Superconductivity in Weyl semimetal NbP: Bulk vs. surface

Transition metal monopnictides belong to the new class of semimetals where the bulk properties are determined by the presence of pairs of nodes with different chirality formed by linear dispersive states in the k-space. Beside the anomaly in the bulk magnetotransport superconductivity is frequently found in some Weyl semimetals. We found signatures of superconductivity in ac and dc magnetization measurements of highly pure and stoichiometric NbP powder. We determined the lower and upper critical field and the Ginzburg-Landau parameter. The relative small superconducting volume fraction is related to either effect of finite grain size and/or surface superconductivity. The last mentioned may originate from either off stoichiometric (Nb-rich) surface layers or a strained surface with different electronic properties. Furthermore the intrinsic normal state susceptibility is determined taking into account a paramagnetic contribution of a few ppm of magnetic impurities

Schr\"odinger's pure-state steering completed

Schroedinger investigated entanglement in two-particle state vectors by assuming measurement finding out if the nearby particle is in a given state vector or not. Without interaction with the distant particle, just on account of the entanglement, the distant particle is steered into a certain state vector. In Schroedinger's finite-dimensional case thus any distant-particle state vector can be reached. This theory was extended to infinite-dimensional spaces by the author. The present article completes the extension by throwing light on the fine structure of steering.Comment: 10 pages, Latex2e, no figure

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Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling

We propose that electron doped nontransition metal-phthalocyanines (MPc) like ZnPc and MgPc, similar to those very recently reported, should constitute novel strongly correlated metals. Due to orbital degeneracy, Jahn-Teller coupling and Hund's rule exchange, and with a large on-site Coulomb repulsion, these molecular conductors should display, particularly near half filling at two electrons/molecule, very unconventional properties, including Mott insulators, strongly correlated superconductivity, and other intriguing phases.Comment: 4 pages, 1 figure, submited to PR

Derivation of the quantum probability law from minimal non-demolition measurement

One more derivation of the quantum probability rule is presented in order to shed more light on the versatile aspects of this fundamental law. It is shown that the change of state in minimal quantum non-demolition measurement, also known as ideal measurement, implies the probability law in a simple way. Namely, the very requirement of minimal change of state, put in proper mathematical form, gives the well known Lueders formula, which contains the probability rule.Comment: 8 page

Coherence of a Josephson phase qubit under partial-collapse measurement

We discuss quantum evolution of a decaying state in relation to a recent experiment of Katz et al. Based on exact analytical and numerical solutions of a simple model, we identify a regime where qubit retains coherence over a finite time interval independently of the rates of three competing decoherence processes. In this regime, the quantum decay process can be continuously monitored via a ``weak'' measurement without affecting the qubit coherence.Comment: 4p., 2eps figure

Mixed-state twin observables

Twin observables, i.e. opposite subsystem observables A+ and A- that are indistinguishable in measurement in a given mixed or pure state W, are investigated in detail algebraicly and geometrically. It is shown that there is a far-reaching correspondence between the detectable (in W) spectral entities of the two operators. Twin observables are state-dependently quantum-logically equivalent, and direct subsystem measurement of one of them ipso facto gives rise to the indirect (i.e. distant) measurement of the other. Existence of nontrivial twins requires singularity of W. Systems in thermodynamic equilibrium do not admit subsystem twins. These observables may enable one to simplify the matrix representing W.Comment: 13 page

State Measurements with Short Laser Pulses and Lower-Efficiency Photon Detectors

It has been proposed by Cook (Phys. Scr. T 21, 49 (1988)) to use a short probe laser pulse for state measurements of two-level systems. In previous work we have investigated to what extent this proposal fulfills the projection postulate if ideal photon detectors are considered. For detectors with overall efficiency less than 1 complications arise for single systems, and for this case we present a simple criterion for a laser pulse to act as a state measurement and to cause an almost complete state reduction.Comment: 13 pages, LaTeX; submitted to J. mod. Op