A principal goal of distributed quantum processing is the ability to generate, manipulate, and transfer quantum states between distant nodes of a quantum network. These protocols generally require connecting photonic and material carriers of quantum information. In this thesis, I present investigations of two experimental realizations of light-matter interfaces that allow for engineered atom-photon interactions in free-space settings. First, we utilize reconfigurable arrays of trapped single atoms to study light scattering in lowdimensional systems. We observe noncollinear phase-matching geometries that have suppressed sensitivity to particle localization. We show that the scattered radiation can be controllably enhanced or diminished as a result of Bragg interference. Such scattering can be used for mapping collective states within an array of neutral atoms onto propagating light fields and for establishing quantum links between separated arrays. Second, we utilize ensembles of trapped Rydberg atoms to study collective many-body phenomena that arise due to strong dipole-dipole interactions. To do so, we employ a magic-wavelength optical lattice that allows for the simultaneous trapping of both ground and Rydberg levels. Using the enhanced coherence times enabled by this trapping scheme, we measure the so-called magic lattice detunings and use them to extract the 6P3/2 − nS1/2 reduced electric dipole matrix elements. Furthermore, we perform precision measurements of differential nuclear-spin dependent light shifts in the Paschen-Back regime in order to determine the hyperfine splitting of Rydberg levels. We create a quasi-two-level system in a regime of Rydberg excitation blockade for a mesoscopic ensemble of several hundred atoms. Using this system, we study Hanbury Brown-Twiss interference between the field radiated by the atoms and an input probe field with a controllable relative phase. Finally, we demonstrate coherent driving and Ramsey interference measurements of light shifts, with timescales on the order of ≃ 10 µs. Whereas the coupling producing the Rabi oscillations is enhanced, there is no corresponding enhancement for the lightshifts. These results may prove useful in applying collective qubits with Rydberg interactions to scalable quantum networking architectures.
A principal goal of distributed quantum processing is the ability to generate, manipulate, and transfer quantum states between distant nodes of a quantum network. These protocols generally require connecting photonic and material carriers of quantum information. In this thesis, I present investigations of two experimental realizations of light-matter interfaces that allow for engineered atom-photon interactions in free-space settings. First, we utilize reconfigurable arrays of trapped single atoms to study light scattering in lowdimensional systems. We observe noncollinear phase-matching geometries that have suppressed sensitivity to particle localization. We show that the scattered radiation can be controllably enhanced or diminished as a result of Bragg interference. Such scattering can be used for mapping collective states within an array of neutral atoms onto propagating light fields and for establishing quantum links between separated arrays. Second, we utilize ensembles of trapped Rydberg atoms to study collective many-body phenomena that arise due to strong dipole-dipole interactions. To do so, we employ a magic-wavelength optical lattice that allows for the simultaneous trapping of both ground and Rydberg levels. Using the enhanced coherence times enabled by this trapping scheme, we measure the so-called magic lattice detunings and use them to extract the 6P3/2 − nS1/2 reduced electric dipole matrix elements. Furthermore, we perform precision measurements of differential nuclear-spin dependent light shifts in the Paschen-Back regime in order to determine the hyperfine splitting of Rydberg levels. We create a quasi-two-level system in a regime of Rydberg excitation blockade for a mesoscopic ensemble of several hundred atoms. Using this system, we study Hanbury Brown-Twiss interference between the field radiated by the atoms and an input probe field with a controllable relative phase. Finally, we demonstrate coherent driving and Ramsey interference measurements of light shifts, with timescales on the order of ≃ 10 µs. Whereas the coupling producing the Rabi oscillations is enhanced, there is no corresponding enhancement for the lightshifts. These results may prove useful in applying collective qubits with Rydberg interactions to scalable quantum networking architectures.
