Ytterbium Atoms Reveal Unique Quantum Interactions with Magnetic Fields

Scientists Masaya Kunimi and Takafumi Tomita at Tokyo University of Science, in collaboration with Institute for Molecular Science, The Graduate University and National Institute, have developed a new set of tools for controlling interactions between alkaline-earth Rydberg atoms. The study utilises strontium-88 and ytterbium-174, revealing interactions that align with an XXZ-type quantum spin model. Ytterbium-174 exhibits markedly different anisotropic behaviour due to strong spin-orbit coupling. Systematic calculation shows that external magnetic fields tune these interaction parameters, potentially enabling the realisation of folded XXZ models and the emergence of a supersolid phase in two-dimensional systems. This advances the exploration of quantum many-body problems and offers key avenues for quantum simulation.

Ytterbium-174 Rydberg atoms exhibit unexpectedly high magnetic anisotropy for enhanced quantum control

The anisotropy parameter for ytterbium-174 Rydberg atoms reaches a value of |δ| ≳10, a substantial increase compared to other atomic species. Achieving such a value previously necessitated precise magnetic field tuning, often requiring complex and highly stable experimental setups. This threshold crossing unlocks new possibilities for quantum simulation, previously hampered by the need for extremely stable and finely adjusted magnetic fields; prior work required careful control to observe similar effects. Calculations reveal that strontium-88 and ytterbium-174 interactions conform to an XXZ-type quantum spin model, a standard framework for understanding magnetic interactions in condensed matter physics. However, a distinct behaviour is observed in ytterbium-174 stemming from its strong spin-orbit coupling, a relativistic effect where the electron’s spin and orbital angular momentum interact. This interaction significantly alters the energy levels and magnetic properties of the atom.

Detailed calculations reveal that the interaction parameters within this model, specifically the exchange interaction and the single-site anisotropy, are tunable via external magnetic fields, offering a degree of control previously unseen in similar systems. The XXZ model describes interactions between spins, with the ‘X’, ‘Y’, and ‘Z’ components representing different types of magnetic coupling. The anisotropy parameter, δ, quantifies the strength of the interaction along the Z-axis relative to the X and Y axes. Ytterbium-174’s strong spin-orbit coupling, where the electron’s spin and orbital motion are linked, results in unique behaviour. Ytterbium-174 actively realises a ‘folded’ XXZ model in one-dimensional chains of ytterbium-174 atoms without requiring precise magnetic field adjustments; this simplification arises from the inherent strong spin-orbit coupling within ytterbium-174, reducing the need for complex field calibrations. This ‘folding’ effectively modifies the interaction landscape, leading to altered quantum properties.

The Rydberg states considered in this study are the and states, where ‘n’ represents the principal quantum number, ‘s’ the orbital angular momentum, ‘S’ the total spin, ‘m_J’ the magnetic quantum number, and the superscript indicates the spin multiplicity. These highly excited states characterise a large electron-electron separation, enhancing their sensitivity to external fields and interatomic interactions. The use of these specific states allows for a clear mapping onto the XXZ model, simplifying the theoretical analysis and facilitating experimental control. The magnetic field dependence of the interaction parameters calculated using a perturbative approach, accounting for the effects of the external field on the atomic energy levels and dipole-dipole interactions between the atoms.

Accurate strontium-ytterbium interactions underpin improved supersolid phase modelling

A strong and tunable platform for quantum simulation remains a central challenge in modern physics. Alkali atoms have long served as a workhorse for these investigations, owing to their relatively simple electronic structure and ease of manipulation. However, achieving complex quantum states like supersolids, exhibiting properties of both solids and superfluids, often demands precise control over atomic interactions and the ability to engineer strong many-body effects. Current modelling of these supersolid phases frequently relies on mean-field approximations, a simplification that inadequately accounts for the complex correlations arising from interactions between many atoms.

Precise mapping of interactions between strontium and ytterbium atoms allows the design and simulation of many-body systems, and the ability to tune these interactions with magnetic fields offers a powerful new avenue for quantum simulation and material design. This could potentially unlock novel states of matter with exotic properties. The combination of strontium-88 and ytterbium-174 offers a unique opportunity to explore these phenomena, as their differing electronic structures and magnetic properties lead to richer interaction landscapes. Further investigation will focus on refining the accuracy of supersolid phase calculations by moving beyond mean-field approximations and incorporating more sophisticated many-body techniques, such as density matrix renormalisation group or quantum Monte Carlo methods.

Utilising strontium-88 and ytterbium-174 Rydberg atoms, the XXZ quantum spin model has successfully modelled their interactions. This extends the applicability of the model beyond alkali atoms, demonstrating its flexible nature in describing a wider range of atomic species. Rydberg atoms are created when electrons are boosted to very high energy levels, increasing their sensitivity to interactions and providing a strong platform for quantum simulation. The findings establish a pathway towards designing novel materials exhibiting exotic states like supersolidity, where matter behaves as both a solid and a superfluid, and the simplified tuning of atomic interactions in ytterbium-174 facilitates this process. The team intends to explore the limits of this model and investigate its potential for simulating more complex quantum systems, including those relevant to condensed matter physics and materials science. Future work will also focus on extending these studies to two-dimensional systems, where the emergence of supersolid phases is predicted to be even more pronounced.

The research successfully modelled interactions between strontium-88 and ytterbium-174 Rydberg atoms using an XXZ quantum spin model. This demonstrates the model’s versatility beyond alkali atoms and provides a means to precisely map and tune atomic interactions with magnetic fields. These findings are important as they offer a platform for simulating complex quantum systems and exploring novel states of matter, such as supersolidity. The authors plan to refine calculations of supersolid phases using more advanced computational techniques.