A recently published paper in Nature Physics, “Time-reversal-based quantum metrology with many-body entangled states”, finds that a new technique to measure atomic vibrations can improve the accuracy of atomic clocks and quantum sensors. This would help scientists find dark matter, gravitational waves or unexpected phenomena. This is a dramatic finding that will push back the possibilities of science. 

The Problem

The paper was written by a group of MIT physicists, Vladan Vuletić being the corresponding author of the paper. The other authors are Simone Colombo, Edwin Pedrozo-Peñafiel, Albert Adiyatullin, Zeyang Li, Enrique Mendez, and Chi Shu.

Independent particles vibrate at a specific, constant frequency, which can serve as the basis for the accurate measurement of time. At the atomic level, quantum mechanics takes over and an atom’s oscillations seem to change at the drop of a hat. It becomes difficult for scientists to measure its oscillations. In order to measure these oscillations, scientists have to take many measurements of an atom. This is what is known as the Standard Quantum Limit. 

So, the most precise atomic clocks achieve their precision by repeatedly measuring the oscillations of thousands of ultracold atoms. Nevertheless, atomic clocks do carry some degree of uncertainty, and there is therefore also a degree of error in atomic time. 

One of the challenges of measuring atomic vibrations and their evolution over time, is the standard quantum limit, which makes it hard to detect these oscillations. According to Vuletić’s team, the standard quantum limit can be overcome through particle entanglement. Atomic entanglement refers to the coercing of particles to act in concert, with a high level of correlation. The frequency of these atoms approach a limit, a common frequency, which makes it easier to measure the oscillations. Nevertheless, entanglement is constrained by the final state readout. In other words, there was a limit to the sensitivity of the process. 

The Technique

The group’s findings are a result of a key decision. Rather than trying to increase the resolution of readout methods, Vuletić’s team decided to amplify the signal coming from any changes in the oscillations, making them readable to existing readout methods.

The authors found that by implementing a time-reversal protocol, they could amplify the signal. The signal amplification through a time-reversed interaction led to the biggest increase in sensitivity over the standard quantum limit. 

Although the idea of time reversal suggests that time has been manipulated, in reality, entanglement implies that atoms have been manipulated to behave as if they are going backward in time. As the scientists evolved the oscillations backward in time, researchers were also able to observe how the oscillations behaved and they were able to measure them.

This technique is known as signal amplification through time reversal (SATIN) is the most effective method for increasing signal amplification.

When you have a group of atoms that are unaffected by classical noise, they will evolve predictably and forward in time, with their oscillations and other interactions determined by the Hamiltonian nature of the system, which refers to the energy within that system.

Around the 1980s, scientists posited that a system’s Hamiltonian nature could be reversed, and its quantum system unwound, which would be akin to de-evolving the system, or evolving it backward in time. 

The Hamiltonian of a system is a marker of a system’s evolution across time, and a quantum system can be made to de-evolving even to its initial state. By reversing a Hamiltonian’s sign, the smallest oscillations that occured in that system will be amplified as that system goes backward in time.

How the Study Was Performed

The study was performed by studying 400 ultra cold ytterbium atoms. These atoms were cooled till they were just near absolute zero, where atomic behaviour becomes quantum. 

The atoms were trapped by lasers and then entangled by a blue-tinged light, coercing the atoms to highly correlated oscillations. 

The atoms were then allowed to evolve forward in time, while they were exposed to a magnetic field, introducing miniscule quantum changes, resulting in changes to the atoms' correlated oscillations.

As we said above, it is impossible to detect these miniscue changes using current readout methods. Tie reversal allowed the scientists to amplify the signal. This was done by beaming in a red-tinged light, which caused atomic disentanglement, mimicking what happens when atoms go backward in time.

The oscillations were measured as the particles reverted to their disentangled states. A difference between the disentangled and initial state was observed, showing the presence of quantum change as the atoms evolved forward.

Vuletić’s team performed this experiment with groups of between 50 and 400 atoms, over thousands of iterations, measuring the expected amplification of the signal. They found that their SATIN technique can increase the accuracy of the best atomic clocks today, by a factor of 15, which means that across the time of the universe’s life, these enhanced atomic clocks would only be off by under 20 milliseconds. 

Prospects after the Study

Given the technique and its results, the researchers believe that they will be able to make advances toward detecting gravitational waves and dark matter, as well as other phenomena.

The technique will be able to improve the science of quantum interference.