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