We study the time and space resolved dynamics of a qubit with an Ohmic coupling to propagating 1D photons, from weak coupling to the ultrastrong coupling regime. A nonperturbative study based on Matrix Product States (MPS) shows the following results
: (i) The ground state of the combined systems contains excitations of both the qubit and the surrounding bosonic field. (ii) An initially excited qubit equilibrates through spontaneous emission to a state, which under certain conditions, is locally close to that ground state, both in the qubit and the field. (iii) The resonances of the combined qubit-photon system match those of the spontaneous emission process and also the predictions of the adiabatic renormalization [A. J. Leggett et al., Rev. Mod. Phys. 59, 1, (1987)]. Finally, a non-perturbative ab-initio calculations show that this physics can be studied using a flux qubit galvanically coupled to a superconducting transmission line.
We investigate creation, manipulation, and steering of entanglement in spin chains from the viewpoint of quantum communication between distant parties. We demonstrate how global parametric driving of the spin-spin coupling and/or local time-dependent
Zeeman fields produce a large amount of entanglement between the first and the last spin of the chain. This occurs whenever the driving frequency meets a resonance condition, identified as entanglement resonance. Our approach marks a promising step towards an efficient quantum state transfer or teleportation in solid state system. Following the reasoning of Zueco et al. [1], we propose generation and routing of multipartite entangled states by use of symmetric tree-like structures of spin chains. Furthermore, we study the effect of decoherence on the resulting spin entanglement between the corresponding terminal spins.
