Search

The energy gap of optimally doped Bi2(Sr,R)2CuOy (R=La and Eu) was probed by angle resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet laser (photon energy 6.994 eV) or He I resonance line (21.218 eV) as photon source. The results show that the gap around the node at sufficiently low temperatures can be well described by a monotonic d-wave gap function for both samples and the gap of the R=La sample is larger reflecting the higher Tc. However, an abrupt deviation from the d-wave gap function and an opposite R dependence for the gap size were observed around the antinode, which represent a clear disentanglement between the antinodal pseudogap and the nodal superconducting gap., Comment: Submitted as the proceedings of LT25

Shenvi, Kempe and Whaley's quantum random-walk search (SKW) algorithm [Phys. Rev. A 67, 052307 (2003)] is known to require $O(\sqrt N)$ number of oracle queries to find the marked element, where $N$ is the size of the search space. The overall time complexity of the SKW algorithm differs from the best achievable on a quantum computer only by a constant factor. We present improvements to the SKW algorithm which yield significant increase in success probability, and an improvement on query complexity such that the theoretical limit of a search algorithm succeeding with probability close to one is reached. We point out which improvement can be applied if there is more than one marked element to find., Comment: 7 pages, 2 figures. Major revision according to referee report

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical transition temperatures (Tc's) of 26 K and 5 K, respectively. We find that high-Tc LaFeAsO:F exhibits a temperature-dependent pseudogap-like feature extending over ~0.1 eV about the Fermi level at 250 K, whereas such a feature is absent in low-Tc LaFePO:F. We also find ~20-meV pseudogap-like features and signatures of superconducting gaps both in LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogap-like features through comparison with the high-Tc cuprates.

We have performed temperature (T) - dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be well explained by the theoretical model and thus represents characteristic bulk electronic structure of the AF SC phase for the first time., Comment: 11 pages, 4 figures

We investigate the doping dependent low energy, low temperature ($T$ = 5 K) properties of nodal quasiparticles in the d-wave superconductor Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+\delta}$ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near $E_{F}$, mean free paths and scattering rates ($\Gamma$) of quasiparticles. For optimally and overdoped, we obtain very sharp quasiparticle peaks of 8 meV and 6 meV full-width at half-maximum, respectively, in accord with terahertz conductivity. For all doping levels, we find the energy-dependence of $\Gamma \sim |\omega |$, while $\Gamma$($\omega =0$) shows a monotonic increase from overdoping to underdoping. The doping dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K $\lsim 0.07T_{\rm c}$), pronounced in the underdoped region.

We analyze the recurrence probability (P\'olya number) for d-dimensional unbiased quantum walks. A sufficient condition for a quantum walk to be recurrent is derived. As a by-product we find a simple criterion for localisation of quantum walks. In contrast to classical walks, where the P\'olya number is characteristic for the given dimension, the recurrence probability of a quantum walk depends in general on the topology of the walk, choice of the coin and the initial state. This allows to change the character of the quantum walk from recurrent to transient by altering the initial state.

We study the motion of two non-interacting quantum particles performing a random walk on a line and analyze the probability that the two particles are detected at a particular position after a certain number of steps (meeting problem). The results are compared to the corresponding classical problem and differences are pointed out. Analytic formulas for the meeting probability and its asymptotic behavior are derived. The decay of the meeting probability for distinguishable particles is faster then in the classical case, but not quadratically faster. Entangled initial states and the bosonic or fermionic nature of the walkers are considered.

Selection of an ensemble of equally prepared quantum systems, based on measurements on it, is a basic step in quantum state purification. For an ensemble of single qubits, iterative application of selective dynamics has been shown to lead to complex chaos, which is a novel form of quantum chaos with true sensitivity to the initial conditions. The Julia set of initial valuse with no convergence shows a complicated structre on the complex plane. The shape of the Julia set varies with the parameter of the dynamics. We present here results for the two qubit case demonstrating how a purification process can be destroyed with chaotic oscillations.

We analyze the realization of a quantum-walk search algorithm in a passive, linear optical network. The specific model enables us to consider the effect of realistic sources of noise and losses on the search efficiency. Photon loss uniform in all directions is shown to lead to the rescaling of search time. Deviation from directional uniformity leads to the enhancement of the search efficiency compared to uniform loss with the same average. In certain cases even increasing loss in some of the directions can improve search efficiency. We show that while we approach the classical limit of the general search algorithm by introducing random phase fluctuations, its utility for searching is lost. Using numerical methods, we found that for static phase errors the averaged search efficiency displays a damped oscillatory behaviour that asymptotically tends to a non-zero value., Comment: 10 pages, 10 figures. Two figures added for clarity, also made improvements to the text

Cobalt oxide superconductor NaxCoO2.yH2O is studied by angle-resolved photoemission spectroscopy (ARPES). We report the Fermi surface (FS) topology and electronic structure near the Fermi level (EF) of NaxCoO2.yH2O. Our result indicates the presence of the hexagonal FS centered at G point, while the small pocket FSs along G-K direction are absent similar to NaxCoO2. The top of the eg' band, which is expected in band calculations to form the small pocket FSs, extends to within 20 - 50 meV below EF, more closer to EF than in NaxCoO2. We discuss its possible role in superconductivity, comparing with other experimental and theoretical results., Comment: 15 pages, 3 figures

