QUROPE Quantum Information Processing and Communication in Europe

Around the globe several observatories are seeking the first direct detection of gravitational waves (GWs). These waves are predicted by Einsteins general theory of relativity and are generated, for example, by black-hole binary systems. Present GW detectors are Michelson-type kilometre-scale laser interferometers measuring the distance changes between mirrors suspended in vacuum. The sensitivity of these detectors at frequencies above several hundred hertz is limited by the vacuum (zero-point) fluctuations of the electromagnetic field.

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Superconducting qubits shine out among other qubit implementations with respect to their excellent scalability. In collaboration with the group of E. Ilichev at IPHT in Jena, the Karlsruhe experimental group of A. Ustinov has performed experiments on a chip containing 7 flux qubits. A newly developed measurement scheme based on frequency multiplexing allowed the researchers to read out all qubits simultaneously, using only a single signal cable.

We have realized a novel device in which a semiconductor double quantum dot is dipole coupled to a GHz-frequency high-quality transmission line resonator. This approach allows us to characterize the properties of the double dot by measuring both its dispersive and dissipative interaction with the resonator. In addition to providing a new readout mechanism, this architecture has the potential to isolate the dots from the environment and to provide long distance coupling between spatially separated dots.

The Toffoli gate is a three-qubit operation that inverts the state of a target qubit conditioned on the state of two control qubits. It enables universal reversible classical computation, forms a universal set of gates in quantum computation together with a Hadamard gate and is also a key element in quantum error correction schemes. Here, Andreas Wallraff and colleagues at ETH-Zurich demonstrated the implementation of a Toffoli gate with three superconducting transmon qubits coupled to a microwave resonator.

Teleportation of a quantum state may be used for distributing entanglement between distant qubits in quantum communication and for quantum computation. In this publication, the group of Andreas Wallraff at ETH Zurich demonstrated the implementation of a teleportation protocol, up to the single-shot measurement step, with superconducting qubits coupled to a microwave resonator. Using full quantum state tomography and evaluating an entanglement witness, they show that the protocol generates a genuine tripartite entangled state of all three qubits.

A team of scientists of the Kavli Institute of Nanoscience of TU Delft and the FOM Foundation has succeeded in very accurately initializing and reading out a mini-quantum computer comprising four quantum bits on a chip of diamond. This breakthrough marks an important step towards a quantum computer and makes it possible to test advanced quantum protocols, such as teleportation, on a chip. The researchers have published their results in Nature 477, 574 (2011).   http://www.nature.com/nature/journal/v477/n7366/full/nature10401.html

We have embedded an artificial atom, a superconducting "transmon" qubit, in an open transmission line and investigated the strong scattering of incident microwave photons (~GHz). When an input coherent state, with an average photon number N<<1 is on resonance with the artificial atom, we observe extinction of up to 90% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT).