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Quantum Science and Technology

Collective vacuum Rabi splitting with an atomic spin wave coupled to a cavity mode

Félix Hoffet, Alexey Vylegzhanin, Emanuele Distante, Lukas Heller, Síle Nic Chormaic, Hugues de Riedmatten

Published Jan 8, 2026• 1 citation
Publisher

Abstract

Abstract A promising platform for quantum information research relies on cavity coupled atomic spin-waves, enabling efficient operations such as quantum memories, quantum light generation and entanglement distribution. In this work, we study the strong coupling between non-classical collective spin excitations generated by Raman scattering in a cold 87 R b atomic ensemble, and a single cavity mode. We report on an intracavity spin wave to single photon conversion efficiency of up to χ = 0.75 ± 0.02 in the quantum domain, as evidenced by a violation of the Cauchy–Schwarz inequality. Our work establishes a relationship between the retrieval of an atomic spin wave in the non-classical regime and the vacuum Rabi splitting. We show that this relationship emerges within the efficiency spectrum, and we finally provide the optimal operational conditions to achieve the maximum intrinsic retrieval efficiency. Our data is well reproduced by simulations based on optical Bloch equations. This work deepens the understanding of cavity-enhanced spin wave readout and its potential applications.

DOI
10.1088/2058-9565/ae3551

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•Aug 15, 2023•0 citations
Abstract A promising platform for quantum information research relies on cavity coupled atomic spin-waves, enabling efficient operations such as quantum memories, quantum light generation and entanglement distribution. In this work, we study the strong coupling between non-classical collective spin excitations generated by Raman scattering in a cold 87 R b atomic ensemble, and a single cavity mode. We report on an intracavity spin wave to single photon conversion efficiency of up to χ = 0.75 ± 0.02 in the quantum domain, as evidenced by a violation of the Cauchy–Schwarz inequality. Our work establishes a relationship between the retrieval of an atomic spin wave in the non-classical regime and the vacuum Rabi splitting. We show that this relationship emerges within the efficiency spectrum, and we finally provide the optimal operational conditions to achieve the maximum intrinsic retrieval efficiency. Our data is well reproduced by simulations based on optical Bloch equations. This work deepens the understanding of cavity-enhanced spin wave readout and its potential applications.
A promising platform for quantum information research relies on cavity coupled atomic spin-waves, enabling efficient operations such as quantum memories, quantum light generation and entanglement distribution. In this work, we study the strong coupling between non-classical collective spin excitations generated by Raman scattering in a cold 87Rb^{87}\mathrm{Rb}87Rb atomic ensemble, and a single cavity mode. We report on an intracavity spin wave to single photon conversion efficiency of up to χ=0.75±0.02χ=0.75 \pm 0.02χ=0.75±0.02 in the quantum domain, as evidenced by a violation of the Cauchy-Schwarz inequality. Our work establishes a relationship between the retrieval of an atomic spin wave in the non-classical regime and the vacuum-Rabi splitting. We show that this relationship emerges within the efficiency spectrum, and we finally provide the optimal operational conditions to achieve the maximum intrinsic retrieval efficiency. Our data is well reproduced by simulations based on optical Bloch equations. This work deepens the understanding of cavity-enhanced spin wave readout and its potential applications.
Abstract By considering linear scattering of laser-driven cold atoms inside an undriven high-finesse optical resonator, we experimentally demonstrate effects unique to a strongly coupled vacuum field. Arranging the atoms in an incommensurate lattice with respect to the radiation wavelength, the Bragg scattering into the cavity can be suppressed by destructive interference: the atomic array is subradiant to the cavity mode under transverse illumination. We show however, that strong collective coupling leads to a drastic modification of the excitation spectrum, as evidenced by well-resolved vacuum Rabi splitting in the intensity of the fluctuations. Furthermore, we demonstrate a significant polarization rotation in the linear scattering off the subradiant array via Raman scattering induced by the strongly coupled vacuum field.
Generating and manipulating multiparticle quantum entangled states is an exciting challenge of modern physics. Along this line, we have built a platform where cold rubidium atoms are strongly coupled to a fiber-based microcavity, under a quantum gas microscope. The atoms are trapped in a lattice at 1560nm, which allows strong and homogeneous coupling to the commensurate probe lattice at 780nm. First, we observe a coherent interaction between the cavity and the atoms, in the strong coupling regime, in spite of a large frequency inhomogeneity induced by the trap. It is the first report of such cavity protection effect with less than 200 emitters. We measure the growing coupling of the dark states to the cavity as the disorder takes over the collective coupling. In the protected regime we engineer frequency-modulated polaritons through a modulation of the atomic frequency. Then, we setup optical tweezers in the microcavity to obtain and manipulate several single atoms. The cavity enables fast and non demolition detection of a single atom in a tweezer. We measure the vacuum Rabi splitting of this atom, demonstrating its strong coupling with an effective cooperativity of 30. Finally, we obtain single atoms in a chain of 9 tweezers. This work opens to way to manipulating many individual atoms strongly coupled to the cavity, with applications in multiparameter quantum metrology, using spatially distributed and locally measurable entanglement as a resource, and quantum simulation of a chain of addressable spins with cavity-mediated infinite range interactions.
Summary form only given. Controlling and characterizing entanglement in large quantum systems is an exciting challenge of modern physics, and a necessary milestone to fulfill the promises of the second quantum revolution. Along this line, we have built a platform where ultracold rubidium atoms are strongly coupled to a fiber-based Fabry-Perot cavity under a quantum gas microscope. Here, the cavity is used to create collective interactions between the atoms, while the microscope is meant to allow for single-qubit manipulation and readout. To maximize the overlap between the atomic distribution and the 780nm cavity mode to which they are coupled, we use an intracavity far off-resonance lattice trap at 1560nm in a configuration where the antinodes of the 1560nm standing wave coincide with antinodes of the 780nm standing wave, which is possible because the two wavelength are commensurate. With this device, we have observed vacuum Rabi frequencies on the order of 60MHz, corresponding to a single-atom cooperativity of about 40, deep into the strong coupling regime. In the short term, we aim at obtaining a 1D chain of single atoms strongly and homogeneously coupled to the cavity, with single atom addressing and detection capabilities. We will discuss at the conference future prospects that we plan to address with this platform, such as using cavity-assisted Raman transitions to implement, within a fully controlled experimental system, spin models with photon-mediated interactions [4], or the Dicke Hamiltonian [5] where many-body entanglement is known to occur in the vicinity of the associated quantum phase transition.
An ensemble of N noninteracting spins being in thermal equilibrium and coupled to the resonant mode of a lossless microwave cavity is studied as the function of the spin temperature τ. Near τ = 0 the system is known to be in a coupled spin-photon state that manifests itself by the splitting of the cavity mode (vacuum Rabi splitting). The cavity emission spectrum is simulated for arbitrary τ. A critical temperature τ C = ω S √N/2, where ω S is the spin excitation energy, is related to the destruction of the strong coupling regime as a consequence of thermal excitations arising within the spin ensemble.
The quantum interface (QI) that generates non-classically-correlated pairs of spin-wave memories and photons is a building block (node) for DLCZ-like quantum repeater. The cross-correlation function, which characterizes quantum correlation of the pairs, and retrieval efficiency together with lifetime are important figures of merits of QIs. In this presented work, we generate highly retrievable spin-wave-photon pairs via cavity-enhanced DLCZ-like interaction between light and a cloud of cold atoms, where, the intrinsic retrieval efficiency reaches to ~77.1% together with a memory lifetime of 0.65 ms. The cross-correlation functions of the pairs as a function of the storage time and excitation probability are measured, respectively. More important, we develop a theoretical model to describe the measured results in quantity. The theoretical model includes the cavity-enhanced noise coming from the imperfect readouts and is in agreement with the measured dependence of the cross-correlation function on the storage time and excitation probability.