The Meaning of Decoherence*

Salman Habib, Yuval Kluger, Emil Mottola +1 more

The time evolution of a closed system of mean fields and fluctuations is Hamiltonian, with the canonical variables parametrizing the general time-dependent Gaussian density matrix of the system. Yet, the evolution manifests both quantum decoherence and apparent irreversibility of energy flow from the coherent mean fields to fluctuating quantum modes. Using scalar QED as an example, we show how this collisionless damping and decoherence may be understood as the result of dephasing of the rapidly varying fluctuations and particle production in the time varying mean field.

Sushanta Dattagupta, Sanjay Puri

Takuji Ishikawa

Decoherence [1] is a kind of non-unitary process and it is a disappearance of quantum interference among different state vectors. Its origin is known as external fluctuations and dissipations [2] (and observations), these are irreversible processes. Decoherence means a ”wave function collapse”. Because when interference vanishes, the system can not go to any other quantum states. It means the system gets classicality. By research of Caldeira-Leggett [3, 4], it was shown that decoherence happens in a main system with heat bath which consisted of a infinite number of harmonic oscillators and which was used as an environment. Although this means that ”temperature” induces decoherence, it is not essence. Because decoherence would arise in a finite system in which it is difficult to define the ”temperature”. In order to define a statistical quantity like temperature, it is said that ergodicity (time average = statistical average) [5] is needed. For ergodicity of a system, chaos seems to be sufficient condition, but it would not be necessary condition. Please imagine a closed system, there three particles are mutually tied with springs which have different angular frequencies respectively. Lagrangean L is as follow. (x1 means position of particle-1, etc.)

Claus Kiefer, Erich Joos

Ricardo Espinoza Masbernat

The description of quantum many-body phenomena that go beyond a mean-field approximation is often difficult or, more typically, impossible to achieve. Experiments in this regime are also challenging due to technological constraints. An exception may be the dissociation of matter-wave breathers, which in 1D are exactly integrable solutions of the mean-field Gross-Pitaevskii equation, by quantum fluctuations. Breathers are coherent superpositions of fundamental solitons. To study these effects on the decoherence of 7Li matter-wave breathers, we first characterize the decoherence due to mean-field phenomena in quasi-1D. We find that mean-field decoherence of breathers is caused primarily by density-dependent atom loss. We also find that the mechanisms of atom loss present in our experiment, i.e. inelastic collisions with background gas and three-body recombination, have different effects on breather coherence. After identifying the sources of mean-field decoherence, we seek to suppress them so that dissociation by quantum fluctuations may be observable. For this purpose, we present a theoretical proposal and its experimental implementation aimed at enhancing the measure of breather dissociation, i.e. the relative distance between constituent solitons, via expansion in an anti-trapping harmonic potential.