A principal goal of distributed quantum processing is the ability to generate, manipulate, and transfer quantum states between distant nodes of a quantum network. These protocols generally require connecting photonic and material carriers of quantum information. Here we present two experimental realizations of light-matter interfaces that allow for engineered atom-photon interactions in free-space settings. First, we utilize reconfigurable arrays of trapped single atoms to study light scattering in low-dimensional systems. We observe noncollinear phase-matching geometries that have suppressed sensitivity to particle localization. We show that the scattered radiation can be controllably enhanced or diminished as a result of Bragg interference. Such scattering can be used for mapping collective states within an array of neutral atoms onto propagating light fields and for establishing quantum links between separated arrays. Second, we create a quasi-two-level system in a regime of Rydberg excitation blockade for a mesoscopic ensemble of several hundred atoms confined in a magic-wavelength optical lattice. We demonstrate coherent driving and Ramsey interference measurements of light shifts, with timescales on the order of ∼ 10 μs. Whereas the coupling producing the Rabi oscillations is enhanced by a factor of √N, there is no corresponding enhancement for the light shifts. These results may prove useful in applying collective qubits with Rydberg interactions to scalable quantum networking architectures.
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.
Rydberg ensembles have many diverse and important applications for quantum information and quantum optics. For example, Rydberg ensembles can generate non-classical states of light which would be useful in a quantum network, and they could be used as qubits in a future quantum computer. For this reason, this thesis will present investigations of light-matter interactions in Rydberg ensembles. Non-classical states of light, such as single-photon states, are important for many quantum communication protocols. We have performed an experiment that demonstrates the generation of single-photon states from a Rydberg ensemble. We then studied the second-order correlations between this single-photon state of light and an incident coherent field. Under these conditions, we observed Hanbury Brown Twiss interference between the emission from a driven super-atom and a coherent field in the absence of stimulated emission. For a universal quantum computing architecture, coherence times must be much longer than gate operations. We observe ground-Rydberg coherence times in excess of 20 microseconds by using a "magic-wavelength" optical lattice to confine the atoms. Using this coherence time, we measured the differential nuclear-spin-dependent light shifts for principal quantum numbers, n, between n = 30 and n = 65, which is relevant for future high-fidelity Rydberg qubits in optical potentials. Also, we measured the hyperfine constant for atomic Rb to be 35.71 GHz with an uncertainty of 0.18 GHz.
The field of quantum computation and communication has prospered over the last few decades because of multiple advances in our understanding of trapping and controlling physical quantum systems like neutral atoms, superconducting qubits, trapped ions, quantum dots etc. using light. A lot of research has gone into development of techniques which facilitate trapping and manipulation of ensembles of neutral atoms by studying how the atomic properties of the system respond to properties of light used to control them. In this dissertation we shall explore some applications of light-matter interactions in ensembles of lambda three level neutral atoms for the purpose of entanglement generation and distribution. The first part of this dissertation focuses on the study of a protocol that can be used to generate multi-particle entangled quantum states called the Greenberger-Horne-Zeilinger (GHZ) states in an ensemble of N neutral atoms. Schemes for creation of N particle entangled Greenberger-Horne-Zeilinger (GHZ) states are important for understanding multi-particle non-classical correlations. A theoretical protocol for creation of a multi-particle GHZ state implemented on a target ensemble of N, three-level Rydberg atoms and a single Rydberg atom as a control using Stimulated Raman Adiabatic Passage (STIRAP) is presented. We work in the Rydberg blockade regime for the ensemble atoms induced due to excitation of the control atom to a high lying Rydberg level. It is shown that using STIRAP, atoms from one ground state of the ensemble can be adiabatically transferred with high fidelity to the other multi-particle ground state, depending on the state of the control atom. Measurement of the control atom in a specific basis after this conditional transfer facilitates one-step creation of a N particle GHZ state. A thorough analysis of adiabatic conditions associated with STIRAP for this scheme and the influence of radiative decay from the excited Rydberg levels is presented. The most important and novel feature of this scheme is that it is immune to the decay rate of the excited level in ensemble atoms and provides a robust way of creating GHZ states. In the second part of this dissertation, we study atomic ensemble based quantum interfaces used in quantum repeater protocols for entanglement distribution. Quantum interfaces provide a platform where in the flying photonic qubits used for information and entanglement transfer can interact with a physical system which stores, processes and releases this information back as photons. The Duan-Lukin-Cirac-Zoller (DLCZ) quantum repeater protocol, which was proposed to realize long distance quantum communication, requires usage of quantum memories or quantum interfaces. Atomic ensembles interacting with optical beams based on off-resonant Raman scattering serve as convenient on-demand quantum memories. Here a complete, free space, three-dimensional theory of the associated read and write process for this quantum memory is worked out with the aim of understanding intrinsic retrieval efficiency. We develop a formalism to calculate the transverse mode structure for the signal and the idler photons and use the formalism to study the intrinsic retrieval efficiency under various configurations. The effects of atomic density fluctuations and atomic motion are incorporated by numerically simulating this system for a range of realistic experimental parameters. Results describe the variation in the intrinsic retrieval efficiency as a function of the memory storage time for skewed beam configuration at a finite temperature, which provides valuable information for optimization of the retrieval efficiency in experiments.
