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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Towards the Observation of Collective Radiance Phenomena in a One-Dimensional Array of Waveguide-Coupled Atoms with Sub $-\lambda/2$ Spacing

Lucas Pache, Hector Letellier, Martin Cordier, Max Schemmer, Philipp Schneeweiß, Jürgen Volz, Arno Rauschenbeutel

Published Jun 23, 2025• 0 citations
Publisher

Abstract

Recently, it has been shown theoretically that the fidelity of photon storage and retrieval in quantum memories scales exponentially better with the number of emitters if one harnesses the collective response of closely spaced atoms ordered in an array [1]. The improved scaling relies on the effect of selective radiance, that is, destructive interference that suppresses scattering into undesired modes. This occurs when the period of an array of emitters is less than half of the atomic resonant wavelength of the probe laser$(d < \lambda/2)$. In order to realize this situation, we trap and optically interface laser-cooled cesium atoms using a two-color nanofiber-based dipole trap [2]. It is composed of a blue-detuned partial standing wave and two red-detuned running wave light fields, which counter-propagate in the fiber to compensate the inhomgenous broadening due to the vector light shift [3]. The resulting trapping potential consists of two 1D-arrays of trapping sites on the opposite sides of the nanofiber, where the axial period is$d\simeq 0.35\lambda$. Radially, the minima of the trapping potential are located at a distance of 300nm from the fibers surface. We characterize the trap by measuring the trap frequencies, the total number of stored atoms, the fraction of sites filled with a single atom in the collisional blockade regime, and the lifetime of the atoms.

