Abstract

Introduction

Decoherence and decay of quantum states in open systems is the main obstacle towards the realization of quantum computing and communications and a major impediment for coherent control (of atomic, molecular and condensed-phase processes) and quantum state engineering (of fields and atoms in cavities and trapped ions). This Focus Issue is an attempt to highlight the development of schemes for the control of quantum decay and decoherence, by presenting the major trends in this area. To this end, I have solicited a number of articles from groups that have been particularly active in the pursuit of these trends.

The articles in this Focus Issue can be grouped in two categories, as detailed below.

Methods for controlling decoherence and decay or reducing their effects

  1. Hofmann, Hess and Mahler show that the decoherence of two-level atom states by spontaneous emission can be measured and subsequently compensated by balanced homodyne detection and a coherent feedback field.
  2. Horoshko and Kilin show that decoherence of a single-mode field in an open cavity can be avoided by making appropriate measurements on the reservoir and adjusting the cavity parameters accordingly. The requirements for conditional and unconditional avoidance of decoherence are discussed.
  3. Harel, Kofman, Kozhekin and Kurizki discuss three methods for the control of non-Markovian decay in cavities and other multi-mode reservoirs: the quantum Zeno effect, location-dependent interference of decay channels and conditionally interfering parallel evolutions.
  4. Law and Eberly describe a method for the preparation of Zeeman-level superpositions by Raman pulses. This method can be regarded as being practically immune to decoherence, since spontaneous emission is negligible at hyperfine transitions.

Methods for characterizing decoherence and decay

  1. Poyatos, Cirac and Zoller show that decoherence in quantum computation can be fully characterized by quantum tomography methods, and apply this description to an ion-trap quantum computer which is coupled to several environments.
  2. Stenholm and Suominen study the decay of molecular vibrational wavepackets by coupling to the continuum (predissociation) and discuss its deviations from exponential decay.

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