Quantum Decoherence of Two Qubits

TL;DR

This paper demonstrates a specific example of true quantum decoherence in a two-qubit system, showing it cannot be explained by random unitary dynamics, thus highlighting a fundamental difference in decoherence mechanisms.

Contribution

The authors construct a feasible two-qubit phase damping channel that cannot be described by random unitary dynamics, providing a new example of genuine quantum decoherence.

Findings

A two-qubit phase damping channel that defies random unitary description

Geometrical measure of the channel's distance to random unitary channels

Agreement between this measure and the Birkhoff defect

Abstract

It is commonly stated that decoherence in open quantum systems is due to growing entanglement with an environment. In practice, however, surprisingly often decoherence may equally well be described by random unitary dynamics without invoking a quantum environment at all. For a single qubit, for instance, pure decoherence (or phase damping) is always of random unitary type. Here, we construct a simple example of true quantum decoherence of two qubits: we present a feasible phase damping channel of which we show that it cannot be understood in terms of random unitary dynamics. We give a very intuitive geometrical measure for the positive distance of our channel to the convex set of random unitary channels and find remarkable agreement with the so-called Birkhoff defect based on the norm of complete boundedness.