Collective versus local measurements on two parallel or antiparallel spins

We give a complete analysis of covariant measurements on two spins. We consider the cases of two parallel and two antiparallel spins, and we consider both collective measurements on the two spins, and measurements which require only Local Quantum Operations and Classical Communication (LOCC). In all cases we obtain the optimal measurements for arbitrary fidelities. In particular we show that if the aim is determine as well as possible the direction in which the spins are pointing, it is best to carry out measurements on antiparallel spins (as already shown by Gisin and Popescu), second best to carry out measurements on parallel spins and worst to be restricted to LOCC measurements. If the the aim is to determine as well as possible a direction orthogonal to that in which the spins are pointing, it is best to carry out measurements on parallel spins, whereas measurements on antiparallel spins and LOCC measurements are both less good but equivalent.Comment: 4 pages; minor revision

Detecting a stochastic background of gravitational radiation: Signal processing strategies and sensitivities

We analyze the signal processing required for the optimal detection of a stochastic background of gravitational radiation using laser interferometric detectors. Starting with basic assumptions about the statistical properties of a stochastic gravity-wave background, we derive expressions for the optimal filter function and signal-to-noise ratio for the cross-correlation of the outputs of two gravity-wave detectors. Sensitivity levels required for detection are then calculated. Issues related to: (i) calculating the signal-to-noise ratio for arbitrarily large stochastic backgrounds, (ii) performing the data analysis in the presence of nonstationary detector noise, (iii) combining data from multiple detector pairs to increase the sensitivity of a stochastic background search, (iv) correlating the outputs of 4 or more detectors, and (v) allowing for the possibility of correlated noise in the outputs of two detectors are discussed. We briefly describe a computer simulation which mimics the generation and detection of a simulated stochastic gravity-wave signal in the presence of simulated detector noise. Numerous graphs and tables of numerical data for the five major interferometers (LIGO-WA, LIGO-LA, VIRGO, GEO-600, and TAMA-300) are also given. The treatment given in this paper should be accessible to both theorists involved in data analysis and experimentalists involved in detector design and data acquisition.Comment: 81 pages, 30 postscript figures, REVTE

Direct detection of neutralino dark matter in supergravity

The direct detection of neutralino dark matter is analysed in general supergravity scenarios, where non-universal soft scalar and gaugino masses can be present. In particular, the theoretical predictions for the neutralino-nucleon cross section are studied and compared with the sensitivity of dark matter detectors. We take into account the most recent astrophysical and experimental constraints on the parameter space, including the current limit on B(Bs-> mu+ mu-). The latter puts severe limitations on the dark matter scattering cross section, ruling out most of the regions that would be within the reach of present experiments. We show how this constraint can be softened with the help of appropriate choices of non-universal parameters which increase the Higgsino composition of the lightest neutralino and minimise the chargino contribution to the b->s transition.Comment: 27 pages, 22 figure

Transfer of quantum states using finite resources

We discuss the problem of transfering a qubit from Alice to Bob using a noisy quantum channel and only finite resources. As the basic protocol for the transfer we apply quantum teleportation. It turns out that for a certain quality of the channel direct teleportation combined with qubit purification is superior to entanglement purification of the channel. If, however, the quality of the channel is rather low one should simply apply an estimation-preparation scheme.Comment: 9 pages RevTeX including 5 figures, replaced with revised version, to appear in Phys. Rev.

