A major challenge in the field of condensed matter physics is to harness the quantum mechanical properties of atomic systems coupled to large environments. Thermal fluctuations destroy quantum information and obstruct the development of quantum technologies such as quantum computers and memory devices. Recent advances in quantum control enable the manipulation of complex quantum states, providing new paths to preserve quantum information and to employ the environment as a resource. In this dissertation, we develop practical quantum control protocols which quickly and efficiently transfer energy to/from an environment. A major contribution of this work is the design of powerful and efficient quantum engines and refrigerators, which use the environment either to generate useful work or to freeze a system to its ground state. In achieving its core objectives, this work has also expanded on several areas of condensed matter quantum physics, including (i) the characterization of special classes of entangled system-environment states, (ii) the discovery of novel quantum chaotic phases of matter, (iii) the design of control schemes which speed-up efficient adiabatic protocols, and (iv) the development of experimentally viable control schemes in trapped ion systems, semiconductors, and nano-diamonds.
A major challenge in the field of condensed matter physics is to harness the quantum mechanical properties of atomic systems coupled to large environments. Thermal fluctuations destroy quantum information and obstruct the development of quantum technologies such as quantum computers and memory devices. Recent advances in quantum control enable the manipulation of complex quantum states, providing new paths to preserve quantum information and to employ the environment as a resource. In this dissertation, we develop practical quantum control protocols which quickly and efficiently transfer energy to/from an environment. A major contribution of this work is the design of powerful and efficient quantum engines and refrigerators, which use the environment either to generate useful work or to freeze a system to its ground state. In achieving its core objectives, this work has also expanded on several areas of condensed matter quantum physics, including (i) the characterization of special classes of entangled system-environment states, (ii) the discovery of novel quantum chaotic phases of matter, (iii) the design of control schemes which speed-up efficient adiabatic protocols, and (iv) the development of experimentally viable control schemes in trapped ion systems, semiconductors, and nano-diamonds.
A major barrier for the development of new quantum technologies is fast and robust methods for preparing and manipulating complex quantum states. To date preparation and manipulation of quantum systems has primarily been done using adiabatic processes. These adiabatic processes while robust have significant disadvantages such as their requiring long process times, a further drawback is that the system-environment interaction over these these long process times can lead to loss of coherence of the state of the system. A new area of study to achieve the robustness of adiabatic schemes but in time-scales much shorter is "Shortcuts to Adiabaticity" (STA). These STA techniques provide new methods for the manipulation of quantum systems that achieve high-fidelity state transfers in much shorter times. The aim of this thesis is to use STA and related methods for many particle systems. A method for manipulating a gas of fermions is developed using Pauli blocking and it’s robustness in the presence of temperature and different particle number investigated. A novel method for trapping and cooling particles using atom-diode and reflecting atomic mirror is investigated, both the classical and quantum cases are simulated. A method for transporting atoms and condensates across an optical lattice using invariant based inverse engineering is developed. Finally we look at non-hermitian potentials and design particular potentials that result in desirable asymmetries in the transmission and reflection of incident particles.
Quantum mechanics has both deepened our understanding of the physical reality and allowed development of quantum technologies beating their classical counterparts in efficiency, security, and sensitivity. As a major downside, the quantum properties behind these technologies are very sensitive to the effects the system’s interaction with its environment. The theory of open quantum systems describes how such interaction influences the system, and as a consequence, how the quantum features are lost, controlled, and possibly returned by manipulating the environment. This thesis begins with a short description of the most relevant features and results of the open quantum system theory. We continue by presenting some commonly used measures to quantify the information encoded in quantum systems and the correlations between spatially or temporally separated parties. The literature review ends with discussion of non-Markovianity and memory effects which combine the fields of open quantum systems and quantum information in a natural way. The rest of the thesis presents the main results of Publications I – VII. We study the memory effects and physicality conditions of open system dynamics described by the vast family of phase-covariant master equations. We show how the open quantum system picture can be exploited to design probing protocols to extract information without knowing how the probe and system interact. Such protocols are considered also from the point-of view of efficiently obtaining the information required in the probing, both in theory and by experimental implementation. The experiment serves also as evidence for applicability of recently developed geometric tools to design informationally incomplete measurements for solving so-called membership problems. We present and experimentally realize a protocol where control over intial correlations between polarization and frequency gives full freedom to implement any dephasing dynamics in the open system polarization, thus dictating how its information content changes in time. We study the two-qubit memory effects of dephasing dynamics and compare them to the effect of such noise in the transmission efficiency in superdense coding. Optimal noise configurations in different situations are found. Finally, we characterize the families of initial states for which the decay of quantum correlations depends or does not depend on the noise location.
