3,073 research outputs found
Coupled Cluster Treatment of the Alternating Bond Diamond Chain
By the analytical coupled cluster method (CCM), we study both the ground
state and lowest-lying excited-state properties of the alternating bond diamond
chain. The numerical exact diagonalization (ED) method is also applied to the
chain to verify the accuracy of CCM results. The ED results show that the
ground-state phase diagram contains two exact spin cluster solid ground states,
namely, the tetramer-dimer (TD) state and dimer state, and the ferrimagnetic
long-range-ordered state. We prove that the two exact spin cluster solid ground
states can both be formed by CCM. Moreover, the exact spin gap in the TD state
can be obtained by CCM. In the ferrimagnetic region, we find that the CCM
results for some physical quantities, such as the ground-state energy, the
sublattice magnetizations, and the antiferromagnetic gap, are comparable to the
results obtained by numerical methods. The critical line dividing the TD state
from the ferrimagnetic state is also given by CCM and is in perfect agreement
with that determined by the ED method.Comment: arXiv admin note: text overlap with arXiv:1502.0680
Freezing motion-induced dephasing in an atomic-ensemble quantum memory
Motion-induced dephasing is a dominant decoherence mechanism for atom-gas
quantum memories. In this paper, we develop a new coherent manipulation
technique which enables arbitrary engineering of the spin-wave momentum with
neglectable noise. By zeroing the spin-wave momentum, motion-induced dephasing
can be frozen completely. We experimentally demonstrate this scheme with
laser-cooled atoms in a DLCZ configuration. By applying the freezing pulses,
memory lifetime gets extended significantly to the limit of atom cloud
expansion and does not depend on the detection angle anymore. The observed high
cross-correlation above 20 proves that high-fidelity memory operation is well
preserved after coherent manipulation.Comment: 4 pages, 4 figure
Analytical and numerical studies of the one-dimensional sawtooth chain
By using the analytical coupled cluster method, the numerical exact
diagonalization method, and the numerical density matrix renormalization group
method, we investigated the properties of the one-dimensional sawtooth chain.
The results of the coupled cluster method based on Neel state demonstrate that
the ground state is in the quasi-Neel-long-range order state when a<ac1. The
translational symmetry of the ground state varies and the ground state evolves
from the quasi-Neel-long-range order state to the dimerized state at the
critical point ac1. The dimerized state is stable in the intermediate parameter
regime and vanishes at another critical point ac2. The results drawn from the
exact diagonalization show that the precise critical point ac1 and ac2 can be
determined by using the spin stiffness fidelity susceptibility and spin gap
separately. We compared the results obtained by using the coupled cluster
method based on canted state with those obtained based on spiral state, and
found that the ground state of the sawtooth chain is in the quasi-canted state
if a>ac2. The results of the coupled cluster method and the density matrix
renormalization group method both disclose that the type of the quantum phase
transition occurring at ac2 belongs to the first-order transition.Comment: accepted versio
1.25 GHz sine wave gating InGaAs/InP single-photon detector with monolithically integrated readout circuit
InGaAs/InP single-photon detectors (SPDs) are the key devices for
applications requiring near-infrared single-photon detection. Gating mode is an
effective approach to synchronous single-photon detection. Increasing gating
frequency and reducing module size are important challenges for the design of
such detector system. Here we present for the first time an InGaAs/InP SPD with
1.25 GHz sine wave gating using a monolithically integrated readout circuit
(MIRC). The MIRC has a size of 15 mm * 15 mm and implements the miniaturization
of avalanche extraction for high-frequency sine wave gating. In the MIRC,
low-pass filters and a low-noise radio frequency amplifier are integrated based
on the technique of low temperature co-fired ceramic, which can effectively
reduce the parasitic capacitance and extract weak avalanche signals. We then
characterize the InGaAs/InP SPD to verify the functionality and reliability of
MIRC, and the SPD exhibits excellent performance with 27.5 % photon detection
efficiency, 1.2 kcps dark count rate, and 9.1 % afterpulse probability at 223 K
and 100 ns hold-off time. With this MIRC, one can further design miniaturized
high-frequency SPD modules that are highly required for practical applications.Comment: 4 pages, 5 figures. Accepted for publication in Optics Letter
Operating Spin Echo in the Quantum Regime for an Atomic-Ensemble Quantum Memory
Spin echo is a powerful technique to extend atomic or nuclear coherence time
by overcoming the dephasing due to inhomogeneous broadening. However, applying
this technique to an ensemble-based quantum memory at single-quanta level
remains challenging. In our experimental study we find that noise due to
imperfection of the rephasing pulses is highly directional. By properly
arranging the beam directions and optimizing the pulse fidelities, we have
successfully managed to operate the spin echo technique in the quantum regime
and observed nonclassical photon-photon correlations. In comparison to the case
without applying the rephasing pulses, quantum memory lifetime is extended by 5
folds. Our work for the first time demonstrates the feasibility of harnessing
the spin echo technique to extend lifetime of ensemble-based quantum memories
at single-quanta level.Comment: 5 pages, 4 figure
Arbitrary Rotation of a Single Spinwave Qubit in an Atomic-Ensemble Quantum Memory
We report the first experimental realization of single-qubit manipulation for
single spinwaves stored in an atomic ensemble quantum memory. In order to have
high-fidelity gate operations, we make use of stimulated Raman transition and
controlled Lamor precession jointly. We characterize the gate performances with
quantum state tomography and quantum process tomography, both of which imply
that high-fidelity operations have been achieved. Our work complements the
experimental toolbox of atomic-ensemble quantum memories by adding the
