88 research outputs found
Optimal correction of independent and correlated errors
We identify optimal quantum error correction codes for situations that do not
admit perfect correction. We provide analytic n-qubit results for standard
cases with correlated errors on multiple qubits and demonstrate significant
improvements to the fidelity bounds and optimal entanglement decay profiles.Comment: 11 pages, 4 figures. Updated to include fidelity analysi
Extended two-level quantum dissipative system from bosonization of the elliptic spin-1/2 Kondo model
We study the elliptic spin-1/2 Kondo model (spin-1/2 fermions in one
dimension with fully anisotropic contact interactions with a magnetic impurity)
in the light of mappings to bosonic systems using the fermion-boson
correspondence and associated unitary transformations. We show that for fixed
fermion number, the bosonic system describes a two-level quantum dissipative
system with two noninteracting copies of infinitely-degenerate upper and lower
levels. In addition to the standard tunnelling transitions, and the transitions
driven by the dissipative coupling, there are also bath-mediated transitions
between the upper and lower states which simultaneously effect shifts in the
horizontal degeneracy label. We speculate that these systems could provide new
examples of continuous time quantum random walks, which are exactly solvable.Comment: 7 pages, 1 figur
Quasiclassical theory for the superconducting proximity effect in Dirac materials
We derive the quasiclassical non-equilibrium Eilenberger and Usadel equations
to first order in quantities small compared to the Fermi energy, valid for
Dirac edge and surface electrons with spin-momentum locking, as relevant for
topological insulators. We discuss in detail several of the key technical
points and assumptions of the derivation, and provide a Riccati-parametrization
of the equations. Solving first the equilibrium equations for S/N and S/F
bilayers and Josephson junctions, we study the superconducting proximity effect
in Dirac materials. Similarly to related works, we find that the effect of an
exchange field depends strongly on the direction of the field. Only components
normal to the transport direction lead to attenuation of the Cooper pair
wavefunction inside the F. Fields parallel to the transport direction lead to
phase-shifts in the dependence on the superconducting phase difference for both
the charge current and density of states in an S/F/S-junction. Moreover, we
compute the differential conductance in S/N and S/F bilayers with an applied
voltage bias, and determine the dependence on the length of the N and F regions
and the exchange field.Comment: 13 pages, 5 figures. Accepted for publication in Phys. Rev.
Critical Temperature and Tunneling Spectroscopy of Superconductor-Ferromagnet Hybrids with Intrinsic Rashba-Dresselhaus Spin-Orbit Coupling
We investigate theoretically how the proximity effect in
superconductor/ferromagnet hybrid structures with intrinsic spin-orbit coupling
manifests in the density of states and critical temperature. To describe a
general scenario, we allow for both Rashba and Dresselhaus type spin-orbit
coupling. Our results are obtained via the quasiclassical theory of
superconductivity, extended to include spin-orbit coupling in the Usadel
equation and Kupriyanov--Lukichev boundary conditions. Unlike previous works,
we have derived a Riccati parametrization of the Usadel equation with
spin-orbit coupling which allows us to address the full proximity regime.
First, we consider the density of states in both SF bilayers and SFS trilayers,
where the spectroscopic features in the latter case are sensitive to the phase
difference between the two superconductors. We find that the presence of
spin-orbit coupling leaves clear spectroscopic fingerprints in the density of
states due to its role in creating spin-triplet Cooper pairs. Unlike SF and SFS
structures without spin-orbit coupling, the density of states in the present
case depends strongly on the direction of magnetization. We show that the
spin-orbit coupling can stabilize singlet superconductivity even in the
presence of a strong exchange field . This leads to the
possibility of a magnetically tunable minigap: changing the direction of the
exchange field opens and closes the minigap. We also determine how the critical
temperature of an SF bilayer is affected by spin-orbit coupling and
demonstrate that one can achieve a spin-valve effect with a single ferromagnet.
We find that displays highly non-monotonic behavior both as a function of
the magnetization direction and the type and direction of the spin-orbit
coupling, offering a new way to exert control over the superconductivity of
proximity structures.Comment: 25 pages, 21 figures. Accepted for publication in Phys. Rev.
High magnetic field superconductivity in a two-band superconductor
When applying an external magnetic field to a superconductor, orbital and
Pauli paramagnetic pairbreaking effects govern the limit of the upper critical
magnetic field that can be supported before superconductivity breaks down.
Experimental studies have shown that many multiband superconductors exhibit
values of the upper critical magnetic field that violate the theoretically
predicted limit, giving rise to many studies treating the underlying mechanisms
that allow this. In this work we consider spin-splitting induced by an external
magnetic field in a superconductor with two relevant bands close to the Fermi
level, and show that the presence of interband superconducting pairing produces
high-field reentrant superconductivity violating the
Pauli-Chandrasekhar-Clogston limit for the value of the upper critical magnetic
field
Cavity-mediated superconductor\unicode{x2013}ferromagnetic insulator coupling
A recent proof of concept showed that cavity photons can mediate
superconducting (SC) signatures to a ferromagnetic insulator (FI) over a
macroscopic distance [Phys. Rev. B, 102, 180506(R) (2020)]. In contrast with
conventional proximity systems, this facilitates long-distance
FI\unicode{x2013}SC coupling, local subjection to different drives and
temperatures, and studies of their mutual interactions without proximal
disruption of their orders. Here we derive a microscopic theory for these
interactions, with an emphasis on the leading effect on the FI, namely, an
induced anisotropy field. In an arbitrary practical example, we find an
anisotropy field of 14 \unicode{x2013} 16 T, which is expected to yield
an experimentally appreciable tilt of the FI spins for low-coercivity FIs such
as Bi-YIG. We discuss the implications and potential applications of such a
system in the context of superconducting spintronics.Comment: 17 pages, 8 figure
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