372 research outputs found
Nonlinear quantum optics in the (ultra)strong light-matter coupling
The propagation of photons in one dimensional waveguides coupled to
qubits is discussed, both in the strong and ultrastrong qubit-waveguide
coupling. Special emphasis is placed on the characterisation of the nonlinear
response and its linear limit for the scattered photons as a function of ,
, qubit inter distance and light-matter coupling. The quantum evolution is
numerically solved via the Matrix Product States technique. Both the time
evolution for the field and qubits is computed. The nonlinear character (as a
function of ) depends on the computed observable. While perfect reflection
is obtained for , photon-photon correlations are still resolved
for ratios . Inter-qubit distance enhances the nonlinear response.
Moving to the ultrastrong coupling regime, we observe that inelastic processes
are \emph{robust} against the number of qubits and that the qubit-qubit
interaction mediated by the photons is qualitatively modified. The theory
developed in this work modelises experiments in circuit QED, photonic crystals
and dielectric waveguides.Comment: Comments are wellcom
Scattering in the ultrastrong regime: nonlinear optics with one photon
The scattering of a flying photon by a two-level system ultrastrongly coupled
to a one-dimensional photonic waveguide is studied numerically. The photonic
medium is modeled as an array of coupled cavities and the whole system is
analyzed beyond the rotating wave approximation using Matrix Product States. It
is found that the scattering is strongly influenced by the single- and
multi-photon dressed bound states present in the system. In the ultrastrong
coupling regime a new channel for inelastic scattering appears, where an
incident photon deposits energy into the qubit, exciting a photon-bound state,
and escaping with a lower frequency. This single-photon nonlinear frequency
conversion process can reach up to 50\% efficiency. Other remarkable features
in the scattering induced by counter-rotating terms are a blueshift of the
reflection resonance and a Fano resonance due to long-lived excited statesComment: 5+4 page
Single photons by quenching the vacuum
Heisenberg's uncertainty principle implies that the quantum vacuum is not
empty but fluctuates. These fluctuations can be converted into radiation
through nonadiabatic changes in the Hamiltonian. Here, we discuss how to
control this vacuum radiation, engineering a single-photon emitter out of a
two-level system (2LS) ultrastrongly coupled to a finite-band waveguide in a
vacuum state. More precisely, we show the 2LS nonlinearity shapes the vacuum
radiation into a nonGaussian superposition of even and odd cat states. When the
2LS bare frequency lays within the band gaps, this emission can be well
approximated by individual photons. This picture is confirmed by a
characterization of the ground and bound states, and a study of the dynamics
with matrix product states and polaron Hamiltonian methods.Comment: 9 pages, 10 figure
One- and two-photon scattering from generalized V-type atoms
The one- and two-photon scattering matrix S is obtained analytically for a
one-dimensional waveguide and a point-like scatterer with N excited levels
(generalized V -type atom). We argue that the two-photon scattering matrix
contains sufficient information to distinguish between different level
structures which are equivalent for single-photon scattering, such as a V -atom
with N = 2 excited levels and two two-level systems. In particular, we show
that the scattering with the V -type atom exhibits a destructive interference
effect leading to two-photon Coupled-Resonator-Induced Transparency, where the
nonlinear part of the two-photon scattering matrix vanishes when each incident
photon fulfills a single-photon condition for transparency
Emergent Causality and the N-photon Scattering Matrix in Waveguide QED
In this work we discuss the emergence of approximate causality in a general
setup from waveguide QED -i.e. a one-dimensional propagating field interacting
with a scatterer. We prove that this emergent causality translates into a
structure for the N-photon scattering matrix. Our work builds on the derivation
of a Lieb-Robinson-type bound for continuous models and for all coupling
strengths, as well as on several intermediate results, of which we highlight
(i) the asymptotic independence of space-like separated wave packets, (ii) the
proper definition of input and output scattering states, and (iii) the
characterization of the ground state and correlations in the model. We
illustrate our formal results by analyzing the two-photon scattering from a
quantum impurity in the ultrastrong coupling regime, verifying the cluster
decomposition and ground-state nature. Besides, we generalize the cluster
decomposition if inelastic or Raman scattering occurs, finding the structure of
the S-matrix in momentum space for linear dispersion relations. In this case,
we compute the decay of the fluorescence (photon-photon correlations) caused by
this S-matrix
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