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Artistic representation of an atomic clock breaking the Standard Quantum Limit. Image credit: Steven Burrows / JILA

Imagine you're trying to keep time by listening to a room full of people clapping. If everyone claps randomly, it’s hard to tell the rhythm. But if they clap in sync, the beat becomes clear and steady. Now imagine you could gently guide them to clap more in unison—not perfectly, but just enough to reduce the noise. That’s what JILA researchers have done with atoms in a clock.

In a new study, researchers led by JILA and NIST Fellow Jun Ye have shown how to make atomic clocks even more precise by leveraging entanglement. This allows the atoms to “tick” more in sync, reducing the randomness that usually limits how precisely we can measure time.

Their results, , show that it’s possible to go beyond what’s known as the��Standard Quantum Limit (SQL)—a fundamental barrier in quantum measurements—by using a technique called��spin squeezing. This work could help improve everything from GPS systems to tests of gravity and the nature of the universe.

What Limits a Clock’s Precision?

Atomic clocks are among the most precise instruments ever built. They work by measuring the frequency of light that causes atoms to jump between energy levels. These transitions are incredibly stable, making them ideal for keeping time. But there’s a catch. Each atom behaves independently, and their random quantum behavior adds noise to the measurement. This randomness is what defines the��Standard Quantum Limit. It’s like trying to hear a single beat in a noisy crowd.

To reduce this noise, scientists often increase the number of atoms. The more atoms you measure, the better your estimate—kind of like averaging more coin flips to get closer to 50/50. But packing too many atoms together causes them to interact in ways that shift the clock frequency, introducing new errors. So instead of adding more atoms, the JILA team tried something different: they made the atoms��entangled.

Entanglement is a quantum connection between particles. When atoms are entangled, their random quantum behavior becomes linked—even if they’re not touching. In this experiment, the researchers used entanglement to make the atoms behave more like a team, reducing the noise in their collective signal.

This approach allows the clock to beat the SQL, achieving better precision without needing more atoms. It’s a clever way to get more information out of the same number of particles.

Entanglement through Nondemolition Measurement

To entangle the atoms, researchers Dr. Yang Yang, Maya Miklos, and their lab mates used a method called��quantum nondemolition (QND) measurement. This means they could measure the atoms without disturbing them too much, like checking the temperature of soup without taking the lid off.

They trapped about 30,000 strontium atoms in a grid of laser light called a��two-dimensional optical lattice. This setup holds the atoms in place and keeps them cold—less than a millionth of a degree above absolute zero. Cold atoms move less, which helps maintain their coherence and reduces unwanted interactions.

The atoms were placed inside an��optical cavity, which bounces light back and forth to enhance its interaction with the atoms. By shining a special probe light into the cavity, the researchers could gently measure the atoms’ collective spin—a property related to their energy state—without collapsing their individual quantum states. The team also used a technique called��spin echo��to cancel out unwanted shifts caused by the probe light. This helped preserve the delicate quantum state of the atoms during the measurement.

This process “squeezes” the uncertainty in one direction, reducing the noise in the measurement. It’s like squeezing a balloon: the uncertainty gets smaller in one direction but bigger in another. For clocks, this trade-off is worth it because it makes the timing signal more precise when one measures along the squeezed direction.

Putting the Squeezed Clock to the Test

To see if their entangled clock really worked better, the researchers compared two groups of atoms in a��“synchronous comparison”��between two atomic ensembles. By comparing two clocks at the same time, they could cancel out common sources of noise—like fluctuations in the laser used to probe the atoms. This allowed them to isolate the improvement due to spin squeezing: they can compare the case where both samples are regular, unentangled atoms (called a coherent spin state, or CSS), to where each sample is prepared in a spin-squeezed state (SSS) to see the improved stability from spin squeezing.

They studied how precisely the clock comparison signal could be measured over time. The spin-squeezed clock showed a��2.0 decibel improvement��beyond the Standard Quantum Limit. That might not sound like much, but in the world of precision measurement, it’s a significant step forward. They found that the spin-squeezed clock not only beat the SQL but also showed a��3.3 dB improvement��over the unentangled clock. This confirms that the entanglement was not just a theoretical benefit—it made a real difference in the clock’s performance.

Over a 43-minute test, the clock reached a��fractional frequency uncertainty of 1.1 × 10⁻¹⁸. That means it could detect a change in time as small as one second over the age of the universe. This is the most precise entanglement-enhanced clock ever demonstrated, proving that such entanglement could in the future help make the world’s best clocks even more precise.

Why Does This Matter?

This research is part of a broader effort at JILA to explore how quantum physics can improve measurement tools. JILA Fellows Adam Kaufman and James Thompson are also exploring the use of entanglement for better measurement precision. Atomic clocks are already used in GPS satellites, telecommunications, and tests of fundamental physics. Making them even more precise opens new possibilities. A key challenge is to demonstrate genuine quantum advantage where an entangled clock can reach a performance level superior to the best clock today.

For example, ultra-precise clocks can measure tiny differences in gravity across short distances. This could help scientists study how gravity affects quantum systems or even searches for new physics beyond Einstein’s theories.

The techniques developed in this study—like spin squeezing and QND measurements—could also be used in other quantum technologies, such as sensors and quantum computers. These tools rely on the same principles of coherence and entanglement to perform tasks that classical systems can’t.

Looking ahead, the team hopes to improve their system by using��three-dimensional optical lattices, which offer even better control over the atoms. They’re also exploring new ways to amplify signals using��time-reversal techniques��and��quantum optimization algorithms.

There is also growing interest in using entangled clocks to probe the interface between��quantum mechanics and gravity. Recent studies together with JILA Fellow Ana Maria Rey and external collaborators at University of Innsbruck have explored how mass-energy equivalence and gravitational gradients affect entangled states, raising fundamental questions about the nature of time and space.

A New Chapter in Quantum Timekeeping

By using entanglement to reduce quantum noise, JILA researchers have taken a meaningful step toward the next generation of atomic clocks. Their work shows that it’s possible to go beyond traditional limits by carefully engineering both the quantum states of atoms and the tools used to measure them.

As clocks become more precise, they also become more sensitive to the world around them. This opens the door to new experiments in gravity, quantum mechanics, and the structure of space-time itself.

In the end, this research isn’t just about keeping better time—it’s about using time to explore the microscopic and macroscopic side of the universe in new ways.

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This research is supported by the US Department of Energy, Office of Science, National Quantum Information, Science Research Centers, Quantum Systems Accelerator; National Science Foundation; V. Bush Fellowship; JILA Physics Frontier Center; and the National Institute of Standards and Technology.��