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arXiv:2503.15034 · Physical Review A

Benchmarking direct and indirect dipolar spin-exchange interactions between two Rydberg atoms

Gabriel Emperauger, Mu Qiao, Guillaume Bornet, Cheng Chen, Romain Martin, Yuki Torii Chew, Bastien Gély, Lukas Klein, Daniel Barredo, Antoine Browaeys, Thierry Lahaye

Published Jun 5, 2025• 10 citations
arXivHTMLPDF

Abstract

We report on the experimental characterization of various types of spin-exchange interactions between two individual atoms, where pseudospin degrees of freedom are encoded in different Rydberg states. For the case of the direct dipole-dipole interaction between states of opposite parity, such as between $|nS\ensuremath{\rangle}$ and $|nP\ensuremath{\rangle}$, we investigate the effects of positional disorder arising from the residual atomic motion on the coherence of spin-exchange oscillations. We then characterize an indirect dipolar spin exchange, i.e., the off-diagonal part of the van der Waals effective Hamiltonian that couples the states $|nS\ensuremath{\rangle}$ and $|(n+1)S\ensuremath{\rangle}$. Finally, we report on the observation of a new type of dipolar coupling, made resonant using addressable light shifts and involving four different Rydberg levels: this exchange process is akin to electrically induced F\"orster resonance, but featuring local control. It exhibits an angular dependence distinct from the usual $1\ensuremath{-}3{cos}^{2}(\ensuremath{\theta})$ form of the resonant dipolar spin-exchange.

DOI
10.1103/physreva.111.062806

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We report on the experimental characterization of various types of spin-exchange interactions between two individual atoms, where pseudospin degrees of freedom are encoded in different Rydberg states. For the case of the direct dipole-dipole interaction between states of opposite parity, such as between ∣nS\ensuremath⟩|nS\ensuremath{\rangle}∣nS\ensuremath⟩ and ∣nP\ensuremath⟩|nP\ensuremath{\rangle}∣nP\ensuremath⟩, we investigate the effects of positional disorder arising from the residual atomic motion on the coherence of spin-exchange oscillations. We then characterize an indirect dipolar spin exchange, i.e., the off-diagonal part of the van der Waals effective Hamiltonian that couples the states ∣nS\ensuremath⟩|nS\ensuremath{\rangle}∣nS\ensuremath⟩ and ∣(n+1)S\ensuremath⟩|(n+1)S\ensuremath{\rangle}∣(n+1)S\ensuremath⟩. Finally, we report on the observation of a new type of dipolar coupling, made resonant using addressable light shifts and involving four different Rydberg levels: this exchange process is akin to electrically induced F"orster resonance, but featuring local control. It exhibits an angular dependence distinct from the usual 1\ensuremath−3cos2(\ensuremathθ)1\ensuremath{-}3{cos}^{2}(\ensuremath{\theta})1\ensuremath−3cos2(\ensuremathθ) form of the resonant dipolar spin-exchange.
We demonstrated a tuned dipole interaction between Rydberg atoms by employing a controllable electric field in a cold cesium ensemble. The | n P 3/2 ⟩ ( n = 38–40) Rydberg pairs are prepared with a three-photon scheme and detected via the state-selective field ionization technique. A weak DC electric field is used to tune the Rydberg pair interaction from the van der Waals interaction regime to the dipole–dipole interaction regime. The Förster resonant interaction and an adiabatic resonance energy transfer between the n P and n S Rydberg states are attained by precisely tuning the electric field. Rydberg excitation blockade with and without the electric field is investigated by changing the excitation pulse duration, which demonstrates that the dipole interaction–induced blockade effect is stronger than the van der Waals interaction–induced blockade effect. The precise control of the Rydberg interaction is of great significance to the coherent interaction in many-body systems and non-radiative collision processes.
We report on the local control of the transition frequency of a spin 1/2 encoded in two Rydberg levels of an individual atom by applying a state-selective light shift using an addressing beam. With this tool, we first study the spectrum of an elementary system of two spins, tuning it from a nonresonant to a resonant regime, where "bright" (super-radiant) and "dark" (subradiant) states emerge. We observe the collective enhancement of the microwave coupling to the bright state. We then show that after preparing an initial single spin excitation and letting it hop due to the spin-exchange interaction, we can freeze the dynamics at will with the addressing laser, while preserving the coherence of the system. In the context of quantum simulation, this scheme opens exciting prospects for engineering inhomogeneous XY spin Hamiltonians or preparing spin-imbalanced initial states.
Resonant laser excitation of multiple Rydberg atoms are prohibited, leading to Rydberg blockade, when the long-range van der Waals interactions are stronger than the laser-atom coupling. Rydberg blockade can be violated, i.e., simultaneous excitation of more than one Rydberg atom, by off-resonant laser excitation, causing an excitation antiblockade. Rydberg antiblockade gives rise to strongly correlated many-body dynamics and spin-orbit coupling and also finds quantum computation applications. Instead of commonly used van der Waals interactions, we investigate antiblockade dynamics of two Rydberg atoms interacting via dipole-dipole exchange interactions. We study typical situations in current Rydberg atom experiments, where different types of dipole-dipole interactions can be achieved by varying Rydberg state couplings. An effective Hamiltonian governing underlying antiblockade dynamics is derived. We illustrate that geometric gates can be realized with the Rydberg antiblockade which is robust against the decay of Rydberg states. Our study may stimulate new experimental and theoretical exploration of quantum optics and strongly interacting many-body dynamics with Rydberg antiblockade driven by dipole-dipole interactions.
Ultracold Rydberg atoms in a static electric field can exchange energy via the dipole-dipole interaction. The Stark effect shifts the energy levels of the atoms which tunes the energy exchange into resonance at specific values of the electric field (F\"orster resonances). We excite rubidium atoms to Rydberg states by focusing either a 480 nm beam from a tunable dye laser or a pair of diode lasers into a magneto-optical trap. The trap lies at the center of a configuration of electrodes. We scan the electric field by controlling the voltage on the electrodes while measuring the fraction of atoms that interact. Dipole-dipole interaction spectra are presented for initially excited rubidium ndndnd states for n=31n=31n=31 to 46 and for four different pairs of initially excited rubidium nsnsns states. We also present the dipole-dipole interaction spectra for individual rubidium 32ddd (j,mj)(j,\phantom{\rule{0.16em}{0ex}}{m}_{j})(j,mj​) fine structure levels that have been selectively excited. The data are compared to calculated spectra.
Rydberg atoms are used in a wide range of applications due to their peculiar properties like strong dipolar and van der Waals interactions. The choice of Rydberg state has a huge impact on the strength and angular dependence of the interactions, and so a detailed understanding of the underlying processes and resulting properties of the interactions is therefore key to select the most suitable states for experiments. We study the van der Waals interactions in alkali-metal atoms in detail and highlight the structures which allow an understanding and exploitation of the various interaction properties. A particular theme is the identification of Förster resonances with n 1 ≠ n 2 , which offer interaction potentials with a wide range of properties that make them particularly interesting for experimental applications. A second theme is a focus on the underlying structures that shape the angular dependency and sign of the interactions. This understanding — instead of brute-force calculations — allows for a much simpler and more systematic search for suitable pair states. These insights can be used for the selection of tailored interaction potentials subject to experimental constraints and requirements. We use rubidium as an example species in this work and also provide data for cesium and pair states that are coupled via two- or three-photon transitions, i.e. up to F states, in the Appendix.