235 research outputs found
A Modified Vivaldi Antenna for Improved Angular-Dependent Fidelity Property
The analysis, design, and realization of a modified Vivaldi antenna optimized for time domain fidelity factor in the half-space located in the direction of the antenna main beam are presented. The proposed antenna shows improved angular-dependent fidelity property, with respect to the signal transmitted in the main beam direction. A substantial increase in the fidelity factor is achieved by utilizing spatial filter effect introduced by adding two dielectric slabs parallel to the antenna substrate. By choosing optimal dimensions and location of the slabs, the signal waveforms in the mentioned half-space are equalized so as to improve the quality of the radiated signal waveform in the main beam direction. As a result, the fidelity property in the half-space is improved greatly. The simulated and measured fidelity factor in the angular operational region is studied and compared with experimental measurements. The ranges with the fidelity factor better than the value of 0.9 are improved by 95% in H-plane and by 14% in E-plane, respectively
Correlations in excited states of local Hamiltonians
Physical properties of the ground and excited states of a -local
Hamiltonian are largely determined by the -particle reduced density matrices
(-RDMs), or simply the -matrix for fermionic systems---they are at least
enough for the calculation of the ground state and excited state energies.
Moreover, for a non-degenerate ground state of a -local Hamiltonian, even
the state itself is completely determined by its -RDMs, and therefore
contains no genuine -particle correlations, as they can be inferred from
-particle correlation functions. It is natural to ask whether a similar
result holds for non-degenerate excited states. In fact, for fermionic systems,
it has been conjectured that any non-degenerate excited state of a 2-local
Hamiltonian is simultaneously a unique ground state of another 2-local
Hamiltonian, hence is uniquely determined by its 2-matrix. And a weaker version
of this conjecture states that any non-degenerate excited state of a 2-local
Hamiltonian is uniquely determined by its 2-matrix among all the pure
-particle states. We construct explicit counterexamples to show that both
conjectures are false. It means that correlations in excited states of local
Hamiltonians could be dramatically different from those in ground states. We
further show that any non-degenerate excited state of a -local Hamiltonian
is a unique ground state of another -local Hamiltonian, hence is uniquely
determined by its -RDMs (or -matrix)
From Economic Cooperation to Strategic Competition:Understanding the US-China Trade Disputes Through the Transformed Relations
This article investigates the escalation of US-China trade disputes and the implications for Sino-US relations. Both structural realism and liberal institutionalism have failed to pay sufficient attention to the evolution of the US-China economic relationship, and this article strives to highlight this crucial issue. The article employs a historical perspective to examine the transformation of US-China economic relations in the twenty-first century. It argues that the US-China economic relationship is evolving from a symbiotic but asymmetric one between 2001 and 2008, toward an increasingly competitive one after the 2008 global financial crisis, especially in the Trump-Xi era. The changing dynamics of US-China economic relations, as well as the shifting perceptions of the top leadership of each country toward the other, create the impetus for the transformation of Sino-US relations. This article suggests that the recent trade tension is embedded in the growing strategic competition between the two countries
Complete Characterization of the Ground Space Structure of Two-Body Frustration-Free Hamiltonians for Qubits
The problem of finding the ground state of a frustration-free Hamiltonian
carrying only two-body interactions between qubits is known to be solvable in
polynomial time. It is also shown recently that, for any such Hamiltonian,
there is always a ground state that is a product of single- or two-qubit
states. However, it remains unclear whether the whole ground space is of any
succinct structure. Here, we give a complete characterization of the ground
space of any two-body frustration-free Hamiltonian of qubits. Namely, it is a
span of tree tensor network states of the same tree structure. This
characterization allows us to show that the problem of determining the ground
state degeneracy is as hard as, but no harder than, its classical analog.Comment: 5pages, 3 figure
Estimating surface carbon fluxes based on a local ensemble transform Kalman filter with a short assimilation window and a long observation window: an observing system simulation experiment test in GEOS-Chem 10.1
We developed a carbon data assimilation system to estimate surface carbon fluxes using the local ensemble transform Kalman filter (LETKF) and atmospheric transport model GEOS-Chem driven by the MERRA-1 reanalysis of the meteorological field based on the Goddard Earth Observing System model, version 5 (GEOS-5). This assimilation system is inspired by the method of Kang et al. (2011, 2012), who estimated the surface carbon fluxes in an observing system simulation experiment (OSSE) as evolving parameters in the assimilation of the atmospheric CO2, using a short assimilation window of 6 h. They included the assimilation of the standard meteorological variables, so that the ensemble provided a measure of the uncertainty in the CO2 transport. After introducing new techniques such as “variable localization”, and increased observation weights near the surface, they obtained accurate surface carbon fluxes at grid-point resolution. We developed a new version of the local ensemble transform Kalman filter related to the “running-in-place” (RIP) method used to accelerate the spin-up of ensemble Kalman filter (EnKF) data assimilation (Kalnay and Yang, 2010; Wang et al., 2013; Yang et al., 2012). Like RIP, the new assimilation system uses the “no cost smoothing” algorithm for the LETKF (Kalnay et al., 2007b), which allows shifting the Kalman filter solution forward or backward within an assimilation window at no cost. In the new scheme a long “observation window” (e.g., 7 d or longer) is used to create a LETKF ensemble at 7 d. Then, the RIP smoother is used to obtain an accurate final analysis at 1 d. This new approach has the advantage of being based on a short assimilation window, which makes it more accurate, and of having been exposed to the future 7 d observations, which improves the analysis and accelerates the spin-up. The assimilation and observation windows are then shifted forward by 1 d, and the process is repeated. This reduces significantly the analysis error, suggesting that the newly developed assimilation method can be used with other Earth system models, especially in order to make greater use of observations in conjunction with models
Ground-State Spaces of Frustration-Free Hamiltonians
We study the ground-state space properties for frustration-free Hamiltonians.
We introduce a concept of `reduced spaces' to characterize local structures of
ground-state spaces. For a many-body system, we characterize mathematical
structures for the set of all the -particle reduced spaces, which
with a binary operation called join forms a semilattice that can be interpreted
as an abstract convex structure. The smallest nonzero elements in ,
called atoms, are analogs of extreme points. We study the properties of atoms
in and discuss its relationship with ground states of -local
frustration-free Hamiltonians. For spin-1/2 systems, we show that all the atoms
in are unique ground states of some 2-local frustration-free
Hamiltonians. Moreover, we show that the elements in may not be the
join of atoms, indicating a richer structure for beyond the convex
structure. Our study of deepens the understanding of ground-state
space properties for frustration-free Hamiltonians, from a new angle of reduced
spaces.Comment: 23 pages, no figur
Quantum Capacity Approaching Codes for the Detected-Jump Channel
The quantum channel capacity gives the ultimate limit for the rate at which
quantum data can be reliably transmitted through a noisy quantum channel.
Degradable quantum channels are among the few channels whose quantum capacities
are known. Given the quantum capacity of a degradable channel, it remains
challenging to find a practical coding scheme which approaches capacity. Here
we discuss code designs for the detected-jump channel, a degradable channel
with practical relevance describing the physics of spontaneous decay of atoms
with detected photon emission. We show that this channel can be used to
simulate a binary classical channel with both erasures and bit-flips. The
capacity of the simulated classical channel gives a lower bound on the quantum
capacity of the detected-jump channel. When the jump probability is small, it
almost equals the quantum capacity. Hence using a classical capacity
approaching code for the simulated classical channel yields a quantum code
which approaches the quantum capacity of the detected-jump channel
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