July 2012

Abstracts of the QSIT Lunch Seminar, July 5, 2012

Observation of entanglement between a quantum dot spin and a single photon

Weibo Gao, Quantum Photonics Group, IQE, ETH Zurich

Entanglement plays a central role in fundamental tests of quantum mechanics as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, a major challenge is the efficient generation of entanglement between stationary (spin) and flying (photon) qubits. Here, we report the observation of quantum entanglement between a semiconductor quantum dot spin and the color of a propagating optical photon. The demonstration of entanglement relies on the use of fast single-photon detection which allows us to project the photon into a superposition of its two frequency components. Our results extend the previous demonstrations of single-spin photon entanglement in trapped ions, neutral atoms and nitrogen vacancy centers to the domain of artificial atoms in semiconductor nano-structures that allow for on-chip integration of electronic and photonic elements. Thanks to fast optical transitions and favorable selection rules, the scheme we implement generates inherently deterministic entangled spin-photon pairs at an unprecedented rate of 76:3 megahertz; the pair-rate we measure experimentally is limited by our photon collection/detection efficiency. Our observation constitutes a key first step towards implementation of a quantum network with nodes consisting of semiconductor spin qubit.

Quantum Hall effect with superconducting electrodes

Peter Rickhaus, Nanoelectronics Group, University of Basel

We report on the realization of an integer quantum Hall system with superconducting electrodes. Graphene was contacted to niobium electrodes that show a critical field of about 4 tesla, where electronic transport passes mainly through quantum Hall edge-states and bulk transport is largely suppressed. We find a magnetic field range of more than one tesla where well developed quantum Hall plateaus coexist with superconductivity in the leads. In high magnetic fields with the electrodes in the normal state we observe plateaus at G=e^2/h for n=2, 4, and 10. Reducing the magnetic field to below the upper critical field of the electrodes, the conductance on the plateaus shows a sudden increase. Whereas the conductance on the n=2 plateau increases only by 10%, the increase on the n=6 and n=10 plateau is considerably larger with 60% and 80%, respectively. We attribute this conductance enhancement to multiple Andreev reflection processes along the graphene-superconductor interface, that lead to the formation of Andreev edge-states. The observed conductance enhancement of the n=6 and 10 plateaus is consistent with a doubling of the conductance contribution of the second and third edge-states. We attribute the small conductance increase on the n=2 plateau to the special nature of the zero energy Landau level, that makes the corresponding edge-state sensitive to the structure of the graphene edge.

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