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Recent advances of femtosecond semiconductor physics at the limits of single electrons and photons down to sub-cycle
time scales are presented. The first part deals with ultrafast measurements on single-electron systems: The transient
quantum dynamics in a single CdSe/ZnSe quantum dot is investigated via femtosecond transmission spectroscopy. A
two-color Er:fiber laser with excellent noise performance is key to these first resonant pump-probe measurements on a
single-electron system. We have observed ultrafast bleaching of an electronic transition in a single quantum dot due to
instantaneous Coulomb renormalization. Since we were also able to invert the two-level system, optical gain due to a
single electron has been detected. By using π-pulses for probing, we could deterministically add or remove a single
photon to or from a femtosecond light pulse, leading to non-classical states of the light field. In order to optimize
electron-photon coupling, nanophotonic concepts like dielectric microresonators and metal optical antennas are explored.
In the second part of the paper, we present multi-terahertz measurements on low-energy excitations in semiconductors.
These studies lead towards a future time-domain quantum optics on a time scale of single cycles of light: Intense multiterahertz
fields of order MV/cm are used to coherently promote optically dark and dense para excitons in Cu2O from the
1s into the 2p state. The nonlinear field response of the intra-excitonic degrees of freedom is directly monitored in the
time domain via ultrabroadband electro-optic sampling. The experimental results are analyzed with a microscopic many-body
theory, identifying up to two internal Rabi cycles. Subsequently, intersubband cavity polaritons in a quantum well
waveguide structure are optically generated within less than one cycle of light by a femtosecond near-infrared pulse.
Mid-infrared probe transients trace the non-adiabatic switch-on of ultrastrong light-matter coupling and the conversion of
bare photons into cavity polaritons directly in the time domain.
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