Abstract (Christian Heide): Precisely controlling the light waveform allows us to manipulate and study processes on a sub-cycle timescale of the laser pulse. Such waveform control opens prospects for technological applications, especially for on-chip signal processing at speeds at optical clock rates. Using a strong light field, it is possible to steer electrons on complex electron trajectories inside 2D materials, such as graphene. Due to the fast timescale involved, the electron dynamics is described fully quantum mechanically, which allows the electron to drive like a matter wave through the band structure. In the vicinity of the band gap, the electron wavefunction can be split into conduction and valence band states. Because these transition events act as electron beam splitters, we have built a light-field-driven electron interferometer akin to a Mach-Zehnder interferometer for light. The quantum phase accumulated in the beam-split state determines in which output port the electron ends up: in the valence or the conduction band, and defines the direction and amount of injected photocurrent. Additionally, by tailoring the electron trajectory with the light field, we can map the accumulated quantum phase and turn the interference on and off, thus, representing an optical analog of a field-effect transistor. 

Although the band structure of graphene allows quantum control most simply, it is even more fascinating to extend the concept of light-field controlled electron dynamics to tailored quantum materials and topologically protected materials. These materials are promising candidates for Floquet engineering or non-dissipative topological quantum electronics.

Research Interests: quantum materials, ultrafast spectroscopy, light-wave electronics, coherent control

Abstract (Sebastien Leger): Quantum impurity problems, that describe the interaction between a degree of freedom (DOF) and an environment, are at the heart of a very rich physics covering fields as diverse as   quantum optics and strongly correlated matter . In this work, we use the tools of circuit QED to address a quantum impurity problem called Boundary Sine Gordon (BSG).

To do so, we wire a highly non-linear SQUID, the DOE, to a multi-mode high impedance cavity, the environment (~4000 modes). The use of a SQUID together with our engineered environment enable us to study the BSG problem from the perturbative regime to a regime where the physics involved remains poorly understood. Thanks to our setup, we could measure the renormalization of the SQUID frequency induced by the interplay between its nonlinearity and the strong interaction with its environment. In addition to this, we have also observed the dissipation induced by the SQUID in the environment modes. The dissipation is materialized by highly non-perturbative photon conversion phenomena where a photon inserted in the cavity can reach a 10% probability of decaying after one round trip. Detailed modeling explains both the dissipation and renormalization quantitatively and confirms that the physics involved is highly non-linear, many-body and quantum.

Research Interests: Circuit QED, Quantum simulation, Many Body Physics

Graduate Student 10 min talk:  Ben Foutty (Feldman Lab)