Ultrafast quantum optics with solid state nanosystems
Semiconductor quantum dots
Semiconductor quantum dots are one of the most promising candidates for implementing robust qubits in quantum information processing. The possibility to optically initialize, manipulate and read out the charge and spin state on ultrafast timescales makes them very attractive. We use femtosecond pump-probe experiments to study the ultrafast dynamics in single semiconductor quantum dots .
To reach the limit of only a single photon manipulating a single electron in a quantum dot the coupling of light with a wavelength of a few hundred nanometers into the object of nanometer dimensions has to be optimized. We mainly study two nanophotonic elements to reach this goal: dielectric microcavities [2, 3] and metallic nanoantennas [4, 5].
Color centers in diamond
Alternatively, we investigate color centers in diamond. Since their first optical characterization on a single emitter level, they have been used as robust single-photon sources. We optically study these nanoscopic light emitters, which operate at room-temperature [6, 7]. Photonic nanostructures are designed and fabricated to increase light coupling.
Atomically thin semiconductors
Graphene is an exceptional two-dimensional material, but a zero band gap semiconductor. In contrast, a monolayer of MoS2 has a band gap and emits photons in the visible regime. We have shown that MoSe2 and WSe2 also shine bright in monolayer form . We found bright and stable single-photon emitters in monolayer WSe2  and could position them on the nanoscale . We coupled a metal nanoantenna to atomically thin WS2 and enhanced the photoluminescence by one order of magnitude . We elucidated the ultrafast valley dynamics for atomically thin WS2 . We tuned the excitons in monolayer WSe2 with uniaxial strain . We studied monolayers in high magnetic fields: we measured the g-factor of excitons in monolayer MoTe2  and explained the magnetic-field-induced rotation of polarized light emission from monolayer WS2  .
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