Arrays Of Quantum Emitters Research Articles

Electroluminescent vertical tunneling junctions based on WSe2 monolayer quantum emitter arrays: Exploring tunability with electric and magnetic fields

We experimentally demonstrate the creation of defects in monolayer WSe2 via nanopillar imprinting and helium ion irradiation. Based on the first method, we realize atomically thin vertical tunneling light-emitting diodes based on WSe2 monolayers hosting quantum emitters at deterministically specified locations. We characterize these emitters by investigating the evolution of their emission spectra in external electric and magnetic fields, as well as by inducing electroluminescence at low temperatures. We identify qualitatively different types of quantum emitters and classify them according to the dominant electron-hole recombination paths, determined by the mechanisms of intervalley mixing occurring in fundamental conduction and/or valence subbands.

Breakdown of steady-state superradiance in extended driven atomic arrays

Recent advances in generating well controlled dense arrangements of individual atoms in free space have generated interest in understanding how the extended nature of these systems influences superradiance phenomena. Here, we provide an in-depth analysis on how space-dependent light shifts and decay rates induced by dipole-dipole interactions modify the steady-state properties of coherently driven arrays of quantum emitters. We characterize the steady-state phase diagram, with particular focus on the radiative properties in the steady state. Interestingly, we find that diverging from the well-established Dicke paradigm of equal all-to-all interactions significantly modifies the emission properties. In particular, the prominent quadratic scaling of the radiated light intensity with particle number in the steady state—a hallmark of steady-state Dicke superradiance—is entirely suppressed, resulting in only linear scaling with particle number. We show that this breakdown of steady-state superradiance occurs due to the emergence of additional dissipation channels that populate not only superradiant states but also subradiant ones. The additional contribution of subradiant dark states in the dynamics leads to a divergence in the time scales needed to achieve steady states. Building on this, we further show that measurements taken at finite times for extended atom ensembles reveal properties closely mirroring the idealized Dicke scenario. Published by the American Physical Society 2024

Metasurface-Based Hybrid Optical Cavities for Chiral Sensing

Quantum metasurfaces, i.e., two-dimensional subwavelength arrays of quantum emitters, can be employed as mirrors towards the design of hybrid cavities, where the optical response is given by the interplay of a cavity-confined field and the surface modes supported by the arrays. We show that stacked layers of quantum metasurfaces with orthogonal dipole orientation can serve as helicity-preserving cavities. These structures exhibit ultranarrow resonances and can enhance the intensity of the incoming field by orders of magnitude, while simultaneously preserving the handedness of the field circulating inside the resonator, as opposed to conventional cavities. The rapid phase shift in the cavity transmission around the resonance can be exploited for the sensitive detection of chiral scatterers passing through the cavity. We discuss possible applications of these resonators as sensors for the discrimination of chiral molecules. Our approach describes a new way of chiral sensing via the measurement of particle-induced phase shifts. Published by the American Physical Society 2024

Optimal subradiant spin wave exchange in dipole-coupled atomic ring arrays

The subwavelength array of quantum emitters provides an ideal platform for exploring rich many-body dynamics, such as super- and subradiance. In this paper, we explore the dynamics of spin wave exchange between two dipole-coupled atomic ring arrays. Subradiant spin waves lead to low-loss and high efficiency of ring-to-ring transfer. The optimal subradiant spin wave exchange occurs at appropriate separations between coplanar rings, despite the fact that the energy transfer efficiency is monotonically enhanced (in the regime ) as the rings’ separation decreases. However, the spin wave will scatter due to the dephasing mechanism of close-by atom pairs, as the separation of two rings is too small. With the increase in the number of atoms on the ring, the subradiant shielding effect also strengthens, leading to a shorter distance for the transfer of spin waves. We investigate the rotation of one of the rings and find that the optimal spin wave exchange corresponds to the scenario where the line connecting the two nearest atoms of the two rings aligns with the center of the circle. Moreover, we study the influence of transition dipole moment orientations on the effective interaction between two atomic rings. We observe that there is a critical point where the effective interaction strength changes dramatically owing to the cooperative effect of the subwavelength atomic array. We believe that our results could be important for quantum information processing based on atomic arrays.

Topological Inverse Band Theory in Waveguide Quantum Electrodynamics.

