100 years of quantum mechanics and quantum information (The Oskar Klein memorial lecture 2025)

by Prof. Reinhard Werner

Thursday 9 Oct 2025, 15:15 → 16:15 Europe/Stockholm

An important principle for Heisenberg's early work was to build a theory on observable quantities alone. The talk will follow this theme from those early days, to the development of appropriate tools for describing quantum operations, on to their role in modern quantum information theory. 

In his "Uncertainty" paper from 1927 Heisenberg used operational definitions in analogy to Einstein's analysis of simultaneity, in an attempt to ground quantum mechanics in directly observable concepts. Bohr initially had some objections, but with the help of Oskar Klein these differences were resolved and Heisenberg's semiclassical dual wave-particle approach became Copenhagen philosophy. Unfortunately, this did not really work as a basis of the new quantum mechanics, as Einstein tried to point out in his famous debates with Bohr.  Apart from back-of-the-envelope heuristics nothing remained of this approach. 

Meanwhile, operational philosophy became part of the mainstream, and operational formulations of quantum mechanics, more systematic and hence more radical than Heisenberg's, were provided by von Neumann (1927), and then axiomatically by Ludwig around 1970. The mathematics of quantum operations, i.e., completely positive maps, was introduced by Kraus, Davies, Lindblad and others. 

These ideas became fundamental for the rise of quantum information theory. We are accustomed to analyze complex information systems into building blocks, each of which has a description as a completely positive map in the formalism, but also as an instruction set for agents like Alice and Bob. A tacit key assumption for that is that the description of each building block makes sense independently of the other parts. For example, in the resource approach to quantum information typically one agent's operation is to be optimized given the others. One might call this assumption "modular causality", and it is apparently the strongest form of causality that Nature lets us get away with.  

About the Speaker: 

Reinhard F. Werner is a professor of physics at Leibniz University Hannover. He received his doctorate from the University of Marburg in 1982, working with Gunther Ludwig, and is interested in the conceptual and mathematical foundations of quantum theory — anything in which the structure of quantum mechanics plays a non-trivial role. In recent years he has focused mostly on quantum information theory, but also quantum statistical mechanics and the theory of time in quantum mechanics.

Throughout his career, Professor Werner has held prestigious positions, grants, and prizes, including an ERC Advanced Grant, Quantum Communication Award 2014 awarded at QCMC conference in Hefei, and Max Planck Medal of the German Physical Society 2025. He was offered the Leigh Trapnell Chair of Quantum Physics at DAMTP, Cambridge before moving to Hannover in 2009.