- J. Kofler, T. Paterek, Č. Brukner, Experimenter's Freedom in Bell's Theorem and Quantum Cryptography, Phys. Rev. A 73, 022104, (2006-02-06), (ID: 330319) Toggle Abstract
Bell's theorem states that no local realistic explanation of quantum mechanical predictions is possible, in which the experimenter has a freedom to choose between different measurement settings. Within a local realistic picture the violation of Bell's inequalities can only be understood if this freedom is denied. We determine the minimal degree to which the experimenter's freedom has to be abandoned, if one wants to keep such a picture and be in agreement with the experiment. Furthermore, the freedom in choosing experimental arrangements may be considered as a resource, since its lacking can be used by an eavesdropper to harm the security of quantum communication. We analyze the security of quantum key distribution as a function of the (partial) knowledge the eavesdropper has about the future choices of measurement settings which are made by the authorized parties (e.g., on the basis of some quasi-random generator). We show that the equivalence between the violation of Bell's inequality and the efficient extraction of a secure key—which exists for the case of complete freedom (no setting knowledge)—is lost unless one adapts the bound of the inequality according to this lack of freedom.
Alternative URL (local restricted copy) - J. Kofler, Č. Brukner, Entanglement distribution revealed by macroscopic observationset, Phys. Rev. A 74, 050304, (2006-11-16), URL DOI: 10.1103/PhysRevA.74.050304 (ID: 420355) Toggle Abstract
What can we learn about entanglement between individual particles in macroscopic samples by observing
only the collective properties of the ensembles? Using only a few experimentally feasible collective properties,
we establish an entanglement measure between two samples of spin-1/2 particles as representatives of twodimensional
quantum systems. This is a tight lower bound for the average entanglement between all pairs of
spins in general and is equal to the average entanglement for a certain class of systems. We compute the
entanglement measures for explicit examples and show how to generalize the method to more than two
samples and multipartite entanglement.t„
- A. Zeilinger, J. Kofler, La dissolution du paradoxe, Sciences et Avenir Hors-Série 148, 54, (2006-10-00), URL (ID: 411778) Toggle Abstract
INFORMATION QUANTIQUE. L’interprétation de la théorie quantique à l’aide de la notion d’information conduit à lever le paradoxe du chat en montrant que l’indétermination
objective est une conséquence du manque fondamental d’information.
Alternative URL (local restricted copy) - J. Kofler, N. Arnold, Axially symmetric focusing as a cuspoid diffraction catastrophe: Scalar and vector cases and comparison with the theory of Mie, Phys. Rev. B 73, 235401, (2006-06-02), doi:10.1103/PhysRevB.73.235401 (ID: 411773) Toggle Abstract
An analytical description of arbitrary strongly aberrated axially symmetric focusing is developed. This is done by matching the solution of geometrical optics with a wave pattern which is universal for the underlying ray structure. The corresponding canonical integral is the Bessoid integral, which is a three-dimensional generalization of the Pearcey integral that approximates the field near an arbitrary two-dimensional cusp. We first develop the description for scalar fields and then generalize it to the vector case. As a practical example the formalism is applied to the focusing of light by transparent dielectric spheres with a few wavelengths in diameter. The results demonstrate good agreement with the Mie theory down to Mie parameters of about 30. Compact analytical expressions are derived for the intensity on the axis and the position of the diffraction focus both for the general case and for the focusing by microspheres. The high intensity region is narrower than for an ideal lens of the same aperture at the expense of longitudinal localization and has a polarization dependent fine structure, which can be explained quantitatively. The results are relevant for aerosol and colloid science where natural light focusing occurs and can be used in laser micro- and nano-processing of materials.
- J. Kofler, V. Vedral, M. S. Kim, Č. Brukner, Entanglement between Collective Operators in a Linear Harmonic Chain, Phys. Rev. A 73, 052107, (2006), archiv:quant-ph/0506236 (ID: 331777) Toggle Abstract
We investigate entanglement between collective operators of two blocks of oscillators in an infinite linear harmonic chain. These operators are defined as averages over local operators (individual oscillators) in the blocks. On the one hand, this approach of "physical blocks" meets realistic experimental conditions, where measurement apparatuses do not interact with single oscillators but rather with a whole bunch of them, i.e., where in contrast to usually studied "mathematical blocks" not every possible measurement is allowed. On the other, this formalism naturally allows the generalization to blocks which may consist of several non-contiguous regions. We quantify entanglement between the collective operators by a measure based on the Peres-Horodecki criterion and show how it can be extracted and transferred to two qubits. Entanglement between two blocks is found even in the case where none of the oscillators from one block is entangled with an oscillator from the other, showing genuine bipartite entanglement between collective operators. Allowing the blocks to consist of a periodic sequence of subblocks, we verify that entanglement scales at most with the total boundary region. We also apply the approach of collective operators to scalar quantum field theory.
Alternative URL (local restricted copy) - M. Lindenthal, J. Kofler, Measuring the absolute photo detection efficiency using photon number correlations, Appl. Opt. 45, 6059, (2006), URL (ID: 411771) Toggle Abstract
We present two methods for determining the absolute detection efficiency of photon-counting detectors directly from their singles rates under illumination from a nonclassical light source. One method is based on a continuous variable analog to coincidence counting in discrete photon experiments, but it does not actually rely on high detector time resolutions. The second method is based on difference detection, which is a typical detection scheme in continuous variable quantum optics experiments. Since no coincidence detection is required with either method, they are useful for detection efficiency measurements of photodetectors with detector time resolutions far too low to resolve coincidence events.
Alternative URL (local restricted copy)