Recursos de colección

Caltech Authors (127.665 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 3.633

  1. A Quantum Version of Schöning's Algorithm Applied to Quantum 2-SAT

    Farhi, Edward; Kimmel, Shelby; Temme, Kristan
    We study a quantum algorithm that consists of a simple quantum Markov process, and we analyze its behavior on restricted versions of Quantum 2-SAT. We prove that the algorithm solves this decision problem with high probability for n qubits, L clauses, and promise gap c in time O(n^2 L^2 c^{-2}). If the Hamiltonian is additionally polynomially gapped, our algorithm efficiently produces a state that has high overlap with the satisfying subspace. The Markov process we study is a quantum analogue of Schöning's probabilistic algorithm for k-SAT.

  2. Precursor problem and holographic mutual information

    Bao, Ning; Kim, Isaac H.
    The recent proposal of Almheiri et al.[http://arxiv.org/abs/1411.7041], together with the Ryu-Takayanagi formula, implies the entanglement wedge hypothesis for certain choices of boundary subregions. This fact is derived in the pure AdS space. A similar conclusion holds in the presence of quantum corrections, but in a more restricted domain of applicability. We also comment on this [http://arxiv.org/abs/1601.05416] and some similarities and differences with this work.

  3. Vacancy-induced low-energy states in undoped graphene

    Sanyal, Sambuddha; Damle, Kedar; Motrunich, Olexei I.
    We demonstrate that a nonzero concentration nv of static, randomly-placed vacancies in graphene leads to a density w of zero-energy quasiparticle states at the band-center ϵ=0 within a tight-binding description with nearest-neighbour hopping t on the honeycomb lattice. We show that w remains generically nonzero in the compensated case (exactly equal number of vacancies on the two sublattices) even in the presence of hopping disorder, and depends sensitively on nv and correlations between vacancy positions. For low, {\em but not-too-low} |ϵ|/t in this compensated case, we show that the density of states (DOS) ρ(ϵ) exhibits a strong divergence of the...

  4. Physics of the Bacon Internal Resonator Banjo

    Politzer, David
    The internal resonator banjo, patented and first sold by Fred Bacon around 1906, remains something of a cult favorite and is still produced by some independent luthiers. According to enthusiasts, the characteristic design elements produce a sound that is mellower, richer, and of greater complexity and presence than without them. Aspects of that sound are studied here, comparing instruments that are otherwise identical and identifying physics mechanisms that are likely responsible.

  5. Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30

    Mondal, Aditya S.; Dewangan, Gulab C.; Pahari, Mayukh; Misra, Ranjeev; Kembhavi, Ajit K.; Raychaudhuri, Biplab
    Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disk. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here we investigate the reality of the broad iron line detected earlier from the neutron star low mass X-ray binary 4U 1820–30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broadband spectral study of the atoll source using Suzaku and simultaneous NuSTAR & Swift observations. We have used different continuum models involving accretion disk emission, thermal...

  6. Tensor networks, p-adic fields, and algebraic curves: arithmetic and the AdS_3/CFT_2 correspondence

    Heydeman, Matthew; Marcolli, Matilde; Saberi, Ingmar; Stoica, Bogdan
    One of the many remarkable properties of conformal field theory in two dimensions is its connection to algebraic geometry. Since every compact Riemann surface is a projective algebraic curve, many constructions of interest in physics (which a priori depend on the analytic structure of the spacetime) can be formulated in purely algebraic language. This opens the door to interesting generalizations, obtained by taking another choice of field: for instance, the p-adics. We generalize the AdS/CFT correspondence according to this principle; the result is a formulation of holography in which the bulk geometry is discrete—the Bruhat–Tits tree for PGL(2, Qp)—but the group of bulk isometries nonetheless agrees with...

  7. Strong CMB Constraint On P-Wave Annihilating Dark Matter

    An, Haipeng; Wise, Mark B.; Zhang, Yue
    We consider a dark sector consisting of dark matter that is a Dirac fermion and a scalar mediator. This model has been extensively studied in the past. If the scalar couples to the dark matter in a parity conserving manner then dark matter annihilation to two mediators is dominated by the P-wave channel and hence is suppressed at very low momentum. The indirect detection constraint from the anisotropy of the Cosmic Microwave Background is usually thought to be absent in the model because of this suppression. In this letter we show that dark matter annihilation to bound states occurs through...

