Department of Physics, National University of Singapore
Condensed Matter Seminar Series
To receive email notifications of this seminar series, please send an email to zouliujun@gmail.com to request.
Condensed Matter Seminar Series
To receive email notifications of this seminar series, please send an email to zouliujun@gmail.com to request.
Previous Seminars
(Both online and in-person seminars were broadcast via Zoom.)
(Click the upper right corner of this page to see the upcoming seminars.)
(YouTube Channel for the seminars: https://www.youtube.com/@NUSPhysicsCondensedMatters-g1z/videos)
(Koushare Page for the seminars: https://www.koushare.com/space/329622/home)
Time: 9am, September 19 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89675000481?pwd=tDWM5CXGfAdPYd3ZexIpWwXx2Nx6bP.1
Password: 546550
Speaker: Yimu Bao
Title: Designs from magic-augmented Clifford circuits
Abstract: In this talk I will introduce magic-augmented Clifford circuits -- architectures in which Clifford circuits are preceded and/or followed by constant-depth circuits of non-Clifford ("magic") gates -- as a resource-efficient way to realize approximate k-designs. We prove that shallow Clifford circuits, when augmented with constant-depth circuits of magic gates, can generate approximate unitary and state k-designs with bounded relative error. The total circuit depth for these constructions improves upon previous results for small k>=4. The required number of magic gates is further reduced when considering k-designs with bounded additive error. We show that shallow Clifford circuits followed by O(k^2) single-qubit magic gates can generate an additive-error state k-design. I will also provide classical statistical mechanics understanding of our results and discuss the no-go theorems on design generation.
Time: 9am, September 12 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89675000481?pwd=tDWM5CXGfAdPYd3ZexIpWwXx2Nx6bP.1
Password: 546550
Speaker: Ruihua Fan
Title: Moore-Read quantum Hall wave function under density decoherence
Abstract: Recent advances in graphene heterostructures bring new experimental motivation to explore topological quantum computation with quantum Hall platform. A natural question is whether the information encoded in a fractional quantum Hall state is robust under local decoherence. Here we give an attempt in a toy setup: quantum Hall model wave function under only density decoherence without any Hamiltonian dynamics. In this talk, I will focus on the Moore-Read wave function, the simplest non-Abelian quantum Hall state, and examine how the fusion space is (not) affected by the density decoherence.
Time: 9am, September 5 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89675000481?pwd=tDWM5CXGfAdPYd3ZexIpWwXx2Nx6bP.1
Password: 546550
Speaker: Marvin Qi
Title: The Symmetry Taco: Equivalences between gapped, gapless, and mixed state SPTs
Abstract: The symmetry topological field theory (SymTFT) offers a unifying framework for describing quantum phases of matter and transitions between them. In this talk, we discuss a natural extension of the SymTFT framework to mixed-state phases by introducing the Symmetry Taco: a bilayer topological order in 2+1d whose folded geometry naturally captures both the strong and weak symmetries of the 1+1d mixed state phase. Using this perspective, we find natural correspondences between certain gapped, gapless, and mixed state SPTs in one dimension. We also identify a new "anomaly" unique to mixed states arising from gauging a weak symmetry.
There is no seminar during the week of August 29 due to various special events.
Time: 11am, August 22 2025, GMT+8
Location: S11-02-07, National University of Singapore
Zoom: https://nus-sg.zoom.us/j/89675000481?pwd=tDWM5CXGfAdPYd3ZexIpWwXx2Nx6bP.1
Password: 546550
Speaker: Peng Ye
Title: Fracton topological orders with boundary Toeplitz braiding
Abstract: In this talk, we explore a class of three-dimensional (3D) fracton topological orders that exhibit exotic boundary phenomena called "Toeplitz braiding" in the thermodynamic limit. These systems are constructed by stacking 2D twisted $Z_N$ topologically ordered layers along the z-direction, where layers are coupled while maintaining translation symmetry. The effective field theory is described by an infinite-component Chern-Simons theory with a block-tridiagonal Toeplitz K-matrix. A key finding is the connection between boundary zero modes in the K-matrix spectrum and the emergence of Toeplitz braiding, where mutual braiding phase angles between boundary anyons oscillate and remain nonzero in the thermodynamic limit. Interestingly, the integer-valued Hamiltonian matrix of the 1D Su-Schrieffer-Heeger model can serve as a nontrivial K-matrix, demonstrating the presence of robust boundary zero modes without relying on symmetry protection. We will also discuss numerical results and potential future directions, including the construction of 3D lattice models to realize this phenomenon. If time permitted, I will present progress on 4D theory where unexpectedly knowledge of non-Hermitian physics plays a critical role.
