Physicists use quantum “time reversal” to measure vibrating atoms

Physicists use quantum

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

The 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 the precision of atomic clocks and quantum sensors, which are systems of atoms whose fluctuations indicate the presence of dark matter, a passing gravitational wave, or even new, unexpected phenomena.

A major hurdle on the way to better quantum measurements is the noise of the classic worldthat can easily overwhelm subtle atomic vibrations, making changes in those vibrations fiendishly difficult to detect.

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

Before you start buying DeLoreans, no, they haven’t figured out a way to reverse time themselves. Rather, the 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 rewound the tape of atomic vibrations, any changes in those vibrations were amplified in a way that could be easily measured.

In a newspaper published today in natural physicsthe team shows that the technique, which they dubbed SATIN (for time-reversal signal amplification), is the most sensitive method for measuring quantum fluctuations developed to date.

The technique could improve the accuracy of the current state of the art atomic clocks by a factor of 15, making their timing so precise that clocks would be off by less than 20 milliseconds throughout the age of the universe. The method could also be used to further focus quantum sensors developed to detect gravitational waves. Dark matterand other physical phenomena.

“We believe 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.”

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

Entangled timekeepers

A certain type of atom vibrates at a specific and constant frequency which, when properly measured, can serve as a very precise pendulum, keeping time in much shorter intervals than the seconds on a kitchen clock. But at the level of a single atom, the laws of quantum mechanics and those of the atom take over vibration changes like the face of a coin with each toss. Only by measuring an atom many times can scientists estimate its actual vibration — a constraint known as the standard quantum limit.

In state-of-the-art atomic clocks, physicists measure the vibrations of thousands ultracold atoms, many times over, to increase their chance of getting an accurate reading. However, these systems have some uncertainty and their timing could be more accurate.

In 2020, Vuletic’s group showed that the precision of current atomic clocks could be improved by atomic entanglement – a quantum phenomenon that forces particles to behave in a collective, highly correlated state. In this entangled state, the vibrations of individual atoms should shift toward a common frequency that would require far fewer experiments to measure accurately.

“Back then, we were still limited by how well we could read the clock phase,” says Vuletic.

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

Reverse the sign

In their new study, instead of trying to improve the resolution of existing readout tools, the team tried to amplify the signal from each change in oscillations so that they could be read by current tools. They did this by taking advantage of another strange phenomenon in quantum mechanics: time reversal.

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

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

“In quantum mechanics, if you know the Hamilton operator, you can follow what the system is doing through time, like a quantum orbit,” explains Pedrozo-Peñafiel. “If this evolution is fully quantum, quantum mechanics tells you that you can reverse engineer or go back and return to the initial state.

“And the idea is that if you could reverse the sign of the Hamilton operator, any small perturbation that occurred after the system had evolved forward would be amplified as you went back in time,” adds Colombo.

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

The team used a laser system to capture the atoms and then sent in a blue-colored “entanglement” light that forced the atoms to vibrate in a correlated state. They allowed the entangled atoms to evolve forward in time and then exposed them to a small magnetic field, which introduced a tiny quantum change and slightly shifted the atoms’ collective vibrations.

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

They then measured the vibrations of the particles as they reverted to their unraveled state and found that their final phase was significantly 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 between 50 and 400 atoms, where the expected amplification of the quantum signal is observed each time. They found that 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.

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

“A cloud of dark matter floating past Earth could change time locally, and some people are comparing clocks in Australia to others in Europe and the US, for example, to see if they can spot sudden changes over time,” says vultic . “Our technology is perfect for this because you have to measure quickly and alternately time Variations as the cloud flies past.”

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

More information:
Simone Colombo et al, Time-reverse based quantum metrology with many-body entanglement states, natural physics (2022). DOI: 10.1038/s41567-022-01653-5

Citation: Physicists Use Quantum “Time Reversal” to Measure Vibrating Atoms (2022, July 14) Retrieved July 15, 2022 from -vibrating.html

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