Quantum States Enhance Calcium Ion Clock Precision and Stability.

Spectroscopy utilising specific quantum states enhances the precision of optical clocks, achieving a fractional frequency instability of 7e-16/√τ. Experiments with calcium-40 ions demonstrate instability below theoretical limits for interrogation times under 100 milliseconds, and outperform clocks utilising classically correlated states, reaching a record low instability.

The pursuit of increasingly precise timekeeping underpins advances in fundamental physics, navigation, and secure communication. Atomic clocks, utilising the consistent frequencies of atomic transitions, currently represent the most accurate time standards, and ongoing research focuses on enhancing their stability and reducing associated uncertainties. A collaborative team, comprising researchers from the Physikalisch-Technische Bundesanstalt and Leibniz Universität Hannover, details a novel approach to optical atomic clock operation utilising quantum entanglement. Their work, published under the title ‘Entanglement-enhanced optical ion clock’, demonstrates improved performance compared to conventional atomic clocks by leveraging the unique properties of entangled states to extend coherence times and accelerate measurement cycles. The research, led by Kai Dietze, Lennart Pelzer, Ludwig Krinner, Fabian Dawel, Johannes Kramer, Nicolas C. H. Spethmann, Timm Kielinski, Klemens Hammerer, Kilian Stahl, Joshua Klose, Sören Dörscher, Christian Lisdat, Erik Benkler, and Piet O. Schmidt, reports a fractional frequency instability of 7 x 10^-16 / √τ, representing a new benchmark for calcium-40 ion clocks.

Optical clocks currently define the forefront of precision timekeeping, with ongoing research consistently refining these technologies to achieve increased accuracy and stability. A recent study presents a detailed comparative analysis of two leading optical atomic clock technologies: a trapped-ion clock utilising Calcium-40 (⁴⁰Ca⁺) and a Strontium-87 (⁸⁷Sr) lattice clock, meticulously assessing their performance characteristics with a specific focus on stability and identification of limiting noise sources. Results demonstrate the ⁴⁰Ca⁺ clock, employing states with reduced magnetic field sensitivity, achieves enhanced coherence times and faster cycle times compared to implementations utilising classically correlated states, establishing a new standard for precision timekeeping.

Researchers rigorously analysed the performance of both clock types, employing advanced techniques to characterise their stability and identify the dominant noise sources limiting accuracy. They prepared two ⁴⁰Ca⁺ ions in a state exhibiting minimal sensitivity to first-order magnetic fields, extending coherence times and achieving probe times approaching the atomic lifetime, reaching up to 550 milliseconds. This preparation allows the ion clock to reach instability levels comparable to uncorrelated ions, but with a reduced probe time, facilitating faster cycle times. Coherence time refers to how long a quantum system maintains its quantum properties, crucial for accurate measurement.

Notably, the research observes instabilities falling below the theoretically predicted projection noise limit for interrogation times under 100 milliseconds, suggesting the implemented techniques effectively mitigate certain quantum noise contributions. Projection noise, a fundamental limit in quantum measurement, arises from the discrete nature of quantum events. A minimum fractional frequency instability of 7 x 10⁻¹⁶ / √τ is achieved at a 250 millisecond probe time, currently representing the lowest instability reported for a ⁴⁰Ca⁺ ion clock and establishing a new benchmark for performance. The symbol τ represents the averaging time, a key parameter in quantifying stability.

Analysis reveals the dominant limitation to clock performance at this level is residual phase noise originating from the probe laser, identifying a key area for future improvement. The study employs the Allan Deviation (OADEV) as a key metric for quantifying frequency stability, utilising the allantools Python package to ensure accurate and reliable results. Furthermore, researchers characterise noise sources through lag-1 autocorrelation functions of the frequency ratio data, distinguishing between white frequency noise, flicker noise, and random walk noise, providing a detailed noise profile. White frequency noise is random and constant across frequencies, flicker noise decreases with increasing frequency, and random walk noise accumulates over time. This comprehensive analysis allows for a thorough understanding of the clock’s performance characteristics and identifies areas for optimisation.

The ⁴⁰Ca⁺ clock exhibits frequency instability below that of a comparable clock operating with classically correlated states across all measured probe times, confirming its superior performance.

The study confirms the potential of entangled states to improve clock performance and pushes the boundaries of precision timekeeping, opening new avenues for research and development. The ability to surpass the projection noise limit, even for short interrogation times, highlights the advantages of utilising quantum correlations in optical clock design, demonstrating a significant advancement in the field. These findings have implications for a range of applications, including fundamental physics research, improved navigation systems, and more accurate synchronisation of global networks, showcasing the broad impact of this research.

Researchers plan to concentrate on reducing the phase noise of the probe laser to further enhance the stability of the ⁴⁰Ca⁺ clock, aiming to achieve even greater precision. Exploring alternative laser stabilisation techniques and investigating novel optical designs could prove beneficial, paving the way for further advancements in optical clock technology. Extending the comparison to include other optical clock technologies, such as ytterbium lattice clocks, will provide a broader understanding of the strengths and weaknesses of each approach, facilitating informed decision-making in the development of future timekeeping systems.