## Hawking Radiation and Non-equilibrium Quantum Critical Current Noise [1]

A new perspective often brings unexpected insights. In theoretical physics such changes of perspective are afforded by transformations to new variables - related to the original variables by some (possibly complicated) mathematical transformation - in terms of which the underlying physics is more transparent. One such change of variables is that between field strengths and lines of force or flux in electrostatics. The quantum mechanical version of this is a relationship between gauge theories such as QCD and string theory that had been speculated upon for some time, and was made concrete in seminal work of Maldecena about a decade ago.

This mapping or duality is now being used to provide some surprising insights in the study of strongly correlated electron systems. In a large number of cases, the complicated interplay of quantum mechanics and the interactions of many electrons lead to emergent simplicity in the group behaviour. This reveals itself in scaling and self-similarity in space and time and on different energy scales. The duality discovered by Maldecena allows this emerging behaviour of many electrons to be mapped to a property - known as general covariance - of the fundamental variables in a related gravitational system. One of the strange aspects of this mapping is that the gravitational system has one extra dimension compared to the electronic system to which it maps. The electronic system can be thought of as a sort of hologram of the gravitational system.

With this mapping in hand, intuition and insights developed on one side may be used to tease out new results on the other. Dr Julian Sonner of DAMTP Cambridge and Professor Andrew Green [2]of the London Centre for Nanotechnology have shown recently how Hawking radiation emitted by a black hole can be mapped onto the current noise measured in a strongly correlated electron system. They have used this insight to reveal some new and possibly (very) general things about the behaviour of strongly correlated quantum systems when they are driven out of equilibrium.

Journal link: "Hawking Radiation and Nonequilibrium Quantum Critical Current Noise" Physical Review Letters 109, 091601 (2012) [3]

**Figure:** Current noise in a quantum critical system provides a mathematical hologram of Hawking radiation in anti-de Sitter space