1,384 research outputs found

    Discord and quantum computational resources

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    Discordant states appear in a large number of quantum phenomena and seem to be a good indicator of divergence from classicality. While there is evidence that they are essential for a quantum algorithm to have an advantage over a classical one, their precise role is unclear. We examine the role of discord in quantum algorithms using the paradigmatic framework of `restricted distributed quantum gates' and show that manipulating discordant states using local operations has an associated cost in terms of entanglement and communication resources. Changing discord reduces the total correlations and reversible operations on discordant states usually require non-local resources. Discord alone is, however, not enough to determine the need for entanglement. A more general type of similar quantities, which we call K-discord, is introduced as a further constraint on the kinds of operations that can be performed without entanglement resources.Comment: Closer to published versio

    Dirac Quantization of the Pais-Uhlenbeck Fourth Order Oscillator

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    As a model, the Pais-Uhlenbeck fourth order oscillator with equation of motion d4q/dt4+(ω12+ω22)d2q/dt2+ω12ω22q=0d^4q/dt^4+(\omega_1^2+\omega_2^2)d^2q/dt^2 +\omega_1^2\omega_2^2 q=0 is a quantum-mechanical prototype of a field theory containing both second and fourth order derivative terms. With its dynamical degrees of freedom obeying constraints due to the presence of higher order time derivatives, the model cannot be quantized canonically. We thus quantize it using the method of Dirac constraints to construct the correct quantum-mechanical Hamiltonian for the system, and find that the Hamiltonian diagonalizes in the positive and negative norm states that are characteristic of higher derivative field theories. However, we also find that the oscillator commutation relations become singular in the ω1ω2\omega_1 \to \omega_2 limit, a limit which corresponds to a prototype of a pure fourth order theory. Thus the particle content of the ω1=ω2\omega_1 =\omega_2 theory cannot be inferred from that of the ω1ω2\omega_1 \neq \omega_2 theory; and in fact in the ω1ω2\omega_1 \to \omega_2 limit we find that all of the ω1ω2\omega_1 \neq \omega_2 negative norm states move off shell, with the spectrum of asymptotic in and out states of the equal frequency theory being found to be completely devoid of states with either negative energy or negative norm. As a byproduct of our work we find a Pais-Uhlenbeck analog of the zero energy theorem of Boulware, Horowitz and Strominger, and show how in the equal frequency Pais-Uhlenbeck theory the theorem can be transformed into a positive energy theorem instead.Comment: RevTeX4, 20 pages. Final version, to appear in Phys. Rev.

    The approach to a superconductor-to-Bose-insulator transition in disordered films

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    Through a detailed study of scaling near the magnetic field-tuned superconductor-to-insulator transition in strongly disordered films, we find that results for a variety of materials can be collapsed onto a single phase diagram. The data display two clear branches, one with weak disorder and an intervening metallic phase, the other with strong disorder. Along the strongly disordered branch, the resistance at the critical point approaches RQ=h/4e2R_Q = h/4e^2 and the scaling of the resistance is consistent with quantum percolation, and therefore with the predictions of the dirty boson model.Comment: 4 pages, 4 figure

    Vanishing quantum discord is not necessary for completely-positive maps

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    The description of the dynamics of a system that may be correlated with its environment is only meaningful within the context of a specific framework. Different frameworks rely upon different assumptions about the initial system-environment state. We reexamine the connections between complete-positivity and quantum discord within two different sets of assumptions about the relevant family of initial states. We present an example of a system-environment state with non-vanishing quantum discord that leads to a completely-positive map. This invalidates an earlier claim on the necessity of vanishing quantum discord for completely-positive maps. In our final remarks we discuss the physical validity of each approach.Comment: close to published versio

    Photorefractive properties of Ce- and Ca-doped Sr0.6Ba0.4Nb2O6

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    We present the results of experimental study of the absorption coefficient, two-beam photorefractive coupling constant, and photorefractive response time of a doubly Ce- and Ca-doped Sr0.6Ba0.4Nb2O6. This crystal displays enhanced photorefractive response at near infrared wavelengths when compared to Ce-doped SBN:60. The temperature dependence of the coupling constant over the range from –30 to 40 °C has also been studied
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