Thesis title: Quantum Dots for Quantum Networks
Single and entangled photon sources are fundamental building blocks of future
light-based quantum technologies such as quantum communication, optical quantum
computing, and, more in general, for quantum networks. Despite the high number
of photon sources proposed over the last years, their exploitation in quantum optical
technologies has been limited so far. One of the main reasons for this is that
the envisioned applications set very stringent requirements on the properties of the
photon source to be used. An ideal photon source should provide single and entangled
photons deterministically, with high purity, high efficiency, high indistinguishability,
and, in the case of entangled photons, a high degree of entanglement. Among the
different photon sources available to date, semiconductor quantum dots (QDs) are
arguably one of the most promising.
A QD is a nanometric crystalline structure capable of confining the wavefunction
of charge carriers in a semiconductor in all three dimensions. The discrete, atom-like
states forming due to confinement can be exploited for the emission of single photons
and, in specific conditions, entangled photon pairs. Quantum dots can be grown in
a variety of semiconductor combinations. This feature, together with the possibility
to tune their optoelectronic properties by changing their physical dimensions and/or
via the application of external fields, allows for controlling their light emission with
high precision over a broad spectral range. Over the years, several demonstrations
of a QD-based source meeting the requirements of the wish-list appeared in the
literature. The best source is yet to be disclosed though, as not all the requirements
were reached simultaneously in the same experiment.
The work performed during my Ph.D. aims to test the suitability of QDs as
sources of entangled photons for future quantum networks. The thesis focuses on
the fabrication and study of near-ideal nanophotonic devices based on GaAs QDs
fabricated by droplet etching and their exploitation in advanced quantum optics
experiments.
In the first part, I will show and discuss the demonstration of quantum telepor-
tation and entanglement swapping using single and entangled photons generated
quasi-deterministically from a single GaAs QDs. More specifically, in the tele-
portation experiment a single quantum state is transferred from one photon to
another via an entangled photon pair. In the entanglement swapping instead, the
interference of two photons coming from two different entangled pairs is exploited
to transfer entanglement to the remaining two, previously uncorrelated, photons.
These two experiments represented the first benchmark to test the suitability of
QDs for quantum communication, as the two quantum protocols lay at the base
of a fundamental element of a quantum network, i.e., the quantum repeater. The
obtained results highlighted that additional improvement of the photon sources is
still needed to optimize the fidelity of the protocols, especially for what concerns
photon-indistinguishability and -extraction efficiency. For this reason, in the second
part of the thesis great efforts have been devoted to the fabrication of photonic
devices.
The photonic structure we investigated, i.e., a circular Bragg resonator, consists
of a single QD in the center of a central cylindrical cavity surrounded by a circular
Bragg grating. It features a modest Purcell enhancement with a broadband resonance
and an improved extraction efficiency. Record-high values of indistinguishability
and brightness were recently reported in the literature using this structure. Building
upon these results, we decided to make an additional step and integrate this photonic
device onto micro-machined piezoelectric actuators. This is needed to achieve full
control over the QD electronic structure and generate entangled photons with near-
unity fidelity and tunable energy. The fabrication of the full device requires several
steps: It starts from the epitaxial growth of the semiconductor sample containing
the GaAs QDs which is reduced into a semiconductor membrane via wet-chemical
etching. A patterned mask is then written on it using electron beam lithography
and transferred on the membrane through reactive ion etching. In parallel, we
also developed an imaging setup to locate the position of the QDs across the wafer
allowing us to position the cavity around a single QD with nanometric precision.
We fabricated the first photonic cavities onto micro-machined piezo-actuators and,
in the very last part of this thesis, we report the first experiments using them as
sources of light.