QUROPE Quantum Information Processing and Communication in Europe

We advocate a Bayesian approach to optimal quantum frequency estimation - an important problem for future quantum enhanced atomic clock operation. The approach provides a clear insight into the interplay between decoherence and the extent of the prior knowledge in determining the optimal interrogation times and optimal estimation strategies.

Accurately controlling a quantum system is a fundamental requirement in quantum information processing and the coherent manipulation of molecular systems. The ultimate goal in quantum control is to prepare a desired state with the highest fidelity allowed by the available resources and the experimental constraints. Here we experimentally implement two optimal high-fidelity control protocols using a two-level quantum system comprising Bose–Einstein condensates in optical lattices.

We present a scheme for achieving coherent spin squeezing of nuclear spin states in semiconductor quantum dots. The nuclear polarization dependence of the electron spin resonance generates a unitary evolution that drives nuclear spins into a collective entangled state. The polarization dependence of the resonance generates an area-preserving, twisting dynamics that squeezes and stretches the nuclear spin Wigner distribution without the need for nuclear spin flips. Our estimates of squeezing times indicate that the entanglement threshold can be reached in current experiments. 

We investigate the thermodynamics of a combined Dicke and Ising model that exhibits a rich phenomenology arising from the second-order and quantum phase transitions from the respective models. The partition function is calculated using mean-field theory, and the free energy is analyzed in detail to determine the complete phase diagram of the system.

It is expected that ion trap quantum computing can be made scalable through protocols that make use of transport of ion qubits between sub-regions within the ion trap. In this scenario, any magnetic field inhomogeneity the ion experiences during the transport, may lead to dephasing and loss of fidelity. Here we demonstrate a scalable way to measure the magnetic field gradient inside a segmented ion trap, by transporting a single ion over variable distances.