QUROPE Quantum Information Processing and Communication in Europe

We extend the Mermin-Wagner theorem to a system of lattice spins which are spin coupled to itinerant and interacting charge carriers. We use the Bogoliubov inequality to rigorously prove that neither (anti-) ferromagnetic nor helical long-range order is possible in one and two dimensions at any finite temperature. Our proof applies to a wide class of models including any form of electron-electron and single-electron interactions that are independent of spin.

The main characteristics of good qubits are long coherence times in combination with fast operating times. It is well known that carbon-based materials could increase the coherence times of spin qubits, which are among the most developed solid-state qubits. Here, we propose how to form spin qubits in graphene quantum dots. A crucial requirement to achieve this goal is to find quantum-dot states where the usual valley degeneracy in bulk graphene is lifted. We show that this problem can be avoided in quantum dots based on ribbons of graphene with armchair boundaries.

Helical modes, conducting opposite spins in opposite directions, are shown to exist in metallic armchair nanotubes in an all-electric setup [1] and in SiGe nanowires [2]. 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.

Tunable microwave resonators in superconducting circuits present ideal devices to implement and investigate parametric resonance phenomena. Josephson junctions incorporated in the resonator allow to tune the resonance frequency over a wide range owing to the junctions intrinsic nonlinearity, which can be designed at a desired strength. By the same means the resonator frequency can be modulated at a rate of the order of the resonance frequency itself. Furthermore, superconducting resonators can be operated in the quantum regime due to small internal losses.

Macroscopic quantum dynamics of Josephson tunnel junctions is analogous to a motion of fictitious particle in an adiabatic Josephson potential. This analogy relies upon equilibrium state of quasiparticles, protected by a wide superconducting energy gap. In junctions containing low energy quasiparticles, the physical picture drastically differs due to a non-­‐adiabatic current component generated by quasiparticles driven far away from equilibrium by the Josephson oscillation.

La simulación cuántica consiste en la reproducción artificial de un comportamiento específico, propio de un sistema físico, en otro sistema al que le es completamente ajeno. De algún modo la simulación cuántica representa la llegada del arte del teatro al mundo cuántico. Presentaremos una introducción pedagógica con ejemplos de propuestas novedosas y experimentos realizados en los últimos años.

A topologically-protected qubit can be encoded using Majorana fermions in a trapped-ion chain. This
qubit is protected against major sources of decoherence, while local operations and measurements can
be easily realized. Furthermore, an efficient quantum interface and memory for arbitrary multiqubit
photonic states can be built, encoding them into a set of entangled Majorana-fermion (MF) qubits
inside cavities. 

We briefly review some landmarks in quantum measurements of the internal and external degrees of freedom in trapped ions. We present also novel developments, some of which have been implemented in recent trapped-ion experiments.