capability of single-qubit manipulation, thus may have important applications
in future scalable quantum networks.Comment: 5 pages, 2 figures, 2 table
Miniaturized high-frequency sine wave gating InGaAs/InP single-photon detector
High-frequency gating InGaAs/InP single-photon detectors (SPDs) are widely
used for applications requiring single-photon detection in the near-infrared
region such as quantum key distribution. Reducing SPD size is highly desired
for practical use, which is favorable to the implementation of further system
integration. Here we present, to the best of our knowledge, the most compact
high-frequency sine wave gating (SWG) InGaAs/InP SPD. We design and fabricate
an InGaAs/InP single-photon avalanche diode (SPAD) with optimized semiconductor
structure, and then encapsulate the SPAD chip and a mini-thermoelectric cooler
inside a butterfly package with a size of 12.5 mm 22 mm 10
mm. Moreover, we implement a monolithic readout circuit for the SWG SPD in
order to replace the quenching electronics that is previously designed with
board-level integration. Finally, the components of SPAD, monolithic readout
circuit and the affiliated circuits are integrated into a single module with a
size of 13 cm 8 cm 4 cm. Compared with the 1.25 GHz SWG
InGaAs/InP SPD module (25 cm 10 cm 33 cm) designed in 2012,
the volume of our miniaturized SPD is reduced by 95\%. After the
characterization, the SPD exhibits excellent performance with a photon
detection efficiency of 30\%, a dark count rate of 2.0 kcps and an afterpulse
probability of 8.8\% under the conditions of 1.25 GHz gating rate, 100 ns
hold-off time and 243 K. Also, we perform the stability test over one week, and
the results show the high reliability of the miniaturized SPD module.Comment: 5 pages, 6 figures. Accepted for publication in Review of Scientific
Instrument
Escaping in couples facilitates evacuation: Experimental study and modeling
In this paper, the impact of escaping in couples on the evacuation dynamics
has been investigated via experiments and modeling. Two sets of experiments
have been implemented, in which pedestrians are asked to escape either in
individual or in couples. The experiments show that escaping in couples can
decrease the average evacuation time. Moreover, it is found that the average
evacuation time gap is essentially constant, which means that the evacuation
speed essentially does not depend on the number of pedestrians that have not
yet escaped. To model the evacuation dynamics, an improved social force model
has been proposed, in which it is assumed that the driving force of a
pedestrian cannot be fulfilled when the composition of physical forces exceeds
a threshold because the pedestrian cannot keep his/her body balance under this
circumstance. To model the effect of escaping in couples, attraction force has
been introduced between the partners. Simulation results are in good agreement
with the experimental ones
The metallicity distribution of F/G dwarfs derived from BATC survey data
Based on synthetic flux spectra calculated from theoretical atmospheric
models, a calibration of temperature and metallicity for the dwarfs observed in
the Beijing-Arizona-Taiwan-Connecticut (BATC) multicolor photometric system is
presented in this paper. According to this calibration, stellar effective
temperatures can be obtained from some temperature-sensitive color indices. The
sample stars have colors and magnitudes in the ranges 0.1<d-i<0.9 and
14.0<i<20.5. The photometric metallicities for these sample stars can be
derived by fitting SEDs. We determine the average stellar metallicity as a
function of distance from the Galactic plane. The metallicity gradient is found
to be d[Fe/H]/dz=-0.37+-0.1dex/kpc for z<4 kpc and
d[Fe/H]/dz=-0.06+-0.09dex/kpc between 5 and 15 kpc. These results can be
explained in terms of different contributions in density distribution for
Galactic models `thin disk', `thick disk' and `halo' components. However, for
the gradient in z>5 kpc, it could not be interpreted according to the different
contributions from the three components because of the large uncertainty. So it
is possible that there is little or no gradient for z>5 kpc. The overall
distribution shows a metallicity gradient d[Fe/H]/dz=-0.17+-0.04dex/kpc for
z<15 kpc.Comment: 22 pages, 9 figure, accepted by Astronomical Journa
Near-field Fourier ptychography: super-resolution phase retrieval via speckle illumination
Achieving high spatial resolution is the goal of many imaging systems.
Designing a high-resolution lens with diffraction-limited performance over a
large field of view remains a difficult task in imaging system design. On the
other hand, creating a complex speckle pattern with wavelength-limited spatial
features is effortless and can be implemented via a simple random diffuser.
With this observation and inspired by the concept of near-field ptychography,
we report a new imaging modality, termed near-field Fourier ptychography, for
tackling high-resolution imaging challenges in both microscopic and macroscopic
imaging settings. The meaning of 'near-field' is referred to placing the object
at a short defocus distance with a large Fresnel number. In our
implementations, we project a speckle pattern with fine spatial features on the
object instead of directly resolving the spatial features via a high-resolution
lens. We then translate the object (or speckle) to different positions and
acquire the corresponding images using a low-resolution lens. A ptychographic
phase retrieval process is used to recover the complex object, the unknown
speckle pattern, and the coherent transfer function at the same time. In a
microscopic imaging setup, we use a 0.12 numerical aperture (NA) lens to
achieve a NA of 0.85 in the reconstruction process. In a macroscale
photographic imaging setup, we achieve ~7-fold resolution gain using a
photographic lens. The final achievable resolution is not determined by the
collection optics. Instead, it is determined by the feature size of the speckle
pattern. The reported imaging modality can be employed in light, coherent
X-ray, and transmission electron imaging systems to increase resolution and
provide quantitative absorption and phase contrast of the object.Comment: 15 pages, 14 figure
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