Clemens Rössler

Spin-Coherent Dot-Cavity Electronics    

C. Rössler, D. Oehri, O. Zilberberg, G. Blatter, M. Karalic, J. Pijnenburg, A. Hofmann, T. Ihn, K. Ensslin, C. Reichl, W. Wegscheider    

Departement of Physics, ETH Zurich, Switzerland   

Quantum physics has profited enormously from combining optical cavities and atoms into a quantum engineering platform, where atoms mediate interactions among photons and photons communicate information between atoms. In mesoscopic physics, the constituents of the optical success story have been independently realized: (i) prototypes of electronic cavities were implemented through structuring of a two-dimensional electron gas (2DEG), yielding extended fermionic modes akin to quantum corrals on metal surfaces, and (ii) artificial atoms in the form of quantum dots have been studied, unraveling numerous interesting phenomena such as the Coulomb blockade and the Kondo effect. The combination of several dots into controlled quantum bits (qubits) has been demonstrated, thus promoting the next challenge of introducing coherent coupling between distant qubits without relying on nearest-neighbor exchange. Analogously to cavity quantum optics, such coherent coupling could be provided by a suitably engineered cavity mode. Here, we report on transport spectroscopy and control of a coherently coupled mesoscopic dot--cavity system, where the cavity is embedded in the drain of the dot. In the weak-coupling regime, we find signatures of standard dot transport enhanced by a tunnel coupling to drain which is modulated by electrostatic control of the cavity. In stark contrast, the strong-coupling regime exhibits the formation of a dot--cavity singlet state testifying to the coherent dynamics of the hybrid system. This singlet competes with and blocks the formation of a Kondo resonance, signaling the presence of a many-body quantum phase transition.  Conjoining our measurements with the large spatial scale of the cavity modes alludes to the dot--cavity hybrid being an original realization of a Kondo box setup. Focussing on applications, it constitutes a tunable and purely electrical component that may serve as a quantum bus for coherently coupling spatially separated qubits.