arXiv subjects
Publications and source records attributed to Goran Wendin.
We discuss some claims that certain UCOMP devices can perform hypercomputation (compute Turing-uncomputable functions) or perform super-Turing computation (solve NP-complete problems in polynomial time). We discover that all these claims rely on the provision of one or more unphysical resources.
We calculate the full frequency spectral density of voltage fluctuations in a Single Electron Transistor (SET), used as an electrometer biased above the Coulomb threshold so that the current through the SET is carried by sequential tunnel events. We consider both a normal state SET and a superconducting SET. The whole spectrum from low frequency telegraph noise to quantum noise at frequencies comparable to the SET charging energy $(E_{C}/\hbar)$, and high frequency Nyquist noise is described. We take the energy exchange between the SET and the measured system into account using a real-time diagrammatic Keldysh technique. The voltage fluctuations determine the back-action of the SET onto the measured system and we specifically discuss the case of superconducting charge qubit read-out and measuring the so-called Coulomb staircase of a single Cooper pair box.
We calculate the spectral density of voltage fluctuations in a Single Electron Transistor (SET), biased to operate in a transport mode where tunneling events are correlated due to Coulomb interaction. The whole spectrum from low frequency shot noise to quantum noise at frequencies comparable to the SET charging energy $(E_{C}/\hbar)$ is considered. We discuss the back-action during read-out of a charge qubit and conclude that single-shot read-out is possible using the Radio-Frequency SET.
We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, external current injection and electromagnetic radiation. In particular we will consider the "giant" Josephson current in "long" SIS tunnel junctions and the regular and anomalous nonequilibrium Josephson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).
We study theoretically the ac Josephson effect in superconducting planar d-wave junctions. The insulating barrier assumed to be present between the two superconductors may have arbitrary strength. Many properties of this system depend on the orientation of the d-wave superconductor: we calculate the ac components of the Josephson current. In some arrangements there is substantial negative differential conductance due to the presence of mid-gap states. We study how robust these features are to finite temperature and also comment on how the calculated current-voltage curves compare with experiments. For some other configurations (for small barrier strength) we find zero-bias conductance peaks due to multiple Andreev reflections through midgap states. Moreover, the odd ac components are strongly suppressed and even absent in some arrangements. This absence will lead to a doubling of the Josephson frequency. All these features are due to the d-wave order parameter changing sign when rotated $90^{\circ}$. Recently, there have been several theoretical reports on parallel current in the d-wave case for both the stationary Josephson junction and for the normal metal-superconductor junction. Also in our case there may appear current density parallel to the junction, and we present a few examples when this takes place. Finally, we give a fairly complete account of the method used and also discuss how numerical calculations should be performed in order to produce current-voltage curves.
We study voltage-biased superconducting planar d-wave junctions for arbitrary transmission and arbitrary orientation of the order parameters of the superconductors. For a certain orientation of the superconductors the odd ac components disappear, resulting in a doubling of the Josephson frequency. We study the sensitivity of this disappearance to orientation and compare with experiments on grain boundary junctions. We also discuss the possibility of a current flow parallel to the junction.
We calculate the x-ray absorption near edge structure (XANES) modulating the Ba 4d-photoabsorption cross section - the giant dipole resonance - centered around 110 eV photon energy for several models of spherical and non-spherical Ba@C_n metallofullerene cage systems. In the cases considered, the XANES interference patterns provide clear structural "fingerprints", distinguishing between center versus off-center Ba position and spherical versus deformed C_n shell.