2,921 research outputs found
Non-Markovian disentanglement dynamics of two-qubit system
We investigated the disentanglement dynamics of two-qubit system in
Non-Markovian approach. We showed that only the couple strength with the
environment near to or less than fine-structure constant 1/137, entanglement
appear exponential decay for a certain class of two-qubit entangled state.
While the coupling between qubit and the environment is much larger, system
always appears the sudden-death of entanglement even in the vacuum environment.Comment: 17 pages, 3 figure
Non-Markovian coherence dynamics of driven spin boson model: damped quantum beat or large amplitude coherence oscillation
The dynamics of driven spin boson model is studied analytically by means of
the perturbation approach based on a unitary transformation. We gave the
analytical expression for the population difference and coherence of the two
level system. The results show that in the weak driven case, the population
difference present damped coherent oscillation (single or double frequency) and
the frequencies depend on the initial state. The coherence exhibit damped
oscillation with Rabi frequency. When driven field is strong enough, the
population difference exhibit undamped large-amplitude coherent oscillation.
The results easily return to the two extreme cases without dissipation or
without periodic driven.Comment: 15 pages,5 figure
Electric-field control of magnetic ordering in the tetragonal BiFeO3
We propose a way to use electric-field to control the magnetic ordering of
the tetragonal BiFeO3. Based on systematic first-principles studies of the
epitaxial strain effect on the ferroelectric and magnetic properties of the
tetragonal BiFeO3, we find that there exists a transition from C-type to G-type
antiferromagnetic (AFM) phase at in-plane constant a ~ 3.905 {\AA} when the
ferroelectric polarization is along [001] direction. Such magnetic phase
transition can be explained by the competition between the Heisenberg exchange
constant J1c and J2c under the influence of biaxial strain. Interestingly, when
the in-plane lattice constant enlarges, the preferred ferroelectric
polarization tends to be canted and eventually lies in the plane (along [110]
direction). It is found that the orientation change of ferroelectric
polarization, which can be realized by applying external electric-field, has
significant impact on the Heisenberg exchange parameters and therefore the
magnetic orderings of tetragonal BiFeO3. For example, at a ~ 3.79 {\AA}, an
electric field along [111] direction with magnitude of 2 MV/cm could change the
magnetic ordering from C-AFM to G-AFM. As the magnetic ordering affects many
physical properties of the magnetic material, e.g. magnetoresistance, we expect
such strategy would provide a new avenue to the application of multiferroic
materials.Comment: 4 pages, 4 figure
On the performance of two protocols: SARG04 and BB84
We compare the performance of BB84 and SARG04, the later of which was
proposed by V. Scarani et al., in Phys. Rev. Lett. 92, 057901 (2004).
Specifically, in this paper, we investigate SARG04 with two-way classical
communications and SARG04 with decoy states. In the first part of the paper, we
show that SARG04 with two-way communications can tolerate a higher bit error
rate (19.4% for a one-photon source and 6.56% for a two-photon source) than
SARG04 with one-way communications (10.95% for a one-photon source and 2.71%
for a two-photon source). Also, the upper bounds on the bit error rate for
SARG04 with two-way communications are computed in a closed form by considering
an individual attack based on a general measurement. In the second part of the
paper, we propose employing the idea of decoy states in SARG04 to obtain
unconditional security even when realistic devices are used. We compare the
performance of SARG04 with decoy states and BB84 with decoy states. We find
that the optimal mean-photon number for SARG04 is higher than that of BB84 when
the bit error rate is small. Also, we observe that SARG04 does not achieve a
longer secure distance and a higher key generation rate than BB84, assuming a
typical experimental parameter set.Comment: 48 pages, 10 figures, 1 column, changed Figs. 7 and
Effects of Zeeman spin splitting on the modular symmetry in the quantum Hall effect
Magnetic-field-induced phase transitions in the integer quantum Hall effect
are studied under the formation of paired Landau bands arising from Zeeman spin
splitting. By investigating features of modular symmetry, we showed that
modifications to the particle-hole transformation should be considered under
the coupling between the paired Landau bands. Our study indicates that such a
transformation should be modified either when the Zeeman gap is much smaller
than the cyclotron gap, or when these two gaps are comparable.Comment: 8 pages, 4 figure
Security proof of a three-state quantum key distribution protocol without rotational symmetry
Standard security proofs of quantum key distribution (QKD) protocols often
rely on symmetry arguments. In this paper, we prove the security of a
three-state protocol that does not possess rotational symmetry. The three-state
QKD protocol we consider involves three qubit states, where the first two
states, |0_z> and |1_z>, can contribute to key generation and the third state,
|+>=(|0_z>+|1_z>)/\sqrt{2}, is for channel estimation. This protocol has been
proposed and implemented experimentally in some frequency-based QKD systems
where the three states can be prepared easily. Thus, by founding on the
security of this three-state protocol, we prove that these QKD schemes are, in
fact, unconditionally secure against any attacks allowed by quantum mechanics.