Quantum computation is an exciting and rapidly developing paradigm of computing, which has the potential to improve or accelerate numerous important applications ranging from cryptography to drug discovery. Inspired by these prospects, researchers and engineers across the world have, in recent years, made tremendous advances in building programmable quantum devices. Nevertheless, the size and error rates in state-of-the-art quantum computers are still several orders of magnitude away from those required to achieve computational speedups using existing quantum algorithms. These developments, along with their limitations, motivate us to consider two important questions: First, are there any useful applications in which current or near-term quantum devices can outperform classical computers? Second, can we develop efficient error-correction schemes to suppress the error rates in near-term quantum devices? In this dissertation, we present novel solutions to both of these questions. To address the first question, we identify an important class of problems in condensed-matter physics, namely the identification and characterization of quantum phases of matter, as one promising application of current and near-term quantum information processors. We develop new algorithms for using such quantum devices to solve these problems, and we demonstrate how our methods substantially outperform existing approaches. In addition, we respond to the second question by developing hardware-efficient quantum error correction protocols. By leveraging the distinctive properties and advantages of a given experimental setup to overcome its particular limitations, our proposals significantly reduce the number of quantum bits and operations required for performing quantum error correction and fault-tolerant quantum computation. Together, the works in this dissertation thus pave the way for the near-term realization of quantum computational advantages.
Quantum communication and quantum computation are anticipated quantum technologies which enable us to tackle problems which are hard or even impossible to solve by classical means. A major challenge in implementing real life applications for quantum communication and quantum computation is to implement efficient interfaces between different kinds of quantum systems which serve as carriers of quantum information, such as single photons and single material quantum systems (i.e. atoms, ions ...). These different kinds of quantum systems have different advantages and disadvantages if it comes to practical applications. Single photons, which may serve as flying qubits, can be transmitted over larger distances without suffering from too much decoherence while single material quantum systems may be used as matter qubits to store and process quantum information locally. For many applications, the particular advantages of both kinds of systems are needed. Hence, we require interfaces between flying qubits and matter qubits. In the first part of this thesis, we develop suitable protocols for implementing interfaces between these flying qubits and matter qubits. The key challenge is to design the interactions to allow the efficient coupling of single photons to single material quantum systems, such as atoms and ions. This is not only of interest for possible practical applications of quantum mechanics but also drives fundamental research, as it is directly connected to a precise control and enhancement of matter-field interactions. The second part of this thesis is concerned with the applications of quantum technologies for simulating complex quantum systems, which cannot be treated by classical means. In many cases, the simulation of complex quantum systems turns out to be a difficult task due to the basic structure of quantum mechanics. The same aspects of quantum mechanics, which allow us to tackle computationally hard problems by using quantum algorithms, create difficulties in understanding the behavior of complex quantum systems. However, the understanding of complex quantum systems is not only of fundamental importance but might also enable us to give answers to problems of practical significance, such as an explanation for high-temperature superconductivity. Here, we develop protocols for implementing such quantum simulations based on ions trapped in state of the art ion traps. The applications of these kinds of quantum simulations are ranging from fundamental research to process design, control, and optimization.
Recent advances in coherent manipulation and control have led to new opportunities to study non-equilibrium dynamics in quantum many-body systems. Using a variety of experimental platforms ranging from cold atoms and ions to solid-state spin defects, small building blocks of fully controllable quantum systems can be assembled into a strongly interacting many-body system. With this approach, one can gain new insights into fundamental questions in basic science such as the mechanism of quantum thermalization or the existence of exotic phases of matter out of equilibrium. At the same time, a fully controllable large-scale quantum device has promising applications for high precision sensing, secure communication, and quantum information processing.\nThis thesis describes recent progress in engineering quantum dynamics that is robust against various imperfections such as environmental noise, disorder, or imprecise controls. We find that non-equilibrium quantum states of strongly interacting systems can be protected from imperfections upon appropriate external manipulations such as time-dependent periodic controls, engineered coherent or dissipative interactions, or an introduction of strong quenched disorder. Furthermore, under certain conditions, these imperfections can be utilized to stabilize non-trivial quantum states. Our results challenge conventional wisdom of statistical mechanics, shedding light on the microscopic mechanisms of thermalization.\nWe also discuss how engineered dynamics can be harnessed for applications such as quantum metrology, simulations, or information processing.
In the past decades, rapid advances in the experimental control of quantum systems have opened up unparalleled capabilities of engineering exotic quantum states. Nowadays, we may consider ourselves witnesses of a second quantum revolution as strongly correlated quantum matter is regularly created, on a daily basis, within the most diverse platforms, e.g. arrays of Rydberg atoms, ultra-cold atoms in optical lattices, superconducting qubits, trapped ions, and quantum dots. In this era, dubbed Noisy-Intermediate Scale Quantum (NISQ) era, the effort in pursuing research for realizing quantum technologies with practical purposes, such as quantum computing, simulations, communication and metrology, has greatly accelerated, and we are just starting to experience the immense progress that could be achieved. The remarkable breakthrough we are facing builds upon seminal theoretical and experimental advances in quantum physics. The key achievements in atomic, molecular, and optical (AMO) physics have opened up the possibility of controlling, trapping, and measuring single atoms, one by one, with high accuracy and reliability. In parallel, from a theoretical point of view, the study of quantum entanglement and correlations has bridged AMO physics and quantum information with crucial proposals for the realization of universal quantum computers. In this context, entanglement has emerged as one of the key tools to characterize and to exploit quantum many-body systems for quantum information purposes. We will study quantum entanglement in several scenarios to investigate and probe complex quantum many-body systems. We will consider examples ranging from generic mixed states in equilibrium to out-of-equilibrium dynamics, with and without dissipation, and topologically non-trivial systems. The leitmotif of this work will be how entanglement and correlations can be exploited to characterize the many-body quantum state describing a physical system and how entanglement can be detected in an experimentally efficient manner.