  8. Inverse Compton X-ray Emissions from TeV blazar Mrk421 during a Historical Low-Flux State Observed with NuSTAR

    Kataoka, Jun; Stawarz, Lukasz
    We report on the detection of excess hard X-ray emission from the TeV BL Lac object Mrk421 during the historical low-flux state of the source in January 2013. NuSTAR observations were conducted four times between MJD56294 and MJD56312 with a total exposure of 80.9 ksec. The source flux in the 3-40 keV range was nearly constant except for MJD56307, when the average flux level increased by a factor of three. Throughout the exposure, the X-ray spectra of Mrk421 were well represented by a steep power-law model with a photon index of 3.1, although a significant excess was noted above 20 keV in the MJD56302 data when the source...

  9. Ab initio phonon coupling and optical response of hot electrons in plasmonic metals

    Brown, Ana M.; Sundararaman, Ravishankar; Narang, Prineha; Goddard, William A., III; Atwater, Harry A.
    Ultrafast laser measurements probe the non-equilibrium dynamics of excited electrons in metals with increasing temporal resolution. Electronic structure calculations can provide a detailed microscopic understanding of hot electron dynamics, but a parameter-free description of pump-probe measurements has not yet been possible, despite intensive research, because of the phenomenological treatment of electron-phonon interactions. We present ab initio predictions of the electron-temperature dependent heat capacities and electron-phonon coupling coefficients of plasmonic metals. We find substantial differences from free-electron and semi-empirical estimates, especially in noble metals above transient electron temperatures of 2000 K, because of the previously-neglected strong dependence of electron-phonon matrix elements...

  10. Quantum voting and violation of Arrow's Impossibility Theorem

    Bao, Ning; Yunger Halpern, Nicole
    We propose a quantum voting system in the spirit of quantum games such as the quantum Prisoner's Dilemma. Our scheme violates a quantum analogue of Arrow's Impossibility Theorem, which states that every (classical) constitution endowed with three innocuous-seeming properties is a dictatorship. Superpositions, interference, and entanglement of votes feature in voting tactics available to quantum voters but not to classical.

  11. A Greenhouse-Gas Information System: Monitoring and Validating Emissions Reporting and Mitigation

    Dimotakis, Paul E.; Walker, Bruce C.; Jonietz, Karl K.; Rotman, Douglas A.
    This study and report focus on attributes of a greenhouse-gas information system (GHGIS) needed to support MRV&V needs. These needs set the function of such a system apart from scientific/research monitoring of GHGs and carbon-cycle systems, and include (not exclusively): the need for a GHGIS that is operational, as required for decision-support; the need for a system that meets specifications derived from imposed requirements; the need for rigorous calibration, verification, and validation (CV&V) standards, processes, and records for all measurement and modeling/data-inversion data; the need to develop and adopt an uncertainty-quantification (UQ) regimen for all measurement and modeling data; and...

  12. A bargaining model of legislative policy-making

    Banks, Jeffrey S.; Duggan, John
    We present a general model of legislative bargaining in which the status quo is an arbitrary point in a multidimensional policy space. In contrast to other bargaining models, the status quo is not assumed to be “bad,” and delay may be Pareto efficient. We prove existence of stationary equilibria. The possibility of equilibrium delay depends on four factors: risk aversion of the legislators, the dimensionality of the policy space, the voting rule, and the possibility of transfers across districts. If legislators are risk averse, if there is more than one policy dimension, and if voting is by majority rule, for example, then delay will almost never occur....

  13. Existence of Nash Equilibria on Convex Sets

    Banks, Jeffrey S.; Duggan, John
    We analyze a non-cooperative game in which the set of feasible strategy profiles is compact and convex but possibly non-rectangular. Thus, a player's feasible strategies may depend on the strategies used by others, as in Debreu's (1952,1982) generalized games. In contrast to the model of Debreu, we do not require preferences to be defined over infeasible strategy profiles, and we do not require a player's feasible strategy correspondence to have non-empty values. We prove existence of Nash equilibria under a lower hemicontinuity condition, and we give examples of classes of games in which this condition is satisfied.

  14. A multidimensional model of repeated elections

    Banks, Jeffrey S.; Duggan, John
    We analyze a discrete-time, infinite-horizon model of elections. In each period, a challenger is chosen from the electorate to run against an incumbent politician in a majority-rule election, and the winner then selects a policy from a multidimensional policy space. Individuals' policy preferences are private information, whereas policy choices are publicly observable. We prove existence and continuity of equilibria in "simple" voting and policy strategies; we provide examples to show the variety of possible equilibrium patterns in multiple dimensions; we analyze the effects of patience and office-holding benefits on the persistence of policies over time; and we identify relationships between equilibrium policies and the core of the underlying...

