Maintaining coherence in Quantum Computers

TL;DR

This paper investigates how external interactions affect quantum computer coherence, emphasizing the importance of coupling strength and calculation duration relative to thermal time scales for maintaining quantum coherence.

Contribution

It provides a quantitative analysis of the conditions necessary to preserve coherence in quantum computations considering environmental coupling and thermal effects.

Findings

Quantum coherence requires weak coupling to the environment.

Calculation time must be shorter than the thermal time scale for coherence.

Stricter conditions on coupling strength arise for longer computation times.

Abstract

The effect of the inevitable coupling to external degrees of freedom of a quantum computer are examined. It is found that for quantum calculations (in which the maintenance of coherence over a large number of states is important), not only must the coupling be small but the time taken in the quantum calculation must be less than the thermal time scale, $\hbar/k_B T$. For longer times the condition on the strength of the coupling to the external world becomes much more stringent.