617 research outputs found
Towards the physical vacuum of cosmic inflation
There have been long debates about the initial condition of inflationary
perturbations. In this work we explicitly show the decay of excited states
during inflation via interactions. For this purpose, we note that the folded
shape non-Gaussianity can be interpreted as the decay of the non-Bunch-Davies
initial condition. The one loop diagrams with non-Bunch-Davies propagators are
calculated to uncover the decay of such excited states. The observed smallness
of non-Gaussianity keeps the window open for probing inflationary initial
conditions and trans-Planckian physics
Echoes of Inflationary First-Order Phase Transitions in the CMB
Cosmological phase transitions (CPTs), such as the Grand Unified Theory (GUT)
and the electroweak (EW) ones, play a significant role in both particle physics
and cosmology. In this letter, we propose to probe the first-order CPTs, by
detecting gravitational waves (GWs) which are generated during the phase
transitions through the cosmic microwave background (CMB). If happened around
the inflation era, the first-order CPTs may yield low-frequency GWs due to
bubble dynamics, leaving imprints on the CMB. In contrast to the nearly
scale-invariant primordial GWs caused by vacuum fluctuation, these
bubble-generated GWs are scale dependent and have non-trivial B-mode spectra.
If decoupled from inflaton, the EWPT during inflation may serve as a probe for
the one after reheating where the baryon asymmetry could be generated via EW
baryogenesis (EWBG). The CMB thus provides a potential way to test the
feasibility of the EWBG, complementary to the collider measurements of Higgs
potential and the direct detection of GWs generated during EWPT.Comment: 5+6 pages, 4 figures. V2 changed title, added one figure about
constraints of Planck2015+BICEP2/Keck data, added references and removed
appendix. Accepted by PL
Tree-based Text-Vision BERT for Video Search in Baidu Video Advertising
The advancement of the communication technology and the popularity of the
smart phones foster the booming of video ads. Baidu, as one of the leading
search engine companies in the world, receives billions of search queries per
day. How to pair the video ads with the user search is the core task of Baidu
video advertising. Due to the modality gap, the query-to-video retrieval is
much more challenging than traditional query-to-document retrieval and
image-to-image search. Traditionally, the query-to-video retrieval is tackled
by the query-to-title retrieval, which is not reliable when the quality of
tiles are not high. With the rapid progress achieved in computer vision and
natural language processing in recent years, content-based search methods
becomes promising for the query-to-video retrieval. Benefited from pretraining
on large-scale datasets, some visionBERT methods based on cross-modal attention
have achieved excellent performance in many vision-language tasks not only in
academia but also in industry. Nevertheless, the expensive computation cost of
cross-modal attention makes it impractical for large-scale search in industrial
applications. In this work, we present a tree-based combo-attention network
(TCAN) which has been recently launched in Baidu's dynamic video advertising
platform. It provides a practical solution to deploy the heavy cross-modal
attention for the large-scale query-to-video search. After launching tree-based
combo-attention network, click-through rate gets improved by 2.29\% and
conversion rate get improved by 2.63\%.Comment: This revision is based on a manuscript submitted in October 2020, to
ICDE 2021. We thank the Program Committee for their valuable comment
Polariton Topological Transition Effects on Radiative Heat Transfer
Twisted two-dimensional bilayer materials exhibit many exotic physical
phenomena. Manipulating the twist angle between the two layers enables fine
control of the physical structure, resulting in development of many novel
physics, such as the magic-angle flat-band superconductivity, the formation of
moire exciton and interlayer magnetism. Here, combined with analogous
principles, we study theoretically the near-field radiative heat transfer
(NFRHT) between two twisted hyperbolic systems. This two twisted hyperbolic
systems are mirror images of each other. Each twisted hyperbolic system is
composed of two graphene gratings, where there is an angle {\phi} between this
two graphene gratings. By analyzing the photonic transmission coefficient as
well as the plasmon dispersion relation of twisted hyperbolic system, we prove
that the topological transitions of the surface state at a special angle (from
open (hyperbolic) to closed (elliptical) contours) can modulate efficiently the
radiative heat transfer. Meanwhile the role of the thickness of dielectric
spacer and vacuum gap on the manipulating the topological transitions of the
surface state and the NFRHT are also discussed. We predict the hysteresis
effect of topological transitions at a larger vacuum gap, and demonstrate that
as thickness of dielectric spacer increase, the transition from the enhancement
effect of heat transfer caused by the twisted hyperbolic system to a
suppression. This technology could novel mechanism and control method for
NFRHT, and may open a promising pathway for highly efficient thermal
management, energy harvesting, and subwavelength thermal imaging
An Anti-Jamming Strategy for Disco Intelligent Reflecting Surfaces Based Fully-Passive Jamming Attacks
Emerging intelligent reflecting surfaces (IRSs) significantly improve system
performance, while also pose a huge risk for physical layer security. A disco
IRS (DIRS), i.e., an illegitimate IRS with random time-varying reflection
properties, can be employed by an attacker to actively age the channels of
legitimate users (LUs). Such active channel aging (ACA) generated by the
DIRS-based fully-passive jammer (FPJ) can be applied to jam multi-user
multiple-input single-output (MU-MISO) systems without relying on either
jamming power or LU channel state information (CSI). To address the significant
threats posed by the DIRS-based FPJ, an anti-jamming strategy is proposed that
requires only the statistical characteristics of DIRS-jammed channels instead
of their CSI. Statistical characteristics of DIRS-jammed channels are first
derived, and then the anti-jamming precoder is given based on the derived
statistical characteristics. Numerical results are also presented to evaluate
the effectiveness of the proposed anti-jamming precoder against the DIRS-based
FPJ
Disco Intelligent Reflecting Surfaces: Active Channel Aging for Fully-Passive Jamming Attacks
Due to the open communications environment in wireless channels, wireless
networks are vulnerable to jamming attacks. However, existing approaches for
jamming rely on knowledge of the legitimate users' (LUs') channels, extra
jamming power, or both. To raise concerns about the potential threats posed by
illegitimate intelligent reflecting surfaces (IRSs), we propose an alternative
method to launch jamming attacks on LUs without either LU channel state
information (CSI) or jamming power. The proposed approach employs an
adversarial IRS with random phase shifts, referred to as a "disco" IRS (DIRS),
that acts like a "disco ball" to actively age the LUs' channels. Such active
channel aging (ACA) interference can be used to launch jamming attacks on
multi-user multiple-input single-output (MU-MISO) systems. The proposed
DIRS-based fully-passive jammer (FPJ) can jam LUs with no additional jamming
power or knowledge of the LU CSI, and it can not be mitigated by classical
anti-jamming approaches. A theoretical analysis of the proposed DIRS-based FPJ
that provides an evaluation of the DIRS-based jamming attacks is derived. Based
on this detailed theoretical analysis, some unique properties of the proposed
DIRS-based FPJ can be obtained. Furthermore, a design example of the proposed
DIRS-based FPJ based on one-bit quantization of the IRS phases is demonstrated
to be sufficient for implementing the jamming attack. In addition, numerical
results are provided to show the effectiveness of the derived theoretical
analysis and the jamming impact of the proposed DIRS-based FPJ
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