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A 'Few-Atom' Quantum Optical Antenna

Andrey Grankin, Denis V. Vasilyev, P.-O. Guimond, Benoît Vermersch, P. Zoller

Published Feb 15, 2018• 0 citations
OpenAlex

Abstract

We describe the design of an artificial `free space' 1D-atom for quantum optics, where we implement an effective two-level atom in a 3D optical environment with a chiral light-atom interface, i.e. absorption and spontaneous emission of light is essentially unidirectional. This is achieved by coupling the atom of interest in a laser-assisted process to a `few-atom' array of emitters with subwavelength spacing, which acts as a phased-array optical antenna. We develop a general quantum optical model based on Wigner-Weisskopf theory, and quantify the directionality of spontaneous emission in terms of a Purcell $\beta$-factor for a given Gaussian (paraxial) mode of the radiation field, predicting values rapidly approaching unity for `few-atom' antennas in bi- and multilayer configurations. Our setup has for neutral atoms a natural implementation with laser-assisted Rydberg interactions, and we present a study of directionality of emission from a string of trapped ions with superwavelength spacing.

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We describe the design of an artificial free space' 1D-atom for quantum optics, where we implement an effective two-level atom in a 3D optical environment with a chiral light-atom interface, i.e. absorption and spontaneous emission of light is essentially unidirectional. This is achieved by coupling the atom of interest in a laser-assisted process to a few-atom' array of emitters with subwavelength spacing, which acts as a phased-array optical antenna. We develop a general quantum optical model based on Wigner-Weisskopf theory, and quantify the directionality of spontaneous emission in terms of a Purcell β\betaβ-factor for a given Gaussian (paraxial) mode of the radiation field, predicting values rapidly approaching unity for `few-atom' antennas in bi- and multilayer configurations. Our setup has for neutral atoms a natural implementation with laser-assisted Rydberg interactions, and we present a study of directionality of emission from a string of trapped ions with superwavelength spacing.
We present the design of a chiral photonic quantum link, where distant atoms interact by exchanging photons propagating in a single direction in free space. This is achieved by coupling each atom in a laser-assisted process to an atomic array acting as a quantum phased-array antenna. This provides a basic building block for quantum networks in free space, i.e., without requiring cavities or nanostructures, which we illustrate with high-fidelity quantum state transfer protocols. Our setup can be implemented with neutral atoms using Rydberg-dressed interactions.
Coherent and efficient light-matter interfaces between an atom and a single mode of the electromagnetic field are essential for quantum technologies. Traditionally, these systems employ optical cavities or waveguides to isolate a specific mode of light, but a more recent approach is to engineer antennas from ordered configurations of trapped ultracold atoms that exhibit controllable and directed scattering. In this work, we propose a method to engineer an antenna from the center-of-mass wavefunction of a single atom, which can produce directed emission and thereby exhibit strong coupling to a target mode of light in free space. We predict that the resulting single-atom cooperativity can be comparable to the current state of the art in optical cavity and waveguide QED experiments. Building on this approach, we show that a wavepacket antenna becomes a single-atom mirror in the linear response regime. We study the modified dipole-dipole interactions and band structure of chains of such emitters, which can exhibit sub- and superradiance at spacings much larger than the wavelength of the light. Extending our approach to multi-level atoms, we propose a method to achieve arbitrary spatial scattering from a single atom, including uni-directional spontaneous emission. Finally, we elucidate how our approach can enhance cooperative scattering in conventional arrays of tightly trapped atoms.
Recent experimental advances in preparing regular defect-free 2D arrays of atoms with optical traps have offered new opportunities to engineer strong collective couplings between light and atoms as quantum optical dipoles. Applications of such collective light-matter interactions include the realization of atom-thick mirrors, retrieval of quantum memories with high efficiency, or the implementation of topological quantum optics. I will present novel applications in the context of quantum networking. First, I will discuss the design of a chiral photonic quantum link, where distant single atoms interact by exchanging photons propagating in a unique direction in free space. This is achieved by coupling both single atoms, representing qubits implemented using a pair of Rydberg states, to bi-layer atomic arrays which act as quantum phased-array antennas. Exploiting the combination of a “Rydberg-dressing” laser and a control laser driving the atomic arrays, an effective Rydberg dipole-dipole interaction can be engineered between atoms and atomic arrays as quantum emitters, allowing to match the spatial modes of spontaneously emitted and absorbed photons to a Gaussian mode of interest. In this way, we realize a chiral quantum interface, i.e., where atoms couple to photonic modes with a unique direction of propagation. This setup provides a basic building block of a novel platform for quantum networks in free space, i.e., without requiring coupling atoms to modes of cavities or nanostructures, which I will illustrate with the deterministic coherent transfer of quantum states between single atoms with high fidelity. Second, I will discuss the collective radiation properties of two distant single-layer atomic arrays acting as quantum memories. This system can support a long-lived non-local superposition state of atomic excitations exhibiting strong subradiance. This “dark” mode consists of an atomic excitation which is delocalized between the two distant atomic arrays, in such a way that the overall radiation is strongly suppressed by quantum interference. I will describe the preparation of these states and their application as resource of non-local entanglement. In particular, by weakly driving the arrays, the state of the quantum memories, localized in each array, can be transferred to this “non-local” subradiant state, allowing again for a deterministic coherent quantum state transfer with high fidelity between the memories. Finally, I will discuss experimental realizations using cold atoms in optical trap arrays, and the effect of experimental imperfections in the arrays, such as finite defect probability, temperature, and depth of the optical traps.
We describe an approach to achieve near-perfect unidirectional light–matter coupling to an effective quantum emitter that is formed by a subwavelength array of atoms in the Rydberg-blockade regime. The nonlinear reflection and transmission of such two-dimensional superatoms are exploited in different interferometric setups for the deterministic generation of tunable single photons and entangling two-photon operations with high fidelities, F ≳0.999. The described setup can function as a versatile nonlinear optical element in a free-space photonic quantum network with simple linear elements and without the need of additional mode confinement, optical resonators, or optical isolators.
We describe an approach to achieve near-perfect unidirectional light-matter coupling to an effective quantum emitter that is formed by a subwavelength array of atoms in the Rydberg-blockade regime. The nonlinear reflection and transmission of such two-dimensional superatoms are exploited in different interferometric setups for the deterministic generation of tunable single photons and entangling two-photon operations with high fidelities, F≳0.999\mathcal{F}\gtrsim0.999F≳0.999. The described setup can function as a versatile nonlinear optical element in a free-space photonic quantum network with simple linear elements and without the need of additional mode confinement, optical resonators, or optical isolators.
Emitter ensembles interact collectively with the radiation field. In the case of a one-dimensional array of atoms near a nanofiber, this collective light-matter interaction does not only lead to an increased photon coupling to the guided modes within the fiber, but also to a drastic enhancement of the chirality in the photon emission. We show that near-perfect chirality can be achieved already for moderately sized ensembles, containing 10 to 15 atoms, by phase matching a superradiant collective guided emission mode via an external laser field. This is of importance for developing an efficient interface between atoms and waveguide structures with unidirectional coupling, with applications in quantum computing and communication such as the development of nonreciprocal photon devices or quantum information transfer channels.