Seminar Schedule

The metric system began with the French revolution, with the lofty ideal that measurements would be tied to the size of the earth, universally available to all. Soon, practical considerations required units of length and mass based on unique physical artifacts, a near-antithesis to universal availability. Now we are experiencing the greatest revolution in measurement since the French revolution, a revolution rooted in the atomic and quantum view of nature, again offering universal availability. The definitions of the kilogram, ampere, kelvin, and mole were all changed on 20 May 2019, and are now based on chosen and fixed values for Planck’s constant, the quantum of electric charge, Boltzmann’s constant, and Avogadro’s number. I will explain how this is possible, why it was necessary, and speculate about future changes in the SI. In this context I will also discuss the role of precision measurement in the history and future of quantum physics.

An ultrafast laser emits vastly multimode light over a broad spectral band, a.k.a. the optical frequency comb (OFC), but the emission happens but one photon at a time, if in a stimulated manner, and no entanglement is created in the light. Changing the gain medium from linear (one-photon) to nonlinear (two-photon) yields an optical parametric oscillator which features massively multipartite entanglement of the OFC modes, as demonstrated experimentally by our group and others. This entanglement can then be exquisitely tailored to cluster states with specific graphs, in particular the two-dimensional ones that are universal for measurement-based, one-way quantum computing. It is worth noting that this requires only sparse experimental resources that are highly compatible with integrated optics, thereby paving the way to the realization of practical quantum computers.

This talk will present our effort to control and use the dipole-dipole interactions between cold atoms in order to implement spin Hamiltonians useful for quantum simulation of condensed matter situations [1]. We trap individual atoms in arrays of optical tweezers separated by few micrometers. We create almost arbitrary geometries of the arrays with unit filling in two and three dimensions up to about 70 atoms. To make the atoms interact, we either excite them to Rydberg states or induce optical dipoles with a near-resonance laser.
We have demonstrated the coherent energy exchange in chains of Rydberg atoms resulting from their resonant dipole-dipole interaction. This interaction realizes the XY spin model and leads to the hopping a spin excitation from a site to another. We use this interaction to study elementary excitations in a dimerized spin chain featuring topological properties (Su-Schrieffer-Heeger model). We have observed the edge states in the topological condition. We probed the regime beyond the linear response by adding several excitations, which act as hard-core bosons [2].
With optical dipoles, we explore light scattering in one dimensional chains of atoms. This system realizes a dissipative spin model, which could find applications in quantum optics to generate optical non-linearities and non-classical states of light [3].

Non-equilibrium quantum many-body systems can display fascinating phenomena relevant for various fields in science ranging from physics, to chemistry, and ultimately, for the broadest possible scope, life itself. The challenge with these systems, however, is that the powerful formalism of statistical physics, which have allowed a classification of quantum phases of matter at equilibrium does not apply. Therefore, using controllable cold atomic systems to shed light on the organizing principles and universal behaviors of dynamical quantum matter is highly appealing. One emerging paradigm is the dynamical phase transition (DPT) characterized by the existence of a long-time-average order parameter that distinguishes two non-equilibrium phases. I will report the observation of a DPT in two different but complementary systems: a trapped quantum degenerate Fermi gas and long lived arrays of atoms in an optical cavity. I will show how these systems can be used to simulate iconic models of quantum magnetism with tunable parameters and to probe the dependence of their associated dynamical phases on a broad parameter space. Besides advancing quantum simulation our studies pave the ground for the generation of metrologically useful entangled states which can enable real metrological gains via quantum enhancement.