Dear Colleagues,

Relativistic quantum field theory, in particular QED, has been developed in the middle of the last century to describe high energy scattering experiments. The extension of the perturbation expansion in quantum mechanics to the transition amplitudes, given in the interaction representation, has been completed by the method using Feynman graphs. Ever since then, there has been a growing need for quantum field theory beyond scattering experiments, and the corresponding perturbation expansion has been introduced by J. Schwinger and bears the name Closed Time Path formalism. This scheme covers the calculation of expectation values and provides a more universal language of quantum theory. The effective theory approach to quantum field theory, developed within this formalism, can cope with open quantum systems. Similar results have been found by L. V. Keldysh by extending Feynman graph techniques to thermal equilibrium.

We believe that the Schwinger–Keldysh formalism is the natural language of quantum field theory since it covers both closed and open interactions in a natural manner. This issue is all the more important because the observed quantum systems tend to be open through interaction channels both at low and at high energy, reflecting the limit of our possibilities and knowledge, respectively. The uncontrollable small energy exchange with the environment leads to the violation of conservation laws, the dynamical breakdown of the time reversal invariance, dissipation, decoherence and relaxation. The last two phenomena are essential to the emergence of classical physics and statistical mechanics. The unknown high energy physics, beyond the current space resolution of the experiments, together with the UV divergences of realistic quantum field theoretical models, force us to use cutoff or effective theories which are valid up to some energy scale. Hence, the usual renormalization group procedure also has to be extended to open dynamics.

The quantum field theory of open systems allows us to access a number of important physical questions:

Why is the conventional quantum description, based on pure states and closed dynamics, so efficient in the presence of the ubiquitous entanglement?

What is the relation between decoherence and dissipation?

In what sense is equilibrium statistical physics the IR fixed point of an infinite open system?

How do the classical, macroscopic averages emerge from the quantum domain?

How can nanophysics be connected to micro- and macro-physics in a quantitative manner?

Contributions are expected to touch these and other issues of open many-body physics.

Prof. Dr. Janos Polonyi
Dr. Sandor Nagy
Guest Editors

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• open quantum system
• dissipation
• decoherence
• relaxation
• renormalization