DOI
10.1109/cleo/europe-eqec65582.2025.11110316

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Towards the Observation of Collective Radiance Phenomena in a One-Dimensional Array of Waveguide-Coupled Atoms with Sub −λ/2-\lambda/2−λ/2 Spacing
Recently, it has been shown theoretically that the fidelity of photon storage and retrieval in quantum memories scales exponentially better with the number of emitters if one harnesses the collective response of closely spaced atoms ordered in an array [1]. The improved scaling relies on the effect of selective radiance, that is, destructive interference that suppresses scattering into undesired modes. This occurs when the period of an array of emitters is less than half of the atomic resonant wavelength of the probe laser(d<λ/2)(d < \lambda/2)(d<λ/2). In order to realize this situation, we trap and optically interface laser-cooled cesium atoms using a two-color nanofiber-based dipole trap [2]. It is composed of a blue-detuned partial standing wave and two red-detuned running wave light fields, which counter-propagate in the fiber to compensate the inhomgenous broadening due to the vector light shift [3]. The resulting trapping potential consists of two 1D-arrays of trapping sites on the opposite sides of the nanofiber, where the axial period isd≃0.35λd\simeq 0.35\lambdad≃0.35λ. Radially, the minima of the trapping potential are located at a distance of 300nm from the fibers surface. We characterize the trap by measuring the trap frequencies, the total number of stored atoms, the fraction of sites filled with a single atom in the collisional blockade regime, and the lifetime of the atoms.
We report on the realization and characterization of a novel magic-wavelength nanofiber-based two-color optical dipole trap for cesium that allows us to generate two diametral periodic one-dimensional arrays of trapping sites with a spacing significantly smaller than half the resonant free-space wavelength of the cesium D 2 transition. This is achieved by launching a blue-detuned partial standing wave and two red-detuned light fields through the nanofiber. We trap and optically interface the atoms in the resulting periodic optical potential and characterize the trap by measuring the lifetime of the trapped atoms, the atom-light coupling strength, the filling factor, and the trap frequencies in the radial and axial directions. The implementation of this nanofiber-based optical interface with magic trapping wavelengths and sub- λ / 2 spacing is an important step towards the exploration of novel collective radiative effects, such as selective radiance.
Integrating the scalability of individually addressable arrays of optical-tweezer-trapped single atoms with the efficient light-matter interface provided by nanophotonic waveguides has been a long-standing challenge in quantum technologies based on atoms and photons. Here we realize a quantum interface between photons guided in an optical nanofiber with a diameter of 310 nm and an array of on average 155 individually addressable atoms. Using a spatial light modulator and an objective lens with NA = 0.45, single cesium atoms are trapped in a one-dimensional array of 200 optical tweezer spots with micrometer-scale trap sizes on the nanofiber. Individual atoms are addressed by spatially scanning an excitation laser beam, focused to a spot size comparable to that of the traps through the same objective lens, along the nanofiber. We confirm the single-atom nature of the individual trapping sites through photon-correlation measurements of the guided fluorescence, observing strong photon antibunching with g(2)(0)≈0.26g^{(2)}(0) \approx 0.26g(2)(0)≈0.26. We measure trap lifetimes of a few hundred milliseconds, with a maximum value of 460 ms, at an atom-surface separation of 670 nm without active cooling, representing an order-of-magnitude improvement over previous nanofiber traps. This platform opens a new regime for atom-photon interfaces, paving the way for scalable distributed quantum computing and quantum networks, as well as for the exploration of collective radiative effects in waveguide QED with individually addressable atoms.
Summary form only given. Developing light-matter interfaces is a crucial capability with unique applications to quantum optics and quantum information networks [1]. In this context, our group focuses on the study of interactions between photons guided by nanoscale waveguides and arrays of trapped atoms. Nanoscale waveguides offer a compelling platform because of the tight transverse confinement of the propagating light, the strong photon-atom coupling in single pass, and the potential long-range atom-atom interaction mediated by the guided photons [2]. Our platform, based on an optical nanofiber, already allowed the demonstration of an all-fibered optical memory with a storage and retrieval efficiency ri = 10% and a high SNR at the single photon level [3]. Then, by using an optical lattice in the evanescent field surrounding the nanofiber with a period close to commensurate with the resonant wavelength, we demonstrated large Bragg reflection by the 1D array of trapped atoms [4].
Summary form only given. Reversible light-matter interfaces are crucial elements in quantum optics and quantum information networks. In particular, the coupling of one-dimensional bosonic nanoscale waveguides and cold atoms appears as a promising pathway to build strong light-matter interaction thanks to the tight transverse confinement of light.Recently, our group has developed an interface where light, tightly guided by a subwavelength-diameter optical fiber (nanofiber), strongly interacts with atoms near its vicinity. In this case, a single atom close to the surface can absorb a non-negligible fraction of the guided light, as the effective area of the mode is comparable with the atomic cross-section. Moreover, using this configuration, it is possible to generate an in-fiber dipole trap for atoms in the vicinity of the waveguide. Using this all-fibered interface we have recently demonstrated the realization of an optical memory at the single photon level [1]. Here, we will describe the most recent result using this new interface: the observation of a large Bragg reflection off a 1D optical lattice [2]. The ability to control photon transport in 1D waveguides coupled to spin systems would allow for novel quantum network capabilities and many-body effects emerging from long-range interactions.
We experimentally and theoretically investigate collective radiative effects in an ensemble of cold atoms coupled to a single-mode optical nanofiber. Our analysis unveils the microscopic dynamics of the system, showing that collective interactions between the atoms and a single guided photon gradually build up along the atomic array in the direction of propagation of light. These results are supported by time-resolved measurements of the light transmitted and reflected by the ensemble after excitation via nanofiber-guided laser pulses, whose rise and fall times are shorter than the atomic lifetime. Superradiant decays more than 1 order of magnitude faster than the single-atom free-space decay rate are observed for emission in the forward-propagating guided mode, while at the same time, no speed-up of the decay rate is measured in the backward direction. In addition, position-resolved measurements of the light that is transmitted past the atoms are performed by inserting the nanofiber-coupled atomic array in a 45-m-long fiber ring resonator, which allow us to experimentally reveal the progressive growth of the collective response of the atomic ensemble. Our results highlight the unique opportunities offered by nanophotonic cold atom systems for the experimental investigation of collective light-matter interaction.
We are in the process of building an experimental setup for trapping and optically interfacing laser-cooled strontium atoms using the evanescent field surrounding an optical nanofiber [1]. The nanofiber is produced from a standard step-index optical fiber in a heat-pull process. It features a waist diameter of 230 nm where light is still efficiently guided while a significant part of the light propagates in the form of an evanescent field surrounding the nanofiber. Atoms are trapped in a one-dimensional (1D) optical lattice formed by two fiber-guided light-fields, red- and blue-detuned with respect to the strong transition at a wavelength of 461 nm. The aim is to realize a compensated trap, where the wavelengths of the trapping fields are magic for the 7.4 kHz wide intercombination line at 689 nm. This will allow us to implement advanced schemes for loading atoms into the 1D optical lattice and to investigate the phenomenon of selective radiance [2], where the atoms themselves can act as the waveguide. Here we will present our compact design for trapping strontium atoms from a laser ablated source with a “hot MOT” (operated at 461 nm wavelength), then transfer them to a “cold MOT” (operated at the intercombination line) and to the nanofiber trap.