QUROPE Quantum Information Processing and Communication in Europe

We propose a method to get experimental access to the physics of the ultrastrong (USC) and deep strong (DSC) coupling regimes of light-matter interaction through the quantum simulation of their dynamics in standard circuit QED. The method makes use of a two-tone driving scheme, using state-of-the-art circuit-QED technology, and can be easily extended to general quantum optical cavity-QED setups. We provide examples of USC/DSC quantum effects that would be otherwise unaccessible. 

Ultrafast quantum gates in circuit QED: we have proposed the design of ultrafast gates in circuit QED by means of the access and suitable manipulation of the ultrastrong coupling regime in the qubit-cavity system.

G. Romero, D. Ballester, Y. M. Wang, V. Scarani, and E. Solano  

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.

A quantum coherent interface between optical and microwave photons can be used as a basic building block within a future quantum information network. The interface is envisioned as an ensemble of rare-earth ions coupled to a superconducting resonator allowing for coherent transfer between optical and microwave photons. Towards this end, we have realized a hybrid device coupling a Er^(3+) doped Y_2 SiO_5 crystal in a superconducting coplanar waveguide cavity. We observe a collective spin coupling of 4 MHz and a spin linewdith of down to 70 MHz. 

The Bloch-Redfield equation is a common tool for studying the evolution of qubit systems weakly coupled to the environment. We investigate the accuracy of the Born approximation underlying this equation. We find that the high-order terms in the perturbative expansion contain accumulating divergences that make straightforward Born approximation inappropriate. We develop a diagrammatic technique to formulate, and solve the improved self-consistent Born approximation.

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. 

The UPV/EHU PI, Enrique Solano, has written a popularizing article in a Spanish newspaper about Quantum Teleportation.