Circuit QED systems of artificial atoms interacting with microwaves are promising tools for quantum simulations and quantum information processing. We investigate whether a circuit QED system containing a large number of artificial atoms can undergo a superradiant phase transition. By applying a no-go theorem for such phase transitions known for cavity QED systems with real atoms to circuit QED systems, we find that the currently accepted standard description of circuit QED fails in an important aspect as it erroneously predicts the possibility of a superradiant phase transition. We generalize the no-go theorem to the case of (artificial) atoms with many energy levels and thus make it more applicable for realistic cavity or circuit QED systems.
[more] Using resonant optical absorption of light
by a self-organized quantum dot, we show that the interactions that give rise to the Kondo effect can be switched off by the absorption of a single photon. We obtain detailed insights into the subsequent decay of
Kondo correlations. In particular, we find signatures of Anderson orthogonality in the absorption lineshapes, with power-law exponents tunable by magnetic field. Our study opens the door toward using quantum optics techniques to study many-body phenomena.
[more] Clebsch-Gordan coefficients (CGCs) arise when decomposing the tensor product of two irreducible representations (irreps) into a direct sum of
irreps. The CGCs of the group SU(N) are also useful for the numerical treatment of models with SU(N) symmetry when exploiting the Wigner-Eckart theorem. We present a numerical algorithm (and a computer
implementation thereof) which produces explicit tables of SU(N) CGCs for two arbitrary irreps and arbitrary N. The algorithm represents a straightforward generalization of a similar scheme for SU(2).
[more] We show that DMRG spectral functions can be calculated very efficiently by expanding the spectral function in terms of Chebyshev polynomials
(CheMPS). The resulting Chebyshev recursion relation for MPS states is numerically stable and can be calculated efficiently using standard
MPS tools. The calculated spectral function is of almost equal accuracy for all frequencies and high spectral resolution is also possible. Compared to previous DMRG standard methods (correction vector) a huge speedup is achieved: the time required to calculate an entire spectral function is typically a factor of 10 times shorter than that needed for a single data point using the correction vector method.
[more]