How Quantum Systems Keep a Memory of Their Environment


Results from research conducted by RWTH physicists published in Physical Review X.


Quantum devices are extremely sensitive to their environment. Depending on a device’s environment, for example, the response to a control pulse may be delayed. Due to the difficulties in modeling quantum devices, the development of applications continues to be highly challenging.

A team from RWTH, Forschungszentrum Jülich, and the JARA-FIT Institute for Quantum Information has now shown how these delayed responses can be modeled more easily. 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. The research results have been published in the current issue of the scientific journal Physical Review X.

In common electrical circuits, damping resistance effects occur. Things get more complicated when the damping effect occurs with a time delay and the system retains a “memory” of its past behavior. In contrast, quantum systems evolve according to the Schrödinger equation, which does not provide for any damping or memory effects. However, since real systems exhibit damping and memory, a great deal of research has sought to understand how these effects arise from Schrödinger's quantum theory. It turns out that damping arises when the environment is taken into account.

Surprisingly, there are then two fundamental, completely different laws describing a quantum system: One law involves memory, while the other law – apparently – does not involve any memory at all. However, both lead to the same, correct description of the quantum system. The Aachen and Jülich scientists have now found the surprisingly simple connection between the 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.


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