7,531 research outputs found
Efficiency of Human Activity on Information Spreading on Twitter
Understanding the collective reaction to individual actions is key to
effectively spread information in social media. In this work we define
efficiency on Twitter, as the ratio between the emergent spreading process and
the activity employed by the user. We characterize this property by means of a
quantitative analysis of the structural and dynamical patterns emergent from
human interactions, and show it to be universal across several Twitter
conversations. We found that some influential users efficiently cause
remarkable collective reactions by each message sent, while the majority of
users must employ extremely larger efforts to reach similar effects. Next we
propose a model that reproduces the retweet cascades occurring on Twitter to
explain the emergent distribution of the user efficiency. The model shows that
the dynamical patterns of the conversations are strongly conditioned by the
topology of the underlying network. We conclude that the appearance of a small
fraction of extremely efficient users results from the heterogeneity of the
followers network and independently of the individual user behavior.Comment: 29 pages, 10 figure
Input-output theory for spin-photon coupling in Si double quantum dots
The interaction of qubits via microwave frequency photons enables
long-distance qubit-qubit coupling and facilitates the realization of a
large-scale quantum processor. However, qubits based on electron spins in
semiconductor quantum dots have proven challenging to couple to microwave
photons. In this theoretical work we show that a sizable coupling for a single
electron spin is possible via spin-charge hybridization using a magnetic field
gradient in a silicon double quantum dot. Based on parameters already shown in
recent experiments, we predict optimal working points to achieve a coherent
spin-photon coupling, an essential ingredient for the generation of long-range
entanglement. Furthermore, we employ input-output theory to identify observable
signatures of spin-photon coupling in the cavity output field, which may
provide guidance to the experimental search for strong coupling in such
spin-photon systems and opens the way to cavity-based readout of the spin
qubit
A Coherent Spin-Photon Interface in Silicon
Electron spins in silicon quantum dots are attractive systems for quantum
computing due to their long coherence times and the promise of rapid scaling
using semiconductor fabrication techniques. While nearest neighbor exchange
coupling of two spins has been demonstrated, the interaction of spins via
microwave frequency photons could enable long distance spin-spin coupling and
"all-to-all" qubit connectivity. Here we demonstrate strong-coupling between a
single spin in silicon and a microwave frequency photon with spin-photon
coupling rates g_s/(2\pi) > 10 MHz. The mechanism enabling coherent spin-photon
interactions is based on spin-charge hybridization in the presence of a
magnetic field gradient. In addition to spin-photon coupling, we demonstrate
coherent control of a single spin in the device and quantum non-demolition spin
state readout using cavity photons. These results open a direct path toward
entangling single spins using microwave frequency photons
Global Patterns of Synchronization in Human Communications
Social media are transforming global communication and coordination. The data
derived from social media can reveal patterns of human behavior at all levels
and scales of society. Using geolocated Twitter data, we have quantified
collective behaviors across multiple scales, ranging from the commutes of
individuals, to the daily pulse of 50 major urban areas and global patterns of
human coordination. Human activity and mobility patterns manifest the synchrony
required for contingency of actions between individuals. Urban areas show
regular cycles of contraction and expansion that resembles heartbeats linked
primarily to social rather than natural cycles. Business hours and circadian
rhythms influence daily cycles of work, recreation, and sleep. Different urban
areas have characteristic signatures of daily collective activities. The
differences are consistent with a new emergent global synchrony that couples
behavior in distant regions across the world. A globally synchronized peak that
includes exchange of ideas and information across Europe, Africa, Asia and
Australasia. We propose a dynamical model to explain the emergence of global
synchrony in the context of increasing global communication and reproduce the
observed behavior. The collective patterns we observe show how social
interactions lead to interdependence of behavior manifest in the
synchronization of communication. The creation and maintenance of temporally
sensitive social relationships results in the emergence of complexity of the
larger scale behavior of the social system.Comment: 20 pages, 12 figures. arXiv admin note: substantial text overlap with
arXiv:1602.0621
Further Improvements in the Understanding of Isotropic Loop Quantum Cosmology
The flat, homogeneous, and isotropic universe with a massless scalar field is
a paradigmatic model in Loop Quantum Cosmology. In spite of the prominent role
that the model has played in the development of this branch of physics, there
still remain some aspects of its quantization which deserve a more detailed
discussion. These aspects include the kinematical resolution of the
cosmological singularity, the precise relation between the solutions of the
densitized and non-densitized versions of the quantum Hamiltonian constraint,
the possibility of identifying superselection sectors which are as simple as
possible, and a clear comprehension of the Wheeler-DeWitt (WDW) limit
associated with the theory in those sectors. We propose an alternative operator
to represent the Hamiltonian constraint which is specially suitable to deal
with these issues in a satisfactory way. In particular, with our constraint
operator, the singularity decouples in the kinematical Hilbert space and can be
removed already at this level. Thanks to this fact, we can densitize the
quantum Hamiltonian constraint in a rigorous manner. Besides, together with the
physical observables, this constraint superselects simple sectors for the
universe volume, with a support contained in a single semiaxis of the real line
and for which the basic functions that encode the information about the
geometry possess optimal physical properties. Namely, they provide a
no-boundary description around the cosmological singularity and admit a
well-defined WDW limit in terms of standing waves. Both properties explain the
presence of a generic quantum bounce replacing the singularity at a fundamental
level, in contrast with previous studies where the bounce was proved in
concrete regimes and focusing on states with a marked semiclassical behavior.Comment: 13 pages, version accepted for publication in Physical Review
Maximum population transfer in a periodically driven two-level system
We study the dynamics of a two-level quantum system under the influence of
sinusoidal driving in the intermediate frequency regime. Analyzing the Floquet
quasienergy spectrum, we find combinations of the field parameters for which
population transfer is optimal and takes place through a series of well defined
steps of fixed duration. We also show how the corresponding evolution operator
can be approximated at all times by a very simple analytical expression. We
propose this model as being specially suitable for treating periodic driving at
avoided crossings found in complex multi-level systems, and thus show a
relevant application of our results to designing a control protocol in a
realistic molecular modelComment: 7 pages, 6 figure
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