Condensed Matter

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.

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.

In non-relativistic systems, the Lieb-Robinson Theorem imposes an emergent speed limit (independent of the relativistic limit set by c), establishing locality under unitary quantum dynamics and constraining the time needed to perform useful quantum tasks. We have extended the Lieb-Robinson Theorem to quantum dynamics with measurements. In contrast to the general expectation that measurements can arbitrarily violate spatial locality, we find at most an (M+1)-fold enhancement to the speed of quantum information, provided the outcomes of M local measurements are known; this holds even when classical communication is instantaneous. Our bound is asymptotically optimal, and saturated by existing measurement-based protocols (the "quantum repeater").  Our bound tightly constrain the resource requirements for quantum computation, error correction, teleportation, generating entangled resource states (Bell, GHZ, W, and spin-squeezed states), and preparing SPT states from short-range entangled states.

Zoom Link: https://pitp.zoom.us/j/95640053536?pwd=Z05oWlFRSEFTZWFRK2dwcHdsWlBBdz09

In this talk I will explore perspectives of quantum entanglement in (1+1)-d systems in presence of defects. The talk consists of two parts. In the first part, I will talk about the ground state entanglement entropy for 1d quantum spin chains with defects, using the transverse field Ising model and the three-state Potts model as examples. In the second part, I will describe the field theoretical replica trick approach to study entanglement entropy in such systems. This talk consists of some reviews about existing work, as well as work in progress with Linnea Grans-Samuelsson, Ananda Roy, Hubert Saleur, and Yifan Wang.

Zoom link: https://pitp.zoom.us/j/93049110080?pwd=QmpneGs2QlpZMlBROUlvU3VzaGtsZz09

 

The Entanglement Bootstrap is a program to derive the universal properties of a phase of matter from a single representative wavefunction on a topologically-trivial region.  Much (perhaps all) of the structure of topological quantum field theory can be extracted starting from a state satisfying two axioms that implement the area law for entanglement.  This talk will focus on recent progress (with Bowen Shi and Jin-Long Huang) using this approach to prove remote detectability of topological excitations in various dimensions.  This is an axiom of topological field theory.  Two key ideas are a quantum avatar of Kirby's torus trick to construct states on closed manifolds, and the new concept of pairing manifold, which is a closed manifold associated with a pair of conjugate excitation types that encodes their braiding matrix.  The pairing manifold also produces Verlinde formulae relating the S-matrix to the structure constants of a generalized symmetry algebra of flexible operators.  

Zoom link:  https://pitp.zoom.us/j/92633473610?pwd=eEhqR3BaQXljQm5ScHZvZm81N2FyZz09

The modular commutator is a recently discovered multipartite entanglement measure that quantifies the chirality of the underlying many-body quantum state. In this Letter, we derive a universal expression for the modular commutator in conformal field theories in 1+1 dimensions and discuss its salient features. We show that the modular commutator depends only on the chiral central charge and the conformal cross ratio. We test this formula for a gapped (2+1)-dimensional system with a chiral edge, i.e., the quantum Hall state, and observe excellent agreement with numerical simulations. Furthermore, we propose a geometric dual for the modular commutator in certain preferred states of the AdS/CFT correspondence. For these states, we argue that the modular commutator can be obtained from a set of crossing angles between intersecting Ryu-Takayanagi surfaces.

Zoom link:  https://pitp.zoom.us/j/94069836709?pwd=RlA2ZUsxdXlPTlh2TStObHFDNUY0Zz09