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arXiv:2109.04540 · Physical Review Letters

Free-Fermion Multiply Excited Eigenstates and Their Experimental Signatures in 1D Arrays of Two-Level Atoms

Yu-Xiang Zhang, Klaus Mølmer

Published Mar 3, 2022• 9 citations
arXivHTMLPDF

Abstract

One-dimensional (1D) subwavelength atom arrays display multiply excited subradiant eigenstates which are reminiscent of free fermions. So far, these states have been associated with subradiant states with decay rates ∝N^{-3}, with N the number of atoms, which fundamentally prevents detection of their fermionic features by optical means. In this Letter, we show that free-fermion states generally appear whenever the band of singly excited states has a quadratic dispersion relation at the band edge and, hence, may also be obtained with radiant and even superradiant states. 1D arrays have free-fermion multiply excited eigenstates that are typically either subradiant or (super)radiant, and we show that a simple transformation acts between the two families. Based on this correspondence, we propose different means for their preparation and analyze their experimental signature in optical detection.

DOI
10.1103/physrevlett.128.093602

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One-dimensional (1D) subwavelength atom arrays display multiply excited subradiant eigenstates which are reminiscent of free fermions. So far, these states have been associated with subradiant states with decay rates ∝N^{-3}, with N the number of atoms, which fundamentally prevents detection of their fermionic features by optical means. In this Letter, we show that free-fermion states generally appear whenever the band of singly excited states has a quadratic dispersion relation at the band edge and, hence, may also be obtained with radiant and even superradiant states. 1D arrays have free-fermion multiply excited eigenstates that are typically either subradiant or (super)radiant, and we show that a simple transformation acts between the two families. Based on this correspondence, we propose different means for their preparation and analyze their experimental signature in optical detection.
Recently, the subradiant states of one-dimensional two-level atom chains coupled to light modes were found to have decay rates obeying a universal scaling, and an unexpected fermionic character of the multiply excited subradiant states was discovered. In this Letter, we theoretically obtain the singly excited subradiant states, and by eliminating the superradiant modes, we demonstrate a relation between the multiply excited subradiant states and the Tonks-Girardeau limit of the Lieb-Liniger model which explains the fermionic behavior. In addition, we identify a new family of subradiant states with correlations different from the fermionic ansatz.
The properties of coupled emitters can differ dramatically from those of their individual constituents. Canonical examples include sub- and super-radiance, wherein the decay rate of a collective excitation is reduced or enhanced due to correlated interactions with the environment. Here, we systematically study the properties of collective excitations for regularly spaced arrays of quantum emitters coupled to a one-dimensional waveguide. We find that, for low excitation numbers, the modal properties are well-characterized by spin waves with a definite wavevector. Moreover, the decay rate of the most subradiant modes obeys a universal scaling with a cubic suppression in the number of emitters. Multi-excitation subradiant eigenstates can be built from fermionic combinations of single excitation eigenstates; such 'fermionization' results in multiple excitations that spatially repel one another. We put forward a method to efficiently create and measure such subradiant states, which can be realized with superconducting qubits. These measurement protocols probe both real-space correlations (using on-site dispersive readout) and temporal correlations in the emitted field (using photon correlation techniques).
We propose a set of subradiant states which can be prepared and detected in a one-dimensional optical lattice. We find that the decay rates are highly dependent on the spatial phases imprinted on the atomic chain, which allows systematic investigations of the subradiance in fluorescence experiments. The time evolution of these states can have a long decay time where up to 100 ms of lifetime is predicted for 100 atoms. They can also show decayed Rabi-like oscillations with a beating frequency determined by the difference of the cooperative Lamb shift in the subspace. Experimental requirements are also discussed for practical implementation of the subradiant states. Our proposal provides a scheme for quantum storage of photons in arrays of two-level atoms through the preparation and detection of subradiant states, which offers opportunities for quantum many-body state preparation and quantum information processing in optical lattices.
The Hermitian part of the field-mediated dipole-dipole interaction in infinite periodic arrays of two-level atoms yields an energy band of the singly excited states. In this Letter, we show that a dispersion relation, ω_{k}-ω_{k_{ex}}∝(k-k_{ex})^{s}, near the band edge of the infinite system leads to the existence of subradiant states of finite one-dimensional arrays of N atoms with decay rates scaling as N^{-(s+1)}. This explains the recently discovered N^{-3} scaling and it leads to the prediction of power law scaling with higher power for special values of the lattice period. For the quantum optical implementation of the Su-Schrieffer-Heeger topological model in a dimerized emitter array, the band gap closing inherent to topological transitions changes the value of s in the dispersion relation and alters the decay rates of the subradiant states by many orders of magnitude.
Abstract Resonant light interacting with matter supports different phases of a polarisable medium, and optical bistability where two phases coexist. Such phases have previously been actively studied in cavities. Here, we identify signatures of optical phase transitions and optical bistability mapped onto scattered light in free-space planar arrays of cold atoms. Methods on how to explore such systems in superradiant and extreme subradiant states are proposed. The cooperativity threshold and intensity regimes for the intrinsic optical bistability, supported by resonant dipole-dipole interactions alone, are derived in several cases of interest analytically. Subradiant states require lower intensities, but stronger cooperativity for the existence of non-trivial phases than superradiant states. The transmitted light reveals phase transitions and bistability that are predicted by mean-field theory as large jumps in coherent and incoherent signals and hysteresis. In the quantum solution, traces of phase transitions are identified in enhanced quantum fluctuations of excited level populations.
When an ensemble of quantum emitters interacts with a common radiation field, their emission becomes collective, giving rise to superradiant and subradiant states, characterized by broadened and narrowed linewidths. In this work, we propose to harness subradiant states for quantum metrology; such states naturally arise in subwavelength-spaced atomic arrays in free space and in small ensembles of emitters coupled to one-dimensional waveguides. We demonstrate that their collective optical response yields sharp, narrow features in the transmittance spectrum, which can be used to enhance sensitivity to external perturbations. This improved sensitivity can be applied to atomic clock operation, spatially resolved imaging of emitter positions, and enables precise detection of both global and spatially varying detunings (such as those induced by electromagnetic fields or gravitational gradients).