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Solid-state photonic interfaces using semiconductor quantum dots



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Boyer de la Giroday, Antoine 


New technologies based on the properties of quantum mechanics promise to revolutionise the way information is processed by outperforming what is possible using classical devices. Examples include massively parallel processing using quantum computers, verifiably secure communication using quantum cryptography, and measurement with sensitivity beyond classical limitation with quantum metrology. Realising the full potential of these technologies necessitates the ability to communicate quantum information over large distances, a key requirement for future quantum networks. However, developing practical implementations of long-distance quantum communication is challenging as it necessitates three major ingredients: light-matter interfaces, elementary quantum operations, and quantum memories. This thesis describes work that has been undertaken to address these requirements using semiconductor nanotechnology. We have first demonstrated that single InAs quantum dots embedded inside conventional diode structures constitute high-fidelity controllable interfaces between optical qubits and solid-state qubits. Indeed, the polarisation state of a photon was transferred into the spin state of an electron-hole pair and eventually restored through radiative recombination of the electron and the hole with a fidelity up to 95%. Moreover, spins were manipulated using subnanosecond modulation of a vertical electric field applied to the quantum dots. By controlling this electrical modulation, we demonstrated elementary phase-shift and spin-flip gate operations with near-unity fidelities. An electron-hole pair confi ned in a single quantum dot has a short radiative lifetime limiting therefore its use as an excitonic quantum memory. The solution we proposed was to use a quantum dot molecule to control the spatial separation of the electron and the hole and therefore prevent their recombination. Comprehensive studies of electric field eff ects upon the photoluminescence of quantum dot molecules lead to a clear understanding and a good control over their physical properties. Single photons were stored in individual quantum dot molecules up to 1μs and read out on a subnanosecond time scale. Moreover, the circular polarisation of individual photons was transferred into the spin state of electron-hole pairs with a fidelity above 90%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment. Our work on single quantum dots could be extended in the near future to allow for two-qubits quantum operations by con fining a second electron-hole pair to be electrically manipulated. Storage of a superposition of spin states in a quantum dot molecule should also be possible if the spin states are made degenerate, which is feasible using the electric fi eld dependence of the energy splitting between the spin states discussed in this thesis. We believe that combining both approaches will lead to the development of a controllable multi-qubit quantum memory for polarised light, a building block for long distance quantum communication based on semiconductor nanotechnology.






Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge