Emerging unitary evolutions in dissipatively coupled systems

Having a broad range of methods available for implementing unitary operations is crucial for quantum information tasks. We study a dissipative process commonly used to describe dissipatively coupled systems and show that the process can lead to pure unitary dynamics on one part of a bipartite system, provided that the process is strong enough. As a consequence of these findings, we discuss within the framework of quantum control theory how the dissipative process can enable universal control of the considered part, thereby turning parts of the system into a system capable of universal quantum information tasks. We characterize the time scales necessary to implement gates with high fidelity through the dissipative evolution. The considered dissipative evolution is of particular importance since it can be engineered in the laboratory in the realm of superconducting circuits. Based on a reservoir that is formed by a lossy microwave mode we present a detailed study of how our theoretical findings can be realized in an experimental setting

Nonreciprocal Photon Transmission and Amplification via Reservoir Engineering

We discuss a general method for constructing nonreciprocal, cavity-based photonic devices, based on matching a given coherent interaction with its corresponding dissipative counterpart; our method generalizes the basic structure used in the theory of cascaded quantum systems, and can render an extremely wide class of interactions directional. In contrast to standard interference-based schemes, our approach allows directional behavior over a wide bandwidth. We show how it can be used to devise isolators and directional, quantum-limited amplifiers. We discuss in detail how this general method allows the construction of a directional, noise-free phase-sensitive amplifier that is not limited by any fundamental gain-bandwidth constraint. Our approach is particularly well-suited to implementations using superconducting microwave circuits and optomechanical systems.Comment: 15 pages, 6 figure

Parametric Couplings in Engineered Quantum Systems

Parametric couplings in engineered quantum systems are a powerful tool to control, manipulate and enhance interactions in a variety of platforms. It allows us to bring systems of different energy scales into communication with each other. This short chapter introduces the basic principles and discusses a few examples of how one can engineer parametric amplifiers with improved characteristics over conventional setups. Clearly, the selected examples are author-biased, and other interesting proposals and implementations can be found in the literature. The focus of this chapter is on parametric effects between linearly coupled harmonic oscillators, however, parametric modulation is also applicable with nonlinear couplings and anharmonic systems.Comment: Submitted to SciPost Lecture Notes. To appear in 'Quantum Information Machines; Lecture Notes of the Les Houches Summer School 2019', eds. M. Devoret, B. Huard, and I. Po

Are Bank Loan Loss Reservers Materiallity Misstated

A bank\u27s estimated credit losses are reflected by its reserve for loan loss account, generally known as the Allowance for Loan and Lease Losses (ALLL). 1 In simple terms, the ALLL is an estimate of the value of loans that probably will not be collected. It has been noted that the estimation of credit loss is often the most difficult element in the preparation of a bank\u27s financial statements. Part of the problem lies in the subjective nature of the loss estimate, but a greater difficulty arises from the conflicting demands of regulatory agencies and the accounting profession. The subject is not simply a mundane accounting issue; the accuracy and sufficiency of the loss reserve is a major factor in gauging the health and survivability of a bank. Unfortunately the terms accuracy and sufficiency appear to be mutually exclusive when assessing the conflicting demands of bank management, the accountants, and bank regulators This project recounts the history and purpose of the ALLL account; examines the divergent objectives of the banking industry, the accounting profession, and the banking regulators; evaluates the conflicting guidance issued by the accounting profession and bank regulators; and analyzes financial data of American banks from 1992 through 2005 to assess whether the ALLL account is an accurate reflection of its intended purpose underGAA

Minimal Models for Nonreciprocal Amplification Using Biharmonic Drives

We present a generic system of three bosonic modes coupled parametrically with a time-varying coupling modulated by a combination of two pump harmonics, and we show how this system provides the minimal platform for realizing nonreciprocal couplings that can lead to gainless photon circulation, and phase-preserving or phase-sensitive directional amplification. Explicit frequency-dependent calculations within this minimal paradigm highlight the separation of amplification and directionality bandwidths, a feature generic to such schemes. We also study the influence of counterrotating interactions that can adversely affect directionality and the associated bandwidth; we find that these effects can be mitigated by suitably designing the properties of the auxiliary mode that plays the role of an engineered reservoir to the amplification mode space.University of Massachusetts at Lowel