Using ‘spooky action at a distance’ to link atomic clocks

The researchers show frequencies of spatially separated clocks can be compared more precisely

September 17, 2022 07:10 pm | Updated 07:10 pm IST

Demonstrated: Researchers have shown that it is possible to generate remote entanglement in a practical way. 

Demonstrated: Researchers have shown that it is possible to generate remote entanglement in a practical way.  | Photo Credit: Special Arrangement

An experiment carried out by the University of Oxford researchers combines two unique and one can say even mind-boggling discoveries, namely, high-precision atomic clocks and quantum entanglement, to achieve two atomic clocks that are “entangled.” This means the inherent uncertainty in measuring their frequencies simultaneously is highly reduced.

While this is a proof-of-concept experiment, it has the potential for use in probing dark matter, precision geodesy and other such applications. The two-node network that they build is extendable to more nodes, the researchers write, in an article on this work published in Nature recently.

Atomic clocks grew in accuracy and became so dependable that in 1967, the definition of a second was revised to be the time taken by 9,19,26,31,770 oscillations of a cesium atom. At the start of the 21st century, the cesium clocks that were available were so accurate that they would gain or lose a second only once in about 20 million years. At present, even this record has been broken and there are “optical lattice clocks” that are so precise that they lose a second only once in 15 billion years. To give some perspective, that is more than the age of the universe, which is 13.8 billion years.

Mundane uses

The more mundane uses to which these clocks can be put include accurate time keeping in GPS, or monitoring stuff remotely on Mars.

“If you can measure the frequency difference between these two clocks that are in different locations, that opens up a host of applications,” says Raghavendra Srinivas, from the Department of Physics, Clarendon Laboratory, University of Oxford, U.K., who is an author of the Nature paper.

Their work is a proof-of-principle demonstration that two strontium atoms separated in space by a small distance, can be pushed into an “entangled state” so that a comparison of their frequencies becomes more precise. Potential applications of this when extended in space and including more nodes than two, are in studying the space-time variation of the fundamental constants and probing dark matter — deep questions in physics.

In quantum physics, entanglement is a weird phenomenon described as a “spooky action at a distance” by Albert Einstein. Normally, when you consider two systems separated in space that are also independent and you wished to compare some physical attribute of the two systems, you would make separate measurements of that attribute and this would involve a fundamental limitation to how precisely you can compare the two — for two separate measurements have to be made.

On the other hand, if the two were entangled, it is a way of saying that their physical attributes, say spin, or in this case, the frequency, vary in tandem. Measuring the attribute on one system, tells you about the other system. This in turn improves the precision of the measurement to the ultimate limit allowed by quantum theory.

Proof of concept

Quantum networks of this kind have been demonstrated earlier, but this is the first demonstration of quantum entanglement of optical atomic clocks.

Dr. Srinivas says, “The key development here is that we could improve the fidelity and the rate of this remote entanglement to the point where it’s actually useful for other applications, like in this clock experiment.”

For their demonstration, the researchers used strontium atoms for the ease in generating remote entanglement. They plan to try this with better clocks such as those that use calcium.

“We showed that you can now generate remote entanglement in a practical way. At some point, it might be useful for state-of-the art systems,” says Dr. Srinivas.

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