Giulia Rubino, University of Vienna
Experimental Entanglement of Temporal Orders
The study of causal relations, a cornerstone of physics, has recently been applied to the quantum realm, leading to the discovery that not all quantum processes have a definite causal structure. While such processes have previously been observed, these observations opened a 'loophole' whereby the observed process could be explained by an underlying theory with a definite causal structure. Here, we present the first experimental demonstration of entangled temporal orders, resulting in a process that is incompatible with a large class of generalized probabilistic theories which are local and have a definite temporal order. We experimentally invalidate this class by violating a Bell inequality. We thus conclude that nature is incompatible with the class of theories requiring a local definite temporal order.
Mikael Parniak, Niels Bohr Institute
Hong-Ou-Mandel intereference of spin waves in a multimode quantum memory
We demonstrate a quantum memory based on the Duan-Lukin-Cirac-Zoller protocol operating in thousands of spatial modes. A cloud of cold rubidium atoms serves as a storage medium for single photon states that take an in-memory form of spin waves – collective atomic spin excitations. We have developed methods to manipulate those collective quasi-particles formed by spins of one billion atoms in the spatial (wavevectors) domain. In particular, by manipulating local phase with an off-resonant ac-Stark shift we realize linear optical operations for spin waves stored in the memory. Our new methods allows both realization of practical protocols such as storage of pulse trains as well as a fundamental demonstration of Hong-Ou-Mandel interreference between a pair of spin-wave Fock states. Our demonstration shows that interference of quasi-particles is possible even in a thermal ensemble of atoms, as long as their spin coherence is preserved.
Peter Harremoes, Copenhagen Business College
Thermodynamic Sufficiency for Generalized Probabilistic Theories
Entropy appear in thermodynamics, in information theory, in statistics, in portfolio theory, and in quantum information theory. We want to answer questions like: On which generalized probabilistic models can thermodynamic quantities be defined? What role does conservation of energy play? Why do similar formulas appear in different areas? For generality we consider an abstract state space, which is a convex set where convex combinations correspond to probabilistic mixing with density matrices as the basic example. The main results are that thermodynamics can be formulated on state spaces that can be represented on Jordan algebras and there are only few other spaces that allow thermodynamics. The complex Hilbert space formalism (including superposition and entanglement) is therefore almost a consequence of thermodynamic constraints alone