How Quantum Systems Keep a Memory of Their Environment

Jülich, 1 June 2021 – Many quantum devices – electrical, optical or otherwise – respond in a delayed manner to a control pulse. This results in damping effects that occur with a time delay – the systems still show traces of past behaviour. Researchers at RWTH Aachen University and Forschungszentrum Jülich have now shown how this “memory” can be modelled more easily. The results are relevant, among other things, for applications in quantum technology, the development of which is still a challenge due to difficulties in modelling.

In physics, the Schrödinger equation is used to describe quantum systems. But this does not provide for any damping and memory effects. But since every real device has damping – quantum systems are no exception – a great deal of research has been done to understand how these effects arise from Schrödinger’s quantum law.

It turned out that the damping and memory effects observed in experiments were due to the influence of the environment. Unexpectedly, when taking this environment into account, two fundamental laws emerge to replace Schrödinger’s equation. One law includes a temporal memory, while the other law – apparently – does not provide for such a memory at all. However, both lead to the same, correct description of the quantum system.

The Aachen and Jülich scientists have now found a surprisingly simple connection between these two laws. This leads to a better understanding of how a quantum system preserves a memory of its environment. Being able to translate between the two laws is also important in practical terms, as they can each answer different questions. The “memory-less” law explains spontaneous jumps of a quantum system, which must be taken into account, for example, in the development of quantum computers. The law capturing memory effects allows for more accurate calculation when the influence of the environment is particularly strong. Neither law answers all questions; only switching between the two laws allows a more complete analysis of quantum systems.

Konstantin Nestmann and Valentin Bruch from RWTH’s Department of Theoretical Physics (Condensed Matter Theory) and Professor Maarten Wegewijs from the RWTH Institute for Theory of Statistical Physics A and the Institute of Theoretical Nanoelectronics at Forschungszentrum Jülich were involved in this work within DFG Research Training Group 1995. The research results have been published in the current issue of the scientific journal Physical Review X.

Original publication:

K. Nestmann, V. Bruch, M. R. Wegewijs

How Quantum Evolution with Memory is Generated in a Time-Local Way

Phys. Rev. X (published 24 May 2021), DOI: 10.1103/PhysRevX.11.021041

/Public Release. This material comes from the originating organization and may be of a point-in-time nature, edited for clarity, style and length. View in full here.