140,158 research outputs found
Asymmetric transmission through a flux-controlled non-Hermitian scattering center
We study the possibility of asymmetric transmission induced by a
non-Hermitian scattering center embedded in a one-dimensional waveguide,
motivated by the aim of realizing quantum diode in a non-Hermitian system. It
is shown that a symmetric non-Hermitian scattering center always
has symmetric transmission although the dynamics within the isolated center can
be unidirectional, especially at its exceptional point. We propose a concrete
scheme based on a flux-controlled non-Hermitian scattering center, which
comprises a non-Hermitian triangular ring threaded by an Aharonov-Bohm flux.
The analytical solution shows that such a complex scattering center acts as a
diode at the resonant energy level of the spectral singularity, exhibiting
perfect unidirectionality of the transmission. The connections between the
phenomena of the asymmetric transmission and reflectionless absorption are also
discussed.Comment: 6 pages, 5 figure
Geometric phase and phase diagram for non-Hermitian quantum XY model
We study the geometric phase for the ground state of a generalized
one-dimensional non-Hermitian quantum XY model, which has
transverse-field-dependent intrinsic rotation-time reversal symmetry. Based on
the exact solution, this model is shown to have full real spectrum in multiple
regions for the finite size system. The result indicates that the phase diagram
or exceptional boundary, which separates the unbroken and broken symmetry
regions corresponds to the divergence of the Berry curvature. The scaling
behaviors of the groundstate energy and Berry curvature are obtained in an
analytical manner for a concrete system.Comment: 6 pages, 3 figure
Momentum-independent reflectionless transmission in the non-Hermitian time-reversal symmetric system
We theoretically study the non-Hermitian systems, the non-Hermiticity of
which arises from the unequal hopping amplitude (UHA) dimers. The
distinguishing features of these models are that they have full real spectra if
all of the eigenvectors are time-reversal (T) symmetric rather than
parity-time-reversal (PT) symmetric, and that their Hermitian counterparts are
shown to be an experimentally accessible system, which have the same
topological structures as that of the original ones but modulated hopping
amplitudes within the unbroken region. Under the reflectionless transmission
condition, the scattering behavior of momentum-independent reflectionless
transmission (RT) can be achieved in the concerned non-Hermitian system. This
peculiar feature indicates that, for a certain class of non-Hermitian systems
with a balanced combination of the RT dimers, the defects can appear fully
invisible to an outside observer.Comment: 9 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1008.5306 by other author
Non-Hermitian anisotropic XY model with intrinsic rotation-time reversal symmetry
We systematically study the non-Hermitian version of the one-dimensional
anisotropic XY model, which in its original form, is a unique exactly solvable
quantum spin model for understanding the quantum phase transition. The
distinguishing features of this model are that it has full real spectrum if all
the eigenvectors are intrinsic rotation-time reversal (RT) symmetric rather
than parity-time reversal (PT) symmetric, and that its Hermitian counterpart is
shown approximately to be an experimentally accessible system, an isotropic XY
spin chain with nearest neighbor coupling. Based on the exact solution,
exceptional points which separated the unbroken and broken symmetry regions are
obtained and lie on a hyperbola in the thermodynamic limit. It provides a nice
paradigm to elucidate the complex quantum mechanics theory for a quantum spin
system.Comment: 7 pages, 3 figure
Partial topological Zak phase and dynamical confinement in non-Hermitian bipartite system
Unlike a Chern number in D and D topological system, Zak phase takes a
subtle role to characterize the topological phase in D. On the one hand, it
is not a gauge invariant, on the other hand, the Zak phase difference between
two quantum phases can be used to identify the topological phase transitions. A
non-Hermitian system may inherit some characters of a Hermitian system, such as
entirely real spectrum, unitary evolution, topological energy band, etc. In
this paper, we study the influence of non-Hermitian term on the Zak phase for a
class of non-Hermitian systems. We show exactly that the real part of the Zak
phase remains unchanged in a bipartite lattice. In a concrete example, D
Su-Schrieffer-Heeger (SSH) model, we find that the real part of Zak phase can
