Shorter Wavelength Boosts Strontium Atomic Clocks and Interferometry Precision

Atomic clocks, the most precise timekeeping devices known, rely on the consistent frequencies of atomic transitions. Optimising these systems demands meticulous control of light interactions with atoms, a process complicated by the phenomenon known as the AC-Stark shift, where light alters atomic energy levels. Xinyuan Ma from the Centre for Quantum Technologies, National University of Singapore, Swarup Das from Nanyang Quantum Hub, Nanyang Technological University, and colleagues report the experimental determination of a ‘magic wavelength’ of 476.82362(8) nanometres for the strontium-88 clock transition, a value at which this disruptive shift is minimised. Published research details a measurement differing slightly from theoretical predictions, and this shorter wavelength, compared to the commonly used 813 nanometres, offers potential benefits for advanced applications such as matter-wave interferometry and the creation of more compact atomic clocks.

Precise control of atomic states underpins advancements in quantum technologies, including quantum computing, simulation, and precision sensing, and neutral atoms, trapped and manipulated with light, offer a versatile platform for these applications. Optical trapping techniques, such as optical lattices and tweezers, confine atoms while enabling coherent control of internal states, yet spatial variations within the trap can introduce unwanted phase shifts that compromise precision. Researchers mitigate these phase shifts by utilising ‘magic wavelengths’, specific wavelengths of trapping light where the light-induced energy shift, known as the AC-Stark shift, remains independent of atomic position, ensuring coherence is maintained for high-precision measurements and computations. Alkaline-earth atoms, like strontium, are particularly attractive due to their narrow clock transitions, which provide exceptional frequency stability and underpin the most accurate timekeeping technology currently available.

Existing optical lattice clocks and quantum simulators commonly operate at a magic wavelength of 813 nanometres for the strontium clock transition, but exploring alternative magic wavelengths expands the possibilities for manipulating atoms and designing novel quantum devices with enhanced capabilities. The search for additional magic wavelengths focuses on optimising specific applications, as shorter wavelengths, for example, enable the creation of optical lattices with smaller inter-trap spacing, increasing the density of trapped atoms and improving signal strength. Furthermore, shorter wavelengths are advantageous for matter-wave interferometry, where atoms are used as waves to measure physical phenomena with high precision.

Researchers pursue increasingly accurate timekeeping with optical lattice clocks, devices utilising the predictable oscillations of atoms trapped within light lattices, and a central challenge in building these clocks is minimising the influence of the trapping light itself on the atomic energy levels, a phenomenon known as the AC-Stark shift. To address this, researchers seek ‘magic wavelengths’ – specific colours of light where this shift is minimised. Recent work details the precise measurement of a new magic wavelength for strontium-88, identified through a technique called AC-Stark shift spectroscopy. This method involves trapping strontium atoms within an optical dipole trap – created by a highly focused laser beam – and meticulously measuring how the frequency of the atomic clock transition shifts as the laser wavelength is varied, providing a precise determination of the wavelength where the shift is minimised. The experimental setup demands exceptional control over the laser light, requiring researchers to carefully tune the laser wavelength across a range encompassing predicted magic wavelengths while simultaneously monitoring the strontium atoms’ response using high-resolution spectroscopy.

The AC-Stark shift manifests as a subtle change in the resonant frequency of the atomic transition, and detecting this shift requires extremely precise measurements, achieved through a sophisticated spectroscopic technique that maps out the AC-Stark shift as a function of wavelength. Researchers determine a measured value of 476.82362(8) nanometres, representing a highly accurate determination of this wavelength for strontium-88, and importantly, this new magic wavelength differs slightly from theoretical predictions, a discrepancy of 0.061(54) nanometres, highlighting the value of experimental validation and suggesting refinements may be needed in the underlying theoretical models. The significance of identifying a shorter magic wavelength lies in its potential to enhance the capabilities of advanced quantum technologies, enabling tighter confinement of the atoms within the optical lattice, leading to improved clock stability and reduced sensitivity to external perturbations. This is particularly beneficial for applications requiring precise control over the atoms’ momentum, such as matter-wave interferometry, where the newly identified magic wavelength facilitates the implementation of Bragg pulses, a specific type of pulse used to manipulate the atoms’ momentum, offering greater control and sensitivity.

Strontium-87 exhibits a newly identified ‘magic wavelength’ at 709 nanometres, significantly expanding the toolkit available for constructing highly precise optical atomic clocks and advancing quantum technologies. Researchers meticulously characterise this wavelength, demonstrating its ability to minimise disruptive light-induced frequency shifts, a critical factor in maintaining clock accuracy. The identification builds upon the established principle of utilising specific wavelengths where atomic transition frequencies remain unaffected by the presence of light, thereby reducing systematic errors. The research employs a combination of theoretical calculation and experimental validation to pinpoint this wavelength. Calculations utilise the Dirac-Hartree-Fock method, a relativistic quantum mechanical approach, to predict wavelengths where the AC-Stark shift – the shift in atomic energy levels caused by an external electromagnetic field – is minimised, and experimental verification involves precise spectroscopy of strontium-87 atoms trapped within an optical dipole trap, confirming the predicted wavelength’s efficacy in suppressing these shifts. This approach ensures the reliability and accuracy of the identified wavelength for practical applications, complementing existing magic wavelengths, such as the commonly used 813 nanometres.

Notably, the shorter wavelength facilitates the implementation of Bragg pulses, enabling more precise control and manipulation of atomic wavefunctions, which is particularly relevant for applications demanding high resolution and sensitivity. Future work should focus on exploring the performance characteristics of this new wavelength in practical applications, investigating its suitability for creating tightly confined optical lattices, and assessing its impact on coherence times in quantum systems. Further theoretical investigations could aim to resolve the observed deviation from predictions, potentially revealing subtle effects not currently accounted for in existing models. Expanding the range of available magic wavelengths allows for greater flexibility in experimental design and optimisation, enabling researchers to tailor their approaches to specific requirements, potentially leading to improvements in the performance of atomic clocks, quantum sensors, and other advanced technologies. This research therefore contributes to a growing toolkit for manipulating matter at the quantum level, and continued investigation into the properties of these wavelengths, alongside the development of more accurate theoretical models, promises to unlock further advancements in the field of atomic physics and quantum technologies. The ability to precisely control and manipulate atoms is fundamental to many emerging technologies, and this work represents a significant step forward in that direction.