Seminar

We set out to combine a mechanical system in which classical mechanics breaks down and quantum mechanics must be used with a seemingly unlikely application, measurement of the strength and effects of gravity. Our optomechanical system consists of a silica microsphere levitated in ultra-high vacuum in a magneto-gravitational trap. The microsphere is trapped in a magnetic field gradient created by permanent magnets and ferromagnetic pole pieces using the weak diamagnetism of the particle. With optical position measurements and feedback, the mechanical motion can be cooled by several orders of magnitude, ideally reaching the quantum ground state. The extreme sensitivity of this optomechanical system to external forces makes it a promising approach to a new measurement of the Newtonian gravitational constant. Furthermore, by measuring the decoherence rate of non-classical motional states of the trapped particle, it may be possible to place limits on theories of gravitational decoherence. This material is based upon work supported by the National Science Foundation under Grant No. 1757005

 In the 1980s, David Mermin derived a simple example of a Bell inequality and showed that it is violated in measurements on entangled quantum systems. In this talk, I reanalyze Mermin’s example, using correlation arrays, the workhorse in Jeffrey Bub’s Bananaworld (2016). For the class of all non-signaling correlations conceivable in the kind of experiment considered by Mermin, I derive both the Bell inequality, a necessary condition for such correlations to be allowed classically, and the Tsirelson bound, a necessary condition for them to be allowed quantum-mechanically. I show that the Tsirelson bound for these experiments follows directly from the geometry going into their quantum-mechanical analysis. I use this example to promote Bubism (not to be confused with QBism though both are information-theoretic approaches to the foundations of quantum mechanics). I do so by comparing the rules for probabilities in quantum mechanics, illustrated by my Bubist reanalysis of Mermin’s example, to the rules for spatio-temporal behavior in special relativity.

The programmed assembly of nanoscale building blocks offers exciting new avenues to creating electronic devices and materials in which structure and functions can be chemically designed and tuned. In this context, the synthesis of inorganic molecular clusters with atomically-defined structures, compositions and surface chemistry provides a rich family of functional building elements. This presentation will describe our efforts to assemble such “designer atoms” into a variety of hierarchical structures and devices, and study the resulting collective properties.

Quantum phase transitions are transitions between distinct many-body ground states, and are of extensive interest in research ranging from condensed matter physics to cosmology. Key features of the phase transitions include a stage with rapidly growing new order, called inflation in cosmology, followed by the formation of topological defects. Based on Bose-Einstein condensates of cesium atoms in a shaken lattices [1], we report the observation of universal scaling of quantum critical dynamics [2], and inflationary dynamics across a quantum phase transition [3]. In particular, the inflation...

Artificial neural networks play a prominent role in the rapidly growing field of machine learning and are recently introduced to quantum many-body systems. This talk will focus on using a machine-learning model, the restricted Boltzmann machine (RBM) to describe entangled quantum states. Both short- and long-range coupled RBM will be discussed. For a short-range RBM, the associated quantum state satisfies an entanglement area law, regardless of spatial dimensions. I will present our recently constructed exact RBM models for nontrivial topological phases, including a 1d...

The extreme surface sensitivity of two-dimensional (2D) materials provides an unprecedented opportunity to engineer the physical properties of these materials via changes to their surroundings, including a substrate, adsorbates, defects, etc.  In particular, the decoration of the 2Dmaterial with adatoms can be utilized to tailor material properties and induce novel quantum phenomena. In this context, first I will discuss the case of 2D semiconducting transition metal dichalcogenides (TMDs), wherein new electronic phenomena such as tunable bandgaps and strongly bound...

A moving fluid can become unstable in the presence of an obstruction, leading to a flow pattern that fluctuates in time due to nonlinear dynamics.  It has been recently found that some materials host electrons that behave like a collective fluid. In particular, experiments with graphene electrons have demonstrated linear hydrodynamic phenomena, such as viscous drag at sample boundaries, but nonlinear effects have yet to be reported.  We observe a AC current instability that develops when driving a DC current through a graphene device in the electron hydrodynamic regime...