ABSTRACT
Coherent interaction between bare quantum states is the basis for any type
of Qubit. Here the focus will be on the realization of flying Qubits realized
by coherent light-matter interaction in solid state. These kind of inter-
action is described in the framework of cavity quantum electrodynamics
(CQED) and can be observed in systems based on high quality microcav-
ities in combination with atom-like emitters [1-3]. CQED addresses in
particular a modification of the emitter’s spontaneous decay in the pres-
ence of confined optical modes in cavities with small mode volumes. In
real cavities, photons are not confined for an infinitive amount of time but
leave the cavity after a characteristic time, which is inversely proportional
to the quality (Q) factor of the latter. The escape of photons through
leaky modes introduces dissipation in the system. Now, if the coupling rate
between an atom-like emitter and the photonic cavity mode is lower than
the photon escape rate irreversible decay dominates, which is described in
terms of weak coupling. It is characteristic for the weak coupling regime
that spontaneous emission of an emitter can be enhanced or reduced com-
pared with its vacuum value by tuning discrete cavity modes in and out of
resonance [4-6]. Recently, due to the enormous progress in nanotechnol-
ogy processing, it has become feasible to realize quantum dot microcavity
systems in which the light-matter coupling rate exceeds any dissipative
decay rate [7-9]. In this case, the conditions for strong coupling are ful-
filled and vacuum field fluctuations initiate a reversible exchange of energy
between the emitter and the cavity. This coherent coupling introduces
entanglement as it is associated with the formation of new ’dressed’ quan-
tum states and may provide a basis of quantum dot-microcavity systems
for future applications in quantum information processing or schemes for
coherent control [10-13]. Evidence of strong coupling is usually manifest
in the emission spectrum that displays anti-crossing between the quantum
dot (QD) exciton and cavity-mode dispersion relations. Strong coupling is
characterized by a vacuum Rabi splitting of up to a few 100 µeV in state-of-
the-art semiconductor structures. In the following, several aspects of strong
coupling will be addressed exemplarily for quantum-dot micropillar cavity
systems.