Quantum Entanglement Boosts Atomic Clock Precision

Optical clocks are now the most precise timekeeping technology available, moving beyond traditional gears and quartz crystals. Instead, these clocks rely on the frequency at which atoms absorb and emit light, with each oscillation representing an incredibly regular “tick.” This consistency allows scientists to define the second with a level of accuracy previously unattainable.

However, even these highly advanced clocks face a fundamental limitation: the inherent statistical noise within atoms at the quantum level. A new experimental study demonstrates a potential way to reduce this limitation by utilizing quantum entanglement between two trapped ions, integrating this strategy into a real optical clock, and directly comparing its performance to a conventional clock.

The Challenge of Quantum Noise

In an optical clock using ions, the reference frequency is determined by probing an extremely stable electronic transition. However, each measurement of the ion’s state yields only a single outcome, and repeating these measurements reveals statistical fluctuations. This phenomenon is known as quantum projection noise.

As the researchers noted, ion clocks “are typically limited by quantum projection noise (QPN).” One direct way to reduce this noise would be to use more ions simultaneously. However, increasing their number without losing control over systematic effects proves technically challenging.

This is where a different idea comes into play: instead of working with independent particles, preparing a state where their properties are strictly correlated at the quantum level. In this scenario, the entire system can behave as a single coherent entity, potentially improving clock stability without multiplying the number of atoms.

A Special State Protected from the Environment

The experiment was conducted with two calcium ions confined in an electromagnetic trap. The goal was to prepare them in a specific entangled state, known as a Bell state, designed to be insensitive—to first order—to fluctuations in the magnetic field.

This protection is achieved by combining two atomic transitions whose magnetic shifts compensate for each other. The result is a decoherence-free subspace, where the evolution relevant to the clock is practically shielded from external variations, allowing the laser to “listen” to the atomic oscillation for a longer period.

The study details that the ions are entangled in “a quantum state with zero first-order sensitivity to the magnetic field, extending the coherence time of the atoms and allowing interrogation times close to the lifetime limit of up to 550 ms.” Simply put, this means the system can remain in superposition for a much longer time before the environment disturbs it.

To verify that the improvement wasn’t solely due to this symmetry against the magnetic field, the researchers also implemented a scheme with classical correlations, but without full entanglement. This direct comparison was essential to evaluate the genuine quantum advantage.

When Entanglement Makes a Difference

The team used the two ions as a frequency reference and compared their performance to that of a strontium optical clock. The experimental conclusion is clear: “the instability of the entangled ion clock is below that of a clock operated with classically correlated states for all interrogation times.”

the authors point out that “we observe instabilities below the theoretically expected limit of quantum projection noise of two uncorrelated ions for interrogation times less than 100 ms.” This means the entangled system can outperform two well-optimized independent ions under certain conditions.

The researchers also note that the obtained result “represents the lowest instability reported to date for a ⁴⁰Ca⁺ ion clock.” While the final performance is still conditioned by residual laser noise, the experiment demonstrates that entanglement is not an abstract resource: it can be integrated into a real metrological device and provide a measurable improvement.

Beyond the Record: New Possibilities

The interest of the study is not limited to breaking a record. Using entangled states modifies the optimal operating time of the clock and allows for faster control cycles. This can facilitate laser stabilization and reduce sensitivity to certain systematic effects.