1,631 research outputs found
Charged and uncharged vortices in quasiclassical theory
The charging effect of a superconducting vortex core is very important for
transport properties of superconducting vortices. The chiral p-wave
superconductor, known as a topological superconductor (SC), has a Majorana
fermion in a vortex core and the charging effect has been studied using
microscopic Bogoliubov-de Gennes (BdG) theory. According to calculations based
on the BdG theory, one type of the vortex is charged as well as the vortex of
the s-wave SC, while the other is uncharged. We reproduce this interesting
charging effect using an augmented quasiclassical theory in chiral p-wave SCs,
by which we can deal with particle-hole asymmetry in the quasiclassical
approximation.Comment: 6 pages, 2 figure
Impurity Effects on Caroli-de Gennes-Matricon Mode in a Vortex Core in Superconductors
We develop a scheme of Gor'kov Green's functions to treat impurity effects on
Caroli-de Gennes-Matricon (CdGM) mode in superconductors (SCs) by improving the
Kopnin-Kravtsov scheme with respect to the coherence factors and applicability
to various SCs. We can study the impurity effects keeping the discreteness of
the energy spectrum in contrast to the quasiclassical theory. We can thus apply
this scheme to the SCs with the small quasiclassical parameter
(which is the product of the Fermi wavenumber
and the coherence length in pure SC at zero
temperature) and/or superclean regime
( and denote, respectively, the level spacing of
the CdGM mode called minigap and the relaxation time for CdGM mode and we take
). We investigate the impurity effects as a white noise for a vortex
in an s-wave SC and two types of vortices in a chiral p-wave SC, for various
values of the quasiclassical parameters and impurity strengths (from moderately
clean regime to superclean regime), and confirm the validity of this scheme.Comment: 8 pages, 6 figure
Impurity Effects on Bound States in the Vortex Core of Topological S-wave Superconductor
We study the impurity effects on the Caroli-de Gennes-Matricon (CdGM) states,
particularly on the level spacings in a vortex core in topological s-wave
superconductor (SC) by two means, numerically and analytically. The topological
s-wave SC belongs to the same class as a chiral p-wave SC and thus there are
two inequivalent vortices in terms of any symmetry operation. We take into
account this inequivalence and numerically calculate the scattering rates based
on an improved version of Kopnin-Kravtsov (iKK) scheme, which enables us to
treat the discrete levels in the presence of white-noise disorder. We also
construct the Andreev equation for the topological s-wave SC and obtain the
Andreev bound states analytically. We use a correspondence between the wave
functions for the Bogoliubov-de Gennes equation and the Andreev equation in the
iKK scheme and deduce the formula of scattering rates described by the wave
function for the Andreev equation. With this formula, we discuss the origin of
impurity scattering rates for CdGM states of topological s-wave SC and the
dependence on the types of vortices related to the inequivalence.Comment: 10 pages, 6 figure
Secure Quantum Network Coding on Butterfly Network
Quantum network coding on the butterfly network has been studied as a typical
example of quantum multiple cast network. We propose secure quantum network
coding on the butterfly network in the multiple unicast setting based on a
secure classical network coding. This protocol certainly transmits quantum
states when there is no attack. We also show the secrecy even when the
eavesdropper wiretaps one of the channels in the butterfly network.Comment: 11 pages, 2 figure
Algebra and Hilbert space structures induced by quantum probes
In the general setting of quantum controls, it is unrealistic to control all
of the degrees of freedom of a quantum system. We consider a scenario where our
direct access is restricted to a small subsystem that is constantly
interacting with the rest of the system . What we investigate here is the
fundamental structure of the Hilbert space that is caused solely by the
restrictedness of the direct control. We clarify the intrinsic space structure
of the entire system and that of the operations which could be activated
through . The structures hereby revealed would help us make quantum control
problems more transparent and provide a guide for understanding what we can
implement. They can be deduced by considering an algebraic structure, which is
the Jordan algebra formed from Hermitian operators, naturally induced by the
setting of limited access. From a few very simple assumptions about direct
operations, we elucidate rich structures of the operator algebras and Hilbert
spaces that manifest themselves in quantum control scenarios.Comment: Main text is the first 12 pages, and the following 24 pages contain
supplementary lemmas and their proofs, including detailed explanations on the
Jordan algebra (with hermitian operators
Singularity for Solutions of Linearized KdV Equations
We investigate the time propagation of singularity of a solution to
linearized KdV equation by using the characterization of wave front sets with
using to the wave packet transform (short time Fourier transform)
Entanglement-assisted classical communication can simulate classical communication without causal order
Phenomena induced by the existence of entanglement, such as nonlocal
correlations, exhibit characteristic properties of quantum mechanics
distinguishing from classical theories. When entanglement is accompanied by
classical communication, it enhances the power of quantum operations jointly
performed by two spatially separated parties. Such a power has been analyzed by
the gap between the performances of joint quantum operations implementable by
local operations at each party connected by classical communication with and
without the assistance of entanglement. In this work, we present a new
formulation for joint quantum operations connected by classical communication
beyond special relativistic causal order but without entanglement and still
within quantum mechanics. Using the formulation, we show that entanglement
assisting classical communication necessary for implementing a class of joint
quantum operations called separable maps can be interpreted to simulate
"classical communication" not respecting causal order. Our results reveal a new
counter-intuitive aspect of entanglement related to spacetime
Secrecy and Robustness for Active Attack in Secure Network Coding and its Application to Network Quantum Key Distribution
In network coding, we discuss the effect of sequential error injection on
information leakage. We show that there is no improvement when the operations
in the network are linear operations. However, when the operations in the
network contains non-linear operations, we find a counterexample to improve
Eve's obtained information. Furthermore, we discuss the asymptotic rate in a
linear network under the secrecy and robustness conditions as well as under the
secrecy condition alone. Finally, we apply our results to network quantum key
distribution, which clarifies the type of network that enables us to realize
secure long distance communication via short distance quantum key distribution.Comment: We fixed several error
Quantum stream cipher by Yuen 2000 protocol: Design and experiment by intensity modulation scheme
This paper shall investigate Yuen protocol, so called Y-00, which can realize
a randomized stream cipher with high bit rate(Gbps) for long distance(several
hundreds km). The randomized stream cipher with randomization by quantum noise
based on Y-00 is called quantum stream cipher in this paper, and it may have
security against known plaintext attacks which has no analog with any
conventional symmetric key ciphers. We present a simple cryptanalysis based on
an attacker's heterodyne measurement and the quantum unambiguous measurement to
make clear the strength of Y-00 in real communication. In addition, we give a
design for the implementation of an intensity modulation scheme and report the
experimental demonstration of 1 Gbps quantum stream cipher through 20 km long
transmission line.Comment: This paper will appear in Phys. Rev.
Probing untouchable environment as a resource for quantum computing
When manipulating a quantum system , its surrounding system, or
\textit{environment}, induces unwanted effects. It is mainly due to its
vastness and the lack of knowledge about the Hamiltonian that governs
the dynamics inside and the interaction with . The detail of is
usually extremely hard to identify, since can hardly be measured or
controlled directly. Nevertheless, here we show that it is possible to probe
and control a part of, if not all, the dynamics involving , within the
timescale in which its effective dimension can be seen finite. That is, we may
be able to let a noisy environment work in our favor as a part of quantum
computer.Comment: 21 pages in tota
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