Advances in the coherent control of Rydberg atoms have led to an explosion in their use in a wide range of topics, including quantum computation, simulation, sensing, and networking. More specifically, Rydberg-coupled ensembles of atoms interfaced with a single mode of light have enabled studies of fundamental light-matter interactions at the single-photon level and have emerged as a promising platform to realize scalable quantum networking nodes. In the first part of this thesis, we utilize Rydberg blockade to generate large ensembles that host a single Rydberg excitation and use the collective state as a resource to efficiently generate single photons. We further investigate the shelving of the collective Rydberg excitations to the ground-state manifold, where we observe an order of magnitude increase in the coherence time of the excitation. In the latter half of the thesis, we use near-resonant microwave fields to generate microwave-dressed Rydberg states, where we utilize the degrees of freedom of the microwave field to tune the strength, form, and angular dependence of the dipole-dipole interactions. To this end, we develop a robust and highly tunable microwave polarization control system. We use a set of in-vacuum electrodes as microwave antennas. We measure the microwave polarizations produced by each source at the positions of the atoms using a Rydberg EIT-based polarimetry technique. By characterizing the three imperfect sources, we find the control settings necessary to produce \sigma_-, \pi, and \sigma_+ polarized microwaves with >99 % fidelity. We extend our purification techniques to frequencies away from Rydberg resonances by utilizing an auxiliary microwave field, generating two-photon microwave resonances. The high-fidelity microwave polarizations are used to resonantly admix opposite-parity Rydberg states, generating dressed states with dipole-dipole interactions that are longer range and stronger than the bare van der Waals interactions. To characterize the strength of the interactions, we measure the photon-number statistics of the light generated by the ensemble of atoms. By varying the cloud length relative to the blockade radii, we demonstrate a clear enhancement of the interaction strength due to microwave dressing. Our platform enables further engineering of interactions by exploiting the polarization and frequency degrees of freedom of the microwave fields, opening pathways for new quantum control strategies in many different experimental platforms.
Scalable quantum computers and quantum networks require the combination of quantum processing nodes with efficient light-matter interfaces to distribute quantum information in local or long-distance quantum networks. Neutral-atom arrays have both been coupled to Rydberg states to enable high-fidelity quantum gates in universal processing architectures, and to optical cavities to realize interfaces to photons. However, combining these two capabilities and coupling atom arrays to highly excited Rydberg states in the mode of an optical cavity has been an outstanding challenge. Here we present a novel cavity-coupled Rydberg array that achieves this long-standing goal. We prepare, detect, and control individual atoms in a scalable optical tweezer array, couple them strongly to the optical mode of a high-finesse optical cavity and excite them in a controlled way to Rydberg states. We show that high-cooperativity coupling to an optical cavity -- demonstrated via the dispersive shift of the resonance of the cavity in presence of the atoms -- and strong Rydberg interactions -- demonstrated via the collective enhancement of Rydberg coupling in the atomic array -- can be achieved in our setup at the same spatial location. Our presented experimental platform opens the path to several new directions, including the realization of quantum network nodes, quantum simulation of long-range interacting, open quantum systems and photonic-state engineering leveraging high-fidelity Rydberg control.