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Friendship paradox disappears under degree biased network sampling
Authors:
Wojciech Roga
Abstract:
We show that in an undirected graph under degree biased sampling the expected degree of vertices is equal to the expected degree of their neighbors. In consequence, under the biased sampling the social network result known as the friendship paradox disappears. The identity is equivalent to the existence of a stationary state of a random walk on the graph or to the conservation of the total flow de…
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We show that in an undirected graph under degree biased sampling the expected degree of vertices is equal to the expected degree of their neighbors. In consequence, under the biased sampling the social network result known as the friendship paradox disappears. The identity is equivalent to the existence of a stationary state of a random walk on the graph or to the conservation of the total flow defined by the difference of the degrees of the vertices.
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Submitted 17 March, 2026;
originally announced March 2026.
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Compact Multi-Spectral Pushframe Camera for Nano-Satellites
Authors:
Yoann Noblet,
Stuart Bennett,
Paul F. Griffin,
Paul Murray,
Stephen Marshall,
Wojciech Roga,
John Jeffers,
Daniel Oi
Abstract:
In this paper we present an evolution of the single-pixel camera architecture, called 'pushframe', which addresses the limitations of pushbroom cameras in space-based applications. In particular, it is well-suited to observing fast moving scenes while retaining high spatial resolution and sensitivity. We show that the system is capable of producing colour images with good fidelity and scalable res…
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In this paper we present an evolution of the single-pixel camera architecture, called 'pushframe', which addresses the limitations of pushbroom cameras in space-based applications. In particular, it is well-suited to observing fast moving scenes while retaining high spatial resolution and sensitivity. We show that the system is capable of producing colour images with good fidelity and scalable resolution performance. The principle of our design places no restriction on the spectral range to be captured, making it suitable for wide infrared imaging.
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Submitted 1 June, 2020;
originally announced June 2020.
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Multipartite entanglement and secret key distribution in quantum networks
Authors:
Masahiro Takeoka,
Eneet Kaur,
Wojciech Roga,
Mark M. Wilde
Abstract:
Distribution and distillation of entanglement over quantum networks is a basic task for Quantum Internet applications. A fundamental question is then to determine the ultimate performance of entanglement distribution over a given network. Although this question has been extensively explored for bipartite entanglement-distribution scenarios, less is known about multipartite entanglement distributio…
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Distribution and distillation of entanglement over quantum networks is a basic task for Quantum Internet applications. A fundamental question is then to determine the ultimate performance of entanglement distribution over a given network. Although this question has been extensively explored for bipartite entanglement-distribution scenarios, less is known about multipartite entanglement distribution.
Here we establish the fundamental limit of distributing multipartite entanglement, in the form of GHZ states, over a quantum network. In particular, we determine the multipartite entanglement distribution capacity of a quantum network, in which the nodes are connected through lossy bosonic quantum channels. This setting corresponds to a practical quantum network consisting of optical links. The result is also applicable to the distribution of multipartite secret key, known as common key, for both a fully quantum network and trusted-node based quantum key distribution network. Our results set a general benchmark for designing a network topology and network quantum repeaters (or key relay in trusted nodes) to realize efficient GHZ state/common key distribution in both fully quantum and trusted-node-based networks. We show an example of how to overcome this limit by introducing a network quantum repeater.
Our result follows from an upper bound on distillable GHZ entanglement introduced here, called the "recursive-cut-and-merge" bound, which constitutes major progress on a longstanding fundamental problem in multipartite entanglement theory. This bound allows for determining the distillable GHZ entanglement for a class of states consisting of products of bipartite pure states.
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Submitted 14 August, 2020; v1 submitted 23 December, 2019;
originally announced December 2019.
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Universal freezing of quantum correlations within the geometric approach
Authors:
Marco Cianciaruso,
Thomas R. Bromley,
Wojciech Roga,
Rosario Lo Franco,
Gerardo Adesso
Abstract:
Quantum correlations in a composite system can be measured by resorting to a geometric approach, according to which the distance from the state of the system to a suitable set of classically correlated states is considered. Here we show that all distance functions, which respect natural assumptions of invariance under transposition, convexity, and contractivity under quantum channels, give rise to…
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Quantum correlations in a composite system can be measured by resorting to a geometric approach, according to which the distance from the state of the system to a suitable set of classically correlated states is considered. Here we show that all distance functions, which respect natural assumptions of invariance under transposition, convexity, and contractivity under quantum channels, give rise to geometric quantifiers of quantum correlations which exhibit the peculiar freezing phenomenon, i.e., remain constant during the evolution of a paradigmatic class of states of two qubits each independently interacting with a non-dissipative decohering environment. Our results demonstrate from first principles that freezing of geometric quantum correlations is independent of the adopted distance and therefore universal. This finding paves the way to a deeper physical interpretation and future practical exploitation of the phenomenon for noisy quantum technologies.
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Submitted 17 March, 2015; v1 submitted 11 November, 2014;
originally announced November 2014.