Topological phases play a crucial role in the fundamental physics of light-matter interaction and emerging applications of quantum technologies. However, the topological band theory of waveguide QED systems is known to break down, because the energy bands become disconnected. Here, we introduce a concept of the inverse energy band and explore analytically topological scattering in a waveguide with an array of quantum emitters. We uncover a rich structure of topological phase transitions, symmetric scale-free localization, completely flat bands, and the corresponding dark Wannier states. Although bulk-edge correspondence is partially broken because of radiative decay, we prove analytically that the scale-free localized states are distributed in a single inverse energy band in the topological phase and in two inverse bands in the trivial phase. Surprisingly, the winding number of the scattering textures depends on both the topological phase of inverse subradiant band and the odevity of the cell number. Our Letter uncovers the field of the topological inverse bands, and it brings a novel vision to topological phases in light-matter interactions.

Single atom in a superoscillatory optical trap

Optical tweezers have become essential tools to manipulate atoms or molecules at a single particle level. However, using standard diffraction-limited optical systems, the transverse size of the trap is lower bounded by the optical wavelength, limiting the application range of optical tweezers. Here we report trapping of single ultracold atom in an optical trap that can be continuously tuned from a standard Airy focus to a subwavelength hotspot smaller than the usual Abbe’s diffraction limit. The hotspot was generated using the effect of superoscillations, by the precise interference of multiple free-space coherent waves. We argue that superoscillatory trapping and continuous potential tuning offer not only a way to generate compact and tenable ensembles of trapped atoms for quantum simulators but will also be useful in single molecule quantum chemistry and the study of cooperative atom-photon interactions within subwavelength arrays of quantum emitters.

Dynamic population of multiexcitation subradiant states in incoherently excited atomic arrays

The authors theoretically investigate the creation of subradiant states with multiple excitations in dense arrays of quantum emitters, which is a long-lasting challenge due to the complexity of the system and the experimental limitations in state-of-the-art platforms. The authors analyze the effects of trapping geometry and correlations on the subradiant-state formation and provide insights for controlling and utilizing subradiant states in quantum technologies.

Field programmable spin arrays for scalable quantum repeaters

The large scale control over thousands of quantum emitters desired by quantum network technology is limited by the power consumption and cross-talk inherent in current microwave techniques. Here we propose a quantum repeater architecture based on densely-packed diamond color centers (CCs) in a programmable electrode array, with quantum gates driven by electric or strain fields. This ‘field programmable spin array’ (FPSA) enables high-speed spin control of individual CCs with low cross-talk and power dissipation. Integrated in a slow-light waveguide for efficient optical coupling, the FPSA serves as a quantum interface for optically-mediated entanglement. We evaluate the performance of the FPSA architecture in comparison to a routing-tree design and show an increased entanglement generation rate scaling into the thousand-qubit regime. Our results enable high fidelity control of dense quantum emitter arrays for scalable networking.

Linear optical elements based on cooperative subwavelength emitter arrays.

We describe applications of two-dimensional subwavelength quantum emitter arrays as efficient optical elements in the linear regime. For normally incident light, the cooperative optical response, stemming from emitter-emitter dipole exchanges, allows the control of the array's transmission, its resonance frequency, and bandwidth. Operations on fully polarized incident light, such as generic linear and circular polarizers as well as phase retarders can be engineered and described in terms of Jones matrices. Our analytical approach and accompanying numerical simulations identify optimal regimes for such operations and reveal the importance of adjusting the array geometry and of the careful tuning of the external magnetic fields amplitude and direction.

Chirality-driven delocalization in disordered waveguide-coupled quantum arrays

We study theoretically the competition between directional asymmetric coupling and disorder in a one-dimensional array of quantum emitters chirally coupled through a waveguide mode. Our calculation reveals a highly nontrivial phase diagram for the eigenstates' spatial profile, nonmonotonously depending on the disorder and directionality strength. The increase of the coupling asymmetry drives the transition from Anderson localization in the bulk through delocalized states to chirality-induced localization at the array edge. Counterintuitively, this transition is not smeared by strong disorder but becomes sharper instead. Our findings could be important for the rapidly developing field of waveguide quantum electrodynamics, where chiral interactions and disorder play crucial roles.

Cooperative subwavelength molecular quantum emitter arrays

Dipole-coupled subwavelength quantum emitter arrays respond cooperatively to external light fields as they may host collective delocalized excitations (a form of excitons) with super- or subradiant character. Deeply subwavelength separations typically occur in molecular ensembles, where in addition to photon-electron interactions, electron-vibron couplings and vibrational relaxation processes play an important role. We provide analytical and numerical results on the modification of super- and subradiance in molecular rings of dipoles including excitations of the vibrational degrees of freedom. While vibrations are typically considered detrimental to coherent dynamics, we show that molecular dimers or rings can be operated as platforms for the preparation of long-lived dark superposition states aided by vibrational relaxation. In closed ring configurations, we extend previous predictions for the generation of coherent light from ideal quantum emitters to molecular emitters, quantifying the role of vibronic coupling onto the output intensity and coherence.