be obtained by an adiabatic process. To demonstrate this finding, we
investigate a scattering problem for a time-dependent scattering center, which
is a magnetic-flux-driven non-Hermitian SSH ring. Owing to the nature of the
Zak phase, the intriguing features of this design are the wave-vector
independence and allow two distinct behaviors, perfect transmission or
confinement, depending on the timing of a flux impulse threading the ring. When
the flux is added during a wavepacket travelling within the ring, the
wavepacket is confined in the scatter partially. Otherwise, it exhibits perfect
transmission through the scatter. Our finding extends the understanding and
broaden the possible application of geometric phase in a non-Hermitian system.Comment: 11 pages, 7 figure
EPR pairing dynamics in Hubbard model with resonant
We study the dynamics of the collision between two fermions in Hubbard model
with on-site interaction strength . The exact solution shows that the
scattering matrix for two-wavepacket collision is separable into two
independent parts, operating on spatial and spin degrees of freedom,
respectively. The S-matrix for spin configuration is equivalent to that of
Heisenberg-type pulsed interaction with the strength depending on and
relative group velocity . This can be applied to create distant
EPR pair, through a collision process for two fermions with opposite spins in
the case of ,\ without the need for
temporal control and measurement process. Multiple collision process for many
particles is also discussed.Comment: 7 pages, 3 figure
The sinusoidal periodicity nature for M>=5 global earthquakes
By using the M>=5 global earthquake data for Jan. 1950 to Dec. 2015, we
performed statistical analyses for the parameters magnitude, time, and depth on
a yearly scale. The magnitude spectrum, which is the earthquake number
accumulated at different magnitudes, had an exponential distribution. For the
first time, we report a very significant characteristic of the sinusoidal
periodic variation in the spectral index. The cycle of the sine function
fitting was 30.98 years. The concept of annual equivalent total magnitude
(AETM) of total released energy for each year was introduced and the trend
variation of AETM year by year was studied. Overall, the global AETM of
earthquakes with M>=5 displayed a certain upward trend as the years elapsed. At
the same time, the change of the average epicenter depth of the global
earthquakes (M>=5) in each year was analyzed
Improving quantum dense key distribution
The capacity of the quantum dense key distribution (QDKD) [Phys. Rev. A69,
032310 (2004)] is doubled by introducing the dense coding. The security of the
improved QDKD against eavesdropping is pointed out to be easily proven. In both
the original QDKD and the present improved QDKD, a strategy to double the
efficiency of generating the secret key with given length is proposed. In
addition, we point out a leak of security of the original QDKD and fix it
Deterministic secure direct communication by using swapping quantum entanglement and local unitary operations
A deterministic direct quantum communication protocol by using swapping
quantum entanglement and local unitary operations is proposed in this paper. A
set of ordered EPR pairs in one of the four Bell states is used. For each pair,
each of the two legitimate users owns a photon of the entangled pair via
quantum channel. The pairs are divided into two types of group, i.e., the
checking groups and the encoding-decoding groups. In the checking groups,
taking advantage of the swapping quantum entanglement and Alice's (the message
sender's) public announcement, the eavesdropping can be detected provided that
the number of the checking groups is big enough. After insuring the security of
the quantum channel, Alice encodes her bits via the local unitary operations on
the encoding-decoding groups. Then she performs her Bell measurements on her
photons and publicly announces her measurement results. After her announcement,
the message receiver Bob performs his Bell measurements on his photons and
directly extracts the encoding bits by using the property of the quantum
entanglement swapping. The security of the present scheme is also discussed:
under the attack scenarios to our best knowledge, the scheme is secure
Generalizing Several Theoretical Deterministic Secure Direct Bidirectional Communications to Improve Their Capacities
Several theoretical Deterministic Secure Direct Bidirectional Communication
protocols are generalized to improve their capacities by introducing the
superdense-coding in the case of high-dimension quantum states
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