QUROPE Quantum Information Processing and Communication in Europe

Diamond spins are an ideal test bed for exploring quantum physics of few well controllable qubit systems. Defect center electron spins show strong coupling to a light field and at the same time interact with few surrounding nuclei in the lattice. As a result the system usually constitutes a few qubit system with excellent coherence and controllability even at room temperature. It fulfills all characteristics of a quantum register including single shot read-out capability.

Spins confined in semiconductor quantum dots offer new possibilities for realizing quantum optical systems with unique properties. Conversely, optical measurement techniques provide a powerful tool for studying mesoscopic condensed-matter systems.

Superconducting quantum circuits with Josephson junctions have shown in the last decade very rich and successful quantum experiments. They appeared as the most promising solid state scalable quantum system for quantum information processor. These superconducting circuits behave as artificial atoms and were extensively studied to test fundamental concepts of quantum mechanics. This research is always in strong development.

We report the observation of photon generation in a microwave cavity with a time-dependent boundary condition. Our system is a microfabricated quarter-wave coplanar waveguide cavity. The electrical length of the cavity is varied by using the tunable inductance of a superconducting quantum interference device. It is measured at a temperature significantly less than the resonance frequency. When the length is modulated at approximately twice the static resonance frequency, spontaneous parametric oscillations of the cavity field are observed.

Helical modes, conducting opposite spins in opposite directions, are shown to exist in metallic armchair nanotubes in an all-electric setup. This is a consequence of the interplay between spin orbit interaction and strong electric fields. The helical regime can also be obtained in chiral metallic nanotubes by applying an additional magnetic field. In particular, it is possible to obtain helical modes at one of the two Dirac points only, while the other one remains gapped.

Diamond defects allow for precise measurement of single electron and nuclear spin quantum states. The excellent controllability of these spins as well as efficient decoupling from environment make them an ideal playground for engineering complex quantum states and development of elaborate control schemes. The talk will describe how nuclear spin states can be efficiently read-out and used as Qbits in spin clusters. Routes towards the controlled engineering of extended spin arrays as well as coupling to control structures will be discussed.

In circuit quantum electrodynamics (QED) the cavity-mediated dispersive interaction is the dominant inter-qubit coupling mechanism when the qubits are detuned from the resonator. This mechanism can be used to realize two-qubit gates. Here, we investigate the strength of this interaction explicitly considering the Fabry-Perot like multi-mode structure of the microwave frequency transmission line resonator. We observe the formation of dark states when the qubits are driven jointly by the same resonator microwave field and tuned into resonance with each other.