Fall 2023

I will talk about:
. New efficient algorithms for quantum state tomography (the quantum analogue of estimating a probability distribution).
. Why you should care about the difference between total variation distance and Hellinger distance and KL divergence and chi-squared divergence.
. Quantum-inspired improvements to the classical problem of independence testing.

Includes joint work with Steven T. Flammia (Amazon)

ATL 3100A and Virtual Via Zoom.

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.

In the last few years, a number of works have proposed and improved provably efficient algorithms for learning the Hamiltonian from real-time dynamics. In this talk, I will first provide an overview of these developments, and then discuss how the Heisenberg limit, the fundamental precision limit imposed by quantum mechanics, can be reached for this task. I will show that reaching the Heisenberg limit requires techniques that are fundamentally different from previous ones.

Abstract: How do we get quantum systems to ‘talk’ to each other? How can we distribute entanglement at global scales? I will describe our work tackling these challenges by using light as a robust mediator of quantum interactions between matter qubits. First, I overview the development of optically-active electron spins in silicon carbide as a platform to realize long-distance quantum links. These qubits uniquely combine world-record spin coherence, noiseless single photon emission, and nanophotonic device integration- all in a wafer-scale semiconductor.

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.

Quantum measurements are critical to virtually any aspect of quantum information processing--for example: quantum error correction, distillation protocols, or state preparation.  We discuss the evolution of quantum information under Pauli measurement circuits.  We define "local reversibility" in context of measurement circuits, which guarantees that quantum information is preserved and remain "localized" after measurement.  We find that measurement circuits can exhibit a richer set of behaviour in comparison to their unitary counterparts.  For example, a finite depth me

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 session, new and current QuICS members will introduce themselves and their research. Organized by Yi-Kai Liu.

Quantum cryptography leverages unique features of quantum mechanics in order to construct cryptographic primitives which are oftentimes impossible for digital computers. Cryptographic applications of quantum computers therefore have the potential for useful quantum advantage – entirely without computational speed-ups. In this talk, I will focus on two fundamental questions: First, is it possible to certify that private data has been deleted? And second, is it possible to revoke a cryptographic key?

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.