Midterm Colloquium by Johannes Borregaard - please note new time – Niels Bohr Institute - University of Copenhagen

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Midterm Colloquium by Johannes Borregaard - please note new time

Entanglement is a property of quantum systems which is highly useful for quantuminformation processing. For instance teleportation and dense coding rely on tanglement between quantum systems and entanglement is also a major advantage for quantum key distribution. It is however not trivial to create and distribute entanglement over long distances. A classical signal can be amplified to compensate losses but this is not possible with a quantum signal due to the nocloning theorem of quantum mechanics. 

Schemes for the creation and distribution of entanglement over long distances are know as quantum repeaters. The key idea is to connect small segments of entanglement to obtain larger and larger distances – a process known as entanglement swapping.

In my midterm colloquium I will give an introduction to entanglement and the basic operations used in quantum repeaters such as entanglement swapping and quantum memories.

I will furthermore go through the quantum repeater scheme suggested by J.B. Brask et al. in Ref [1]. Here entanglement is created using single photon detection and quantum memories. The entangled states are then grown into squeezed cat states using homodyne detection. Finally it is shown how segments of entangled cat states can be swapped using only linear optics and homodyne detection. This last step of entanglement swapping can be made near-deterministic using auxiliary cat states.

The quantum repeater suggested by J.B. Brask et al. is shown to have a rate of 0.3 entangled pairs pr. minute at a distance of 1000 km. It may be possible to improve this rate using a different method to create entanglement. This will be the final subject of my colloquium.

[1] “A Hybrid Long-Distance Entanglement Distribution Protocol”, J.B. Brask, I.

Rigas, E. S. Polzik, U. L. Andersen and A. S. Sørensen, Phys. Rev. Lett. 105,

160501 (2010)