms is determined solely by quantum effects.

The team used a system of lasers to trap the atoms and then sent out “entangled” light with a blue tint that caused the atoms to vibrate in a correlated state. They allowed the entangled atoms to evolve forward in time and then subjected them to a small magnetic field, which introduced a small quantum change, slightly shifting the collective vibrations of the atoms.

Such a shift would be impossible to detect with existing measurement tools. Instead, the team applied time reversal to amplify this quantum signal. To do this, they sent another laser with a red tint, which stimulated the disentangling of the atoms, as if they were evolving backwards in time.

They then measured the oscillations of the particles as they returned to their entangled state and found that their final phase was markedly different from their initial phase—clear evidence that a quantum change had taken place somewhere in their direct evolution.

The team repeated this experiment thousands of times with clouds ranging from 50 to 400 atoms, each time seeing the expected amplification of the quantum signal. They found that their entangled system was 15 times more sensitive than similar unentangled atomic systems. If their system is applied to modern atomic clocks, it will 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 quantum sensors, for example, for dark matter.

“A cloud of dark matter floating past the Earth can change time locally, and some people are comparing clocks in, say, Australia with others in Europe and the US to see if they can detect sudden changes in how time passes,” says Vuletic. . “Our technique is just right for this, because you have to measure rapidly changing time variations as the cloud flies by.”

A new type of atomic clock could help scientists detect dark matter and study how gravity affects time

Additional Information:
Simone Colombo et al., Quantum Metrology Based on Time Reversal with Entangled Many-Body States, Physics of nature (2022). DOI: 10.1038/s41567-022-01653-5

Provided by the Massachusetts Institute of Technology

Quote: Physicists use quantum “time reversal” to measure vibrating atoms (2022 July 14), retrieved July 15, 2022 from vibrating.html.

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