Universality of Dicke superradiance in arrays of quantum emitters

Dicke superradiance is an example of emergence of macroscopic quantum coherence via correlated dissipation. Starting from an initially incoherent state, a collection of excited atoms synchronizes as they decay, generating a macroscopic dipole moment and emitting a short and intense pulse of light. While well understood in cavities, superradiance remains an open problem in extended systems due to the exponential growth of complexity with atom number. Here we show that Dicke superradiance is a universal phenomenon in ordered arrays. We present a theoretical framework – which circumvents the exponential complexity of the problem – that allows us to predict the critical distance beyond which Dicke superradiance disappears. This critical distance is highly dependent on the dimensionality and atom number. Our predictions can be tested in state of the art experiments with arrays of neutral atoms, molecules, and solid-state emitters and pave the way towards understanding the role of many-body decay in quantum simulation, metrology, and lasing.

Efficient nano-photonic antennas based on dark states in quantum emitter rings.

Nanoscopic arrays of quantum emitters can feature highly sub-radiant collective excitations with a lifetime exponentially growing with emitter number. Adding an absorptive impurity as an energy dump in the center of a ring shaped polygon allows to exploit this feature to create highly efficient single photon antennas. Here among regular polygons with an identical center absorbing emitter, a nonagon exhibits a distinct optimum of the absorption efficiency. This special enhancement originates from the unique emergence of a subradiant eigenstate with dominant center occupation. Only for nine emitters the sum of coupling strengths of each emitter to all others matches the center to the ring coupling. Analogous to a parabolic mirror the antenna ring then concentrates incoming radiation at its center without being significantly excited itself. Similar large efficiency enhancements, which even prevail for broadband excitation, can also be engineered for other antenna sizes by tailoring the frequency and magnitude of the central absorber. Interestingly, for very small structures a quantum treatment predicts an even stronger enhancement for the single photon absorption enhancement than a classical dipole model. As natural light harvesting structures are often based on ring shaped structures, the underlying principle might be exploited there as well.

Stationary excitation waves and multimerization in arrays of quantum emitters

We explore the features of an equally-spaced array of two-level quantum emitters, that can be either natural atoms (or molecules) or artificial atoms, coupled to a field with a single continuous degree of freedom (such as an electromagnetic mode propagating in a waveguide). We investigate the existence and characteristics of bound states, in which a single excitation is shared among the emitters and the field. We focus on bound states in the continuum, occurring in correspondence of excitation energies in which a single excited emitter would decay. We characterize such bound states for an arbitrary number of emitters, and obtain two main results, both ascribable to the presence of evanescent fields. First, the excitation profile of the emitter states is a sinusoidal wave. Second, we discuss the emergence of multimers, consisting in subsets of emitters separated by two lattice spacings in which the electromagnetic field is approximately vanishing.

Prospects and challenges of quantum emitters in 2D materials

The search for an ideal single-photon source has generated significant interest in discovering emitters in materials as well as developing new manipulation techniques to gain better control over the emitters' properties. Quantum emitters in atomically thin two-dimensional (2D) materials have proven to be very attractive with high brightness, operation under ambient conditions, and the ability to be integrated with a wide range of electronic and photonic platforms. This Perspective highlights some of the recent advances in quantum light generation from 2D materials, focusing on hexagonal boron nitride and transition metal dichalcogenides. Efforts in engineering and deterministically creating arrays of quantum emitters in 2D materials, their electrical excitation, and their integration with photonic devices are discussed. Finally, we address some of the challenges the field is facing and the near-term efforts to tackle them. We provide an outlook toward efficient and scalable quantum light generation from 2D materials to controllable and addressable on-chip quantum sources.

Scalable and Deterministic Fabrication of Quantum Emitter Arrays from Hexagonal Boron Nitride.

We demonstrate the fabrication of large-scale arrays of single-photon emitters (SPEs) in hexagonal boron nitride (hBN). Bottom-up growth of hBN onto nanoscale arrays of dielectric pillars yields corresponding arrays of hBN emitters at the pillar sites. Statistical analysis shows that the pillar diameter is critical for isolating single defects, and diameters of ∼250 nm produce a near-unity yield of a single emitter at each pillar site. Our results constitute a promising route toward spatially controlled generation of hBN SPEs and provide an effective and efficient method to create large-scale SPE arrays. The results pave the way to scalability and high throughput fabrication of SPEs for advanced quantum photonic applications.

Van der Waals heterostructure polaritons with moiré-induced nonlinearity.

