Sachdev's research describes the connection between physical properties of modern quantum materials and the nature of quantum entanglement in the many-particle wavefunction. Sachdev has worked extensively on the description of the diverse varieties of entangled states of quantum matter. These include states with topological order, with and without an energy gap to excitations, and critical states without quasiparticle excitations. Many of these contributions have been linked to experiments, especially to the rich phase diagrams of the high temperature superconductors.
Strange metals and black holes
Extreme examples of complex quantum entanglement arise in metallic states of matter without quasiparticle excitations, often called strange metals. Remarkably, there is an intimate connection between the quantum physics of strange metals found in modern materials (which can be studied in tabletop experiments), and quantum entanglement near black holes of astrophysics.
This connection is most clearly seen by first thinking more carefully about the defining characteristic of a strange metal: the absence of quasiparticles. In practice, given a state of quantum matter, it is difficult to completely rule out the existence of quasiparticles: while one can confirm that certain perturbations do not create single quasiparticle excitations, it is almost impossible to rule out a non-local operator which could create an exotic quasiparticle in which the underlying electrons are non-locally entangled. Sachdev argued (book, paper) instead that it is better to examine how rapidly the system loses quantum phase coherence, or reaches local thermal equilibrium in response to general external perturbations. If quasiparticles existed, dephasing would take a long time during which the excited quasiparticles collide with each other. In contrast, states without quasiparticles reach local thermal equilibrium in the fastest possible time, bounded below by a value of order (Planck constant)/((Boltzmann constant) x (absolute temperature)). Sachdev proposed a solvable model of a strange metal (a variant of which is now called the Sachdev-Ye-Kitaev (SYK) model), which was shown to saturate such a bound on the time to reach quantum chaos.
We can now make the connection to the quantum theory of black holes: quite generally, black holes also thermalize and reach quantum chaos in a time of order (Planck constant)/((Boltzmann constant) x (absolute temperature)), where the absolute temperature is the black hole's Hawking temperature. And this similarity to quantum matter without quasiparticles is not a co-incidence: for the SYK models, Sachdev had argued that the strange metal has a holographic dual description in terms of the quantum theory of black holes in a curved spacetime with 1 space dimension.
This connection, and other related work by Sachdev and collaborators, have led to valuable insights on the properties of electronic quantum matter, and on the nature of Hawking radiation from black holes. Solvable models of strange metals obtained from the gravitational mapping have inspired analyses of more realistic models of strange metals in the high temperature superconductors and other compounds. Such predictions (paper 1, paper 2) have been connected to experiments, including some that are in good quantitative agreement with observations on graphene (paper 1, paper 2).________________________________
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