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.