A novel approach could enhance the precision of atomic clocks and quantum detectors used to detect dark matter or gravitational waves.
Atomic quantum vibrations contain a little universe of information. If scientists can precisely measure these atomic oscillations and how they change over time, they will be able to improve the accuracy of atomic clocks and quantum sensors, which are systems of atoms whose fluctuations can indicate the presence of dark matter, a passing gravitational wave, or even new, unexpected phenomena.
Noise from the classical world is a major obstacle on the route to better quantum measurements, as it may easily overpower small atomic vibrations, making it difficult to detect any changes in those vibrations.
Now, MIT physicists have demonstrated that quantum entanglement and time reversal can greatly increase quantum alterations in atomic vibrations.
No, they have not discovered a means to reverse the flow of time. Rather, the physicists altered quantum-entangled atoms so that the particles acted as though they were evolving backwards in time. As the researchers essentially rewound the tape containing atomic oscillations, any alterations to those oscillations were amplified in a manner that was easily quantifiable.
In an article published today in Nature Physics, the team demonstrates that SATIN (for signal amplification via time reversal) is the most sensitive method discovered to date for quantifying quantum fluctuations.
The approach might increase the accuracy of present state-of-the-art atomic clocks by a factor of 15, making their timing so precise that it would be within 20 milliseconds across the whole age of the universe. Quantum sensors designed to detect gravitational waves, dark matter, and other scientific phenomena could also benefit from this technique.
“We believe this to be the paradigm of the future,” said Lester Wolfe Professor of Physics at MIT and primary author Vladan Vuletic. This method can benefit any quantum interference that involves several atoms.
Simone Colombo, Edwin Pedrozo-Peafiel, Albert Adiyatullin, Zeyang Li, Enrique Mendez, and Chi Shu are among the MIT co-authors of the paper.
A certain type of atom vibrates at a specific and constant frequency, which, if well measured, can function as a highly accurate pendulum, keeping time in intervals much less than a kitchen clock’s second. At the level of a single atom, however, the laws of quantum physics take control, and the atom’s oscillation alters with each flip of a coin. Scientists may only determine an atom’s real oscillation by conducting numerous observations, a limitation known as the Standard Quantum Limit.
In modern atomic clocks, researchers repeatedly measure the oscillation of thousands of ultracold atoms to maximize the likelihood of obtaining an accurate measurement. Nonetheless, these systems contain a degree of ambiguity, and their timekeeping may be more accurate.
In 2020, Vuletic’s team demonstrated that the precision of existing atomic clocks may be enhanced by entangling the atoms, a quantum phenomena in which particles are forced to act in a highly correlated collective form. In this condition of entanglement, the oscillations of individual atoms should gravitate toward a common frequency that can be precisely measured with significantly fewer attempts.
“At the time, we were constrained by our ability to accurately detect the clock phase,” Vuletic explains. In other words, the instruments employed to monitor atomic oscillations were insufficiently sensitive to detect or measure even the most minute variations in the atoms’ collective oscillations.
Reverse the sign
In their new study, rather than seeking to improve the resolution of existing readout methods, the team sought to amplify the signal from any shift in oscillations so that they could be detected by existing tools. They did it by utilizing another peculiar quantum mechanical phenomenon: time reversal.
It is believed that a purely quantum system, such as a group of atoms completely isolated from classical noise, should evolve forward in time in a predictable manner, and the atoms’ interactions (such as their oscillations) should be precisely described by the system’s “Hamiltonian” — a mathematical description of the system’s total energy. In the 1980s, theorists hypothesized that if the Hamiltonian of a system were inverted and the same quantum system was made to de-evolve, it would be as if the system were moving back in time.
Pedrozo-Peafiel notes, “In quantum physics, if you know the Hamiltonian, you can monitor what the system is doing in time, similar to a quantum trajectory.” “If this evolution is entirely quantum, quantum mechanics indicates that it is possible to de-evolve, or return to the initial condition.”
Colombo continues, “And the idea is that if you could flip the sign of the Hamiltonian, every minor perturbation that occurred after the system went ahead would be exacerbated if you traveled back in time.”
For their latest study, the team examined 400 ultracold ytterbium atoms, one of the two types of atoms used in modern atomic clocks. They cooled the atoms to a temperature just a hair above absolute zero, when classical effects such as heat are negligible and the atoms’ behavior is determined only by quantum effects.
The scientists utilized a set of lasers to capture the atoms, and then they introduced blue-tinged “entangling” light, which forced the atoms to oscillate in a correlated state. They allowed the entangled atoms to evolve forward in time before exposing them to a minuscule magnetic field, which created a tiny quantum transition and shifted the collective oscillations of the atoms.
It would be impossible to detect such a change with the currently available measurement instruments. Instead, the team reversed time to amplify this quantum signal. To do this, they sent in a second laser with a reddish hue that stimulated the atoms to separate, as if they were evolving in reverse.
The researchers next analyzed the oscillations of the particles as they returned to their unentangled states and discovered that their final phase was significantly different from their beginning phase, providing conclusive proof that a quantum change had happened at some point in their history.
The scientists repeated this experiment thousands of times with clouds containing 50 to 400 atoms, observing the expected quantum signal amplification each time. The researchers discovered that their entangled system was up to 15 times more sensitive than comparable unentangled atomic systems. If their approach were applied to current state-of-the-art atomic clocks, the number of measurements required by these clocks would be reduced by a factor of 15.
The researchers intend to test their technology on atomic clocks and quantum sensors, such as those for dark matter, in the future.
“A cloud of dark matter passing by Earth could alter local time, and some people compare clocks in Australia, Europe, and the United States to detect rapid changes in the passage of time,” explains Vuletic. “Our method is ideally suited for this since you must measure rapidly varying time variations as the cloud goes by.”