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Johan Brännlund

Publications and source records attributed to Johan Brännlund.

5 recordsLinked to original sources

Optimized network clustering by jumping sub-optimal dendrograms

We propose a method to improve community division techniques in networks that are based on agglomeration by introducing dendrogram jumping. The method is based on iterations of sub-optimal dendrograms instead of optimization of each agglomeration step. We find the algorithm to exhibit excellent scaling behavior of its computational complexity. In its present form the algorithm scales as $\mathcal{O} (N^{2})$, but by using more efficient data structures it is possible to achieve a scaling of $\mathcal{O} (N \log^{2} N)$. We compare our results with other methods such as the greedy algorithm and the extremal optimization method. We find modularity values larger than the greedy algorithm and values comparable to the extremal optimization method.

physics.soc-ph↗

Density Analysis of Network Community Divisions

We present a compact matrix formulation of the modularity, a commonly used quality measure for the community division in a network. Using this formulation we calculate the density of modularities, a statistical measure of the probability of finding a particular modularity for a random but valid community division into $C$ communities. We present our results for some well--known and some artificial networks, and we conclude that the general features of the modularity density are quite similar for the different networks. From a simple model of the modularity we conclude that all nnected networks must show similar shapes of their modularity densities. The general features of this density may give valuable information in the search for good optimization schemes of the modularity.

cond-mat.stat-mech↗

Mixed state geometric phases, entangled systems, and local unitary transformations

The geometric phase for a pure quantal state undergoing an arbitrary evolution is a ``memory'' of the geometry of the path in the projective Hilbert space of the system. We find that Uhlmann's geometric phase for a mixed quantal state undergoing unitary evolution not only depends on the geometry of the path of the system alone but also on a constrained bi-local unitary evolution of the purified entangled state. We analyze this in general, illustrate it for the qubit case, and propose an experiment to test this effect. We also show that the mixed state geometric phase proposed recently in the context of interferometry requires uni-local transformations and is therefore essentially a property of the system alone.

quant-ph↗

Generalization of geometric phase to completely positive maps

We generalize the notion of relative phase to completely positive maps with known unitary representation, based on interferometry. Parallel transport conditions that define the geometric phase for such maps are introduced. The interference effect is embodied in a set of interference patterns defined by flipping the environment state in one of the two paths. We show for the qubit that this structure gives rise to interesting additional information about the geometry of the evolution defined by the CP map.

quant-ph↗