Time: 11am, August 15 2025, GMT+8
Location: S11-02-07, National University of Singapore
Zoom: https://nus-sg.zoom.us/j/89675000481?pwd=tDWM5CXGfAdPYd3ZexIpWwXx2Nx6bP.1
Password: 546550
Speaker: Han Ma
Title: Switch between continuous and discontinuous transition at 2d Nematic transition/Stoner Instabilities via Field-Theoretical RG
Abstract: I will talk about my work on the fixed-point structure of an interacting two-flavor fermionic system, applicable to Moiré systems near the valley polarization transition, where each flavor possesses a dispersion with a tunable real exponent "a". Starting from the Fermi liquid phase at weak interaction, we identify two possible Pomeranchuk/Stoner-type instabilities at stronger interaction. These instabilities lead to spontaneous spatial rotational or flavor symmetry breaking. Our key discovery is that one of these instabilities exhibits an RG fixed point that is attractive for "a<1", indicating a continuous transition. Conversely, for "a>1", the fixed point becomes repulsive, resulting in a discontinuous transition. We also investigate the collective modes driving these instabilities of Fermi liquids. This work predicts a universal ratio between collective modes and Fermi surface sizes at the continuous transition instability, which is experimentally observable.
The seminar series in the spring semester of 2025 is over, and the next seminar will take place in the fall semester of 2025.
Time: 9am, April 15 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Rui Wen
Title: Gapless SPT and topological holography
Abstract: Topological holography sheds light on a comprehensive understanding of gapless quantum phases. In this talk I discuss generalities of gapless symmetry-protected topological phases(gSPT) and how they fit into the topological holography paradigm. By employing the decorated domain wall (DDW) construction, I give a classification of G-gSPTs. I compare with previously proposed classifications in low dimensions and show they agree with the DDW approach. I then show a perfect correspondence between certain class of condensable algebras in the Symmetry Topological Field Theory(SymTFT) and gSPTs in one lower dimension. Moreover, the twist of the condensed phase in the SymTFT is found to be exactly the emergent anomaly of the corresponding gSPT.
Time: 11am, April 8 2025, GMT+8
Location: Conference room S11-02-07, National University of Singapore
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Yidun Wan
Title: Nonabelian anyon condensation in 2+1d topological orders: A string-net model realization
Abstract: In this talk, we develop a comprehensive framework for realizing anyon condensation of topological orders within the string-net model by constructing a Hamiltonian that bridges the parent string-net model before and the child string-net model after anyon condensation. Our approach classifies all possible types of bosonic anyon condensation in any parent string-net model and identifies the basic degrees of freedom in the corresponding child models. Compared with the traditional UMTC perspective of topological orders, our method offers a finer categorical description of anyon condensation at the microscopic level. We also explicitly represent relevant UMTC categorical entities characterizing anyon condensation through our model-based physical quantities, providing practical algorithms for calculating these categorical data.
Time: 9am, April 1 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Haoyu Guo
Title: Field theory of monitored, interacting fermion dynamics with charge conservation
Abstract: Measurement-induced phase transitions (MIPTs) in monitored quantum dynamics are non-equilibrium phase transitions between quantum-chaotic (volume-law entangled) and entanglement-suppressed, area-law phases. Here we reveal how monitored dynamics are situated within the framework of general far-from-equilibrium, quantum condensed-matter physics. Measurement-induced heating effects scramble the distribution function in generic (interacting) monitored fermion systems, and this enables a simplified symmetry-based description of the dynamics. We demonstrate the equivalence of the Keldysh technique with the conventional Statistical-Mechanics Model for circuits, resulting from a doubled Hilbert-space (Choi-Jamiolkowski) mapping. We illustrate this using the monitored dynamics of interacting fermions with a conserved charge, deriving a unified effective field theory that captures all phases and phase transitions. The non-interacting counterpart in 1D space only has an area-law phase, with no MIPT. This was explained via an effective non-linear sigma model replica field theory possessing a very large symmetry. We show that other phases and phase transitions emerge when the replica symmetry is reduced by interactions. The reduced symmetry combines a replica permutation symmetry and charge-conservation within each replica. The former and its spontaneous breaking govern the MIPT, which can be recognized via a separatrix in the renormalization group flow. The replica-resolved charge conservation dictates the ``charge-sharpening" transition between two kinds of dynamics, where the global charge information is either hidden or reconstructible from the measurements. The field theory explains why the charge-sharpening transition should occur only in the volume-law phase. Our framework provides a template for other classes of MIPTs and situates these within the arena of non-equilibrium condensed matter physics.
