We consider an infinite waveguide supporting a single propagating mode for which the excitation (say the electric or the magnetic field) is assigned in a given section assumed as the origin of the coordinate z along its axis. In steady state the state evolution along z is the solution of the spatial Cauchy problem along such coordinate. As soon as the radiation involves a low number of photons, or even reduces to a single one, the classical treatment must be replaced by a quantum one. Moreover the effect of dissipation must be taken into account by either a microscopic treatment of the properties of the dielectric material and of the metallic boundaries, or, as long as only the mathematical form of the equations be of interest, by using a spatial version of Lindblad equations. We choose here the second alternative. From the obtained equations it is possible to extend the treatment to multimode finite terminated waveguides.
We consider an infinite waveguide supporting a single propagating mode for which the excitation (say the electric or the magnetic field) is assigned in a given section assumed as the origin of the coordinate z along its axis. In steady state the state evolution along z is the solution of the spatial Cauchy problem along such coordinate. As soon as the radiation involves a low number of photons, or even reduces to a single one, the classical treatment must be replaced by a quantum one. Moreover the effect of dissipation must be taken into account by either a microscopic treatment of the properties of the dielectric material and of the metallic boundaries, or, as long as only the mathematical form of the equations be of interest, by using a spatial version of Lindblad equations. We choose here the second alternative. From the obtained equations it is possible to extend the treatment to multimode finite terminated waveguides.
Light-matter interaction at the single-quantum level is the heart of many regimes of high fundamental importance to modern quantum technologies. Strong interaction of a qubit with a single photon of an electromagnetic field mode is described by the cavity/circuit electrodynamics (QED) regime which is one of the most advanced platforms for quantum computing. The opposite regime of the waveguide QED, where qubits interact with a continuum of modes in an infinite one-dimensional space, is also at the focus of recent research revealing novel quantum phenomena. Despite the demonstration of several key features of waveguide QED, the transition from an experimentally realizable finite-size system to the theoretically assumed infinite device size is neither rigorously justified nor fully understood. In this paper, we formulate a unifying theory which under a minimal set of standard approximations accounts for physical boundaries of a system in all parameter domains. Considering two qubits in a rectangular waveguide which naturally exhibits a low frequency cutoff we are able to account for infinite number of modes and obtain an accurate description of the waveguide transmission, a life-time of a qubit-photon bound state and the exchange interaction between two qubit-photon bounds states. For verification, we compare our theory to experimental data obtained for two superconducting qubits in a rectangular waveguide demonstrating how the infinite size limit of waveguide QED emerges in a finite-size system. Our theory can be straightforwardly extended to other waveguides such as the photonic crystal and coupled cavity arrays.
We introduce a picture to describe and interpret waveguide-QED problems in the non-Markovian regime of long photonic retardation times resulting in delayed coherent feedback. The framework is based on an intuitive spatial decomposition of the waveguide into blocks. Among these, the block directly coupled to the atoms embodies an effective lossy multimode cavity leaking into the rest of the waveguide, in turn embodying an effective white-noise bath. The dynamics can be approximated by retaining only a finite number of cavity modes, which grows with the time delay. This description captures the atomic as well as the field’s dynamics, even with many excitations, in both emission and scattering processes. As an application, we show that the recently identified non-Markovian steady states can be understood by retaining very few or even only one cavity mode.
The aim of this work is to study how losses modify quantum effects in a system governed by the spatial nonlinear Schrodinger equation. Quantum effects in space, such as quantum noise, squeezing, and bunching, have been studied in a lossless case.1 We will consider the propagation of a field through a planar nonlinear waveguide,2,3 where confinement is provided in one transverse dimension (y direction) by a linear refractive index and in the orthogonal dimension (x direction) by the intensity dependence of the refractive index.1,4 Attenuation is given to the spatial distribution in the x-z plane. In this work we want to extend the dissipation approach described in Ref. 5 to a system governed by the spatiai-nonlinear Sclirodinger equation (SNLS). We use a quantum-field description1 in which the spatial variable of propagation z plays the role of the time in the standard quantum theory, and we apply it to the case of a strong plane wave that can be described as a classical field propagating through a nonlinear waveguide in the z direction.4 By using both paraxial and slowly-varying-envelope approximations, the propagation of the field in a planar waveguide is described by the NLS.1
In this paper, we explore quantum control systems tuned by spatially varying functions, which are different from the broadly studied quantum control systems with time-dependent controls. Such control problems exist in the design of quantum waveguides and materials (e.g., fibers, photonic crystals, and metamaterials) and have not been considered from a control theoretic point of view. The spatial quantum control model is built from quantized Maxwell equations for dielectric waveguides, where the spatially varying dielectric parameters are taken as controls. The model can be applied to both passive and active dielectrics or their mixtures such as metamaterials. We derive the iterative equation for slab structures, which is analogous to a discrete-time control system. For homogeneous waveguides, the model is parallel with the well-known Langevin equation in the time domain. However, for inhomogeneous and dispersive waveguides, the control model becomes bilinear even for linear dielectric materials and hence is more complicated. The resulting input-output relation is derived in terms of irrational transfer functions, and the statistical properties of quantum noises are analyzed. This work provides a new perspective on modeling and analyzing quantum control systems, on the basis of which further design and optimization with spatial parameters can be explored.
We present a full multimode treatment of the interaction of the quantized radiation field with a single two-level atom in a one-dimensional waveguide configuration. Starting with an incident pulse consisting of an arbitrary (finite) number of photons in a general initial state, we derive the equations of motion and a closed-form expression for the shape of the pulse after its interaction with the atom. We then specialize our results to the two-photon case where a number of analytical results can be derived, for both unidirectional and bidirectional systems. We study the effects of different pulse shapes, the manifestations of the entangled, so-called bound state of the two photons, single- and two-photon detection probabilities, and provide simple approximate results for the strong-coupling (or adiabatic, or long-pulse) regime. We also discuss the requirements for a true unidirectional setup, and the application of such a setup to photon sorting proposed by Witthaut, Lukin, and S\o{}rensen [Europhys. Lett. 97, 50007 (2012)].
A quantum description of the spatial behavior of a field that satisfies a spatial nonlinear Schrödinger equation (e.g., a single-mode field in a nonlinear planar waveguide) is studied with the assumption that quantum effects are small. An analytical expression for dissipation in systems governed by the spatial nonlinear Schrödinger equation is presented. Dissipation is found to have detrimental effects on the behavior of quantum phenomena such as quantum noise, squeezing, and bunching. A detecting scheme for the spatial frequency components of the nonclassical properties is also given.