Critical couplings for chiral symmetry breaking via instantons

Using an instanton effective action formalism, we compute the critical coupling for the nonperturbative formation of a dynamical mass via instantons in non-Abelian gauge theories with N_f massless fermions. Only continuous phase transitions are considered. For large values of N_f the critical couplings are found to be much smaller than the equivalent critical couplings obtained from gauge exchange calculations in the ladder approximation.Comment: 11 pages, 3 figures, signs and i's correcte

Infinitesimal propagation equation for atmospheric decoherence with multiphoton correlations

Previously a set of coupled first order differential equations were derived for the decoherence of a pair of spatial mode entangled photons, propagating along different paths through turbulence. Here we extend this analysis to the situation where both photons travel along the same path, which introduces the effect of multiple photon correlations. The resulting equation now contains additional terms that take these multiphoton correlations into account. At the same time, we provide a more thorough formulation of the quantized field, starting from a Lorentz invariant formulation, which is then explicitly broken by the choice of a particular propagation direction. The effect of the latter improvement in the quantization on the form of the final equation is minimal.Comment: 10 pages, 1 Figures, extension of work done in arXiv:1102.5166v

Lindblad equation for the decay of entanglement due to atmospheric scintillation

The quantum state for the spatial degrees of freedom of photons propagating through turbulence is analyzed. The turbulent medium is modeled by a single phase screen for weak scintillation conditions and by multiple phase screens for general scintillation conditions. In the former case the process is represented by an operator product expansion, leading to an integral expression that is consistent with current models. In the latter case the evolution of the density operator is described by a first order differential equation with respect to the propagation distance. It is shown that this differential equation has the form of a Lindblad master equation. Additionally, it is shown that this differential equation can take on the form of the infinitesimal propagation equation.Comment: 17 pages, 1 figure. Accepted for publication in J. Phys.

Combining spatio-temporal and particle-number degrees of freedom

Quadrature bases that incorporate spatio-temporal degrees of freedom are derived as eigenstates of momentum dependent quadrature operators. The resulting bases are shown to be orthogonal for both the particle-number and spatio-temporal degrees of freedom. Using functional integration, we also demonstrate the completeness of these quadrature bases.Comment: 14 pages, no figures, accepted versio

Optical vortex trajectories in an astigmatic and elliptical Gaussian beam

An optical vortex, produced at one point in an optical beam, would propagate through an optical system to another point where the vortex can be used for some purpose. However, asymmetrical optical elements in such a system can cause astigmatism or at least distroy the rotational symmetry of the beam, which may affect the propagation of the vortex in an undesirable way. While an optical vortex in a rotationally symmetric, stigmatic Gaussian beam retains its initial morphology for as far as it propagates, the morphology of an optical vortex in an asymmetric or astigmatic Gaussian beam changes. The vortex can even be replaced by another with the opposite topological charge. We consider the behavior of single noncanonical vortices propagating in Gaussian beams that are asymmetric and/or astigmatic. General expressions for the vortex trajectories are provided. The locations of the flip planes and the evolution of the anisotropy of the vortex are considered for different non-ideal situations.Comment: 8 pages, 4 figure

Infinitesimal propagation equation for decoherence of an OAM entangled biphoton in atmospheric turbulence

We derive a first order differential equation for the decoherence of an orbital angular momentum entangled biphoton state propagating through a turbulent atmosphere. The derivation is based on the distortion that orbital angular momentum states experience due to propagation through a thin sheet of turbulent atmosphere. This distortion is treated as an infinitesimal transformation leading to a first order differential equation, which we call an infinitesimal propagation equation. The equation is applied to a simple qubit case to show how the entanglement decays.Comment: 12 pages, 3 figures, expanded version of arXiv:1009.1956. Minor corrections to improve clarit

Evolution equation for multi-photon states in turbulence

The recently developed Wigner functional theory is used to formulate an evolution equation for arbitrary multi-photon states, propagating through a turbulent atmosphere under arbitrary conditions. The resulting evolution equation, which is obtained from an infinitesimal propagation approach, is in the form of a Fokker-Planck equation for the Wigner functional of the state and therefore incorporates functional derivatives. We show consistency with previously obtained solutions from different approaches and consider possible ways to find additional solutions for this equation.Comment: 22 pages, no figures, to appear in J. Phys.

Non-Markovian evolution of photonic quantum states in atmospheric turbulence

The evolution of the spatial degrees of freedom of a photon propagating through atmospheric turbulence is treated as a non-Markovian process. Here, we derive and solve the evolution equation for this process. The turbulent medium is modeled by a sequence of multiple phase screens for general turbulence conditions. The non-Markovian perspective leads to a second-order differential equation with respect to the propagation distance. The solution for this differential equation is obtained with the aid of a perturbative analysis, assuming the turbulence is relatively weak. We also provide another solution for more general turbulence strength, but where we introduced a simplification to the differential equation.Comment: 12 pages, no figure

Decoherence of orbital angular momentum entanglement in a turbulent atmosphere

The evolution of an entangled photon state propagating through a turbulent atmosphere is formulated in terms of a set of coupled first order differential equations, by using an infinitesimal propagation approach. The orbital angular momentum (OAM) basis is used to described the density matrix of the state. Although the analysis is done in the paraxial limit for a monochromatic optical field, the formalism is comprehensive in the sense that it does not require any assumptions about the strength of the turbulence and it can incorporate any spectral model for the turbulence. As a comparative example the case of entangled qubit OAM biphoton states is considered.Comment: 4 pages, 1 figure, a few minor change

Quantifying entanglement of parametric down-converted states in all degrees of freedom

The amount of entanglement that exists in a parametric down-converted state is investigated in terms of all the degrees of freedom of the state. We quantify the amount of entanglement by the Schmidt number of the state, represented as a pure bipartite state by tagging the down-converted photons in terms of orthogonal states of polarization with the aid of type II phase-matching. To facilitate our calculations, we use a Wigner functional approach, which allows the incorporation of the full infinite dimensional spatiotemporal degrees of freedom. A quantitative example with reasonably achievable experimental conditions is considered to demonstrate that extremely large Schmidt numbers are achievable.Comment: 6 pages, 2 figure