Quantum entanglement and information processing via excitons in optically-driven quantum dots

We show how optically-driven coupled quantum dots can be used to prepare maximally entangled Bell and Greenberger-Horne-Zeilinger states. Manipulation of the strength and duration of the selective light-pulses needed for producing these highly entangled states provides us with crucial elements for the processing of solid-state based quantum information. Theoretical predictions suggest that several hundred single quantum bit rotations and Controlled-Not gates could be performed before decoherence of the excitonic states takes place.Comment: 3 separate PostScript Figures + 7 pages. Typos corrected. Minor changes added. This updated version is to appear in PR

Cross-Correlation of the Cosmic Microwave Background with the 2MASS Galaxy Survey: Signatures of Dark Energy, Hot Gas, and Point Sources

We cross-correlate the Cosmic Microwave Background (CMB) temperature anisotropies observed by the Wilkinson Microwave Anisotropy Probe (WMAP) with the projected distribution of extended sources in the Two Micron All Sky Survey (2MASS). By modelling the theoretical expectation for this signal, we extract the signatures of dark energy (Integrated Sachs-Wolfe effect;ISW), hot gas (thermal Sunyaev-Zeldovich effect;thermal SZ), and microwave point sources in the cross-correlation. Our strongest signal is the thermal SZ, at the 3.1-3.7 \sigma level, which is consistent with the theoretical prediction based on observations of X-ray clusters. We also see the ISW signal at the 2.5 \sigma level, which is consistent with the expected value for the concordance LCDM cosmology, and is an independent signature of the presence of dark energy in the universe. Finally, we see the signature of microwave point sources at the 2.7 \sigma level.Comment: 35 pages (preprint format), 8 figures. In addition to minor revisions based on referee's comments, after correcting for a bug in the code, the SZ detection is consistent with the X-ray observations. Accepeted for publication in Physical Review

Quantum Characterization of a Werner-like Mixture

We introduce a Werner-like mixture [R. F. Werner, Phys. Rev. A {\bf 40}, 4277 (1989)] by considering two correlated but different degrees of freedom, one with discrete variables and the other with continuous variables. We evaluate the mixedness of this state, and its degree of entanglement establishing its usefulness for quantum information processing like quantum teleportation. Then, we provide its tomographic characterization. Finally, we show how such a mixture can be generated and measured in a trapped system like one electron in a Penning trap.Comment: 8 pages ReVTeX, 8 eps figure

Tackling Systematic Errors in Quantum Logic Gates with Composite Rotations

We describe the use of composite rotations to combat systematic errors in single qubit quantum logic gates and discuss three families of composite rotations which can be used to correct off-resonance and pulse length errors. Although developed and described within the context of NMR quantum computing these sequences should be applicable to any implementation of quantum computation.Comment: 6 pages RevTex4 including 4 figures. Will submit to Phys. Rev.

Gate errors in solid state quantum computer architectures

We theoretically consider possible errors in solid state quantum computation due to the interplay of the complex solid state environment and gate imperfections. In particular, we study two examples of gate operations in the opposite ends of the gate speed spectrum, an adiabatic gate operation in electron-spin-based quantum dot quantum computation and a sudden gate operation in Cooper pair box superconducting quantum computation. We evaluate quantitatively the non-adiabatic operation of a two-qubit gate in a two-electron double quantum dot. We also analyze the non-sudden pulse gate in a Cooper-pair-box-based quantum computer model. In both cases our numerical results show strong influences of the higher excited states of the system on the gate operation, clearly demonstrating the importance of a detailed understanding of the relevant Hilbert space structure on the quantum computer operations.Comment: 6 pages, 2 figure

Implications of the Muon Anomalous Magnetic Moment for Supersymmetry

We re-examine the bounds on supersymmetric particle masses in light of the E821 data on the muon anomalous magnetic moment. We confirm, extend and supersede previous bounds. In particular we find (at one sigma) no lower limit on tan(beta) or upper limit on the chargino mass implied by the data at present, but at least 4 sparticles must be lighter than 700 to 820 GeV and at least one sparticle must be lighter than 345 to 440 GeV. However, the E821 central value bounds tan(beta) > 4.7 and the lighter chargino mass by 690 GeV. For tan(beta) < 10, the data indicates a high probability for direct discovery of SUSY at Run II or III of the Tevatron.Comment: 20 pages LaTeX, 14 figures; references adde