Time: 9am, March 25 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Bowen Shi
Title: Robust entanglement bootstrap: Motivations and puzzles
Abstract: Entanglement bootstrap is the idea of using entanglement-based conditions to derive nontrivial constraints on quantum many-body systems. It provides nontrivial physical insights into the emergence of anyons and their fusion spaces in gapped many-body systems in 2+1D systems. More generally speaking, it is a candidate theory for explaining the emergence of TQFT in gapped systems and with a possible extension to CFT. In the context of gapped states, previous proofs were given in the context of exact entanglement area law. We explain the physical motivation to consider a robust version allowing the errors of the area law axioms, including a no-go theorem for accommodating chirality in finite Hilbert spaces. We discuss a few ongoing thoughts toward robustness, including open problems and partial answers.
There is no seminar during the week of March 17 2025 due to the APS March meeting.
Time: 9am, March 11 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Carolyn Zhang
Title: Enforced gaplessness from states with exponentially decaying correlations
Abstract: It is well known that a local Hamiltonian must be gapless if its ground state has algebraic correlations. We show that even exponentially decaying correlations can imply gaplessness. Our first result says that certain loop correlations in a 2+1D state imply that any local parent Hamiltonian must either be gapless or have a ground state degeneracy scaling with system size. Examples of states with such loop correlations include a subset of Rokhsar-Kivelson type states. Our second result applies to frustration-free parent Hamiltonians and bounds the finite-size gap above the ground state subspace by $\mathcal{O}(1/L^3)$ on periodic boundary conditions, through the construction of variational low energy states. Strikingly, these variational states look like loop waves---non-quasiparticle analogs of spin waves---generated from the ground state by non-local loop operators. A corollary of our first result is that Glauber dynamics for the ordered phase of the two-dimensional classical Ising model on the torus must have a gapless Markov transition matrix, with our second result bounding its gap.
This is based on work with Rahul Sahay, Curt von Keyserlingk, and Ruben Verresen.
Time: 9am, March 4 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Taige Wang
Title: Continuous transitions out of topological phases of matter
Abstract: Quantum phase transitions beyond the Landau paradigm are a focal point of condensed matter research, as they challenge conventional classification schemes. A prominent example of such a transition is the bosonic Laughlin state to superfluid transition. Despite several proposed field theories, this transition has evaded both experimental and numerical verification. In this talk, I will present our numerical results using infinite density matrix renormalization group, demonstrating a periodic-potential-driven continuous transition from the bosonic Laughlin state to the superfluid in the half-filled lowest Landau level. The emergence of an SO(3) symmetry at the critical point provides strong evidence for an underlying QED3-Chern-Simons theory. I will also discuss how the insights from this transition can be generalized to other transitions that have recently attracted attention.
There is no seminar during the week of February 24 2025 because it is the reading week at NUS.
Time: 10:15am, February 18 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Zheng Zhou
Title: Exploring 3d CFTs in the fuzzified world: Critical gauge theories, conformal defects and boundaries
Abstract: Conformal field theories have provided invaluable insight into critical phenomena and fundamental structures of QFTs. Meanwhile, many open questions on 3d CFTs bearing theoretical and practical significance remain largely unexplored, such as the properties of various critical gauge theories and conformal defects and boundaries. The fuzzy (non-commutative) sphere has emerged as a powerful method and has shown exceptional efficiency in studying 3d CFTs, bridging them with quantum Hall physics. In this talk, I will introduce a series of findings through the fuzzy sphere, including (1) the conformal symmetry and pseudo-criticality of deconfined criticality, (2) a new series of CFTs with Sp(N) global symmetry closed related to Chern-Simons-matter theories, (3) the magnetic line defect in 3d Ising CFT including its RG-monotonic g-function, and (4) various conformal boundaries in 3d Ising theory. I will also envisage the future of this journey.
