How quantum devices keep their memory
Quantum devices are well-known to be extremely sensitive to their environment, which continues to pose challenges to modeling their behavior and engineering their applications. One well-known consequence is that a quantum device may not necessarily respond instantly when one tries to control it, because its environment delays the response. Physicists from RWTH Aachen and Forschungszentrum Jülich involved in the JARA-FIT Institute for Quantum Information, have now shown how one may nevertheless pretend that there is no such time-delay at all and still get things right.
In ordinary mechanical systems, such as a child on a swing, there is a competition between ideal behavior (swing) and unwanted friction or damping (necessitating parental driving). A similar thing happens in electric circuits where one may want to sustain a voltage oscillation in competition with resistive effects. This becomes even more complicated if the damping is time-delayed causing ‘sluggish’ behavior: the system now keeps some ‘memory’ of its past behavior.
It has been well-established since 1925 that quantum systems evolve in time according to the Schrödinger equation, a law in which there is no damping or memory at all. Since actual systems do show damping and memory, much effort has gone into understanding how they emerge from this quantum law. It turns out that damping arises when one takes the environment into account. In fact, in this case, a quantum system obeys two fundamental yet entirely different looking laws. One law includes memory while the other law – seemingly – has no memory at all. They still, however, lead to precisely the same, correct description of the quantum device. The researchers have now found the surprisingly simple connection between these two laws. The results have been published in the renowned journal Physical Review X.
Two fundamental laws are better than one
The law with memory takes into account that a quantum device really has to ‘drag its environment along’ to change its state, similar to ordinary mechanical systems and electric circuits. What is surprising is that the other law without memory still predicts the same ‘sluggish’ behavior. Both laws have been studied for a long time and have been used to successfully model and explain countless experiments in quantum optics, quantum electronics and quantum computing. However, it remained unclear how such different laws give the same correct result. The researchers’ newly discovered connection shows precisely how this works, clarifying how a quantum system keeps a ‘memory’ of its environment. The two leading authors of the article, Konstantin Nestmann and Valentin Bruch (RWTH University), have shown the usefulness of this basic result for a variety of problems.
As their supervisor Prof. Maarten Wegewijs (Peter Grünberg Institute, Forschungszentrum Jülich and member of JARA-FIT) explains, this new connection is important because each law can only be used to answer a limited set of questions. For example, understanding how a quantum system makes spontaneous ‘jumps’ from time to time is easy using the law without memory, which is the way that errors are accounted for in the design of quantum computers, often with the hope that the influence of the environment is small. This is practically impossible using the other law. But the situation is reversed if one tries to find out how strong the influence of environments can actually be. In such case typically the law with memory allows one to do much more accurate calculations. Due to the presence of the system’s environment there is no single fundamental law that provides ‘easy’ answers to all questions.
Two communities are better than one
As a result, the newly found recipe for switching between both laws enables a more complete analysis of quantum systems which suffer from ‘noise’ caused by their environment. This does not mean that the problem has suddenly become easy. It does, however, connect two communities of researchers which have already established many insights and techniques by just concentrating on one of the two laws. This kind of synergy reflects the strategy of DFG Research Training Group
(RTG) within which the researchers from Jülich and Aachen are collaborating. The specific aim of RTG 1995, coordinated by Prof. Volker Meden (RWTH University), is to train doctoral students in the use of several advanced methods to solve outstanding problems in many-body quantum physics.
Physical Review X (published online 24 May 2021), DOI: https://doi.org/10.1103/PhysRevX.11.021041
Peter Grünberg Institute, Theoretical Nanoelectronics (PGI-2)
DFG Research Training Group RTG 1995, Quantum Many-Body Methods in Condensed Matter Systems, www.rtg1995.rwth-aachen.de
The study was funded within the framework Research Training Group RTG 1995 of the German Research Foundation (DFG).
Prof. M. R. Wegewijs
Theoretical Nanoelectronics (PGI-2) Tel: +49 2461 61-3137