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Title: Solid-state photonic interfaces using semiconductor quantum dots
Author: Boyer de la Giroday, Antoine
ISNI:       0000 0004 2716 9340
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2012
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New technologies based on the properties of quantum mechanics promiseto revolutionise the way information is processed by outperforming what ispossible using classical devices. Examples include massively parallel processingusing quantum computers, verifiably secure communication using quantumcryptography, and measurement with sensitivity beyond classical limitationwith quantum metrology. Realising the full potential of these technologiesnecessitates the ability to communicate quantum information over largedistances, a key requirement for future quantum networks. However, developingpractical implementations of long-distance quantum communicationis challenging as it necessitates three major ingredients: light-matter interfaces,elementary quantum operations, and quantum memories. This thesisdescribes work that has been undertaken to address these requirements usingsemiconductor nanotechnology. We have first demonstrated that single InAs quantum dots embedded insideconventional diode structures constitute high-fidelity controllable interfacesbetween optical qubits and solid-state qubits. Indeed, the polarisationstate of a photon was transferred into the spin state of an electron-hole pairand eventually restored through radiative recombination of the electron andthe hole with a fidelity up to 95%. Moreover, spins were manipulated usingsubnanosecond modulation of a vertical electric field applied to the quantumdots. By controlling this electrical modulation, we demonstrated elementaryphase-shift and spin-flip gate operations with near-unity fidelities. An electron-hole pair confi ned in a single quantum dot has a short radiativelifetime limiting therefore its use as an excitonic quantum memory. The solution we proposed was to use a quantum dot molecule to control thespatial separation of the electron and the hole and therefore prevent theirrecombination. Comprehensive studies of electric field eff ects upon the photoluminescenceof quantum dot molecules lead to a clear understanding anda good control over their physical properties. Single photons were stored inindividual quantum dot molecules up to 1?s and read out on a subnanosecond time scale. Moreover, the circular polarisation of individual photons wastransferred into the spin state of electron-hole pairs with a fidelity above90%, which does not degrade for storage times up to the 12.5 ns repetitionperiod of the experiment. Our work on single quantum dots could be extended in the near future toallow for two-qubits quantum operations by con fining a second electron-holepair to be electrically manipulated. Storage of a superposition of spin statesin a quantum dot molecule should also be possible if the spin states are madedegenerate, which is feasible using the electric fi eld dependence of the energysplitting between the spin states discussed in this thesis. We believe thatcombining both approaches will lead to the development of a controllablemulti-qubit quantum memory for polarised light, a building block for long distancequantum communication based on semiconductor nanotechnology.
Supervisor: Shields, Andrew ; Ritchie, David Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral