Subir Sachdev

Subir Sachdev is Herchel Smith Professor of Physics at Harvard University specializing in condensed matter.

He was elected to the U.S. National

He held professional positions at Bell Labs (1985–1987) and at Yale University (1987–2005), where he was a Professor of Physics, before returning to Harvard, where he is now the Herchel Smith Professor of Physics.

He has also held visiting positions as the Cenovus Energy James Clerk Maxwell Chair in Theoretical Physics at the Perimeter Institute for Theoretical Physics, and the Dr. Homi J. Bhabha Chair Professorship at the Tata Institute of Fundamental Research.

Academy of Sciences in 2014, received the Lars Onsager Prize from the American Physical Society and the Dirac Medal from the ICTP in 2018, and was elected Foreign Member of the Royal Society ForMemRS in 2023.

He was a co-editor of the Annual Review of Condensed Matter Physics 2017–2019, and is Editor-in-Chief of Reports on Progress in Physics 2022-.

Sachdev's research describes the consequences of quantum entanglement on the macroscopic properties of natural systems: watch his Institute Lecture at IIT Delhi in January 2024.

Sachdev has made extensive contributions to the description of the diverse varieties of entangled states of quantum matter.

Many of these contributions have been linked to experiments, especially to the rich phase diagrams of the high temperature superconductors.

Sachdev's research has exposed remarkable connections between the nature of quantum entanglement in certain laboratory materials, and the quantum entanglement in astrophysical black holes, and these connections have led to new insights on the entropy and radiation of black holes proposed by Stephen Hawking.

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 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 have been connected to experiments, including some that are in good quantitative agreement with observations on graphene.

These topics are discussed in more detail in Research.

P.W. Anderson proposed that Mott insulators realize antiferromagnets which could form resonating valence bond (RVB) or quantum spin liquid states with an energy gap to spin excitations without breaking time-reversal symmetry.

It was conjectured that such RVB states have excitations with fractional quantum numbers, such as a fractional spin 1/2.

The existence of such RVB ground states, and of the deconfinement of fractionalized excitations was first established by Read and Sachdev and Wen by the connection to a Z2 gauge theory.

Sachdev was also the first to show that the RVB state is an odd Z2 gauge theory, as described in Research.

An odd Z2 spin liquid has a background Z2 electric charge on each lattice site (equivalently, translations in the x and y directions anti-commute with each other in the super-selection sector of states associated with a Z2 gauge flux (also known as the m sector)).

Sachdev showed that antiferromagnets with half-integer spin form odd Z2 spin liquids, and those with integer spin form even Z2 spin liquids.

Using this theory, various universal properties of the RVB state were understood, including constraints on the symmetry transformations of the anyon excitations.

Sachdev also obtained many results on the confinement transitions of the RVB state, including restrictions on proximate quantum phases and the nature of quantum phase transitions to them.

Sachdev attended school at St. Joseph's Boys' High School, Bangalore and Kendriya Vidyalaya, ASC, Bangalore.

He attended college at Indian Institute of Technology, Delhi for a year.

He transferred to Massachusetts Institute of Technology where he received a B.S. in Physics.

He received his Ph.D. in theoretical physics from Harvard University.

He has also been on the Physical Sciences jury for the Infosys Prize from 2018.

Subir Sachdev has made profound contributions to theoretical condensed matter physics research.

His main interests have been in quantum magnetism, quantum criticality, and perhaps most innovative of all, links between the nature of quantum entanglement in black holes and strongly interacting electrons in materials.

Professor Subir Sachdev is a world renowned condensed matter theorist, with many seminal contributions to the theory of strongly interacting condensed matter systems.