Time: 9am, February 11 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Jian-Hao Zhang
Title: A Universe of Mixed-State Quantum Phases
Abstract: Recently the exploration of phases of matter in open quantum systems has garnered substantial interest across various disciplines including condensed matter physics and quantum information science, especially those phenomena that are intrinsically mixed without analogues in closed systems. In particular, the energy gap is a crucial concept in physics, particularly in the study of solid-state physics and quantum mechanics.
In this talk, based on the correlation behaviors of density matrix, we first generalize the concept of “gap” to mixed quantum states. For gapped mixed states, we introduce the concept of “average symmetry-protected topological (ASPT) phases”, together with a systematic construction and classification paradigm. In particular, a plethora of ASPT states that cannot be realized in pure states are provided, dubbed “intrinsic ASPT” phases.
For “gapless” mixed states, we systematically introduce a new concept of “strong-to-weak spontaneous symmetry breaking” (SWSSB) in mixed quantum states. As a universal mixed-state quantum phase, we demonstrate the universality of SWSSB through the stability against symmetric finite-depth local channel perturbation, and the spontaneity implies that a local perturbation may lead to a global change of the whole density matrix.
Time: 9am, February 4 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Zhaoyu Han
Title: Frustration-free Hamiltonians: two unifying schemes
Abstract: I will describe two general schemes for the construction of quantum many-body models whose exact ground state(s) can be solved.
The first one is a generalized Witten’s conjugation method. I will show that, when applied to interacting boson systems, it unifies many known solvable models, points to their natural generalizations, and provides systematic construction for more exotic phases of matter, e.g. supersolid, Bose-Einstein insulator and Bose metal with “bose surfaces”.
The second one is called “common commuting order” method. I will describe how it helps define the novel concept of “quantum geometric nesting” for flat-band electronic systems, whose satisfaction permits the construction of an infinite class of local interacting Hamiltonians with ground states hosting certain (symmetry-breaking) bilinear orders suggested by the electronic structure.
There is no seminar during the week of Janurary 27 2025 due to the Chinese New Year.
Time: 9am, Janurary 21 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Margarita Davydova
Title: A local automaton for the 2D toric code
Abstract: Realizing a stable and scalable topological quantum memory in two and three dimensions is an open challenge that is theoretically and practically interesting. The existing solutions are, in fact, not truly scalable.
This talk is based on joint work [arXiv:2412.19803] with Shankar Balasubramanian and Ethan Lake, where we propose a local (measurement and feedback) circuit that preserves the 2D toric code for an exponentially long time in the presence of arbitrary weak noise. Our construction is inspired by the classical cellular automata of Tsirelson and Gács, which, by renormalization group-like dynamics, realize a stable one-dimensional ordered phase with local interactions. Our results solve the open problem of constructing a local and scalable topological quantum memory below 4D.
Time: 9am, January 14 2025, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/81446736724?pwd=nUypAWg4s8hIaOceWLkubfacUbxQbz.1
Password: 740238
Speaker: Qicheng Tang
Title: A critical state under weak measurement is not critical
Abstract: Critical states under certain measurement protocols exhibit typical signatures of criticality, including power-law decaying correlation functions and logarithmic entanglement entropy. However, by considering free Dirac fermions as a concrete example, we show that the post-measurement states are generally not critical, hosting a gapped and long-ranged entanglement Hamiltonian even in the single-interval case. This is distinct from the critical ground state, where the entanglement Hamiltonian for a single interval is gapless and local.
The seminar series in the fall semester of 2024 is over, and the next seminar will take place in the spring semester of 2025.
Time: 9am, November 12 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Speaker: Arkya Chatterjee
Title: Quantized axial charge and the chiral anomaly in a Hamiltonian lattice model
Abstract: Realizing chiral global symmetries on a finite lattice is a long-standing challenge in lattice gauge theory, with potential implications for non-perturbative regularization of the Standard Model. One of the simplest examples of such a symmetry is the axial U(1) symmetry of the 1+1d massless Dirac fermion field theory: it acts by equal and opposite phase rotations on the left- and right-moving Weyl components of the Dirac field. This field theory also has a vector U(1) symmetry which acts identically on left- and right-movers. These two U(1) symmetries exhibit a mixed anomaly, commonly known as the Schwinger anomaly. In this talk, I will discuss how both symmetries are realized exactly in a "tight-binding" lattice Hamiltonian with a finite-dimensional Hilbert space. Intriguingly, the continuum anomaly of the abelian U(1) symmetries is matched on the lattice by a non-Abelian Lie algebra. The lattice symmetry forces the low-energy phase to be gapless, closely paralleling the effects of the anomaly in the continuum field theory.
