Scientists at the University of Oxford have been able to demonstrate a network of two entangled optical atomic clocks for the first time. They have also succeeded in demonstrating how the entanglement between the remote clocks can be used to improve their measurement precision, according to a press release by the institution published on Thursday.

“Improving the precision of frequency comparisons between multiple atomic clocks offers the potential to unlock our understanding of all sorts of natural phenomena. It is essential, for example, in measuring the space-time variation of fundamental constants, for geodesy where the frequency of the atomic clocks is used to measure the heights of two locations, and even in the search for dark matter,” wrote the statement.

# The key to reaching the fundamental limit of precision that’s determined by quantum theory

Entanglement is a quantum phenomenon in which two or more particles become linked together so that they can no longer be described independently, even at vast distances. The process is the key to reaching the fundamental limit of precision that’s determined by quantum theory. In the past, previous experiments have demonstrated that entanglement between clocks in the same system can be used to improve the quality of measurements.

However, this is the first time researchers have been able to achieve this between clocks in two separate remotely entangled systems. “Thanks to years of hard work from the whole team at Oxford, our network apparatus can produce entangled pairs of ions with high fidelity and high rate at the push of a button. Without this capability this demonstration would not have been possible,” said Bethan Nichol, one of the authors of the paper published in *Nature.*

# Demonstrating versatility

The Oxford team used a state-of-the-art quantum network to achieve their results. The two clocks used for the experiment were only 2 meters apart, but in principle such networks can be scaled up to cover much larger distances.

“While our result is very much a proof-of-principle, and the absolute precision we achieve is a few orders of magnitude below the state of the art, we hope that the techniques shown here might someday improve state-of the art systems,” explained Dr Raghavendra Srinivas, another of the paper’s authors. “At some point, entanglement will be required as it provides a path to the ultimate precision allowed by quantum theory.”

Professor David Lucas, whose team at Oxford were responsible for the experiment, said, “Our experiment shows the importance of quantum networks for metrology, with applications to fundamental physics, as well as to the more well-known areas of quantum cryptography and quantum computing.”

The devices for the experiment were developed by the UK’s Quantum Computing and Simulation (QCS) Hub, a consortium of 17 universities led by the University of Oxford that was designed for quantum computing and for communication rather than for quantum-enhanced metrology.

Abstract

Optical atomic clocks are our most precise tools to measure time and frequency. Precision frequency comparisons between clocks in separate locations enable one to probe the space–time variation of fundamental constants and the properties of dark matter, to perform geodesy and to evaluate systematic clock shifts. Measurements on independent systems are limited by the standard quantum limit; measurements on entangled systems can surpass the standard quantum limit to reach the ultimate precision allowed by quantum theory—the Heisenberg limit. Although local entangling operations have demonstrated this enhancement at microscopic distances, comparisons between remote atomic clocks require the rapid generation of high-fidelity entanglement between systems that have no intrinsic interactions. Here we report the use of a photonic link to entangle two 88Sr+ ions separated by a macroscopic distance19 (approximately 2 m) to demonstrate an elementary quantum network of entangled optical clocks. For frequency comparisons between the ions, we find that entanglement reduces the measurement uncertainty by nearly 2–√, the value predicted for the Heisenberg limit. Today’s optical clocks are typically limited by dephasing of the probe laser; in this regime, we find that entanglement yields a factor of 2 reduction in the measurement uncertainty compared with conventional correlation spectroscopy techniques. We demonstrate this enhancement for the measurement of a frequency shift applied to one of the clocks. This two-node network could be extended to additional nodes, to other species of trapped particles or—through local operations—to larger entangled systems.

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