Physicists harness quantum 'time reversal' to measure vibrating atoms
MIT researchers used a system of lasers to first entangle, then reverse the evolution of a cloud of ultracold atoms. Credit: Simone Colombo

There is a world of information in the atoms. If scientists can accurately measure these atomic oscillations, and how they evolve over time, they can hone the precision of atomic clocks as well as quantum sensors, which are systems of atoms whose fluctuations can indicate the presence of dark matter.

Classical world noise can easily overwhelm subtle atomic vibrations, making it hard to detect any changes to them.

MIT physicists have shown that they can amplify quantum changes by putting the particles through two processes.

They haven't figured out a way to reverse time. Physicists manipulated quantumly entangled atoms to behave as if they were evolving backwards in time. The researchers rewound the tape of atomic oscillations in a way that made it easy to measure any changes to them.

The most sensitive method for measuring quantum fluctuations has been demonstrated by the team in a paper.

The technique could make the current state-of-the-art atomic clocks so precise that they would be less than 20 milliseconds off. The method could be used to focus quantum sensors that are meant to detect physical phenomena.

The lead author of the paper says that they think this is the future. Any quantum interference that works with a lot of atoms can make money.

The study's MIT co-authors include Simone Colombo.

Timekeepers areangled

If measured correctly, a given type of atom can serve as a very precise pendulum, keeping time in shorter intervals than a kitchen clock's second. At the scale of a single atom, the laws of quantum mechanics take over, and the atom's behavior changes like the face of a coin. The Standard Quantum Limit is a limitation on how much an atom can be measured.

Physicists use state-of- the-art atomic clocks to increase their chance of getting an accurate measurement. The systems have some uncertainty and could be more precise.

In 2020, Vuletic's group showed that the precision of current atomic clocks could be improved by entangling the atoms. In this entangled state, the individual atoms should shift their frequencies in a way that would make it much easier to measure them.

We were limited by how well we could read the clock phases.

The tools used to measure atomic oscillations were not sensitive enough.

If you reverse the sign, you'll get a different sign.

Instead of trying to improve the resolution of existing tools, the team looked to boost the signal from any change in oscillations, so that they could be read by current tools. Time reversal is one of the curiosities in quantum mechanics.

A group of atoms that are completely isolated from classical noise should evolve forward in time in a predictable manner and their interactions should be described precisely by the system's "Hamiltonian"

If a system's Hamiltonian were reversed, the same quantum system would go back in time.

If you know the Hamiltonian, you can track what the system is doing through time. If this evolution is completely quantum, you can either de-evolve or go back to the beginning.

The idea is that if you could reverse the sign of the Hamiltonian, there would be more changes in the system.

400 ultracold atoms of ytterbium were studied by the team for their new study. The atoms were cooled to just a hair above absolute zero, where classical effects like heat fade away and the atoms' behavior is governed by quantum effects.

The team used a system of lasers to trap the atoms, then sent in a blue-tinged "entangling" light, which forced the atoms to move in a correlated state. They exposed the atoms to a small magnetic field, which caused a small quantum change.

Existing measurement tools can't detect such a shift. Time reversal was used to boost the signal. They used a red-tinged laser that stimulated the atoms to untangle, as if they were evolving back in time.

They found that the particles' final phase was different from their initial one, and that a quantum change had taken place somewhere in their evolution.

The team repeated the experiment thousands of times, with clouds ranging from 50 to 400 atoms. Their system was up to 15 times more sensitive than similar systems. If their system is applied to current state-of-the-art atomic clocks, it would reduce the number of measurement by 15.

The researchers want to test their method on atomic clocks and quantum sensors 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 US, to see if they can spot sudden changes in how time passes." The technique we use is suited to that because you have to measure quickly changing time variations.

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