Creation of quantum entangled networks of atomic clocks and accelerometers

Sequence of atomic sensors. Credit: Nature (2022). DOI: 10.1038/s41586-022-05363-z

Researchers affiliated with the Q-NEXT Quantum Research Center show how to create quantum entangled networks of atomic clocks and accelerometers, and they demonstrate the superior, high-precision performance of the setup.

For the first time, scientists have entangled atoms for use as networked quantum sensors, specifically atomic clocks and accelerometers.

The research team’s experimental setup resulted in ultra-precise measurements of time and acceleration. Compared to a similar setup that doesn’t rely on quantum entanglement, their time measurements were 3.5 times more accurate and acceleration measurements 1.2 times more accurate.

The results are published in Nature. The research was conducted by scientists from Stanford University, Cornell University and the DOE’s Brookhaven National Laboratory.

“The impact of using entanglement in this setup was that it produced better sensor network performance than would have been available had quantum entanglement not been used as a resource,” said Mark Kasevich, lead author of the paper, Q-NEXT fellow, Professor William R. Kenan, Jr. at the Stanford School of Humanities and Sciences and professor of physics and applied physics. “For atomic clocks and accelerometers, ours is a pioneering demonstration.”

Greater sensitivity of atomic clocks and accelerometers would lead to more accurate timing and navigation systems, such as those used in global positioning systems, defense, and broadcast communications. Ultra-precise clocks are also used in finance and commerce.

“The GPS is telling me where I am at about a meter right now,” Kasevich said. “But what if I wanted to know where I was within 10 centimeters? That’s what the impact of better clocks would be.”

We can mark the passage of time by counting the number of pulses in an electromagnetic wave, just as we would count the ticks of a clock. If you know that a particular wave beats 6 billion times per second, you know that once you count 6 billion peaks of the wave, one second has passed. So, knowing the exact frequency of a microwave gives an accurate way to track time.

The rubidium atoms, trapped inside a cavity, are separated into two groups of about 100,000 atoms each. The groups are seated between two mirrors. The light is made to bounce between the mirrors, working its way through the clusters of atoms with each shot. The ricocheting light entangles them.

A microwave ripples through the two groups of atoms. The atoms that resonate with the particular frequency of the microwave react by changing state, like the wine glass that vibrates when a soprano hits just the right note.

Similarly, when a particular acceleration is applied to the atomic groups, a part of the atoms of each group reacts by changing state.

The two entangled atomic groups behave like two sides of the same clock, or like two readings of an accelerometer.

The research team measured the number of atoms that changed state, those that vibrated like a wine glass, in each group.

Then they used the numbers to calculate the difference in the microwave frequencies applied to the two groups, and therefore the difference in the time or acceleration readings of the groups.

Kasevich’s team found that entanglement improves the accuracy of the frequency or acceleration difference read by displays.

In their setup, measuring time at two places was 3.5 times more accurate when the clocks were entangled than when they were running independently. For acceleration, the measurement was 1.2 times more accurate with entanglement.

Impact

“If you want to know how long something is, you can look at one clock as a starting point and then run to another room to look at another clock as the end point,” Kasevich said. “Our method exploits the principle of entanglement to make this comparison as accurate as possible.”

The researchers were also able to network four clusters of atoms in four separate locations using this configuration.

In the team’s experiment, the two groups of atoms were separated by about 20 micrometers, nearly the average width of a human hair.

Their work means that time or acceleration can be compared, with unprecedented sensitivity, between four distinct, yet close to each other, locations.

“In the future, we want to push them longer distances. The world wants clocks whose time can be compared. It’s the same with accelerometers. There are sensing setups where you want to be able to read the difference in acceleration of one group versus another. We were able to show how to do that,” Kasevich said.

“It’s a tour de force from Mark and his team,” said JoAnne Hewett, deputy director of Q-NEXT, who is also associate director of fundamental physics at SLAC National Accelerator Laboratory and director of research. as well as a professor of particle physics at Stanford. and astrophysics. “This means we can harness entanglement to develop much more powerful sensors than the ones we use today. We’re getting one step closer to using quantum phenomena to improve our daily lives.”

What is quantum entanglement? How does it apply to sensors?

• Entanglement, a special property of nature at the quantum level, is a correlation between two or more objects. When two atoms are entangled, we can measure the properties of the two atoms by observing only one. This is true regardless of the distance – even if it is light years – that separates the entangled atoms.
• A useful everyday analogy: a red marble and a blue marble are placed in a box. If you pull a red marble from the box, you know, without having to look at the other one, that it is blue. The color of the beads is correlated, or entangled.
• In the quantum realm, the entanglement is more subtle. An atom can take several states (colors) at the same time. If our marbles were like atoms, each marble would be both red and blue. Neither is entirely red or blue when in the box. The quantum ball only “decided” on its color at the time of revelation. And once you draw a “decided” color ball, you know the color of its entangled partner.
• Taking a measurement of one member of an entangled pair is effectively taking a reading of both at the same time.
• To go further: two entangled clocks are practically equivalent to a single clock with two displays. Time measurements taken using entangled clocks can be more accurate than measurements from two separate, synchronized clocks.