CHAPTER 8 Measurement and Decoherence, Universality Revisited

Roland L. Omnes

Thomas Ihn

Quantum information processing is a modern research topic in which many different disciplines within and beyond physics interact. This chapter first makes contact with basic concepts of classical information theory, which is often not familiar to physicists. It then introduces Shannon's entropy and related entropic quantities, revisits the sampling theorem, discusses the relation between thermodynamics, measurement, and information theory, and looks into Landauer's principle. The chapter looks into the very basics of quantum information, the description of qubits, and operations of qubits. Finally, qubit implementations are discussed based on charge and spin qubits in quantum dots. Experimental results on single and two-qubit operations, and issues of decoherence are discussed.

Hervé Zwirn C

Soulas, Antoine

The aim of this thesis is to better understand the features of quantum matter that makes quantum gravity so hard to reach. In Chapter 3, we present two general models of decoherence, without specifying any Hamiltonian. The first one makes decoherence naturally appear as a geometrical phenomenon between reservoirs of dimensions. The second one specifically tackles the question of why, in a universe where decoherence is ubiquitous, things are not frozen by the quantum Zeno effect. In Chapter 4, we discuss the ontological problem of quantum physics, usuallyreferred to as the ‘measurement problem’. Our aim in this chapter is not to give a solution, but rather to formulate the problem independently of any interpretation of quantum mechanics, in the light of the theory of decoherence. In Chapter 5, a new ingredient is introduced in the discussion: spacetime. After investigating the apparent incompatibility between quantum mechanics and special relativity, we present a program of research and a toy model designed to build a backgroundless notion of position within the quantum formalism.