2,425 research outputs found
Single-photon-detection attack on the phase-coding continuous-variable quantum cryptography
The phase-coding quantum cryptographic scheme using the homodyne detection
and weak coherent state [Hirano et al.,Phys. Rev. A 68, 042331 (2003)] provides
the simplest continuous-variable quantum key distribution scheme from the
experimental side. However, the inherent loss of the practical system will not
only increase the bit error rate (BER) but also affect the security of the
final key. In this paper, we propose a single-photon-detection attack, and then
the security of the final key will be compromised in some parameter regimes.
Our results show that the BER induced by Eve can be lower than the inherent BER
induced by the loss of the system in some parameter regimes. Furthermore, our
attack gives the maximal communication distance of this scheme for given
experimental parameters.Comment: 6oages,4figure
Quantum Superposition States of Two Valleys in Graphene
A system in a quantum superposition of distinct states usually exhibits many
peculiar behaviors. Here we show that putting quasiparticles of graphene into
superpositions of states in the two valleys can complete change the properties
of the massless Dirac fermions. Due to the coexistence of both the quantum and
relativistic characteristics, the superposition states exhibit many oddball
behaviors in their chiral tunneling process. We further demonstrate that a
recently observed line defect in graphene could be used to generate such
superposition states. A possible experimental device to detect the novel
behaviors of the relativistic superposition states in graphene is proposed.Comment: 4 Figure
Robust quantum repeater with atomic ensembles against phase and polarization instability
We propose an alternative scheme for quantum repeater without phase
stabilization and polarization calibration of photons transmitted over
long-distance channel. We introduce time-bin photonic states and use a new
two-photon interference configuration to robustly generate entanglement between
distant atomic-ensemble-based memory qubits. Our scheme can be performed with
current experimental setups through making some simple adjustments.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
Topology-dependent quantum dynamics and entanglement-dependent topological pumping in superconducting qubit chains
We propose a protocol using a tunable Xmon qubit chain to construct
generalized Su-Schrieffer-Heeger (SSH) models that support various topological
phases. We study the time evolution of a single-excitation quantum state in a
SSH-type qubit chain and find that such dynamics is linked to topological
winding number. We also investigate the adiabatic transfer of a
single-excitation quantum state in a generalized SSH-type qubit chain and show
that this process can be connected with topological Chern number and be used to
generate a novel entanglement-dependent topological pumping. All results have
been demonstrated to be robust against qubit coupling imperfections and can be
observed in a short Xmon qubit chain. Our study provides a simple method to
directly measure topological invariants rooted in momentum space using quantum
dynamics in real space.Comment: 7 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1711.0775
Practical decoy-state measurement-device-independent quantum key distribution
Measurement-device-independent quantum key distribution (MDI-QKD) is immune
to all the detection attacks; thus when it is combined with the decoy-state
method, the final key is unconditionally secure, even if a practical weak
coherent source is used by Alice and Bob. However, until now, the analysis of
decoy-state MDI-QKD with a weak coherent source is incomplete. In this paper,
we derive, with only vacuum+weak decoy state, some tight formulas to estimate
the lower bound of yield and the upper bound of error rate for the fraction of
signals in which both Alice and Bob send a single-photon pulse to the untrusted
third party Charlie. The numerical simulations show that our method with only
vacuum+weak decoy state can asymptotically approach the theoretical limit of
the infinite number of decoy states. Furthermore, the statistical fluctuation
due to the finite length of date is also considered based on the standard
statistical analysis.Comment: 5 pages, 3 figure
Robust quantum state transfer via topological edge states in superconducting qubit chains
Robust quantum state transfer (QST) is an indispensable ingredient in
scalable quantum information processing. Here we present an experimentally
feasible mechanism for realizing robust QST via topologically protected edge
states in superconducting qubit chains. Using superconducting Xmon qubits with
tunable couplings, we construct generalized Su-Schrieffer-Heeger models and
analytically derive the wave functions of topological edge states. We find that
such edge states can be employed as a quantum channel to realize robust QST
between remote qubits. With a numerical simulation, we show that both
single-qubit states and two-qubit entangled states can be robustly transferred
in the presence of sizable imperfections in the qubit couplings. The transfer
fidelity demonstrates a wide plateau at the value of unity in the imperfection
magnitude. This approach is general and can be implemented in a variety of
quantum computing platforms.Comment: 7 pages, 4 figure
Wavelength attack on practical continuous-variable quantum-key-distribution system with a heterodyne protocol
We present the wavelength attack on a practical continuous-variable
quantum-key-distribution system with a heterodyne protocol, in which the
transmittance of beam splitters at Bob's station is wavelength-dependent. Our
strategy is proposed independent of but analogous to that of Huang et al.
