In the late 1980s, Anton Zeilinger became interested in quantum entanglement. This work resulted in his most significant accomplishments and opened up the new fields of quantum teleportation, quantum information, quantum communication and quantum cryptography.
Together with Daniel Greenberger and Michael Horne, Zeilinger wrote the first paper ever on entanglement beyond two particles. The resulting GHZ theorem (see Greenberger-Horne-Zeilinger state) is fundamental for quantum physics, as it provides the most succinct contradiction between local realism and the predictions of quantum mechanics. Also, GHZ states were the first instances of multi-particle entanglement ever investigated. Such states have become essential in quantum information science. GHZ states are now even an individual entry in the PACS code.
As a professor at the University of Innsbruck, Zeilinger started experiments on entangled photons, as the low phase space density of neutrons produced by reactors precluded their use in such experiments. His goal from the early 1990s on, to demonstrate the GHZ contradiction, was achieved finally in 1998.
Along the road, Zeilinger developed many novel tools for entangled photon physics, for example a bright source for polarization-entangled photons, ways to identify Bell states and methods for producing coherent emission of more than one entangled pair from one crystal. The resulting technology allowed him to perform a number of first quantum information experiments with entangled photons. The first ever use of entanglement in any quantum information protocol was his demonstration of hyperdense coding. His achievements also include the first entanglement-based quantum cryptography, the first quantum teleportation experiment of an independent photon, the first realization of entanglement swapping and an experiment closing the communication loophole in a test of Bell’s inequality.
Since 2000, Zeilinger’s research has focused on all-optical quantum computation, the development of entanglement-based quantum cryptography systems, and experiments with entangled photon pairs over very large distances. In all-optical quantum computation, Zeilinger with his group were the first to demonstrate a number of basic procedures, like entanglement purification and certain quantum gates. This culminated in the first demonstrations of one-way quantum computation, including most recently, ultra-fast active feed-forward. The one-way quantum computation scheme was used to realize Grover’s search algorithm and various quantum games, including prisoner’s dilemma.
In quantum cryptography, Zeilinger’s group is developing a prototype in collaboration with industry. While most of the community was working on the much easier scheme of using weak laser pulses, Zeilinger based his approach exclusively on the more demanding scheme using entangled photons. A recent proof that entanglement is a necessary condition for the security of the quantum channel confirms that this choice is correct.
Zeilinger’s experiments on the distribution of entanglement over large distances began with both free-space and fiber-based quantum communication and teleportation between laboratories located on the different sides of the river Danube. This was then extended to larger distances across the city of Vienna and most recently over 144 km between two Canary Islands, resulting in a successful demonstration that quantum communication with satellites is feasible. His dream was to bounce entangled light off of satellites in orbit. This was achieved during an experiment at the Italian Matera Laser Ranging Observatory.
An important fundamental spin-off of these experiments was the first test in 2007 of a non-local realistic theory proposed by Leggett which goes significantly beyond Bell's theorem. While Bell showed that a theory which is both local and realistic is at variance with quantum mechanics, Leggett considered nonlocal realistic theories where the individual photons are assumed to carry polarization. The resulting inequality was shown to be violated in the experiments of the Zeilinger group.
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