Atomic Clocks and Quantum Time Reversal

^{By Amal Pushp, Affiliate Physicist at the Resonance Science Foundation}

The quantum world essentially contains a myriad of intriguing phenomena and continues to add up to the imagination of science explorers. One such phenomenon concerns the oscillations at the level of atoms which forms the basis for the creation of quantum devices like atomic clocks and sensors. The elements that are used in modern day atomic clocks involve ytterbium, caesium among others. A significant part of the advances in contemporary atomic clocks research is mainly because of its usability in certain scenarios like dark matter and gravitational wave detections. 

Due to the subtle nature of these physical events, sometimes unwanted noise from the surrounding environment can cause distortions in the signal and negatively impact the results. In order to overcome this major challenge, physicists from the Massachusetts Institute of Technology (MIT) have come up with a viable proposition and that is to use a combination of two key phenomena: entanglement and time reversal [1]. This strategy is supposed to help amplify the microscopic level changes in atomic oscillations and thus combat the associated problem.

It is important to emphasize here and bring attention to the fact that time reversal doesn’t mean finding a way to reverse time. Time reversal is basically a mathematical operation to describe any particular event in time wherein its causality has been reversed. In other words, all motions associated with that event are reversed. So, the MIT researchers worked out with entangled atoms in such a way that the behavior of particles was analogous to evolving backwards in time. This process could help enhance the atomic vibrations and thus result in accurate measurements.

Atomic clocks are sensitive devices that measure time making use of the transitions that usually take place between quantum states of atoms and the frequency of radiation emitted as a result of the transition. Generally, at the level of individual atoms, the laws of quantum mechanics dominate and one needs to take a lot of measurements in order to get a realistic estimate of atomic oscillations. This is a limitation referred to as the standard quantum limit. Now, this is exactly where the role of time-reversal comes in. The MIT team irradiated the entangled atoms with a laser beam and this action caused the atoms to lose their quantum correlation which appeared as if they were advancing backward in time. It was also found that the final phase was way different from the initial phase thus confirming the occurrence of a quantum change. 

The novel technique developed by MIT scientists has been assigned the acronym SATIN (Stands for signal amplification through time reversal) and is regarded as the most sensitive method for measuring quantum fluctuations developed as yet and can possibly enhance the accuracy of futuristic atomic clocks by a factor of 15. Given the circumstances, it is quite certain that this innovation would provide an extra boost to the science of metrology which would in turn serve fundamental as well as technological research in the physical sciences.

^{The Graph represents a historical account of the accuracy of atomic clocks. Credit: National Institute of Standards and Technology (NIST)}

References

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