15,582 research outputs found
Photonic Crystal Architecture for Room Temperature Equilibrium Bose-Einstein Condensation of Exciton-Polaritons
We describe photonic crystal microcavities with very strong light-matter
interaction to realize room-temperature, equilibrium, exciton-polariton
Bose-Einstein condensation (BEC). This is achieved through a careful balance
between strong light-trapping in a photonic band gap (PBG) and large exciton
density enabled by a multiple quantum-well (QW) structure with moderate
dielectric constant. This enables the formation of long-lived, dense 10~m
- 1~cm scale cloud of exciton-polaritons with vacuum Rabi splitting (VRS) that
is roughly 7\% of the bare exciton recombination energy. We introduce a
woodpile photonic crystal made of CdMgTe with a 3D PBG of 9.2\%
(gap to central frequency ratio) that strongly focuses a planar guided optical
field on CdTe QWs in the cavity. For 3~nm QWs with 5~nm barrier width the
exciton-photon coupling can be as large as \hbar\Ome=55~meV (i.e., vacuum
Rabi splitting 2\hbar\Ome=110~meV). The exciton recombination energy of
1.65~eV corresponds to an optical wavelength of 750~nm. For 106 QWs
embedded in the cavity the collective exciton-photon coupling per QW,
\hbar\Ome/\sqrt{N}=5.4~meV, is much larger than state-of-the-art value of
3.3~meV, for CdTe Fabry-P\'erot microcavity. The maximum BEC temperature is
limited by the depth of the dispersion minimum for the lower polariton branch,
over which the polariton has a small effective mass where
is the electron mass in vacuum. By detuning the bare exciton
recombination energy above the planar guided optical mode, a larger dispersion
depth is achieved, enabling room-temperature BEC
Distributed quantum sensing enhanced by continuous-variable error correction
A distributed sensing protocol uses a network of local sensing nodes to estimate a global feature of the network, such as a weighted average of locally detectable parameters. In the noiseless case, continuous-variable (CV) multipartite entanglement shared by the nodes can improve the precision of parameter estimation relative to the precision attainable by a network without shared entanglement; for an entangled protocol, the root mean square estimation error scales like 1/M with the number M of sensing nodes, the so-called Heisenberg scaling, while for protocols without entanglement, the error scales like 1√M. However, in the presence of loss and other noise sources, although multipartite entanglement still has some advantages for sensing displacements and phases, the scaling of the precision with M is less favorable. In this paper, we show that using CV error correction codes can enhance the robustness of sensing protocols against imperfections and reinstate Heisenberg scaling up to moderate values of M. Furthermore, while previous distributed sensing protocols could measure only a single quadrature, we construct a protocol in which both quadratures can be sensed simultaneously. Our work demonstrates the value of CV error correction codes in realistic sensing scenarios
Diagnosing Degenerate Higgs Bosons at 125 GeV
We develop diagnostic tools that would provide incontrovertible evidence for
the presence of more than one Higgs boson near 125 GeV in the LHC data.Comment: 4 pages, 2 figure
Nonlinear Dual-Mode Control of Variable-Speed Wind Turbines with Doubly Fed Induction Generators
This paper presents a feedback/feedforward nonlinear controller for
variable-speed wind turbines with doubly fed induction generators. By
appropriately adjusting the rotor voltages and the blade pitch angle, the
controller simultaneously enables: (a) control of the active power in both the
maximum power tracking and power regulation modes, (b) seamless switching
between the two modes, and (c) control of the reactive power so that a
desirable power factor is maintained. Unlike many existing designs, the
controller is developed based on original, nonlinear,
electromechanically-coupled models of wind turbines, without attempting
approximate linearization. Its development consists of three steps: (i) employ
feedback linearization to exactly cancel some of the nonlinearities and perform
arbitrary pole placement, (ii) design a speed controller that makes the rotor
angular velocity track a desired reference whenever possible, and (iii)
introduce a Lyapunov-like function and present a gradient-based approach for
minimizing this function. The effectiveness of the controller is demonstrated
through simulation of a wind turbine operating under several scenarios.Comment: 14 pages, 9 figures, accepted for publication in IEEE Transactions on
Control Systems Technolog
Achieving the Heisenberg limit in quantum metrology using quantum error correction
Quantum metrology has many important applications in science and technology,
ranging from frequency spectroscopy to gravitational wave detection. Quantum
mechanics imposes a fundamental limit on measurement precision, called the
Heisenberg limit, which can be achieved for noiseless quantum systems, but is
not achievable in general for systems subject to noise. Here we study how
measurement precision can be enhanced through quantum error correction, a
general method for protecting a quantum system from the damaging effects of
noise. We find a necessary and sufficient condition for achieving the
Heisenberg limit using quantum probes subject to Markovian noise, assuming that
noiseless ancilla systems are available, and that fast, accurate quantum
processing can be performed. When the sufficient condition is satisfied, a
quantum error-correcting code can be constructed which suppresses the noise
without obscuring the signal; the optimal code, achieving the best possible
precision, can be found by solving a semidefinite program.Comment: 16 pages, 2 figures, see also arXiv:1704.0628
Isospin-violating dark-matter-nucleon scattering via two-Higgs-doublet-model portals
We show that in a multi-Higgs model in which one Higgs fits the LHC 125 GeV
state, one or more of the other Higgs bosons can mediate DM-nucleon
interactions with maximal DM isospin violation being possible for appropriate
Higgs-quark couplings, independent of the nature of DM. We then consider the
explicit example of a Type II two-Higgs-doublet model, identifying the h or H
as the 125 GeV state while the H or h, respectively, mediates DM-nucleon
interactions. Finally, we show that if a stable scalar, S, is added then it can
be a viable light DM candidate with correct relic density while obeying all
direct and indirect detection limits.Comment: Two subsections are added to address the collider bounds from direct
search for heavy Higgs bosons and from jet plus missing energy final states.
The LUX (2013) bound considered in the previous version is replaced by the
latest LUX (2016) bound and the SuperCDMS limit is taken into account. The
conclusions remain unchanged. A very minor change made in the title and new
references include
Two-Higgs-Doublet Models and Enhanced Rates for a 125 GeV Higgs
We examine the level of enhancement that can be achieved in the ZZ and
\gamma\gamma channels for a two-Higgs-doublet model Higgs boson (either the
light h or the heavy H) with mass near 125 GeV after imposing all constraints
from LEP data, B physics, precision electroweak data, vacuum stability,
unitarity and perturbativity. The latter constraints restrict substantially the
possibilities for enhancing the gg -> h -> \gamma\gamma or gg -> H ->
\gamma\gamma signal relative to that for the SM Higgs, hSM. Further, we find
that a significant enhancement of the gg -> h -> \gamma\gamma or gg -> H ->
\gamma\gamma signal in Type II models is possible only if the gg -> h -> ZZ or
gg -> H -> ZZ mode is even more enhanced, a situation disfavored by current
data. In contrast, in the Type I model one can achieve enhanced rates in the
\gamma\gamma final state for the h while having the ZZ mode at or below the SM
rate - the largest [gg -> h -> \gamma\gamma]/[gg -> hSM -> \gamma\gamma] ratio
found is of order ~1.3 when the two Higgs doublet vacuum expectation ratio is
tan\beta = 4 or 20 and the charged Higgs boson has its minimal LEP-allowed
value of m_{H^\pm} = 90 GeV.Comment: 15 pages, 9 figure
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