Controlling matter-light interactions with cavities is of fundamental importance in modern science and technology1. This is exemplified in the strong-coupling regime, where matter-light hybrid modes form, with properties that are controllable by optical-wavelength photons2,3. By contrast, matter excitations on the nanometre scale are harder to access. In two-dimensional van der Waals heterostructures, a tunable moiré lattice potential for electronic excitations may form4, enabling the generation of correlated electron gases in the lattice potentials5-9. Excitons confined in moiré lattices have also been reported10,11, but no cooperative effects have been observed and interactions with light have remained perturbative12-15. Here, by integrating MoSe2-WS2 heterobilayers in a microcavity, we establish cooperative coupling between moiré-lattice excitons and microcavity photons up to the temperature of liquid nitrogen, thereby integrating versatile control of both matter and light into one platform. The density dependence of the moiré polaritons reveals strong nonlinearity due to exciton blockade, suppressed exciton energy shift and suppressed excitation-induced dephasing, all of which are consistent with the quantum confined nature of the moiré excitons. Such a moiré polariton system combines strong nonlinearity and microscopic-scale tuning of matter excitations using cavity engineering and long-range light coherence, providing a platform with which to study collective phenomena from tunable arrays of quantum emitters.

Theory of moiré localized excitons in transition metal dichalcogenide heterobilayers

Transition-metal dichalcogenide heterostructures exhibit moir\'e patterns that spatially modulate the electronic structure across the material's plane. For certain material pairs, this modulation acts as a potential landscape with deep, trigonally symmetric wells capable of localizing interlayer excitons, forming periodic arrays of quantum emitters. Here, we study these moir\'e localized exciton states and their optical properties. By numerically solving the two-body problem for an interacting electron-hole pair confined by a trigonal potential, we compute the localized exciton spectra for different pairs of materials. We derive optical selection rules for the different families of localized states, each belonging to one of the irreducible representations of the potential's symmetry group $C_{3v}$, and numerically estimate their polarization-resolved absorption spectra. We find that the optical response of localized moir\'e interlayer excitons is dominated by states belonging to the doubly-degenerate $E$ irreducible representation. Our results provide new insights into the optical properties of artificially confined excitons in two-dimensional semiconductors.

Highly energy-tunable quantum light from moiré-trapped excitons.

Photon antibunching, a hallmark of quantum light, has been observed in the correlations of light from isolated atomic and atomic-like solid-state systems. Two-dimensional semiconductor heterostructures offer a unique method to create a quantum light source: Moiré trapping potentials for excitons are predicted to create arrays of quantum emitters. While signatures of moiré-trapped excitons have been observed, their quantum nature has yet to be confirmed. Here, we report photon antibunching from single moiré-trapped interlayer excitons in a heterobilayer. Via magneto-optical spectroscopy, we demonstrate that the discrete anharmonic spectra arise from bound band-edge electron-hole pairs trapped in moiré potentials. Last, we exploit the large permanent dipole of interlayer excitons to achieve large direct current (DC) Stark tuning up to 40 meV. Our results confirm the quantum nature of moiré-confined excitons and open opportunities to investigate their inhomogeneity and interactions between the emitters or energetically tune single emitters into resonance with cavity modes.

A subradiant optical mirror formed by a single structured atomic layer.

Versatile interfaces with strong and tunable light-matter interactions are essential for quantum science1 because they enable mapping of quantum properties between light and matter1. Recent studies2-10 have proposed a method of controlling light-matter interactions using the rich interplay of photon-mediated dipole-dipole interactions in structured subwavelength arrays of quantum emitters. However,a key aspect of this approach-the cooperative enhancement of the light-matter coupling strength and the directional mirror reflection of the incoming light using an array of quantum emitters-has not yet been experimentally demonstrated. Here we report the direct observation of the cooperative subradiant response of a two-dimensional square array of atoms in an optical lattice. We observe a spectral narrowing of the collective atomic response well below the quantum-limited decay of individual atoms into free space. Through spatially resolved spectroscopic measurements, we show that the array acts as an efficient mirror formed by a single monolayer of a few hundred atoms. By tuning the atom density in the array and changing the ordering of the particles, we are able to control the cooperative response of the array and elucidate the effect of the interplay of spatial order and dipolar interactions on the collective properties of the ensemble. Bloch oscillations of the atoms outside the array enable us to dynamically control the reflectivity of the atomic mirror. Our work demonstrates efficient optical metamaterial engineering based on structured ensembles of atoms4,8,9 and paves the way towards controlling many-body physics with light5,6,11 and light-matter interfaces at the single-quantum level7,10.