The main task in our proof is to upper bound the phase error rate of the qubits
given the bit error rates observed. Unconditional security can then be proved
not only for the ideal case of a single-photon source and perfect detectors,
but also for the realistic case of a phase-randomized weak coherent light
source and imperfect threshold detectors. Our result on the phase error rate
upper bound is independent of the loss in the channel. Also, we compare the
three-state protocol with the BB84 protocol. For the single-photon source case,
our result proves that the BB84 protocol strictly tolerates a higher quantum
bit error rate than the three-state protocol; while for the coherent-source
case, the BB84 protocol achieves a higher key generation rate and secure
distance than the three-state protocol when a decoy-state method is used.Comment: 10 pages, 3 figures, 2 column
An experimental study on (2) modular symmetry in the quantum Hall system with a small spin-splitting
Magnetic-field-induced phase transitions were studied with a two-dimensional
electron AlGaAs/GaAs system. The temperature-driven flow diagram shows the
features of the (2) modular symmetry, which includes distorted
flowlines and shiftted critical point. The deviation of the critical
conductivities is attributed to a small but resolved spin splitting, which
reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62,
10278.] Universal scaling is found under the reduction of the modular symmetry.
It is also shown that the Hall conductivity could still be governed by the
scaling law when the semicircle law and the scaling on the longitudinal
conductivity are invalid. *corresponding author:[email protected]: The revised manuscript has been published in J. Phys.: Condens.
Matte
Chiral Quantum Walks
Given its importance to many other areas of physics, from condensed matter
physics to thermodynamics, time-reversal symmetry has had relatively little
influence on quantum information science. Here we develop a network-based
picture of time-reversal theory, classifying Hamiltonians and quantum circuits
as time-symmetric or not in terms of the elements and geometries of their
underlying networks. Many of the typical circuits of quantum information
science are found to exhibit time-asymmetry. Moreover, we show that
time-asymmetry in circuits can be controlled using local gates only, and can
simulate time-asymmetry in Hamiltonian evolution. We experimentally implement a
fundamental example in which controlled time-reversal asymmetry in a
palindromic quantum circuit leads to near-perfect transport. Our results pave
the way for using time-symmetry breaking to control coherent transport, and
imply that time-asymmetry represents an omnipresent yet poorly understood
effect in quantum information science.Comment: 9 pages, 4 figures, REVTeX 4.1 - published versio
Phase-Remapping Attack in Practical Quantum Key Distribution Systems
Quantum key distribution (QKD) can be used to generate secret keys between
two distant parties. Even though QKD has been proven unconditionally secure
against eavesdroppers with unlimited computation power, practical
implementations of QKD may contain loopholes that may lead to the generated
secret keys being compromised. In this paper, we propose a phase-remapping
attack targeting two practical bidirectional QKD systems (the "plug & play"
system and the Sagnac system). We showed that if the users of the systems are
unaware of our attack, the final key shared between them can be compromised in
some situations. Specifically, we showed that, in the case of the
Bennett-Brassard 1984 (BB84) protocol with ideal single-photon sources, when
the quantum bit error rate (QBER) is between 14.6% and 20%, our attack renders
the final key insecure, whereas the same range of QBER values has been proved
secure if the two users are unaware of our attack; also, we demonstrated three
situations with realistic devices where positive key rates are obtained without
the consideration of Trojan horse attacks but in fact no key can be distilled.
We remark that our attack is feasible with only current technology. Therefore,
it is very important to be aware of our attack in order to ensure absolute
security. In finding our attack, we minimize the QBER over individual
measurements described by a general POVM, which has some similarity with the
standard quantum state discrimination problem.Comment: 13 pages, 8 figure
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