57 research outputs found
Quantum Mutual Information Capacity for High Dimensional Entangled States
High dimensional Hilbert spaces used for quantum communication channels offer
the possibility of large data transmission capabilities. We propose a method of
characterizing the channel capacity of an entangled photonic state in high
dimensional position and momentum bases. We use this method to measure the
channel capacity of a parametric downconversion state, achieving a channel
capacity over 7 bits/photon in either the position or momentum basis, by
measuring in up to 576 dimensions per detector. The channel violated an
entropic separability bound, suggesting the performance cannot be replicated
classically.Comment: 5 pages, 2 figure
Nanocalorimetric Evidence for Nematic Superconductivity in the Doped Topological Insulator SrBiSe
Spontaneous rotational-symmetry breaking in the superconducting state of
doped has attracted significant attention as an
indicator for topological superconductivity. In this paper, high-resolution
calorimetry of the single-crystal
provides unequivocal evidence of a two-fold rotational symmetry in the
superconducting gap by a \emph{bulk thermodynamic} probe, a fingerprint of
nematic superconductivity. The extremely small specific heat anomaly resolved
with our high-sensitivity technique is consistent with the material's low
carrier concentration proving bulk superconductivity. The large basal-plane
anisotropy of is attributed to a nematic phase of a two-component
topological gap structure and caused by a
symmetry-breaking energy term .
A quantitative analysis of our data excludes more conventional sources of this
two-fold anisotropy and provides the first estimate for the symmetry-breaking
strength , a value that points to an onset transition of
the second order parameter component below 2K
Unexpected Enhancement of Three-Dimensional Low-Energy Spin Correlations in Quasi-Two-Dimensional FeTeSe System at High Temperature
We report inelastic neutron scattering measurements of low energy ( meV) magnetic excitations in the "11" system
FeTeSe. The spin correlations are two-dimensional (2D) in
the superconducting samples at low temperature, but appear much more
three-dimensional when the temperature rises well above K, with a
clear increase of the (dynamic) spin correlation length perpendicular to the Fe
planes. The spontaneous change of dynamic spin correlations from 2D to 3D on
warming is unexpected and cannot be naturally explained when only the spin
degree of freedom is considered. Our results suggest that the low temperature
physics in the "11" system, in particular the evolution of low energy spin
excitations towards %better satisfying the nesting condition for mediating
superconducting pairing, is driven by changes in orbital correlations
Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconducting FeTeSe
It has recently been demonstrated that dynamical magnetic correlations
measured by neutron scattering in iron chalcogenides can be described with
models of short-range correlations characterized by particular {choices of
four-spin plaquettes, where the appropriate choice changes as the} parent
material is doped towards superconductivity. Here we apply such models to
describe measured maps of magnetic scattering as a function of two-dimensional
wave vectors obtained for optimally superconducting crystals of
FeTeSe. We show that the characteristic antiferromagnetic wave
vector evolves from that of the bicollinear structure found in underdoped
chalcogenides (at high temperature) to that associated with the stripe
structure of antiferromagnetic iron arsenides (at low temperature); {these can
both be described with the same local plaquette, but with different
inter-plaquette correlations}. While the magnitude of the low-energy magnetic
spectral weight is substantial at all temperatures, it actually weakens
somewhat at low temperature, where the charge carriers become more itinerant.
The observed change in spin correlations is correlated with the dramatic drop
in the electronic scattering rate and the growth of the bulk nematic response
on cooling. Finally, we also present powder neutron diffraction results for
lattice parameters in FeTeSe indicating that the tetrahedral bond
angle tends to increase towards the ideal value on cooling, in agreement with
the increased screening of the crystal field by more itinerant electrons and
the correspondingly smaller splitting of the Fe orbitals
Collapse of superconductivity in cuprates via ultrafast quenching of phase coherence
The possibility of driving phase transitions in low-density condensates
through the loss of phase coherence alone has far-reaching implications for the
study of quantum phases of matter. This has inspired the development of tools
to control and explore the collective properties of condensate phases via phase
fluctuations. Electrically-gated oxide interfaces, ultracold Fermi atoms, and
cuprate superconductors, which are characterized by an intrinsically small
phase-stiffness, are paradigmatic examples where these tools are having a
dramatic impact. Here we use light pulses shorter than the internal
thermalization time to drive and probe the phase fragility of the
BiSrCaCuO cuprate superconductor, completely melting
the superconducting condensate without affecting the pairing strength. The
resulting ultrafast dynamics of phase fluctuations and charge excitations are
captured and disentangled by time-resolved photoemission spectroscopy. This
work demonstrates the dominant role of phase coherence in the
superconductor-to-normal state phase transition and offers a benchmark for
non-equilibrium spectroscopic investigations of the cuprate phase diagram.Comment: 24 pages, 9 figures, Main Text and Supplementary Informatio
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