Master Thesis Defense by Ivan Iakoupov – Niels Bohr Institute - University of Copenhagen

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Master Thesis Defense by Ivan Iakoupov

Quantum computers operate with qubits, which are entities that can be in the state |0⟩, state |1⟩ or any superposition (linear combination) of those. One of the essential parts of a quantum computer is a controlled two-qubit gate. An example of such gate is the controlled phase gate, which imparts a phase of π (i.e. multiplies by -1) to the state, where both qubits are in |1⟩. 

In this thesis there were considered three different schemes to implement a controlled phase gate for qubits stored in photons. The photons are special in the sense that they normally do not interact with each other, which makes it hard to make controlled two-qubit gates with them. This is why the usual role of the photons in quantum computing is to carry quantum information over long distances instead of being the registers of a quantum computer that the quantum algorithms are run on directly. However, the single-qubit gates for photons work very well, and if an efficient two-qubit gate was constructed, the photons could also possibly function in the latter role.

To make an effective interaction between the photons, atoms with a certain energy level structure were chosen. In the first of the three controlled phase gate schemes, only one such atom was used. Afterwards two modifications were considered that make use of an ensemble of those atoms and the collective enhancement effects that they can give.