Time: 9am, November 5 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Speaker: Mengxing Ye
Title: Coadjoint-orbit Bosonization of a Fermi surface in a magnetic field
Abstract: I will present a bosonized effective field theory for a 2d Fermi surface in a weak magnetic field. Using the recently proposed coadjoint-orbit nonlinear bosonization approach, where the boson fields living in phase space describe the fluctuations of the Fermi surface, we show that on the one hand, the quantum physics of Landau levels and Landau level degeneracy naturally arises in the theory. On the other hand, in the semiclassical limit, for a parabolic dispersion, the resulting theory describes free chiral bosons propagating in momentum space. In addition, the action contains a topological θ-term, which was overlooked but has significant consequence on the oscillatory behavior of magnetization. Our theory paves the way toward understanding correlated gapless fermionic systems in a magnetic field using the powerful approach of bosonization. Generalization to a Fermi surface in multi-band fermion systems will also be discussed.
References:
[1] Mengxing Ye, Yuxuan Wang, arXiv:2408.06409
[2] Mengxing Ye, Yuxuan Wang, in preparation
Time: 9am, October 29 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Speaker: Subhayan Sahu
Title: Symmetry enforced entanglement in maximally mixed states
Abstract: Entanglement in quantum many-body systems is typically fragile to interactions with the environment. Generic unital quantum channels, for example, have the maximally mixed state with no entanglement as their unique steady state. However, we find that for a unital quantum channel that is 'strongly symmetric', i.e. it preserves a global on-site symmetry, the maximally mixed steady state in certain symmetry sectors can be highly entangled. For a given symmetry, we analyze the entanglement and correlations of the maximally mixed state in the invariant sector (MMIS), and show that the entanglement of formation and distillation are exactly computable and equal for any bipartition. For all Abelian symmetries, the MMIS is separable, and for all non-Abelian symmetries, the MMIS is entangled. Remarkably, for non-Abelian continuous symmetries described by compact semisimple Lie groups (e.g. SU(2)), the bipartite entanglement of formation for the MMIS scales logarithmically ∼logN with the number of qudits N.
Time: 10:30am, October 24 2024, GMT+8
Location: Conference room S11-02-07, National University of Singapore
Zoom: https://nus-sg.zoom.us/j/87680496984?pwd=DmdkykY4yIzWh5JuA07XAf65BFKr0l.1
Password: 975914
Speaker: Vir Bulchandani
Title: Random-matrix models of monitored quantum circuits
Abstract: We study the competition between Haar-random unitary dynamics and measurements for unstructured systems of qubits. For projective measurements, we derive various properties of the statistical ensemble of Kraus operators analytically, including the purification time and the distribution of Born probabilities. The latter generalizes the Porter–Thomas distribution for random unitary circuits to the monitored setting and is log-normal at long times. We also consider weak measurements that interpolate between identity quantum channels and projective measurements. In this setting, we derive an exactly solvable Fokker–Planck equation for the joint distribution of singular values of Kraus operators, analogous to the Dorokhov–Mello–Pereyra–Kumar (DMPK) equation modelling disordered quantum wires. We expect that the statistical properties of Kraus operators we have established for these simple systems will serve as a model for the entangling phase of monitored quantum systems more generally.
Time: 9am, October 15 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Speaker: Chao Yin
Title: Robust many-body localized and error-correcting phases of matter in low-density parity-check codes
Abstract: Low-density parity-check (LDPC) codes play an important role in everyday electronic devices, and in quantum error correction where “good” quantum LDPC codes are able to achieve extremely robust quantum error correction with only a constant overhead. In this talk, I will show that from a physics point of view, many LDPC codes represent stable phases of matter living in infinite dimensions. First, certain classical LDPC codes, in the presence of local quantum perturbations, form the first provably robust example of many-body localization [1], where low-energy-density eigenstates cluster near one codeword and violate the eigenstate thermalization hypothesis. Similarly, the error-correction ground states of quantum LDPC codes are stable against perturbations [2], which robustly violate the third law of thermodynamics. Our result also implies stability of the 2d toric code under spatially nonlocal perturbations.
References: [1] Chao Yin, Rahul Nandkishore, Andrew Lucas. PRL 133, 137101, Editor's Suggestion (2024)
[2] Chao Yin, Andrew Lucas. To appear.
