QUROPE Quantum Information Processing and Communication in Europe

Quantum-state engineering is of critical importance to the development of quantum technologies. One promising platform is thermal atomic vapours, because they offer long coherence times with reproducible and scalable hardware. However, the inability to address isolated atomic states in a controlled manner, due to multi-level degeneracy and motional broadening, is a major obstacle to their wider application.

We show how the entanglement contained in states of spins arranged on a lattice may be quantified with observables arising in scattering experiments. We focus on the partial differential cross-section obtained in neutron scattering from magnetic materials but our results are sufficiently general such that they may also be applied to, e.g., optical Bragg scattering from ultracold atoms in optical lattices or from ion chains.

We introduce an operational entanglement classification of symmetric mixed states for an arbitrary number of qubits under stochastic local operations and classical communication (SLOCC). We define families of entanglement classes successively embedded into each other, prove that they are of non-zero measure, and construct witness operators to distinguish them. Moreover, we discuss how arbitrary symmetric mixed states can be realized in the lab via a one-to-one correspondence between well-defined sets of controllable parameters and the corresponding entanglement families.

The generation of entanglement between two oscillators that interact via a common reservoir is theoretically studied. The reservoir is modeled by a one-dimensional harmonic crystal initially in thermal equilibrium. Starting from a separable state, the oscillators can become entangled after a transient time, that is of the order of the thermalization time scale. This behavior is observed at finite temperature even when the oscillators are at a distance significantly larger than the crystal's interparticle spacing.