We have investigated the low energy electronic state of a boron-doped diamond thin film by the ultrahigh resolution laser-excited photoemission spectroscopy. We observed a clear shift of the leading edge below 11 K indicative of a superconducting gap opening ($\Delta \sim 0.78$ meV at 4.5 K). The gap feature is significantly broad and the well-defined quasiparticle peak is not recognizable even at the lowest temperature of measurement 4.5 K. We discuss our result in terms of possible disorder effect on superconductivity in this system., Comment: 12 pages, 4 figures

We analyze the consequences of iterative measurement-induced nonlinearity on the dynamical behavior of qubits. We present a one-qubit scheme where the equation governing the time evolution is a complex-valued nonlinear map with one complex parameter. In contrast to the usual notion of quantum chaos, exponential sensitivity to the initial state occurs here. We calculate analytically the Lyapunov exponent based on the overlap of quantum states, and find that it is positive. We present a few illustrative examples of the emerging dynamics., Comment: 4 pages, 3 figures

We study the electronic structure of Mott-Hubbard systems SrVO$_{3}$ and CaVO$_3$ with bulk and surface-sensitive high-resolution photoemission spectroscopy (PES), using a VUV laser, synchrotron radiation and a discharge lamp ($h\nu$ = 7 - 21 eV). A systematic suppression of the density of states (DOS) within $\sim$ 0.2 eV of the Fermi level ($E_F$) is found on decreasing photon energy i.e. on increasing bulk sensitivity. The coherent band in SrVO$_{3}$ and CaVO$_3$ is shown to consist of surface and bulk derived features, separated in energy. The stronger distortion on surface of CaVO$_{3}$ compared to SrVO$_{3}$ leads to higher surface metallicity in the coherent DOS at $E_F$, consistent with recent theory., Comment: 4 pages 5 figures (including 2 auxiliary figures); A complete analysis of the spectra based on the surface and bulk analysis shows in auxiliary figures Fig. A1 and A2

Angle-resolved photoemission spectroscopy of ZrTe3 has been performed from room temperature(T) down to 6 K (across Tcdw = 63K) to study the charge-density-wave (CDW) fluctuation effects in a metallic CDW compound having Fermi surface (FS) sheets with differing dimensionality. While spectra on the 3-dimensional(D) FS show typical Fermi-Dirac function-like T-dependence, those along the quasi 1-D FS show formation of a pseudogap, starting at a much higher T than Tcdw. Simultaneously, a van-Hove singularity consisting of the quasi 1D FS intersecting the 3D FS shows increase of coherence. This demonstrates the role of CDW fluctuations on the spectral function, and relation to the dimensionality of the states, in a metallic CDW system., Comment: 14 pages, 4 figures (to appear as PRB Rapid Communication)

``Sub-meV'' resolution photoemission spectroscopy was used to study carbon-substitution dependence on the multiple superconducting gap of Mg(B$_{1 - x}$C$_{x})_{2}$. Two features corresponding to $\sigma $ and $\pi $ gaps are clearly observed in the raw spectra up to carbon concentration x = 7.5 {%}. The observed x dependence of the two gaps shows a qualitatively different behavior: a marked change of the $\sigma $ gap proportional to the $T_{c}$ variation and a negligible one of the $\pi $ gap. This as well as the temperature dependence can be explained with the two-band mean-field theory. Implications from the present study are discussed., Comment: 4 figures, PDF only

We have studied the temperature-dependent electronic structure near the Fermi level (EF) of the layered cobaltate superconductor, Na0.35CoO2.1.3H2O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap with an energy scale of ~ 20 meV in Na0.35CoO2.1.3H2O and Na0.35CoO2.0.7H2O, which is clearly absent in Na0.7CoO2. The energy scale of the pseudogap is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low temperatures. We discuss implications of the pseudogap in relation to available transport and magnetic susceptibility results., Comment: 4 pages, 3 figures

One can use transsonic Bose-Einstein condensates of alkali atoms to establish the laboratory analog of the event horizon and to measure the acoustic version of Hawking radiation. We determine the conditions for supersonic flow and the Hawking temperature for realistic condensates on waveguides where an external potential plays the role of a supersonic nozzle. The transition to supersonic speed occurs at the potential maximum and the Hawking temperature is entirely determined by the curvature of the potential.

