Quantum Control in Open and Periodically Driven Systems

Quantum technology resorts to efficient utilization of quantum resources to realize technique innovation. The systems are controlled such that their states follow the desired manners to realize different quantum protocols. However, the decoherence caused by the system-environment interactions causes the states deviating from the desired manners. How to protect quantum resources under the coexistence of active control and passive decoherence is of significance. Recent studies have revealed that the decoherence is determined by the feature of the system-environment energy spectrum: Accompanying the formation of bound states in the energy spectrum, the decoherence can be suppressed. It supplies a guideline to control decoherence. Such idea can be generalized to systems under periodic driving. By virtue of manipulating Floquet bound states in the quasienergy spectrum, coherent control via periodic driving dubbed as Floquet engineering has become a versatile tool not only in controlling decoherence, but also in artificially synthesizing exotic topological phases. We will review the progress on quantum control in open and periodically driven systems. Special attention will be paid to the distinguished role played by the bound states and their controllability via periodic driving in suppressing decoherence and generating novel topological phases.Comment: A review articl

Quantum Measurement and Bath Engineering for Superconducting Qubits via Multiple Parametric Couplings

Quantum computers have huge potential applications, but do not currently exist. It has already been proven that a quantum computer would outperform the best classical supercomputers in certain problems, some of which have vital connections with our daily lives. For example, quantum computers efficiently solve the prime number factoring problem, which in turn is the foundation of the RSA algorithm behind most online transactions. There is a great deal of current effort to implement quantum computers, and we have seen good progress in platforms including superconducting circuits, ion traps, and photons in cavity QED systems and spins in semiconductors. These machines include up to roughly 50 quantum bits at present, but they are not very useful as quantum errors quickly decohere the computer's state and prevent computation. These errors can be mitigated via quantum error correction at the cost of additional size and complexity. Progress in the field towards error corrected, large-scale quantum machines requires us to require new tools for controlling, coupling, and reading out qubits. In this thesis, I will focus on such explorations in superconducting circuits. In this thesis, we seek to expand the already flexible toolkit of quantum circuits by exploring the uses of parametric couplings based on third-order nonlinearities. This type of nonlinearities has only been used in quantum-limited amplifiers before, here we try to further explore their applications by creating new methods for controlling and measuring qubits that based on it. In the first experiment, we address the problem of implementing a highly efficient quantum non-demolition qubit readout. With the use of two-mode squeezed (TMS) light and combined with phase-preserving parametric amplifiers into an interferometer for dispersive qubit readout, we demonstrate a measurement scheme with a 44$\%$ improvement in power signal-to-noise ratio. We also investigate the back-action of the measurement scheme. In the second experiment, we create an effective chemical potential for photons with parametric system-bath coupling. In particular, we use a lossy Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL) as both the bath and coupler. The bath engineering is realized by combining the multiple parametric drives and the dissipation together

Quantum magnonics: when magnon spintronics meets quantum information science

Spintronics and quantum information science are two promising candidates for innovating information processing technologies. The combination of these two fields enables us to build solid-state platforms for studying quantum phenomena and for realizing multi-functional quantum tasks. For a long time, however, the intersection of these two fields was limited. This situation has changed significantly over the last few years because of the remarkable progress in coding and processing information using magnons. On the other hand, significant advances in understanding the entanglement of quasi-particles and in designing high-quality qubits and photonic cavities for quantum information processing provide physical platforms to integrate magnons with quantum systems. From these endeavours, the highly interdisciplinary field of quantum magnonics emerges, which combines spintronics, quantum optics and quantum information science.Here, we give an overview of the recent developments concerning the quantum states of magnons and their hybridization with mature quantum platforms. First, we review the basic concepts of magnons and quantum entanglement and discuss the generation and manipulation of quantum states of magnons, such as single-magnon states, squeezed states and quantum many-body states including Bose-Einstein condensation and the resulting spin superfluidity. We discuss how magnonic systems can be integrated and entangled with quantum platforms including cavity photons, superconducting qubits, nitrogen-vacancy centers, and phonons for coherent information transfer and collaborative information processing. The implications of these hybrid quantum systems for non-Hermitian physics and parity-time symmetry are highlighted, together with applications in quantum memories and high-precision measurements. Finally, we present an outlook on the opportunities in quantum magnonics.Comment: 93 pages, 35 figures, Physics Reports (in press

Towards optical quantum information processing using Rydberg dark-state polaritons

