We model a close-knit community of friends and enemies as a fully connected network with positive and negative signs on its edges. Theories from social psychology suggest that certain sign patterns are more stable than others. This notion of social “balance” allows us to define an energy landscape for such networks. Its structure is complex: numerical experiments reveal a landscape dimpled with local minima of widely varying depths. We derive rigorous bounds on the energies of these local minima and prove that they have a modular structure that can be used to classify them.
INTELLIGENCE chiefs have warned that China may have gained the capability to shut down Britain by crippling its telecoms and utilities.
They have told ministers of their fears that equipment installed by Huawei, the Chinese telecoms giant, in BT’s new communications network could be used to halt critical services such as power, food and water supplies.
The warnings coincide with growing cyberwarfare attacks on Britain by foreign governments, particularly Russia and China.
A confidential document circulating in Whitehall says that while BT has taken steps to reduce the risk of attacks by hackers or organised crime, “we believe that the mitigating measures are not effective against deliberate attack by China”.
The world’s first “unbreakable” quantum encryption network was unveiled this week at a science conference in Vienna.
The EU-sponsored network (called SECOQC) uses 200 km of fiber optic cables, provided by Siemens, to interconnect six locations in Vienna and the neighboring town of St. Poelten.
Quantum cryptography is markedly different from current security schemes, which are based on complex mathematical procedures that are extremely difficult, but not impossible, for outsiders to crack.
Instead, quantum systems harness the inherently unbreakable laws of quantum theory. The notion of quantum cryptography was established 25 years ago by Charles Bennett of IBM and Gilles Brassard of Montreal University, who was in attendance this week in Vienna to observe the new network in action.
“All quantum security schemes are based on the Heisenberg Uncertainty Principle, on the fact that you cannot measure quantum information without disturbing it,” Brassard told BBC News.
“Because of that, one can have a communications channel between two users on which it’s impossible to eavesdrop without creating a disturbance. An eavesdropper would create a mark on it. That was the key idea.”
We model a close-knit community of friends and enemies as a fully connected network with positive and negative signs on its edges. Theories from social psychology suggest that certain sign patterns are more stable than others. This notion of social “balance” allows us to define an energy landscape for such networks. Its structure is complex: numerical experiments reveal a landscape dimpled with local minima of widely varying depths. We derive rigorous bounds on the energies of these local minima and prove that they have a modular structure that can be used to classify them.
