QUROPE Quantum Information Processing and Communication in Europe

The authors establish methods for quantum state tomography based on compressed sensing. These methods are specialized for quantum states that are fairly pure, and they offer a significant performance improvement on large quantum systems: they require only simple Pauli measurements, use fast convex optimization, are stable against noise, and can be applied to states that are only approximately low rank.

As part of the WP3 activities QUIE2T presented an exhibit at the ICT 2010 conference in Brussels, 27 – 29 September 2010. The event was very well-attended both by specialists and non-specialists including well-known media people.

We report the creation of Greenberger-Horne-Zeilinger states with up to 14 qubits. By investigating the coherence of up to 8 ions over time, we observe a decay proportional to the square of the number of qubits. The observed decay agrees with a theoretical model which assumes a system affected by correlated, Gaussian phase noise. This model holds for the majority of current experimental systems developed towards quantum computation and quantum metrology.

Strong coupling between a microwave photon and electron spins, which could enable a long-lived quantum memory element for superconducting qubits, is possible using a large ensemble of spins. This represents an inefficient use of resources unless multiple photons, or qubits, can be orthogonally stored and retrieved. Here we employ holographic techniques to realize a coherent memory using a pulsed magnetic field gradient and demonstrate the storage and retrieval of up to 100 weak 10 GHz coherent excitations in collective states of an electron spin ensemble.

Scientists at the Max Planck Institute of Quantum Optics succeed in recording single-atom resolved images of a highly correlated quantum gas.

Device-Independent Quantum Key Distribution protocols aim at establishing a secret key between two honest parties whose security is independent of the details of the devices used in the protocol. In this work, the authors propose an implementation of this idea using a noiseless photon amplifier. The proposal suggest that positive secret-key rates may be obtained using near-future technology.

The german science magazine Spektrum der Wissenschaften publishes a special edition on Quantum Information.

The uncertainty principle bounds the uncertainties about the outcomes of two incompatible measurements on a particle. However, if the particle is prepared entangled with a quantum memory it is possible to predict the outcomes for both measurement choices precisely. In this work, the authors extend the uncertainty principle to this scenario, providing a lower bound on the uncertainties which depends on the amount of entanglement between the particle and the quantum memory.

Physicists at MPQ control the optical properties of a single atom!

Due to the continued miniaturization of computer chip components, we are about to cross a fundamental boundary where technology can no longer rely on the laws of the macroscopic world. With this in mind, scientists all over the world are researching technologies based on quantum effects that can be used to communicate and process information. One of the most promising developments in this direction are quantum networks in which single photons communicate the information between different nodes, e.g. single atoms. There the information can be stored and processed. A key element in these systems is Electromagnetically Induced Transparency (EIT), an effect that allows to radically change the optical properties of an atomic medium by means of light. Previously, scientists have studied this effect and its amazing properties, using atomic ensembles with hundreds of thousands of atoms. Now, scientists in the group of Prof. Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics (MPQ) in Garching and Head of the Quantum Dynamics Division, have managed to control the optical response of a single atom using laser light (Nature, Advanced Online Publication, DOI: 10.1038 /nature09093). While representing a corner stone in the development of new quantum based technologies, these results are also fundamental for the understanding of how the quantum behaviour of single atoms can be controlled with light.

Complex networks represent one of the most active research topics, as they found application in many different scenarios, from studied of internet to propagation of diseases. In this work, the authors extend the concept of a random network, arguably the simplest example of complex network, to the quantum domain. They show that the obtained model has a completely distinct behavior of the critical probabilities at which different subgraphs appear.