We propose and discuss quantum spin lenses, where quantum states of delocalized spin excitations in an atomic medium are focused in space in a coherent quantum process down to (essentially) single atoms.These can be employed to create controlled interactions in a quantum light-matter interface, where photonic qubits stored in an atomic ensemble are mapped to a quantum register represented by single atoms.We propose Hamiltonians for quantum spin lenses as inhomogeneous spin models on lattices, which can be realized with Rydberg atoms in 1D, 2D, and 3D, and with strings of trapped ions.We discuss both linear and nonlinear quantum spin lenses: in a nonlinear lens, repulsive spin-spin interactions lead to focusing dynamics conditional to the number of spin excitations.This allows the mapping of quantum superpositions of delocalized spin excitations to superpositions of spatial spin patterns, which can be addressed by light fields and manipulated.Finally, we propose multifocal quantum spin lenses as a way to generate and distribute entanglement between distant atoms in an atomic lattice array.
We propose and discuss quantum spin lenses, where quantum states of delocalized spin excitations in an atomic medium are focused in space in a coherent quantum process down to (essentially) single atoms.These can be employed to create controlled interactions in a quantum light-matter interface, where photonic qubits stored in an atomic ensemble are mapped to a quantum register represented by single atoms.We propose Hamiltonians for quantum spin lenses as inhomogeneous spin models on lattices, which can be realized with Rydberg atoms in 1D, 2D, and 3D, and with strings of trapped ions.We discuss both linear and nonlinear quantum spin lenses: in a nonlinear lens, repulsive spin-spin interactions lead to focusing dynamics conditional to the number of spin excitations.This allows the mapping of quantum superpositions of delocalized spin excitations to superpositions of spatial spin patterns, which can be addressed by light fields and manipulated.Finally, we propose multifocal quantum spin lenses as a way to generate and distribute entanglement between distant atoms in an atomic lattice array.
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.
Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light. By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems. In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection. Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter. Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2). Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.
While artificially fabricated patterned metasurfaces are providing paradigm-shifting optical components for classical light manipulation, strongly interacting, controllable, and deterministic quantum interfaces between light and matter in free space remain an outstanding challenge.Here, we theoretically demonstrate how to achieve quantum control of both the electric and magnetic components of an incident single-photon pulse by engineering the collective response of a two-dimensional atomic array.High-fidelity absorption and storage in a long-lived subradiant state, and its subsequent retrieval, are achieved by controlling classically or quantum-mechanically the ac Stark shifts of the atomic levels and suppressing the scattering during the absorption.Quantum wave-front control of the transmitted photon with nearly zero reflection is prepared by coupling the collective state of the array to another photon in a cavity and by engineering a Huygens' surface of atoms using only a single coherent standing wave.The proposed schemes allow for the generation of entanglement between the cavity, the lattice, and hence the state of the stored, reflected, or transmitted light, and for quantum-state transfer between the cavity and propagating photons.Bipartite entanglement generation is explicitly calculated between a stored single-photon excitation of the array and the cavity photon.We illustrate the control by manipulating the phase, phase superposition, polarization, and direction of a transmitted or reflected photon, providing quantum-optical switches and functional quantum interfaces between light and atoms that could form links in a larger quantum information platform.
These lecture notes discuss applications of atom-light interactions in cavities to quantum metrology, simulation, and computation. A focus is on nonlocally interacting spin systems realized by coupling many atoms to a delocalized mode of light. We will build up from the fundamentals: understanding how a cavity enables light to coherently imprint information on atoms and atoms to imprint information on the light, enabling quantum non-demolition measurements that constitute a powerful means of engineering nonclassical states. By extension, letting the intracavity light act back on the atoms enables coherent photon-mediated interactions. I start by discussing collective spin models, emphasizing applications in entanglement-enhanced metrology, before proceeding to richer many-body physics enabled by incorporating spatiotemporal control or employing multiple cavity modes. I will highlight opportunities for leveraging these tools for quantum simulations inspired by problems in condensed matter and quantum gravity. Along the way, I provide a pedagogical introduction to criteria for strong atom-light coupling, illustrate how the corresponding figure of merit -- the cooperativity -- sets fundamental limits on the coherence of atom-light interactions, and discuss prospects for harnessing high-cooperativity cavity QED in quantum simulation and computation.
Devices that harness the surprising properties of quantum systems at scale hold the possibility of revolutionary advances in information technology. Algorithms for quantum computers can take advantage of quantum parallelism to solve problems that are intractable with classical hardware, including prime factorization and simulating chemical processes. A quantum communication network that carries qubits instead of bits can leverage the irreversibility of projective measurement to guarantee secure communication. Because quantum states are fragile, building such devices requires exquisite control over many isolated quantum systems. One of the most critical capabilities is the interaction of stationary and flying qubits. In this thesis we consider how to engineer atom-light interactions as an interface for networked quantum systems. Quantum atom-light couplers require the efficient and reversible interaction of atomic and optical qubits, but the natural interaction of atoms and photons is weak. It is this very lack of interaction between light and its environment that makes light such an excellent carrier of quantum information. There are three promising strategies for stronger coherent atom-light interactions. First, the light field may be confined to a cavity so that it interacts with the atom over a longer time. Second, the light field may be focussed to a small spot with high aperture optics and matched to the natural atomic radiation pattern. Third, we may pass the light field through an ensemble of many atoms and store the qubit in a distributed state. In practice, atom-light couplers employ some combination of all three strategies. We will focus our research on the second two approaches: first by coupling one and two trapped ions with high aperture optics, and then operating a quantum optical memory with an ensemble of approximately 10 billion neutral atoms. Atoms that have been ionized and trapped in a common electromagnetic potential are an advanced few-qubit computation platform. Qubit states are associated with an electronic excitation at each atom, controlled coherently with lasers, and made to interact in two (or many) qubit gates by mutual Coulomb repulsion. One path to quantum advantage with ion-trap quantum processors is to scale small qubit registers by distributing entanglement with photonic interconnects. We consider the feasible efficiency of free-space atom-light couplers for atomic dipole transitions, and derive the atomic image by common free-space couplers. We couple a trapped-ion register with high-aperture lenses and operate a single trapped ion as a photon source. We model the dynamic character of the source and show spin-orbit coupling in the single-photon spatial mode. Although the total collection efficiency is only approximately 0.01, the source has exceptional photon-number purity A = (1.9 +/- 0:2) * 10^-3 such that the higher-order quantum nature of the field persists even after most of the field has been discarded. We derive an efficient quantum non-Gaussian witness and surpass it, the first such demonstration for a trapped-atom single-photon source. We entangle two trapped atoms by single-photon detection, and observe an interference pattern in the spatial mode of the bipartite state. Constructive interference between the entangled components enhances the emission probability by up to 29%, an example of collective enhancement by a two-atom ensemble. The efficiency of free-space atom-light couplers is limited by our capability to engineer high-precision, high-aperture optics. We fabricate ultra-precise hemispheric mirrors with numerical aperture 0.996 by single point diamond turning. The mirrors are amongst the smoothest hemispheric surfaces ever manufactured with root-mean-square (RMS) errors consistently below 25 nm. The smoothest of our mirrors has a RMS error of 14 nm and peak-to-valley error of 88 nm. A mirror with this surface is capable of suppressing or enhancing the spontaneous emission of an atom into free space by 96% in a proposed quantum electrodynamics experiment. We show how these mirrors, with a simple modification, can shape the spatial mode of a trapped atom by similar vacuum-mode engineering. We derive a near-hemispheric mirror coupling scheme that should be 72% efficient with mirrors as precise as ours. We design an ion trap for use with such high-aperture optics. With careful conditioning and control, ensembles of many atoms can be made to store and release photonic qubits on demand. Such optical quantum memories leverage the collective interaction of a light field with many billions of atoms to achieve storage and recall efficiencies approaching unity. We show how stationary light fields for photonic phase gates can be generated in optically deep ensembles, an effect which has since been observed. We implement the gradient echo memory (GEM) scheme in an ultra-high optical depth cold-atom ensemble. The performance of a practical quantum memory is contingent on the chosen qubit encoding. We extend the GEM protocol to allow the simultaneous storage of frequency separated signals and demonstrate that this `dual-rail' memory is suitable for high-fidelity frequency qubits. Dual-rail signals are recalled with 35% efficiency, 82% interference fringe visibility and 6 degrees of phase stability. We describe how the fidelity of the scheme is limited by frequency-dependent polarization rotation and how this may be addressed in an improved configuration. Finally, we demonstrate single-rail storage by GEM with 87% efficiency and 1 ms memory lifetime. Our memory surpasses the no-cloning limit for up to 600 us of storage, out-performing an optical fibre delay line by a factor of six. This is the first quantum memory to beat this important benchmark.