This thesis proposes a novel method to implement universal quantum gates for photonic qubits using the strong dipole-dipole interactions present in a cold gas of Rydberg atoms and the control offered by microwave fields. By means of electromagnetically induced transparency (EIT) we store the information encoded in photonic qubits as Rydberg excitations, and then couple these to neighbouring states using microwaves. Microwaves alter the range of the dipole-dipole interactions between the excitations, and a suitable geometrical arrangement of the excitations in the cloud leads to a controlled $\pi$ phase shift in the system's wavefunction, the basis of the universal gates proposed. After processing, the excitations in the medium are later retrieved as photons. A theoretical description of the implementation of a 2-qubit universal gate is presented and a numerical analysis shows the feasibility of its implementation in a cold cloud of Rubidium atoms. A scheme is also proposed to construct more general gates with applications in quantum information processing. These schemes have been made possible by the analysis of recent experiments performed in the group. This analysis is repeated here, along with the characterization of parts of the detection system required to obtain them

Colloquium: Quantum Batteries

Recent years have witnessed an explosion of interest in quantum devices for the production, storage, and transfer of energy. In this Colloquium, we concentrate on the field of quantum energy storage by reviewing recent theoretical and experimental progress in quantum batteries. We first provide a theoretical background discussing the advantages that quantum batteries offer with respect to their classical analogues. We then review the existing quantum many-body battery models and present a thorough discussion of important issues related to their open nature. We finally conclude by discussing promising experimental implementations, preliminary results available in the literature, and perspectives.Comment: 36 pages, 12 figures, 311 references. Review and perspective article on quantum batteries. Commissioned for Reviews of Modern Physics. Comments and feedback are welcom

Diverse applications of the Quantum Walk model in Quantum Information: a theoretical and experimental analysis in the optical framework

Quantum Walks have been a very important model in the last thirty years, after their first definition and rigorous description. The analysis of the many possible variations of their behavior has delivered a plethora of solutions and platforms for the many diverse fields of investigation. The applications of the Quantum Walk model spreads from the development of Quantum Algorithm, to the modeling and simulation of systems of the most diverse nature, such as solid state or biological systems. In general, it helped developing a well-established quantum (or coherent) propagation model, which is useful both inside and outside physics. In this thesis, we focus on the study of disordered Quantum Walks, in order to get better understanding of the inuence of Quantum Walk disordered dynamics to non-classical correlations and propagating quantum information. Afterwards, we generalize this dynamical approach to Quantum Information processing, developing a Quantum Receiver for Quantum State Discrimination featuring a time multiplexing structure and we investigate the potentiality of this Quantum Walk inspired framework in the field of Quantum State Discrimination, through the developing and realization of experimental protocols characterized by increasing complexity. We also report on some apparent deviations from this path, although still aimed at the transfer of our expertise, built in previous investigations, to the study of new models and more complex quantum systems

Coherent and dissipative dynamics at quantum phase transitions

The many-body physics at quantum phase transitions shows a subtle interplay between quantum and thermal fluctuations, emerging in the low-temperature limit. In this review, we first give a pedagogical introduction to the equilibrium behavior of systems in that context, whose scaling framework is essentially developed by exploiting the quantum-to-classical mapping and the renormalization-group theory of critical phenomena at continuous phase transitions. Then we specialize to protocols entailing the out-of-equilibrium quantum dynamics, such as instantaneous quenches and slow passages across quantum transitions. These are mostly discussed within dynamic scaling frameworks, obtained by appropriately extending the equilibrium scaling laws. We review phenomena at first-order quantum transitions as well, whose peculiar scaling behaviors are characterized by an extreme sensitivity to the boundary conditions, giving rise to exponentials or power laws for the same bulk system. In the last part, we cover aspects related to the effects of dissipative interactions with an environment, through suitable generalizations of the dynamic scaling at quantum transitions. The presentation is limited to issues related to, and controlled by, the quantum transition developed by closed many-body systems, treating the dissipation as a perturbation of the critical regimes, as for the temperature at the zero-temperature quantum transition. We focus on the physical conditions giving rise to a nontrivial interplay between critical modes and various dissipative mechanisms, generally realized when the involved mechanism excites only the low-energy modes of the quantum transitions.Comment: Review paper, 138 pages. Final version to appear in Physics Report

Quantum walks: a comprehensive review

Quantum walks, the quantum mechanical counterpart of classical random walks, is an advanced tool for building quantum algorithms that has been recently shown to constitute a universal model of quantum computation. Quantum walks is now a solid field of research of quantum computation full of exciting open problems for physicists, computer scientists, mathematicians and engineers. In this paper we review theoretical advances on the foundations of both discrete- and continuous-time quantum walks, together with the role that randomness plays in quantum walks, the connections between the mathematical models of coined discrete quantum walks and continuous quantum walks, the quantumness of quantum walks, a summary of papers published on discrete quantum walks and entanglement as well as a succinct review of experimental proposals and realizations of discrete-time quantum walks. Furthermore, we have reviewed several algorithms based on both discrete- and continuous-time quantum walks as well as a most important result: the computational universality of both continuous- and discrete- time quantum walks.Comment: Paper accepted for publication in Quantum Information Processing Journa