The Charge-density-wave(CDW) in 2H-NbSe2(TCDW ~ 33 K), which coexists with superconductivity(Tc = 7.2 K), is investigated using Angle-resolved photoemission spectroscopy as a function of temperature. Across TCDW, energy and momentum(k) distribution curves show changes associated with the primary and secondary CDW vectors. The CDW is due to hidden charge order occurring at specific k-points on Fermi surfaces in the 2-dimensional Brillouin zone(2D BZ) with spectral weight suppression, but no gap formation. The study rules out nesting in the 2D BZ and the van Hove singularity driven CDW transition in 2H-NbSe2., Comment: This paper has been withdrawn

Artificial black holes, such as sonic holes in Bose-Einstein condensates, may give insights into the role of the physics at the event horizon beyond the Planck scale. We show that sonic white holes give rise to a discrete spectrum of instabilities that is insensitive to the analogue of trans-Planckian physics for Bose-Einstein condensates.

Artificial black holes may demonstrate some of the elusive quantum properties of the event horizon, in particular Hawking radiation. One promising candidate is a sonic hole in a Bose-Einstein condensate. We clarify why Hawking radiation emerges from the condensate and how this condensed-matter analog reflects some of the intriguing aspects of quantum black holes.

Like classical fluids, quantum gases may suffer from hydrodynamic instabilities. Our paper develops a quantum version of the classical stability analysis in fluids, the Bogoliubov theory of elementary excitations in unstable Bose-Einstein condensates. In unstable condensates the excitation modes have complex frequencies. We derive the normalization conditions for unstable modes such that they can serve in a mode decomposition of the non-condensed component. Furthermore, we develop approximative techniques to determine the spectrum and the mode functions. Finally, we apply our theory to a sonic white hole and find that the spectrum of unstable modes is intrinsically discrete.

We study the single particle density of states (DOS) across the superconducting transition (Tc = 31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pile-up in the DOS, Delta(5.3 K) = 5.2 meV and 2Delta(0)/kBTc = 3.9. In addition, we observe a pseudogap below and above Tc, occurring as a suppression in intensity over an energy scale up to the breathing mode phonon(~ 70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0.67K0.33BiO3., Comment: 5 pages with 4 figures, submitted to Phys. Rev. Lett

We study the new binary intermetallic superconductor MgB2 using high-resolution photo-emission spectroscopy. The superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a simple isotropic-gap opening. The spectrum can be well reproduced using the weighted sum of two Dynes functions with the gap sizes of 1.7 and 5.6 meV. Temperature-dependent study shows that both gaps close at the bulk transition temperature. These results provide spectroscopic evidence for a multiple gap of MgB2., Comment: 4 pages in RevTeX format (3 Figures) submitted to PRL

A beam of diatomic molecules scattered off a standing wave laser mode splits according to the rovibrational quantum state of the molecules. Our numerical calculation shows that single state resolution can be achieved by properly tuned, monochromatic light. The proposed scheme allows for selecting non-vibrating and non-rotating molecules from a thermal beam, implementing a laser Maxwell's demon to prepare a rovibrationally cold molecular ensemble., Comment: 11 pages, LaTeX. To appear in the European Physical Journal D

The role of the off-diagonal density matrix elements of the entangled pair is investigated in quantum teleportation of a qbit. The dependence between them and the off-diagonal elements of the teleported density matrix is shown to be linear. In this way the ideal quantum teleportation is related to an entirely classical communication protocol: the one-time pad cypher. The latter can be regarded as the classical counterpart of Bennett's quantum teleportation scheme. The quantum-to-classical transition is demonstrated on the statistics of a gedankenexperiment., Comment: 11 pages, 1 figure, accepted for publication in J. Phys. A (Math. Gen.)

In our paper we estimated the quantum backaction of dispersive imaging with off-resonant light on Bose-Einstein condensates. We have calculated the rates of the two processes involved, phase diffusion and depletion of the condensate. We compare here the depletion rate obtained within our model limitations to the Rayleigh scattering rate, both having the same physical origin: dispersive interaction of light with matter. We show that residual absorption sets indeed the limit of dispersive imaging., Comment: 1 page (Reply to comment)

Impressive pictures of moving Bose-Einstein condensates have been taken using phase-contrast imaging M. R. Andrews et al., Science 273, 84 (1996). We calculate the quantum backaction of this measurement technique. We find that phase-contrast imaging is not a quantum nondemolition measurement of the atomic density. Instead, the condensate gets gradually depleted at a rate that is proportional to the light intensity and to the inverse cube of the optical wave length. The fewer atoms are condensed the higher is the required intensity to see a picture, and, consequently, the higher is the induced backaction. To describe the quantum physics of phase-contrast imaging we put forward a new approach to quantum-optical propagation. We develop an effective field theory of paraxial optics in a fully quantized atomic medium., Comment: 11 pages RevTex, 2 ps figures, revised. European Physical Journal D (in press)

The authors of the Comment [G. M. D'Ariano and C. Macchiavello to be published in Phys. Rev. A, quant-ph/9701009] tried to reestablish a 0.5 efficiency bound for loss compensation in optical homodyne tomography. In our reply we demonstrate that neither does such a rigorous bound exist nor is the bound required for ruling out the state reconstruction of an individual system [G. M. D'Ariano and H. P. Yuen, Phys. Rev. Lett. 76, 2832 (1996)]., Comment: LaTex, 2 pages, 1 Figure; to be published in Physical Review A