Bell and Leggett-Garg tests of local and macroscopic realism
Dr. Johannes Kofler (Max-Planck-Institut für Quantenoptik, Garching)

Quantum physics is in conflict with a classical world view both conceptually and mathematically. The assumptions of a genuine classical world – local realism and macroscopic realism – are at variance with quantum mechanical predictions as characterized by the violation of experimentally testable inequalities. While Bell’s inequality is used to demonstrate quantum entanglement between two or more quantum systems that are distant in space, the Leggett-Garg inequality was designed to witness quantum superpositions of macroscopic objects. The former phenomenon – quantum entanglement – is of great importance for the new field of quantum information science, where it is used in quantum cryptography or quantum computation. The latter – macroscopic superpositions (“Schrödinger cats”) – is not yet experimentally verified and is intimately linked to the famous measurement problem and the quantum-to-classical transition.
In this talk, I will discuss some recent results regarding conceptual issues in Bell tests [1,2] (which are also important for security proofs in quantum cryptography), foundational entanglement experiments [3-4], as well as theoretical developments of the Leggett-Garg inequality [5-7]. This paves the way towards a completely loophole-free Bell test and towards witnessing quantum superpositions in more and more macroscopic and complex systems, potentially in biological organisms.

[1] T. Scheidl, R. Ursin, J. Kofler, S. Ramelow, X. Ma, T. Herbst, L. Ratschbacher, A. Fedrizzi, N. Langford, T. Jennewein, and A. Zeilinger, PNAS 107, 19708 (2010)

[2] M. Giustina, A. Mech, S. Ramelow, B. Wittmann, J. Kofler, A. Lita, B. Calkins, T. Gerrits, S. W. Nam, R. Ursin, and A. Zeilinger, Nature 497, 227 (2013)

[3] X. Ma, T. Herbst, T. Scheidl, D. Wang, S. Kropatschek, W. Naylor, A. Mech, B. Wittmann, J. Kofler, E. Anisimova, V. Makarov, T. Jennewein, R. Ursin, and A. Zeilinger, Nature 489, 269 (2012)

[4] X. Ma, S. Zotter, J. Kofler, R. Ursin, T. Jennewein, Č. Brukner, and A. Zeilinger, Nature Phys. 8, 480 (2012)

[5] J. Kofler and Č. Brukner, Phys. Rev. Lett. 99, 180403 (2007)

[6] J. Kofler and Č. Brukner, Phys. Rev. Lett. 101, 090403 (2008)

[7] J. Kofler and Č. Brukner, Phys. Rev. A 87, 052115 (2013)