The unique properties of two-dimensional moiré systems have been widely studied from many perspectives. However, relatively little work has investigated how the real-space structure of moiré systems can directly engender novel properties and functionalities. In this Letter, we explore how intriguing cooperative optical effects can emerge from the real-space superlattice formed by moiré excitons. Particularly, we show that the collective moiré exciton states can have either strongly enhanced (superradiant) or suppressed (subradiant) radiative decay rates, depending on their in-plane wave vector. These super- and subradiant states can be efficiently switched by a gate-induced electric field gradient. Moreover, the cooperative transmittance T of the atomically thin moiré system can be switched from $T\approx 0$ (opaque) to $T \approx 1$ (transparent) with $\lesssim ~1\%$ heterostrain or a $1^{\circ}$ adjustment in the twist angle $θ$. These features are robust against nonradiative losses and disorders, making the moiré system a highly versatile platform for cooperative quantum optics with potential applications in, e.g., single photon storage and switching.
The unique properties of two-dimensional moiré systems have been widely studied from many perspectives. However, relatively little work has investigated how the real-space structure of moiré systems can directly engender novel properties and functionalities. In this Letter, we explore how intriguing cooperative optical effects can emerge from the real-space superlattice formed by moiré excitons. Particularly, we show that the collective moiré exciton states can have either strongly enhanced (superradiant) or suppressed (subradiant) radiative decay rates, depending on their in-plane wave vector. These super- and subradiant states can be efficiently switched by a gate-induced electric field gradient. Moreover, the cooperative transmittance T of the atomically thin moiré system can be switched from T≈0 (opaque) to T≈1 (transparent) with ≲1% heterostrain or a 1∘ adjustment in the twist angle θ. These features are robust against nonradiative losses and disorders, making the moiré system a highly versatile platform for cooperative quantum optics with potential applications in, e.g., single photon storage and switching.
Solid-state platforms are particularly attractive for quantum optics because they facilitate on-chip integration and are compatible with established semiconductor and photonic technologies. However, a major challenge in solid-state quantum optics is the fabrication of arrays of identical emitters, such as quantum dots. In this work, we propose moire superlattices as a novel solid-state platform for manipulating light at the single-photon level. Moire superlattices form arrays of artificial-atom states characterized by nearly identical optical transition energies, tunable spacing, and highly adjustable electronic structures. They naturally operate as atomically thin, scalable, periodic emitters, making them ideal for quantum applications. Additionally, the extensive materials database of moire superlattices offers spectral coverage spanning a broad range of optical wavelengths.
In this dissertation we focused on two topics: 1. atomic cooperative effect, including superradiance, subradiance and collective Lamb shift in an atomic ensemble; 2. novel topological effect in quantum optics system, such as Haldane model, synthetic magnetic field in superradiance lattice and synthetic spin-orbit interaction in Fock-state lattices. \n\nIn the first part, we start from the Dicke state, an N-particle atomic ensemble excited by a single photon. Due to the interaction of the atoms with a continuum of vacuum modes, the Dicke state has a larger decay rate compared with an isolated atom, termed the single photon superradiance. In the “opposite” case, the single-photon subradiance state does not decay because of the destructive interference of the atomic transitions. We study the protection of subradiant states by the symmetry of the atomic distributions in the Dicke limit, in which collective Lamb shifts cannot be neglected. We find that antisymmetric states are subradiant states for distributions with reflection symmetry. Continuous symmetry can also be used to achieve subradiance in an extended ensemble. This study is relevant to the problem of robust quantum memory with long storage time and fast readout.\n\nIn the second part, we start from introducing the concepts, methods, and models from quantum optics or condensed matter physics. They include the timed-Dicke states, electromagnetically induced transparency, coupled wave equations, tight-binding model, band theory, Floquet theory, superradiance lattice, integer quantum Hall effect, Haldane model, etc.. Based upon these tools, we study three topological quantum optics systems: (i) a Haldane model with in situ tunable topological properties in a two-dimensional momentumspace superradiance lattice composed of timed Dicke states in electromagnetically induced transparency; (ii) a spin-orbit coupled Fock-state lattice, which introduce a novel quantum operation to generate mesoscopic superposition states, e.g., the NOON states and cat states; (iii) a synthetic magnetic field created in a quasi one dimensional superradiance lattice, which is predicted to be observed in thermal vapours of alkali atoms instead of only in cold atoms (as in previous experiments). This study aims to provide a novel, highly tunable platform simulating exotic phenomena in condensed matter physics and offers a basis of topological quantum optics and novel photonic devices.
