Master Thesis Defense: Rasmus Bruhn Nielsen
Towards cluster state generation using charged quantum dot
Quantum computers are receiving an increasing amount of attention from the public due to the promises of unlocking the ability to solve problems which, up until this point, has been unsolvable on classical computers. This includes applications in cryptography, physics and the medical industry. However, to get to the point of solving these problems a scalable, fault tolerant quantum computer must be developed. This requires, among other things, long lived qubits resistant to noise, efficient single qubit operations and efficient and scalable 2-qubit operations for realizing quantum entanglement between qubits. This thesis explores the possibility of using photon based qubits as a building block for a quantum computer. The system of a negatively charged InGaAs quantum dot embedded within a GaAs waveguide is presented along with an excitation scheme to generate highly indistinguishable time-bin encoded single photons entangled with the electron spin. A theoretical derivation of the optical spin control and photon generation protocol will be given while also discussing possible error sources and the significance of these. The spin-echo and nuclear spin narrowing sequences are also discussed as possible solutions to minimizing the decoherence of the spin. The experimental setup is described including the polarization and power control of the lasers used for spin control and photon generation, the temperature stabilization of the setup and the self stabilizing interferometer which is used in both the generation and measurement phase of the time-bin encoding. A simulation approach to speed up the optimization of PID parameters for PID's with a large input delay is also discussed. The main focus of this thesis is the generation protocol for GHZ states and linear cluster states using time-bin encoded photons and an implementation of Deutsch's algorithm in these. The theoretical background for measurement based quantum computations is described along with the implementation of Deutsch's algorithm used in the experimental work. A 3-qubit GHZ state was realized in the experiment with an extracted fidelity of F = 0.56 +- 0.02. A three qubit version of Deutsch's algorithm was implemented in this state yielding the fidelity 0.88 +- 0.02 (0.85 +- 0.01) for the constant (balanced) version of the algorithm. Finally a Monte Carlo simulation was set up with realistic error parameters achieving a fidelity of 0.53. Simple improvements to the error parameters were added improving the fidelity to 0.66 suggesting a huge possible improvement to the fidelity. Using the improved parameter a 4-qubit linear cluster state was simulated to yield a fidelity of 0.48.