Talks

Quantum dot arrays are an emerging system to synthesize controlled many-body quantum states for quantum simulation and computation. When cooled to a low temperature, each quantum dot acts as a site on which the number of half-integer spin particles can be controlled using voltages applied to gates, not unlike the gates on classical transistors. Moreover, the spin can be controlled and measured with the help of patterns of gates. It has recently been shown that tunnel couplings between individual sites can be controlled using the same gates to emulate a Hubbard model (unlike other systems i.e., superconductors, trapped ions, Rydberg atoms, etc), making it possible to program a many-body system using only voltages applied to gates, and that the spin can be initialized, controlled and read out on arrays of 4 to 6 quantum dots like a conventional quantum computer (unlike cold atoms in optical lattices). It has also recently been shown that the coherence times of the spin degree of freedom can be as long as 10 ms in this material system, and that the quantum dots can be proximized to superconductors. In this talk I will describe our efforts towards synthesis of interesting quantum states using this platform, at our lab in University of British Columbia.

We develop a field-theoretic functional renormalization group formalism for field theories of metals that include all gapless modes around the Fermi surface. Due to the presence of intrinsic scales (Fermi momenta), the usual notion of scale invariance and renormalizable field theory need to be generalized. The formalism is applied to the non-Fermi liquid that arises at the anti-ferromagnetic quantum critical point in two space dimensions. We identify the interacting non-Fermi liquid fixed point in the space of coupling functions, and extract the universal scaling behaviour of the normal state and the pathway to the superconducting state at low energies.

We discuss emergent non-Fermi liquid behaviors in multipolar Kondo systems, where conduction electrons interact with the non-Kramers local moments carrying higher-rank multipolar moments such as quadrupolar and octupolar moments. We first show that unexpected non-Fermi liquid states arise in the single impurity multipolar Kondo system using the renormalization group and conformal field theory. Next, we study the competition between the Kondo and RKKY interactions in the Bose-Fermi Kondo systems, where the RKKY interaction between multipolar moments is represented by a bosonic degree of freedom. We present the renormalization group solution of this problem and describe the quantum critical behaviors. If time permits, we also discuss possible superconducting states arising from the multipolar Kondo interactions. We compare the theoretical results with existing experimental data on some cubic f-electron systems.

3:15PM Arnab Adhikary 3:18PM Anjishnu Bose 3:21PM Matthew Duschenes 3:24PM SangEun Han & Daniel Schultz 3:27PM Andrew Hardy 3:30PM Daniel Huerga 3:33PM Vedangi Pathak 3:36PM Shengqi Sang 3:39PM Joseph Tindall 3:42PM Tarun Tummuru 3:45PM Ryohei Weil 3:48PM Rui Wen 3:51PM Ye Weicheng 3:54PM Emily Zhang

Unlike the half spin Kitaev honeycomb model which can be solved by an exact parton construction, the higher spin analogue of it is not solvable and it is still controversial if it exhibits a quantum spin liquid phase. In this talk, I will present a generalized parton construction where each spin-S is represented by 8S Majorana fermions. This framework naturally leads to a Z2 spin liquid when S is a half integer and gives a trivial ground state when S is an integer. Particularly, in the Z2 spin liquid, the Z2 charge is carried by a product of 2S Majorana fermions. In the anisotropic limit, say the interaction on the z bond is much stronger than others, the charges are gapped and the higher spin Kitaev model is reduced to a Wen-plaquette model exhibiting Z2 topological order. However, it is expected that at certain interaction strength on the x,y,z bond, the charges become gapless which results in a gapless Z2 spin liquid.

A brief introduction to Celestial Holography

We introduce the origins of holography and illustrate in broad strokes the theory of celestial holography. We discuss the development of asymptotic symmetries from soft theorems and how these symmetries point to a codimension 2 boundary on which would live the dual CFT. We show the connection between predicted asymptotic symmetries and observable memory effects, completing the famous infrared triangle. We conclude with some applications and current problems we are thinking about, in particular with respect to bulk reconstruction.

I will discuss our recent work on the use of autoregressive neural networks for many-body physics. In particular, I will discuss two approaches to represent quantum states using these models and their applications to the reconstruction of quantum states, the simulation of real-time dynamics of open quantum systems, and the approximation of ground states of many-body systems displaying long-range order, frustration, and topological order. Finally, I will discuss how annealing in these systems can be used for combinatorial optimization where we observe solutions to problems that are orders of magnitude more accurate than simulated and simulated quantum annealing.

"The relatively weak van der Waals bond in 2D materials has ushered in a rich new era of stacking engineering. We recently found in rhombohedrally stacked MoS2, a Berry phase contrast between layers can induce an asymmetric interlayer coupling and an out-of-plane spontaneous electrical polarization (1). The polarization direction can be switched via interlayer sliding, forming a new type of ferroelectricity. In addition, we demonstrated that such a polarization can lead to a spontaneous photovoltaic effect without any pn junctions (2). Compared to conventional PV effects, our device shows a similar quantum efficiency with an ultrafast speed and potentially a programmable polarity. The rhombohedrally stacked transition metal dichalcogenides therefore provide a new platform for studying spontaneous polarization at the atomic scale. (1) Jing Liang, et al, arXiv:2209.06966 (2022). (2) Dongyang Yang, et al, Nature Photonics, 16, 469–474 (2022)."

Structures composed of two monolayer-thin d-wave superconductors with a twist angle close to 45° are predicted to form a robust, fully gapped topological superconducting phase with spontaneously broken time-reversal symmetry and protected chiral edge modes. In this talk I will briefly review the theory behind the topological phase and discuss recent experimental efforts to fabricate and probe twisted flakes of high-Tc cuprate Bi2Sr2CaCu2O8+δ. Signatures of d-wave symmetry and of spontaneous T-breaking are indeed visible in the device Josephson current response, as detected through Fraunhofer pattern and Shapiro step analysis, and, very recently, a pronounced superconducting diode effect observed in samples near 45° twist but absent in untwisted samples.