[arXiv: 1206.6550v1 [quant-ph]], but in that paper the shot noise of the two
beams that Eve sends to Bob, transmitting after the homodyne detector, is
unconsidered. However, shot noise is the main contribution to the deviation of
Bob's measurements from Eve's when implementing the wavelength attack, so it
must be considered accurately. In this paper, we firstly analyze the solutions
of the equations specifically that must be satisfied in this attack, which is
not considered rigorously by Huang et al. Then we calculate the shot noise of
the homodyne detector accurately and conclude that the wavelength attack can be
implemented successfully in some parameter regime.Comment: 6 pages, 3 figure
Local oscillator fluctuation opens a loophole for Eve in practical continuous-variable quantum-key-distribution systems
We consider the security of practical continuous-variable quantum key
distribution implementation with the local oscillator (LO) fluctuating in time,
which opens a loophole for Eve to intercept the secret key. We show that Eve
can simulate this fluctuation to hide her Gaussian collective attack by
reducing the intensity of the LO. Numerical simulations demonstrate that, if
Bob does not monitor the LO intensity and does not scale his measurements with
the instantaneous intensity values of LO, the secret key rate will be
compromised severely.Comment: 8 pages, 5 figure
Impossibility Criterion for Obtaining Pure Entangled States From Mixed States By Purifying Protocols
Purifying noisy entanglement is a protocol which can increase the
entanglement of a mixed state (as a source)at expense of the entanglement of
others(as an ancilla)by collective measurement. A protocol with which one can
get a pure entangled state from a mixed state is defined as purifying mixed
states. We address a basic question: can one get a pure entangled state from a
mixed state? We give a necessary and sufficient condition of purifying a mixed
state by fit local operations and classical communication and show that for a
class of source states and ancilla states in arbitrary bipartite systems
purifying mixed states is impossible by finite rounds of purifying protocols.
For systems, it is proved that arbitrary states cannot be purified
by individual measurement. The possible application and meaning of the
conclusion are discussed.Comment: 5 pages,to be published in PR
Analysis of s-wave, p-wave and d-wave holographic superconductors in Ho\v{r}ava-Lifshitz gravity
In this work, the s-wave, p-wave and d-wave holographic superconductors in
the Ho\v{r}ava-Lifshitz gravity are investigated in the probe limit. For the
present approach, it is shown that the equations of motion for different wave
states in Einstein gravity can be written into a unified form, and condensates
take place in all three cases. This scheme is then generalized to
Ho\v{r}ava-Lifshitz gravity, and an unified equation for multiple holographic
states is obtained. Furthermore, the properties of the condensation and the
optical conductivity are studied numerically. It is found that, in the case of
Ho\v{r}ava-Lifshitz gravity, it is always possible to find some particular
parameters in the corresponding Einstein case where the condensation curves are
identical. For fixed scalar field mass , a non-vanishing becomes
the condensation easier than in Einstein gravity for s-wave superconductor.
However, the p-wave and d-wave superconductors have greater than s-wave
one.Comment: published versio
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