Kondo Resonance of a Microwave Photon

We emulate renormalization group models, such as the Spin-Boson Hamiltonian or the anisotropic Kondo model, from a quantum optics perspective by considering a superconducting device. The infra-red confinement involves photon excitations of two tunable transmission lines entangled to an artificial spin-1/2 particle or double-island charge qubit. Focusing on the propagation of microwave light, in the underdamped regime of the Spin-Boson model, we identify a many-body resonance where a photon is absorbed at the renormalized qubit frequency and reemitted forward in an elastic manner. We also show that asymptotic freedom of microwave light is reached by increasing the input signal amplitude at low temperatures which allows the disappearance of the transmission peak.Comment: Final Version: Main text and Supplementary Materia

Shot Noise in SU(N) Quantum Dot Kondo Effects

We study shot noise in the current of quantum dots whose low-energy behaviour corresponds to an SU(N) Kondo model, focusing on the case N=4 relevant to carbon nanotube dots. For general N, two-particle Fermi liquid interactions have two distinct effects: they can enhance the noise via back-scattering processes with an N-dependent effective charge, and can also modify the coherent partition noise already present without interactions. For N=4, in contrast to the SU(2) case, interactions enhance shot noise solely through an enhancement of partition noise. This leads to a non-trivial prediction for experiment.Comment: 4+ pages; error in numerical prefactor describing interaction effect on noise correcte

Neutral scalar Higgs bosons in the USSM at the LHC

We study the possibility of discovering neutral scalar Higgs bosons in the $U(1)'$-extended supersymmetric standard model (USSM) at the CERN Large Hadron Collider (LHC), by examining their productions via the exotic quark loop in the gluon fusion process at leading order. It is possible in some parameter region that the neutral scalar Higgs bosons may have stronger couplings with the exotic quarks than with top quark. In this case, the exotic quarks may contribute more significantly than top quark in productions of the neutral scalar Higgs bosons in the gluon fusion process. We find that there is indeed some parameter region in the USSM that supports our speculations.Comment: 18 pages; changed content; JPhys

Double symmetry breaking and 2D quantum phase diagram in spin-boson systems

The quantum ground state properties of two independent chains of spins (two-levels systems) interacting with the same bosonic field are theoretically investigated. Each chain is coupled to a different quadrature of the field, leading to two independent symmetry breakings for increasing values of the two spin-boson interaction constants $\Omega_C$ and $\Omega_I$. A phase diagram is provided in the plane ($\Omega_C$,$\Omega_I$) with 4 different phases that can be characterized by the complex bosonic coherence of the ground states and can be manipulated via non-abelian Berry effects. In particular, when $\Omega_C$ and $\Omega_I$ are both larger than two critical values, the fundamental subspace has a four-fold degeneracy. Possible implementations in superconducting or atomic systems are discussed

Quantum criticality in spin chains with non-ohmic dissipation

We investigate the critical behavior of a spin chain coupled to bosonic baths characterized by a spectral density proportional to $\omega^s$, with $s>1$. Varying $s$ changes the effective dimension $d_\text{eff} = d + z$ of the system, where $z$ is the dynamical critical exponent and the number of spatial dimensions $d$ is set to one. We consider two extreme cases of clock models, namely Ising-like and U(1)-symmetric ones, and find the critical exponents using Monte Carlo methods. The dynamical critical exponent and the anomalous scaling dimension $\eta$ are independent of the order parameter symmetry for all values of $s$. The dynamical critical exponent varies continuously from $z \approx 2$ for $s=1$ to $z=1$ for $s=2$, and the anomalous scaling dimension evolves correspondingly from $\eta \gtrsim 0$ to $\eta = 1/4$. The latter exponent values are readily understood from the effective dimensionality of the system being $d_\text{eff} \approx 3$ for $s=1$, while for $s=2$ the anomalous dimension takes the well-known exact value for the 2D Ising and XY models, since then $d_{\rm{eff}}=2$. A noteworthy feature is, however, that $z$ approaches unity and $\eta$ approaches 1/4 for values of $s < 2$, while naive scaling would predict the dissipation to become irrelevant for $s=2$. Instead, we find that $z=1,\eta=1/4$ for $s \approx 1.75$ for both Ising-like and U(1) order parameter symmetry. These results lead us to conjecture that for all site-dissipative $Z_q$ chains, these two exponents are related by the scaling relation $z = \text{max} {(2-\eta)/s, 1}$. We also connect our results to quantum criticality in nondissipative spin chains with long-range spatial interactions.Comment: 8 pages, 6 figure

Hidden Caldeira-Leggett dissipation in a Bose-Fermi Kondo model

We show that the Bose-Fermi Kondo model (BFKM), which may find applicability both to certain dissipative mesoscopic qubit devices and to heavy fermion systems described by the Kondo lattice model, can be mapped exactly onto the Caldeira-Leggett model. This mapping requires an ohmic bosonic bath and an Ising-type coupling between the latter and the impurity spin. This allows us to conclude unambiguously that there is an emergent Kosterlitz-Thouless quantum phase transition in the BFKM with an ohmic bosonic bath. By applying a bosonic numerical renormalization group approach, we thoroughly probe physical quantities close to the quantum phase transition.Comment: Final version appearing in Physical Review Letter