MIT Physicists Harness Quantum “Time Reversal” for Detecting Gravitational Waves and Dark Matter

Atomic clocks and quantum sensors for spotting dark matter or gravitational waves may become more accurate with the development of a novel method to monitor vibrating atoms.

The quantum oscillations of atoms hold a tiny world of information. If researchers can precisely detect these atomic oscillations and how they change over time, they will be able to improve the accuracy of both atomic clocks and quantum sensors. Quantum sensors—systems of atoms that may be employed as detectors thanks to fluctuations—can reveal the existence of dark matter, the passage of gravitational waves, or even brand-new, unanticipated phenomena.

A major obstacle to more accurate quantum measurements is the noise from the classical world, which may quickly overwhelm minuscule atomic vibrations and make any changes to those oscillations very impossible to detect.

However, by treating the particles to two crucial processes—quantum entanglement and time reversal—MIT researchers have recently shown that they can greatly enhance quantum alterations in atomic vibrations.

Let me reassure you that they haven't discovered a way to turn back time before you go out and buy a DeLorean. Instead, the researchers made quantum-entangled atoms behave as though they were going backward in time. The researchers effectively rewound the tape of atomic oscillations, magnifying any changes and making them simple to see.

The team of scientists shows that the technology, which they termed SATIN (for signal amplification via time reversal), is the most sensitive method yet created for monitoring quantum fluctuations in research published on July 14, 2022 in the journal Nature Physics.

By sending the particles through two crucial processes—quantum entanglement and time reversal—MIT researchers have demonstrated that they can dramatically enhance quantum alterations in atomic vibrations. Jose-Luis Olivares of MIT provided the statistics with assistance from iStockphoto.

The method might increase the precision of today's most sophisticated atomic clocks by a factor of 15, making them so precise that their time would be wrong by no more than 20 milliseconds throughout the history of the universe. The method may potentially be used to improve quantum sensors that are intended to find dark matter, gravitational waves, and other scientific phenomena.

According to main author Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, "We think this is the paradigm of the future." This method may be used to benefit any quantum interference that involves many of atoms.

First author Simone Colombo, Edwin Pedrozo-Peafiel, Albert Adiyatullin, Zeyang Li, Enrique Mendez, and Chi Shu are MIT co-authors on the work.

A certain kind of atom vibrates at a specific, constant frequency that, if well measured, may act as an extremely accurate pendulum and keep time at intervals considerably smaller than a kitchen clock's second. At the size of a single atom, however, quantum mechanical principles take control, and the atom's oscillation alters every time a coin is flipped. The Standard Quantum Limit is a restriction that prevents scientists from estimating an atom's real oscillation without making several observations of it.

Modern atomic clocks allow physicists to detect the oscillation of thousands of ultracold atoms repeatedly, increasing the likelihood that their measurements will be correct. However, there is some uncertainty in these systems, and they might improve the accuracy of their timekeeping.

The growth of a cloud of extremely cold atoms was first entangled, then reversed, by MIT researchers using a laser device. 

In 2020, Vuletic's team demonstrated that entangling the atoms—a quantum phenomena in which particles are forced to act in a collective, highly correlated state—could increase the accuracy of existing atomic clocks. The oscillations of the various atoms should change toward a common frequency in this entangled state, which would need far fewer efforts to precisely measure.

Vuletic explains that at the time, "we were still constrained by how effectively we could read out the clock phase."

In other words, the equipment used to monitor atomic oscillations was not sensitive enough to detect even a little change in the oscillations of the entire atom.                                                                                                                  

In their new work, the team tried to increase the signal from any change in oscillations so that it could be read by existing equipment instead of trying to improve the precision of existing readout techniques. They achieved this by taking use of time reversal, another intriguing quantum mechanical phenomena.

It is believed that a purely quantum system, such as a collection of atoms, should evolve forward in time in a predictable way and that the interactions between the atoms (such as their oscillations) should be precisely described by the system's "Hamiltonian" — essentially, a mathematical description of the system's total energy.

Theorists asserted in the 1980s that if a quantum system's Hamiltonian were reversed and the system was made to de-evolve, it would be the equivalent of the system travelling back in time.

As explained by Pedrozo-Peafiel, "In quantum physics, if you know the Hamiltonian, then you can follow what the system is doing over time, like a quantum trajectory. "Quantum physics teaches you that you may de-evolve or go back and go to the beginning condition if this evolution is totally quantum."

Colombo continues, "And the notion is that if you could turn the Hamiltonian around, any little disturbance that happened after the system moved ahead would get exacerbated if you go back in time.

The team's latest research used 400 ytterbium ultracold atoms, one of the two atom kinds utilized in modern atomic clocks. They cooled the atoms to only a few degrees above absolute zero, when most classical influences, such as heat, vanish and the behavior of the atoms is solely controlled by quantum processes.

The scientists first trapped the atoms with a laser system before entangling them with light by forcing them to vibrate in a correlated state. They allowed the entangled atoms to advance in time before exposing them to a weak magnetic field, which brought about a minute quantum transition and marginally altered the atoms' collective oscillations.

With the measuring techniques available today, such a change would be hard to identify. The scientists instead used time reversal to strengthen this quantum signal. To do this, scientists sent in a second, red-tinged laser that encouraged the atoms to detangle by stimulating them to evolve retrogradely.

The oscillations of the particles were then measured as they returned to their unentangled states, and they discovered that their final phase differed significantly from their initial phase. This was unmistakable proof that a quantum change had taken place at some point in the particles' forward evolution.

The scientists performed this experiment hundreds of times using clouds with 50 to 400 atoms, and they saw the anticipated quantum signal amplification in each instance. They discovered that compared to comparable unentangled atomic systems, their entangled system was up to 15 times more sensitive. Their method would significantly reduce the number of measurements needed by today's top-of-the-line atomic clocks—by a factor of 15.

In the future, the researchers intend to test their methodology on atomic clocks and in quantum sensors, such as those used to detect dark matter.

According to Vuletic, "a cloud of dark matter passing by Earth may modify local time, and what some people do is compare clocks, say, in Australia with others in Europe and the United States to see if they can notice dramatic changes in how time passes." Because you need to monitor rapidly changing time variations as the cloud moves by, our approach is ideal for this.

By JENNIFER CHU, MASSACHUSETTS INSTITUTE OF TECHNOLOGY