PhD Defence by Jonatan Bohr Brask – Niels Bohr Institute - University of Copenhagen

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PhD Defence by Jonatan Bohr Brask

Entanglement is one of the curious counter-intuitive features which sets quantum mechanics apart from classical physics. It lies at the foundations of quantum theory, and at the same it can be harnessed for applications such as quantum teleportation and secure communication schemes.

This thesis is concerned with the study and theoretical development of methods for generating entangled states between objects at distant locations. Quantum states are fragile creatures which are easily degraded by interactions with a noisy environment. The most viable carrier of quantum states over long distances is light, because it moves fast and couples relatively weakly to its surroundings, but even light travelling e.g. in optical fibres is subject to losses which grow exponentially with distance. For classical communication this problem is solved by amplification, but for quantum signals the so-called no-cloning theorem, which stipulates that an unknown quantum state cannot be copied, implies that noiseless amplification is impossible. Instead, the problem can be solved by first generating entanglement over short distances and then ``swapping'' the entanglement to longer distances via quantum teleportation. Schemes which implement this approach are known as quantum repeaters.

The thesis presents two new proposals for quantum repeater schemes. In my talk I will give an introduction to quantum repeaters and describe the ideas behind the new proposals.

The defense will be followed by a reception.