Time: 9am, October 8 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Koushare streaming: https://www.koushare.com/live/details/37717
Speaker: Seth Musser
Title: Quantum charge liquids and minimal topological order
Abstract: In this talk I will introduce and motivate the concept of a “quantum charge liquid” (QCL), an incompressible insulating state at partial filling of a lattice which nonetheless does not break translation symmetry. A gapped QCL must necessarily exhibit non-trivial topological order, prompting the natural question “what is the smallest (or minimal) topological order consistent with symmetry and filling constraints?” I will formalize the concept of minimal order and prove a lower bound on the anyon count of a fermionic QCL. I will close by commenting both on possible realizations of fermionic QCLs as well as on the extension of minimal order to the fractional quantum Hall setting.
Time: 9am, October 1 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Koushare streaming: https://www.koushare.com/live/details/37617
Speaker: Evelyn Tang
Title: Robust dynamics and function in stochastic topological systems
Abstract: Living systems exhibit various robust dynamics during system regulation, growth, and motility. However, how robustness emerges from stochastic components remains unclear. Towards understanding this, I develop topological theories that support robust edge currents and localization, effectively reducing the system function to a lower-dimensional subspace. I will introduce stochastic networks in molecular reaction space that model long and emergent time scales, such as the circadian rhythm. More generally, we find that unlike in their quantum counterparts, stochastic topological systems require non-reciprocity for edge states and strong localization. Our work provides design guidelines for the synthesis and control of active systems.
There is no seminar during the week of September 23, because it is a reading week at NUS.
Time: 11am, September 16 2024, GMT+8
Location: Conference room S11-02-07, National University of Singapore
Zoom: https://nus-sg.zoom.us/j/83795365442?pwd=Juzw26IGxauQf6X7BMaHzBrW5bIg45.1
Password: 055942
Koushare: https://www.koushare.com/live/details/37276
Speaker: Hoi Chun (Adrian) Po
Title: Entanglement renormalization and tensor network representation of Chern insulators
Abstract: Tensor networks are a powerful framework for describing quantum many-body states. They effectively overcome the challenge posed by the exponential size of the quantum many-body Hilbert space by combining local data into a globally defined state. However, certain quantum materials have ground states that cannot be easily captured using tensor network states. A prime example of this is the Chern insulator, which exhibits the quantum anomalous Hall effect and can support dissipationless current at its boundary. In this talk, we will explore how concepts derived from entanglement renormalization can be employed to facilitate the construction of a tensor network representation for Chern insulators.
Time: 9am, September 10 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Speaker: Rui Wen
Title: Topological quantum memory from gapped boundaries in twisted MoTe2
Abstract: Recent experiments in twisted homobilayer MoTe2 (tb-MoTe2) at band filling fraction ν = 3 suggest the presence of fractional quantum spin Hall (FQSH) states. In this talk I discuss how one could utilize such a FQSH state to construct a topological quantum memory by creating gapped boundaries. I will discuss schemes for detecting the topological ground-space degeneracy (GSD) via electrical measurements of thermal entropy and for reading out the state of Cheshire qudits through supercurrent transport. Additionally, I will discuss a full classification of different types of gapped boundaries, and parafermionic twist defects for various Abelian and non-Abelian candidate FQSH orders that are consistent with the transport data.
Time: 9am, September 3 2024, GMT+8
Location: Online
Zoom: https://nus-sg.zoom.us/j/89860451618?pwd=lSouTW3ZYoabu24ltzdps9n5qhbKyu.1
Password: 726763
Speaker: Yijian Zou
Title: Approximate quantum error correcting codes from conformal field theory
Abstract: The low-energy subspace of a conformal field theory (CFT) can serve as a quantum error correcting code, with important consequences in holography and quantum gravity. We consider generic 1+1D CFT codes under extensive local dephasing channels and analyze their error correctability in the thermodynamic limit. We show that (i) there is a finite decoding threshold if and only if the minimal nonzero scaling dimension in the fusion algebra generated by the jump operator of the channel is larger than 1/2 and (ii) the number of protected logical qubits k≥Ω(loglogn), where n is the number of physical qubits. As an application, we show that the one-dimensional quantum critical Ising model has a finite threshold for certain types of dephasing noise. Our general results also imply that a CFT code with continuous symmetry saturates a bound on the recovery fidelity for covariant codes.