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MP: Theoretische und Mathematische Grundlagen der Physik

MP 12: Symposium: Quanteninformationsverarbeitung (gemeinsam mit Q)

MP 12.1: Hauptvortrag

Montag, 15. März 1999, 10:15–10:55, TE1

Violation of Bell’s inequality by photons 10 km apart — •Nicolas Gisin — Group of Applied Physics, University of Geneva, 20, rue de l’Ecole de Medecine, CH-1211 Geneva 4, Switzerland

We live in a quantum world - something physicists have considered with amazement for more than seventy years. But we now realize that quantum physics is more than a radical departure from classical physics. It also offers many new possibilities for information processing.

In particular, quantum theory is non-local: it predicts entanglement between distant systems leading to correlations that cannot be explained by any theory based only on local variables, as demonstrated by Bell’s inequality. All experiments are in remarkable agreement with quantum theory. Hence, the physics community faces a very strange worldview: in theory, everything is entangled, but, in practice, decoherence makes it impossible to reveal this entanglement. In addition to its "experimental metaphysics" aspects, quantum entanglement has recently gained much interest and respect because it is at the heart of quantum information processing. The general idea is that entanglement provides means to carry out tasks that are either impossible classically (quantum cryptography) or that would require significantly more steps to perform on a classical computer (searching a database, factorization) .

However, how robust is quantum entanglement? How long can one maintain it under control? Over which distances? Can one really exploit it? As a step towards answering these questions, this contribution will present an experiment carried out in Geneva, using the fiber optics network of Swisscom, showing that photons with a spatial separation of more than 10 km can still be non-separated.