In this Thesis we design radiation patterns capable of creating effective light-matter interactions suited to applications in quantum computing, quantum simulation and quantum sensing. On the one hand, we have used dynamical decoupling techniques to design quantum operations that are robust against errors in environmental and control fields, achieving high-fidelity quantum logic in trapped ions and energy-efficient nuclear magnetic resonance at the nanoscale with nitrogen-vacancy centers in diamond. On the other hand, we have studied generalised models of light-matter interaction, leading to the discovery of selective multi-photon interactions in the Rabi-Stark model and a proposal for preparing non-classical quantum states using the nonlinear quantum Rabi model. Finally, we have shown how the appropriate tailoring of interactions among ultracold atoms in optical lattices could lead to solve the boson sampling problem faster than the best supercomputers, thus demonstrating quantum supremacy. In this manner, we believe the results presented here significantly expand our knowledge on the control of light-matter interactions, and provide optimal scenarios for current quantum devices to generate the next-generation of quantum applications.
In this Thesis we design radiation patterns capable of creating effective light-matter interactions suited to applications in quantum computing, quantum simulation and quantum sensing. On the one hand, we have used dynamical decoupling techniques to design quantum operations that are robust against errors in environmental and control fields, achieving high-fidelity quantum logic in trapped ions and energy-efficient nuclear magnetic resonance at the nanoscale with nitrogen-vacancy centers in diamond. On the other hand, we have studied generalised models of light-matter interaction, leading to the discovery of selective multi-photon interactions in the Rabi-Stark model and a proposal for preparing non-classical quantum states using the nonlinear quantum Rabi model. Finally, we have shown how the appropriate tailoring of interactions among ultracold atoms in optical lattices could lead to solve the boson sampling problem faster than the best supercomputers, thus demonstrating quantum supremacy. In this manner, we believe the results presented here significantly expand our knowledge on the control of light-matter interactions, and provide optimal scenarios for current quantum devices to generate the next-generation of quantum applications.
Quantum computing and quantum information processing are fast developing fields. As more and more qubits are integrated into the quantum computing systems, a deeper understanding and more careful modeling of the dynamics of these quantum systems in presence of driving and/or loss are necessary, not only for optimizing existing quantum devices, but also for designing and achieving novel method for more precise manipulation of these quantum systems. Our work investigating these quantum systems is inspired by quantum optics, a field focused on the quantum description of light and light-matter interactions. In this thesis, we use the theoretical toolboxes provided by quantum optics as well as condensed matter theory to investigate and model realistic quantum systems. Specifically, we focus on the Nitrogen-vacancy centers in diamond crystal, a Josephson parametric amplifier (that uses Josephson junctions as its non-linear elements), and plasmonic nanowire system. With the knowledge of the dynamics of these systems, we proposed single-photon heralded two-NV center quantum gates; designed and optimized the superconducting circuit for a Josephson parametric amplifier to improve the amplifier's saturation power; and designed new methods for robust light manipulation using topologically protected plasmonic modes. All the devices that we study are either ready to be implemented in experiments or have already been built. Further, as discussed in this thesis, our theoretical analysis of NV-centers can be extended to similar device types like solid-state defect centers, while our analysis of Josephson parametric amplifiers can be extended to other types of superconducting circuit systems.
Simulation of the interaction of light with matter, including at the few-photon level, is important for understanding the optical and optoelectronic properties of materials and for modeling next-generation nonlinear spectroscopies that use entangled light. At the few-photon level the quantum properties of the electromagnetic field must be accounted for with a quantized treatment of the field, and then such simulations quickly become intractable, especially if the matter subsystem must be modeled with a large number of degrees of freedom, as can be required to accurately capture many-body effects and quantum noise sources. Motivated by this we develop a quantum simulation framework for simulating such light-matter interactions on platforms with controllable bosonic degrees of freedom, such as vibrational modes in the trapped ion platform. The key innovation in our work is a scheme for simulating interactions with a continuum field using only a few discrete bosonic modes, which is enabled by a Green's function (response function) formalism. We develop the simulation approach, sketch how the simulation can be performed using trapped ions, and then illustrate the method with numerical examples. Our work expands the reach of quantum simulation to important light-matter interaction models and illustrates the advantages of extracting dynamical quantities such as response functions from quantum simulations.
