Using "spooky action at a distance" to link atomic clocks

Using “spooky action at a distance” to link atomic clocks

The researchers show that spatially separated clock frequencies can be compared more precisely

The researchers show that spatially separated clock frequencies can be compared more precisely

An experiment carried out by researchers at the University of Oxford combines two unique and arguably mind-boggling discoveries, namely high-precision atomic clocks and quantum entanglement, to produce two atomic clocks that are “entangled”. This means that the uncertainty inherent in the simultaneous measurement of their frequencies is greatly reduced.

Although this is a proof-of-concept experiment, it has the potential to be used in dark matter probing, precision geodesy, and other such applications. The two-node network they build is scalable to multiple nodes, the researchers write in an article about this work published in The nature recently.

Atomic clocks increased in precision and became so reliable 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 beginning of the 21st century, available cesium clocks were so accurate that they would only gain or lose a second once every 20 million years or so. Today, even this record has been broken, and there are “optical lattice clocks” that are so precise that they miss a second only once in 15 billion years. To give some perspective, this is more than the age of the universe, which is 13.8 billion years.

Mundane uses

More mundane uses that these clocks can be put to include keeping accurate GPS time or monitoring things from afar on Mars.

“If you can measure the difference in frequency between these two clocks that are in different locations, it opens up a number of applications,” says author Raghavendra Srinivas of the Department of Physics, Clarendon Laboratory, University of Oxford, UK. of The nature paper.

Their work is a proof of principle 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 accurate. Potential applications of this when extended in space and including more nodes of two are in studying the space-time variation of fundamental constants and in probing dark matter—profound questions in physics.

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

On the other hand, if the two have been entangled, it is a way of saying that their physical attributes, say spin, or in this case 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 before, but this is the first demonstration of quantum crossing of optical atomic clocks.

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

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

“We’ve shown that you can now generate entanglement at a distance in a practical way. At some point, it could be useful for next-generation systems,” says Dr. Srinivas.

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