### Steven Girvin

*Yale University*

### "Schrödinger Cats, Maxwell’s Demon and Quantum Error Correction (That Works)"

A revolution is underway in the construction of ‘artificial atoms’ out of superconducting electrical circuits. These macroscopic ‘atoms’ have quantized energy levels and can emit and absorb quanta of light (in this case microwave photons), just like ordinary atoms. The large size of these ‘atoms’ yields remarkably strong atom-photon coupling and has given us the ability to completely control and measure the quantum state of the electromagnetic field in a cavity at the level of individual microwave photons.

In particular, it is now possible to create record large ‘Schrödinger cat’ photon states. In addition to being used to study quantum mechanics at macroscopic scales, these cat states form a novel basis for quantum information processing in a ‘photonic’ quantum computer. Recent experiments at Yale have successfully demonstrated quantum error correction that reaches the break-even point for the first time. This talk will present an elementary introduction to the field as well as an overview of recent experimental progress.

**Background Reading:**

**Elementary**:

[1] ‘Wiring up quantum systems,’ R.J. Schoelkopf and S.M. Girvin, Nature **451**, 664 (2008).

[2] ‘Superconducting Circuits for Quantum Information: An Outlook,’ M.H. Devoret and R.J. Schoelkopf, Science **339**, 1169 (2013).

**Advanced:**

[3] `Deterministically encoding quantum information in 100-photon Schrödinger cat states,' Vlastakis, B., et al. Science **342**, 607 (2013).

[4] ‘A Schro¨dinger Cat Living in Two Boxes,’ Chen Wang, et al., Science **352**, 1087 (2016).

[5] ‘Extending the lifetime of a quantum bit with error correction in superconducting circuits,' Nissim Ofek, et al., Nature **536**, 441445 (2016).

**About the speaker**

Steven Girvin is a theoretical physicist who studies the quantum mechanics of large collections of atoms, molecules and electrons such as are found in superconductors, magnets and transistors. Of particular current interest to him is the engineering question of whether it is possible to build a quantum computer. He is collaborating with experimentalists Rob Schoelkopf and Michel Devoret in Applied Physics who are constructing superconducting circuit elements which might someday form the basis for a quantum computer. Such a computer could in principle solve problems which are impossible on ordinary computers. However in order to build a quantum computer it is necessary to create circuit devices which behave quantum mechanically (like individual atoms) despite the fact that they are macroscopic and consist of a very large number of atoms. In addition to potential practical applications, this difficult challenge will help us better understand the connections between the microscopic quantum world and the macroscopic classical world of everyday experience.

Professor Girvin is interested in quantum many-body physics, and quantum and classical phase transitions, particularly in disordered systems. A quantum phase transition is one which occurs at zero temperature as some parameter in the Hamiltonian is varied. Using path integral techniques, one can often show that a quantum critical point in a d-dimensional system is in the same universality class as some other classical system in (d+1)-dimensions. He is interested in finding quantities that are universal properties of the system near the critical point and are independent of all microscopic details. For example, the electrical conductivity of some two-dimensional conductors at critical points is a universal dimensionless number of order unity times the quantum of conductance, e2/h.

Much of his work has been on the quantum Hall effect, but he has also worked on the superconductor-insulator transition, the vortex glass transition in high Tc superconductors, superfluid helium in fractal aerogel, the Anderson localization problem, the Coulomb blockade problem in mesoscopic device physics, and on quantum spin chains.

His work is approximately evenly divided between analytical theories and numerical simulations.