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Atomic ensembles in a microcavity : from cavity protection to single atom control

Pierre-Antoine Bourdel

Published Dec 5, 2022• 0 citations
Publisher

Abstract

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.

DOI
10.70675/edd43b3bz5161z428fza4d8z2bc3bc29aa51

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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.
Strong atom-photon interactions in optical cavities are a key resource for quantum information processing, quantum networking, and the exploration of quantum optical effects. Optical tweezer arrays offer scalable, site-resolved control of neutral atoms, but their integration with high-cooperativity cavity QED systems remains challenging. Here we combine a twelve-site 87^{87}87Rb optical tweezer array with a high-cooperativity fiber Fabry-Pérot microcavity. The array is positioned within the cavity mode and individual sites are controlled with subwavelength precision, enabling continuous tuning of the single-atom coupling strength via deterministic displacement through the standing-wave field. For up to five atoms coupled to the cavity, we measure collectively enhanced vacuum Rabi splitting and implement cavity-based non-destructive readout of the number of coupled atoms. These results establish a scalable architecture for cavity-mediated entanglement generation and many-body cavity QED with single-atom control, and they lay the foundation for fiber-integrated quantum network nodes.
A major focus in modern experimental quantum physics is to create many-particle entangled states with single-particle control. In this Thesis we present a new platform that aims at achieving this purpose by combining two main ingredients: the collective coupling of cold rubidium atoms to an optical cavity and the individual control of the particles via a highresolution microscope. At the heart of our experimental system is a miniaturized fiber Fabry-Perot cavity that allows accessing the strong coupling regime of Cavity Quantum Electrodynamics (CQED). In addition to the CQED probe, a second cavity mode with twice the wavelength creates a lattice potential for identically and maximally coupling the atoms to the cavity. The spatial overlap between the two intra-cavity standing waves plays an essential role and we present methods to characterize and optimize it. We also report a technique for integrating the fiber cavity with a high-numerical aperture objective in a compact and stable setup. For transporting cold atoms from the capture region to the fiber resonator, we developed a system based on a movable optical dipole trap, driven by an acousto-optical deflector, which realizes a fast and efficient loading of the intracavity lattice. The combination of these elements allowed the observation of strong coupling between the cavity mode and the atoms in the 1D lattice via vacuum-Rabi-splitting measurements. This constitutes an important step towards generating many-body entanglement in our system, where the interplay between collective operations (performed via the cavity) and local control on the atomic qubits (provided by the microscope) is a promising approach for new entanglement schemes.
Scalable quantum computers and quantum networks require the combination of quantum processing nodes with efficient light-matter interfaces to distribute quantum information in local or long-distance quantum networks. Neutral-atom arrays have both been coupled to Rydberg states to enable high-fidelity quantum gates in universal processing architectures, and to optical cavities to realize interfaces to photons. However, combining these two capabilities and coupling atom arrays to highly excited Rydberg states in the mode of an optical cavity has been an outstanding challenge. Here we present a novel cavity-coupled Rydberg array that achieves this long-standing goal. We prepare, detect, and control individual atoms in a scalable optical tweezer array, couple them strongly to the optical mode of a high-finesse optical cavity and excite them in a controlled way to Rydberg states. We show that high-cooperativity coupling to an optical cavity -- demonstrated via the dispersive shift of the resonance of the cavity in presence of the atoms -- and strong Rydberg interactions -- demonstrated via the collective enhancement of Rydberg coupling in the atomic array -- can be achieved in our setup at the same spatial location. Our presented experimental platform opens the path to several new directions, including the realization of quantum network nodes, quantum simulation of long-range interacting, open quantum systems and photonic-state engineering leveraging high-fidelity Rydberg control.
The cavity quantum electrodynamics (QED) system is a promising platform for quantum optics and quantum information experiments.Its core is the strong coupling between atoms and optical cavity, which causes difficulty in the overlap between the atoms and the antinode of optical cavity mode.Here, we use a programmable movable optical dipole trap to load a cold atomic ensemble into an optical fiber microcavity and realize the strong coupling between the atoms and the optical cavity in which the coupling strength can be improved by polarization gradient cooling and adiabatic loading.By the measurement of vacuum Rabi splitting, the coupling strength can be as high as g N = 2π × 400 MHz, which means the effective atom number is N eff = 16 and the collective cooperativity is C N = 1466.These results show that this experimental system can be used for cold atomic ensemble and cold molecule based cavity QED research.
In this video, I present the latest results obtained with our cavity QED setup where an ensemble of cold atoms is trapped in a fiber-based Fabry-Perot cavity, deep in the strong coupling regime. We have observed cavity protected polaritons in the form of two transmissions peaks of the atoms-cavity system, their linewidth being much narrower than the inhomogeneously-broadened atomic frequency distribution. This effect, which was previously observed in solid-state systems, can be reproduced here with tunable experimental parameters and holds great prospects for preserving coherence in large inhomogeneous quantum systems with possible applications to the field of quantum memories.