Friday Quantum Seminar

Abstract: Optical spectroscopy is used to study a material by measuring the intensity of light modes that scatter off it. In this work, we develop a theory for G2 spectroscopy of correlated materials, where instead of measuring the intensity of scattered photons, one measures the second order coherence between pairs of photons scattered off a material. We map this correlation function of the photons to the correlation functions of the material being probed.

We use time-dependent density functional theory to investigate the possibility of hosting organic color centers in (6, 6) armchair single-walled carbon nanotubes, which are known to be metallic. Our calculations show that in short segments of (6, 6) nanotubes ∼5 nm in length there is a dipole-allowed singlet transition related to the quantum confinement of charge carriers in the smaller segments. The introduction of sp3 defects to the surface of (6, 6) nanotubes results in new dipole-allowed excited states.

Qubit-based noise spectroscopy (QNS) techniques, where the dephasing of a probe qubit is exploited to study a system of interest, underlie some of the most common quantum sensing and noise characterization protocols. They have a variety of applications, ranging from designing effective quantum control protocols to investigating properties (phase transitions, thermodynamics, etc.) of quantum many-body systems.

The two most prevalent classes of ordered states in quantum materials are those arising from spontaneous symmetry breaking (SSB) and from topological order. However, a systematic study for their coexistence in interacting systems is still lacking. In this talk, I will discuss how the topological configuration in order parameter spaces from SSB (classical topology) interplays with the symmetry protected/enriched topological orders (quantum topology) in two spatial dimensions (2d). Three examples of such systems will be given.

At the heart of modern quantum technologies is the interference in the radiation of quantum emitters mediated by common vacuum modes. When there are many atoms interfering in the emission process, one observes enhancement or suppression of decay rate coefficient, which is called superradiance and subradiance, respectively [1]. When there are transitions from different excited levels interfering in the emission process, the intensity of the emitted light is modulated at the frequency of the excited level splittings, which is called quantum beats.

The fundamental assumption of statistical mechanics is that the long-time average of any observable is equal to its average over the microcanonical ensemble. In classical mechanics, this stems from Boltzmann’s ergodic hypothesis, by which a generic initial state in an ergodic system visits the neighborhood of all states in phase space with the same energy. However, wavelike effects in quantum mechanics have made it difficult to identify what it even means for a quantum system to be ergodic, except on a case-by-case basis for individual observables.

A key goal in modern quantum science is to harness the complex behavior of quantum systems to develop new technologies. While precisely engineered platforms with ultracold atoms and trapped ions have emerged as powerful tools for this task, our limited ability to theoretically and computationally probe these systems poses immense challenges for their improved control and characterization.

In this talk, I will present a theory of interaction-induced band-flattening in strongly correlated electron systems. I will begin by illustrating an inherent connection between flat bands and index theorems and presenting a generic prescription for constructing flat bands by periodically repeating local Hamiltonians with topological zero modes. Specifically, a Dirac particle in an external, spatially periodic magnetic field can be cast in this form.

Abstract: In this talk, I will present a theory of interaction-induced band-flattening in strongly correlated electron systems. I will begin by illustrating an inherent connection between flat bands and index theorems and presenting a generic prescription for constructing flat bands by periodically repeating local Hamiltonians with topological zero modes. Specifically, a Dirac particle in an external, spatially periodic magnetic field can be cast in this form.