Quantum theory of light and noise polarization in nonlinear optics

We present a consistent quantum theory of the electromagnetic field in nonlinearly responding causal media, with special emphasis on $\chi^{(2)}$ media. Starting from QED in linearly responding causal media, we develop a method to construct the nonlinear Hamiltonian expressed in terms of the complex nonlinear susceptibility in a quantum mechanically consistent way. In particular we show that the method yields the nonlinear noise polarization, which together with the linear one is responsible for intrinsic quantum decoherence.Comment: 4 pages, no figure

Conditional quantum-state transformation at a beam splitter

Using conditional measurement on a beam splitter, we study the transformation of the quantum state of the signal mode within the concept of two-port non-unitary transformation. Allowing for arbitrary quantum states of both the input reference mode and the output reference mode on which the measurement is performed, we show that the non-unitary transformation operator can be given as an $s$-ordered operator product, where the value of $s$ is entirely determined by the absolute value of the beam splitter reflectance (or transmittance). The formalism generalizes previously obtained results that can be recovered by simple specification of the non-unitary transformation operator. As an application, we consider the generation of Schr\"odinger-cat-like states. An extension to mixed states and imperfect detection is outlined.Comment: 7 Postscript figures, using Late

Three-Dimensionally Confined Optical Modes in Quantum Well Microtube Ring Resonators

We report on microtube ring resonators with quantum wells embedded as an optically active material. Optical modes are observed over a broad energy range. Their properties strongly depend on the exact geometry of the microtube along its axis. In particular we observe (i) preferential emission of light on the inside edge of the microtube and (ii) confinement of light also in direction of the tube axis by an axially varying geometry which is explained in an expanded waveguide model.Comment: 5 pages, 4 figure

Measuring the Density Matrix by Local Addressing

We introduce a procedure to measure the density matrix of a material system. The density matrix is addressed locally in this scheme by applying a sequence of delayed light pulses. The procedure is based on the stimulated Raman adiabatic passage (STIRAP) technique. It is shown that a series of population measurements on the target state of the population transfer process yields unambiguous information about the populations and coherences of the addressed states, which therefore can be determined.Comment: 4 pages, 1 figur

Local-field correction to one- and two-atom van der Waals interactions

Based on macroscopic quantum electrodynamics in linearly and causally responding media, we study the local-field corrected van der Waals potentials and forces for unpolarized ground-state atoms placed within a magnetoelectric medium of arbitrary size and shape. We start from general expressions for the van der Waals potentials in terms of the (classical) Green tensor of the electromagnetic field and the atomic polarizability and incorporate the local-field correction by means of the real-cavity model. In this context, special emphasis is given to the decomposition of the Green tensor into a medium part multiplied by a global local-field correction factor and, in the single-atom case, a part that only depends on the cavity characteristics. The result is used to derive general formulas for the local-field corrected van der Waals potentials and forces. As an application, we calculate the van der Waals potential between two ground-state atoms placed within magnetoelectric bulk material.Comment: 9 pages, 2 figures, corrections according to erratu

Patchy Reconnection in a Y-Type Current Sheet

We study the evolution of the magnetic field in a Y-type current sheet subject to a brief, localized magnetic reconnection event. The reconnection produces up- and down-flowing reconnected flux tubes which rapidly decelerate when they hit the Y-lines and underlying magnetic arcade loops at the ends of the current sheet. This localized reconnection outflow followed by a rapid deceleration reproduces the observed behavior of post-CME downflowing coronal voids. These simulations support the hypothesis that these observed coronal downflows are the retraction of magnetic fields reconnected in localized patches in the high corona.Comment: 4 pages, 3 figure

Dual-grating dielectric accelerators driven by a pulse-front-tilted laser

Dielectric laser-driven accelerators (DLAs) can provide high accelerating gradients in the GV/m range due to their higher breakdown thresholds than metals, which opens the way to miniaturize our next-generation particle accelerator facility. However, the electron energy gain is limited by the short interaction length between the laser pulses and the electron bunch for previously reported DLAs. This paper numerically investigates the dual-grating DLAs driven by a pulse-front-tilted (PFT) laser which extends the interaction length and boosts the electrons energy gain. The optical system to generate the PFT laser beam is also studied in detail. By two-dimensional (2D) particle-in-cell simulations we show that such a PFT laser effectively increases the energy gain by more than 100% as compared to that of a normally incident laser beam

A GAS JET BEAM HALO MONITOR FOR LINACS

The gas jet beam profile monitor is a non-invasive beam monitor that is currently being commissioned at the Cockcroft Institute. It utilises a supersonic gas curtain which traverses the beam perpendicular to its propagation and measures beam-induced ionisation interactions of the gas. A 2D transverse beam profile image is created by orientating the gas jet 45 degrees to obtain both X and Y distributions of the beam. This paper builds upon previously used single-slit skimmers and improves their ability to form the gas jet into a desired distribution for imaging beam halo. A skimmer device removes off-momentum gas particles and forms the jet into a dense thin curtain, suitable for transverse imaging of the beam. The use of a novel double-slit skimmer is shown to provide a mask-like void of gas over the beam core, increasing the relative intensity of the halo interactions for measurement. Such a non-invasive monitor would be beneficial to linacs by providing real time beam characteristic measurements without affecting the beam. More specifically, beam halo behaviour is a key characteristic associated with beam losses within linacs