Matyas Mechler

Entanglement criteria based on local uncertainty relations are strictly stronger than the computable cross norm criterion

We show that any state that violates the computable cross norm (or realignment) criterion for separability also violates the separability criterion of the local uncertainty relations. The converse is not true. The local uncertainty relations provide a straightforward construction of nonlinear entanglement witnesses for the cross norm criterion.

Optimization of periodic single-photon sources

We introduce a theoretical framework which is suitable for the description of all spatial and time-multiplexed periodic single-photon sources realized or proposed thus far. Our model takes into account all possibly relevant loss mechanisms. This statistical analysis of the known schemes shows that multiplexing systems can be optimized in order to produce maximal single-photon probability for various sets of loss parameters by the appropriate choice of the number of multiplexed units of spatial multiplexers or multiplexed time intervals and the input mean photon pair number, and reveals the physical reasons of the existence of the optimum. We propose a novel time-multiplexed scheme to be realized in bulk optics, which, according to the present analysis, would have promising performance when experimentally realized. It could provide a single-photon probability of 85\% with a choice of experimental parameters which are feasible according to the experiments known from the literature.

Near-field diffraction in a two-dimensional V-groove and its role in SERS

Optical field distribution in micro-nano geometries of miniaturized optical devices is often significantly different from that in identical but macroscopic geometries. Plasmon effects and near-field diffraction can enhance the local field intensity, leading to enhanced cross section for light absorption and scattering, which can be utilized in substrate-enhanced spectroscopies for the detection of trace amounts of adsorbed chemicals. A specific problem is an ingenious but only empirically described way to enhance signal intensity in Raman spectroscopy by the use of a substrate patterned with gold coated micron size pyramidal pits. While Raman enhancement on nanostructured substrates is generally attributed to surface plasmons, here the micron size, and thus the sub-wavelength to near-wavelength dimensions suggest that resonant enhancement emanating from optical near-field diffraction might also play a role. To answer this question, light diffraction in a projection of the pyramidal pit: a V-groove, was modelled with a modified Neerhoff-Mur formalism suitable to calculate electromagnetic field distribution in sub-wavelength structures. Under the boundary conditions a perfect conductor screen was assumed, which excludes plasmon effects. The calculations show that interference in the cavity causes a modest resonant increase in local intensity and that near-field diffraction strongly influences the field distribution, which is explained with the electrodynamic edge effect. The magnitude of the resonant electric field on its own cannot account for the experimentally observed Raman enhancement. However, a resonant enhancement of a similar magnitude is expected for the emitted Stokes frequencies. In this case the geometry implements the conditions for the classical electromagnetic Raman enhancement, ~E(4), in a good agreement with experimental results.

Optimization of periodic single-photon sources based on combined multiplexing

We consider periodic single-photon sources with combined multiplexing in which the outputs of several time-multiplexed sources are spatially multiplexed.We give a full statistical description of such systems in order to optimize them with respect to maximal single-photon probability.We carry out the optimization for a particular scenario which can be realized in bulk optics and its expected performance is extremely good at the present state of the art. We find that combined multiplexing outperforms purely spatially or time-multiplexed sources for certain parameters only, and we characterize these cases. Combined multiplexing can have the advantages of possibly using less nonlinear sources, achieving higher repetition rates, and the potential applicability for continuous pumping. We estimate an achievable single-photon probability between 85% and 89%. DOI: 10.1103/

Construction of quantum states by special superpositions of coherent states

We consider the optimal approximation of certain quantum states of a harmonic oscillator with the superposition of a finite number of coherent states in phase space placed either on an ellipse or on a certain lattice. These scenarios are currently experimentally feasible. The parameters of the ellipse and the lattice and the coefficients of the constituent coherent states are optimized numerically, via a genetic algorithm, in order to obtain the best approximation. It is found that for certain quantum states the obtained approximation is better than the ones known from the literature thus far.

Prospects of semiconductor terahertz pulse sources

Extremely high pump-to-terahertz (THz) conversion efficiencies up to 0.7% were demonstrated in recent experiments with ZnTe THz pulse sources. Such high efficiencies could be achieved by pumping at an infrared wavelength sufficiently long to suppress both two- and three-photon absorption and the associated free-carrier absorption at THz frequencies. Here, high-field high-energy THz pulse generation by optical rectification in semiconductor nonlinear materials is investigated by numerical simulations. Basic design aspects of infrared-pumped semiconductor THz sources are discussed. Optimal pumping and phase-matching conditions are given. Multicycle THz pulse generation for particle acceleration is discussed.

Prospects of Semiconductor Terahertz Pulse Sources

In the low-frequency THz range optical rectification (OR) in LiNbOs (LN) has been providing the highest THz pulse energies, but limitations became apparent [I]. The potential of semiconductor nonlinear optical materials for high-energy high-field THz pulse generation by OR has been recently demonstrated [2-4]. Whereas pumping OR in ZnTe at 0.8 μm, near its collinear phase-matching wavelength, resulted in maximum 1.5 μJ THz pulse energy at 3.1×105 efficiency [5], pumping at 1.7 μm wavelength recently resulted in more than two orders of magnitude higher efficiency, as high as 0.7%, and 14 μJ THz pulse energy [4]. The reason for the enormous increase in efficiency was the elimination of lower-order (2nd- and 3rd-order) multiphoton pump absorption at the longer pump wavelength and the associated free-carrier absorption in the THz range. An important advantage of semiconductors over LN is the much smaller required pulse-front tilt angle of <30°, in contrast to 63° in LN. This enabled the realization of a monolithic contact-grating THz source easily scalable to high energies [3].

Forward-scattered wave analysis of an optical Hardy-like setup

We analyze a photon interferometric scenario that is closely similar to that of the gedanken experiment of Hardy, where the annihilation of a particle–antiparticle pair is replaced by the interference of two photons on a beam splitter. We discuss its relation to Hardys paradox. We calculate the forward-scattered waves of the output beam splitters for this setup and analyze their entanglement-like structure.

Generation and Application Possibilities of Terahertz Pulses with Extremely High Field Strength

Generation and application possibilities of terahertz pulses with 10 MV/cm peak electric field strength is discussed. Production of single-cycle or shaped EUV pulses with sub-nJ energy by coherent Thomson-scattering on ultrashort electron bunches is predicted.