Dynamic scaling in natural swarms

Collective behavior in biological systems presents theoretical challenges beyond the borders of classical statistical physics. The lack of concepts such as scaling and renormalization is particularly problematic, as it forces us to negotiate details whose relevance is often hard to assess. In an attempt to improve this situation, we have presented experimental evidence of the emergence of dynamic scaling laws in natural swarms of midges. We found that spatio–temporal correlation functions in different swarms can be rescaled by using a single characteristic time, which grows with the correlation length with a dynamical critical exponent z ≈ 1, a value not found in any other standard statistical model. To check whether out–of–equilibrium effects may be responsible for this anomalous exponent, we run simulations of the simplest model of self–propelled particles and find z ≈ 2, suggesting that natural swarms belong to a novel dynamic universality class. This conclusion is strengthened by experimental evidence of the presence of non–dissipative modes in the relaxation, indicating that previously overlooked inertial effects are needed to describe swarm dynamics. The absence of a purely dissipative regime suggests that natural swarms undergo a near–critical censorship of hydrodynamics.

Contact person: Andrea Cavagna, ISC Rome

Complex active optical networks as a new laser concept

We have investigated complex optical networks containing one or more gain sections, and we have reported the evidence of lasing action; the emission spectrum reflects the topological disorder induced by the connections. A theoretical description compares well with the measurements, mapping the networks to directed graphs and showing the analogies with the problem of quantum chaos on graphs. We have shown that the interplay of chaotic diffusion and amplification leads to an emission statistic with characteristic heavy tails: for different topologies, we have provided an unprecedented experimental demonstration of Lévy statistics, expected for random lasers, for a continuous–wave pumped system.

Contact person: Stefano Lepri, ISC-CNR Sesto Fiorentino

Local equilibrium in bird flocks

We have introduced a novel dynamical inference technique, based on the principle of maximum entropy, which accommodates network rearrangements and overcomes the problem of slow experimental sampling rates. We have used this method to infer the strength and range of alignment forces from data of starling flocks. We have found that local bird alignment occurs on a much faster timescale than neighbour rearrangement. Accordingly, equilibrium inference, which assumes a fixed interaction network, gives results consistent with dynamical inference. We conclude that bird orientations are in a state of local quasi–equilibrium over the interaction length scale, providing firm ground for the applicability of statistical physics in certain active systems.

Contact person: Massimiliano Viale, ISC–CNR Univ. Roma La Sapienza

Electrospun amplified fiber optics

We have reported on near–infrared polymer fiber amplifiers working over a band of about 20 nm. The fibers are cheap, spun with a process entirely carried out at room temperature, and shown to have amplified spontaneous emission with good gain coefficients and low levels of optical losses (a few cm–1). The amplification process is favored by high fiber quality and low self–absorption. The found performance metrics appear to be suitable for short–distance operations, and the large variety of commercially available doping dyes might allow for effective multi–wavelength operations by electrospun amplified fiber optics.

Contact person: Dario Pisignano, NANO–CNR Pisa