The interaction between light and matter has captivated physicists for centuries, from early studies of vision and refraction in ancient Greece to the development of quantum mechanics and quantum electrodynamics in the past century. While the response of a single quantum emitter to light is well understood, the radiative properties of an ensemble of closely spaced emitters are far more intricate. Coupling to a shared electromagnetic environment induces coherent and dissipative interactions between emitters, giving rise to a collective response that cannot be captured by treating them independently. In the regime of few excitations, the system hosts delocalized subradiant states, that is, coherent superpositions that are largely decoupled from the electromagnetic field and thus decay at suppressed rates. While this weak coupling makes subradiant states attractive for quantum technologies, it also renders them difficult to manipulate. At higher excitation densities, the intrinsic nonlinearity of emitters and the exponential growth of the Hilbert space make theoretical and numerical descriptions of the system and its dynamics increasingly challenging. This thesis explores two fundamental questions: How can subradiant and dark states be selectively accessed and harnessed for practical applications in quantum technologies? And how can interacting ensembles of quantum emitters be efficiently simulated to uncover their many-body physics? The first part of the thesis develops protocols for controlling and addressing dark states in free-space and waveguide-coupled atomic arrays, demonstrating their utility in quantum storage and the deterministic generation of entangled photonic states. Incorporating atomic motion, we further show that collective subradiant states can enhance cooling in dense atomic arrays, offering new avenues for controlling motional dynamics. In the second part, we introduce cumulant expansions of the equations of motion as a powerful tool to analytically and numerically investigate collective decay in the many-body regime. We first examine the collective decay of fully excited atomic arrays in free space, characterizing the onset and scaling of the superradiant burst across different geometries. In collaboration with experiments on ultracold erbium atoms in optical lattices, we provide the first direct observations of many-body collective effects in free-space ordered arrays, including early-time superradiant bursts, late-time subradiant tails, and the emergence of atomic correlations throughout the dynamics. Finally, we theoretically and numerically explore the transient formation of multi-excitation subradiant states, and demonstrate how the existence of multiple dissipation channels suppresses steady-state superradiance in extended arrays.
Twisted atomically thin layers have attracted much attention for Moiré potential and correlated quantum phenomena. However, existing Moiré superlattices have largely been limited to extensive wavefunction without lateral confinement. Here we introduce a new platform where 1D nanoribbons of 2D MoS2 grown by vapor deposition can be easily superposed at various angles from stacking and transferring, to form Moiré quantum dots at their intersections with unique exciton physics. Angle-dependent Moiré intersections show enhanced exciton emission at commensurate angle 22 deg, which demonstrates faster relaxation at the cryogenic temperature. A size-dependent study further exhibits a reduced exciton energy and soften out-of-plane interlayer coupling for smaller Moiré areas. Our results reveal exciton physics turnability via precise overlapping of 1D nanoribbons.
Cooperative effects such as super(or sub)radiance in quantum systems arise from the interplay among quantum emitters. While bright superradiant states have been extensively studied and have yielded important insights into cooperative phenomena, subradiant states remain less explored due to their inherently dark state nature. However, subradiance holds significant potential as a valuable quantum resource exploiting long-lived and large-scale entanglement, which is a key for advancing quantum information technologies. Here, we demonstrate strong collective emission from a cavity-mediated steady-state subradiant state. In a tailored photonic environment with balanced cavity dissipation, emitter-field coupling strength, and incoherent pumping, two quantum dots coupled to a low-Q cavity exhibit a steady-state population in a subradiant state with a highly negative cooperativity parameter among the emitters. As a key signature of steady-state subradiance, the system shows strong photon bunching ( g(2)(0)>8 ) and suppressed single-photon decay (36 ns). Furthermore, we investigate that such collective interactions can be manipulated by controlling various system parameters, such as detuning and dephasing, supported by numerical simulations. Our approach to inducing cavity-mediated subradiance paves the way for generating and harnessing quantum correlations among quantum emitters via controlled dissipation. Subradiance is a cooperative effect in systems of quantum emitters, which is the opposite of superradiance, or enhanced emission, but has been less explored. Here, the authors demonstrate a steady-state subradiant state in a system of two InAs quantum dots coupled to a low-Q nanophotonic cavity.
Moiré lattices formed by superimposing rotated two-dimensional (2D) periodic sublattices, such as twisted bilayer graphene, can exhibit fascinating properties not observed in their individual constituent layers. Despite extensive research on 2D moiré lattices, the physics of three-dimensional (3D) moiré lattices—formed by superimposing rotated 3D periodic sub-lattices with crystallographic symmetries, which exhibit unique 3D potentials determined by twisting angles—remains largely unexplored. In this work, we demonstrate that depending on the choice of rotation angles, moiré potentials composed of two cubic sub-lattices can exhibit three different phases: they can be fully 3D incommensurate, incommensurate in only one spatial direction (while remaining periodic in the orthogonal plane), or fully periodic. These incommensurate potentials, which can be created for condensates of non-interacting atoms using judiciously combined laser beams, are shown to support compact states of three distinct types: fully localized in space, confined to a line, or confined to a specific plane. Our findings establish a foundation for controlling wave localization in incommensurate 3D moiré systems, with potential applications in cold atom systems, optics, and beyond. 3D moiré lattices can exhibit distinct incommensurate phases depending on twist angles. Here, authors demonstrate phase-controlled emergence of fully localized, line-localized, and plane-localized states, enabling tunable 3D transport in cold atom and optical systems.