  15. Scanning probe microscopy of thermally excited mechanical modes of an optical microcavity

    Kippenberg, T. J.; Rokhsari, H.; Vahala, K. J.
    The resonant buildup of light within optical microcavities elevates the radiation pressure which mediates coupling of optical modes to the mechanical modes of a microcavity. Above a certain threshold pump power, regenerative mechanical oscillation occurs causing oscillation of certain mechanical eigenmodes. Here, we present a methodology to spatially image the micro-mechanical resonances of a toroid microcavity using a scanning probe technique. The method relies on recording the induced frequency shift of the mechanical eigenmode when in contact with a scanning probe tip. The method is passive in nature and achieves a sensitivity sufficient to spatially resolve the vibrational mode pattern...

  16. Purcell factor enhanced scattering efficiency in optical microcavities

    Kippenberg, T. J.; Tchebotareva, A. L.; Kalkman, J.; Polman, A.; Vahala, K. J.
    Scattering processes in an optical microcavity are investigated for the case of silicon nanocrystals embedded in an ultra-high Q toroid microcavity. Using a novel measurement technique based on the observable mode-splitting, we demonstrate that light scattering is highly preferential: more than 99.8% of the scattered photon flux is scattered into the original doubly-degenerate cavity modes. The large capture efficiency is attributed to an increased scattering rate into the cavity mode, due to the enhancement of the optical density of states over the free space value and has the same origin as the Purcell effect in spontaneous emission. The experimentally determined...

  17. The Mathematics of Entanglement

    Brandão, Fernando G. S. L.; Christandl, Matthias; Harrow, Aram W.; Walter, Michael
    These notes are from a series of lectures given at the Universidad de Los Andes in Bogotá, Colombia on some topics of current interest in quantum information. While they aim to be self-contained, they are necessarily incomplete and idiosyncratic in their coverage. For a more thorough introduction to the subject, we recommend one of the textbooks by Nielsen and Chuang or by Wilde, or the lecture notes of Mermin, Preskill or Watrous. Our notes by contrast are meant to be a relatively rapid introduction into some more contemporary topics in this fast-moving field. They are meant to be accessible to...

  18. Amplifying the randomness of weak sources correlated with devices

    Wojewódka, H.; Brandão, F.; Grudka, A.; Horodecki, M.; Horodecki, K.; Horodecki, P.; Pawłowski, M.; Ramanathan, R.
    The problem of device-independent randomness amplification against no-signaling adversaries has so far been studied under the assumption that the weak source of randomness is uncorrelated with the (quantum) devices used in the amplification procedure. In this work, we relax this assumption, and reconsider the original protocol of Colbeck and Renner, Nature Physics 8, 450-454 (2012), on randomness amplification using a Santha-Vazirani (SV) source. To do so, we introduce an SV-like condition for devices, namely that any string of SV source bits remains weakly random conditioned upon any other bit string from the same SV source and the outputs obtained when...

  19. Estimating operator norms using covering nets

    Brandão, Fernando G. S. L.; Harrow, Aram W.
    We present several polynomial- and quasipolynomial-time approximation schemes for a large class of generalized operator norms. Special cases include the 2→q norm of matrices for q>2, the support function of the set of separable quantum states, finding the least noisy output of entanglement-breaking quantum channels, and approximating the injective tensor norm for a map between two Banach spaces whose factorization norm through ℓ^n_1 is bounded. These reproduce and in some cases improve upon the performance of previous algorithms by Brandão-Christandl-Yard and followup work, which were based on the Sum-of-Squares hierarchy and whose analysis used techniques from quantum information such as...

  20. Randomness amplification against no-signaling adversaries using two devices

    Ramanathan, Ravishankar; Brandão, Fernando G. S. L.; Horodecki, Karol; Horodecki, Michał; Horodecki, Paweł; Wojewódka, Hanna
    Recently the first physically realistic protocol amplifying the randomness of Santha-Vazirani sources using a finite number of no-signaling devices and with a constant rate of noise has been proposed, however there still remained the open question whether this can be accomplished under the minimal conditions necessary for the task. Namely, is it possible to achieve randomness amplification using only two no-signaling devices and in a situation where the violation of a Bell inequality implies only an upper bound for some outcome probability for some setting combination? Here, we solve this problem and present the first device-independent protocol for the task...

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