This thesis offers novel strategies for the measurement of quantum correlations present in controllable quantum systems, as well as for a full-fledged implementation of the models of light-matter interaction through which these correlations can be generated. We propose the use of an ancillary qubit to efficiently access both time-correlation functions and entanglement monotones, and we provide two experimental demonstrations of our methods, measuring time correlations in an NMR setup and entanglement monotones in a photonic system. Moreover, we explain how time-correlation functions could be exploited for the quantum simulation of open quantum dynamics, and we provide an experimental recipe for the measurement of entanglement monotones in trapped ion technologies. On the other hand, we explore the quantum simulation of quantum optical models of light-matter interaction for inaccessible coupling regimes, providing experimental proposals for their implementation, both in ions and superconducting circuits. Finally, we also provide an experimental proposal for the quantum simulation of spin models in trapped ions following a digital-analog simulation scheme.
The tools of atomic physics enable researchers to explore rich quantum mechanical phenomena by way of light-matter interactions. In some applications, light is used to manipulate or modify the properties of a material medium, for instance, laser-cooling and optical trapping of neutral atoms and ions; electromagnetically-induced-transparency (EIT); and Bose- Einstein-Condensation (BEC). Likewise, there are applications where matter is used to manipulate the properties of light, as for example, non-linear optical processes; atomic quantum memories; and quantum-information-processing. This thesis explores two routes towards manipulating light using matter and vice versa in our lab. The work presented in this thesis falls under two categories, one is the experimental side where we have designed and built a laser system to cool a thermal gas of 39K atoms to sub-Doppler temperatures, using the ‘Gray Molasses’ technique. This work is motivated by the long term goal of performing quantum simulation experiments with degenerate quantum gas of K. The second category is on the theoretical (numerical) side, where we introduce a novel quantum memory protocol for coherent storage and processing of broadband light pulses, based on a fundamental physical process in atomic physics, called the ‘Autler-Townes’ effect. Ultracold quantum gases are one of the most versatile systems in the field of cold atom physics; these atomic systems can be experimentally controlled and manipulated with high degree of precision and are thus, widely used to model the behavior of other complex quantum mechanical systems. Laser cooling is the first and indeed a crucial stage towards creating an ultracold gas of atoms. In this thesis, we describe an optical setup for producing a laser-cooled and trapped gas of bosonic 39K atoms, using D1 and D2 atomic transition-lines. The optical setup is built to generate appropriate laser frequencies for implementing various stages of the experiment: red- detuned two and three-dimensional magneto-optical traps (MOT) on the D2-line; a blue-detuned gray molasses phase on the D1-line; as well as optical pumping, imaging and push beams on the D2-line. We also highlight the underlying physics behind the blue-detuned gray molasses cooling and the advantages it offers in cooling species like K and Li. We also discuss in detail, the techniques used for frequency locking our lasers, namely, saturated absorption spectroscopy (SAS) and beatnote locking. The second part of the thesis describes a new atomic quantum-memory scheme for storing broadband pulses of light, which we call the ‘Autler- Townes Splitting (ATS)’ protocol. We show the light storage and retrieval operation in three different configurations, via numerical simulation of Maxwell-Bloch equations. We also discuss the strategies for getting high storage and retrieval efficiency in the ATS memory in terms of only two phenomenological parameters, both of which are experimentally accessible. We have shown that a near-unity efficiency is achievable for a wideband signal, under moderate values of atomic density and laser powers; this significantly relaxes the technical requirements of our protocol when compared to other broadband storage schemes.
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.
In the field of quantum information, a major challenge is to protect and shield quantum systems from environmental influences, which cause dissipation and decoherence. One approach to address this problem is called dynamical decoupling, where we act on a quantum system with a series of fast and strong local control operations. If done correctly, the system's state gets rotated in discrete steps in its state space in such a way that the effects of the environment cancel up to a certain order. In the first part of this thesis, we will introduce the basics of dynamical decoupling and present a new approach for constructing dynamical decoupling schemes based on sequences of Weyl operators on networks of qudits. This approach goes beyond merely protecting a quantum system, as it is also capable of altering existing Hamiltonian interactions in ways suitable for quantum simulation purposes. We will also investigate how imperfect controls influence the effectiveness of dynamical decoupling and focus particularly on stochastic noise. In the second part of the thesis, we study concrete scenarios for the application of dynamical decoupling. First, we consider the task of quantum state transfer on a linear chain of qubits and use decoupling to protect the transfer from detrimental influences caused by a bend in the chain. We also look at how to design the specific interaction strengths required on the chain to make the state transfer work and extend this result to chains of qudits. Another chapter deals with decoupling the atomic centre-of-mass motion of a trapped atom or ion in a cavity interacting with a radiation field. Finally, we use decoupling to execute sequences of one- and two-qubit quantum gates on a chain of coupled qubits. Particular care is taken to ensure high fidelity operations in the presence of imperfect decoupling pulses.