Chiral Magnetic Effect and Inverse Cascade in relativistic and non-relativistic systems

• Alexey Boyarsky (Leiden U.)

We discuss how the instability towards the grows of helical magnetic fields appear in relativistic and in non-relativistic systems. We discuss many applications in physics and astrophysics and their importance for fundamental physics questions.

Chiral anomaly in Euler fluid

• Alexander Abanov (Stony Brook)

We show that the chiral anomaly of quantum field theories with Dirac fermions subject to an axial background field is an inherent property of kinematics of a perfect classical fluid. We construct a variational principle generating classical equations of motion in the presence of background gauge fields. A prominent effect of the chiral anomaly is the transport electric current at equilibrium.

A new form of ergodicity breaking from quantum many-body scars

• Maksym Serbyn (IST Austria)

In my introductory lectures I will review a new mechanism of the weak ergodicity breaking relevant for the experimentally realized Rydberg-atom quantum simulator [1]. This mechanism arises from the presence of special eigenstates in the many-body spectrum that are reminiscent of quantum scars in chaotic non-interacting systems [2]. In the single-particle case, quantum scars correspond to wave functions concentrated in the vicinity of unstable periodic classical trajectories. I will demonstrate that many-body scars appear in the Fibonacci chain, a model with a constrained local Hilbert space which can be realized by a Rydberg chain. The quantum scarred eigenstates are embedded throughout the otherwise thermalizing many-body spectrum but lead to direct experimental signatures, as I show for periodic recurrences that reproduce those observed in the experiment [1]. If time permits, I will also discuss the relation between scars and TDVP, and consider the interplay between quantum scars and quantum many-body control problems [3]. [1] Bernien, H. et al., Nature 551, 579–584 (2017), arXiv:1707.04344 [2] C. J. Turner, A. A. Michailidis, D. A. Abanin, M. Serbyn, Z. Papić, Nature Physics (May 2018), arXiv:1711.03528 and Phys. Rev. B 98, 155134 (2018) arXiv:1806.10933 [3] M. Ljubotina, B.Roos, D. Abanin, M. Serbyn, arXiv:2204.02899

QFTs emerging from many-body quantum physics

• Jörg Schmiedmayer (TU-Wien)

Quantum Field theories are a natural way to describe quantum many body systems. Irrelevant details of the microscopic physics are ignored and new relevant degrees of freedom, described by the QFT, emerge at a larger scale. I will illustrate this in the example of the emergence of the Sine-Gordon quantum field theory from the microscopic description of two tunnel coupled super fluids [1] and in the emergence of Pauli blocking in a weakly interacting Bose gas [2]. Special emphasis will be put on how to verify such emergent quantum simulators and how to characterize them. Thereby I will present two tools: High order correlation functions and their factorization [1], the evaluation of the quantum effective action and the momentum dependence of propagators and vertices (running couplings, renormalization of mass etc ..) of the emerging quantum field theory [3] and quantum field tomography that points to a new way to read out quantum simulators [4]. Together they establish general methods to analyse quantum systems through experiments and thus represents a crucial ingredient towards the implementation and verification of quantum simulators. As an example, I will report on the progress towards measuring area laws of mutual information and entanglement entropy in a QFT, and the study of information fronts in curved space time. Work performed in collaboration with the groups of Th. Gasenzer und J. Berges (Heidelberg), Jens Eisert (FU Berlin) and E. Demler (Harvard). Supported by the DFG-FWF: SFB ISOQUANT: and the EU: ERC-AdG QuantumRelax [1] T. Schweigler et al., Nature 545, 323 (2017), arXiv:1505.03126; [3] F. Cataldine et al. arXiv:2111.13647; [3] T. Zache et al. Phys. Rev. X 10, 011020 (2020); [4] M. Gluza et al., Communication Physics 3, 12 (2020).

Applied Fractons

• Andrey Gromov (Brown)

Fractons are a class of quasiparticles that cannot freely propagate through space. They were first introduced in a model of quantum (almost) self-correcting memory. Later it became clear that fractons, as well as, adjacent ideas such as tensor gauge theories and multipole or subsystem conservation laws provide a language to describe some known and some new phenomena. In this talk I will explain what fractons are, what kind of systems are known to support them and what kind of problems they will help to elucidate in the future.

Renormalization and thermodynamics of quantum membranes

• Achille Mauri (Radboud U.)

Thermally-fluctuating membranes have been the subject of extensive investigations, from biological systems to graphene and other atomically-thin 2D materials. This presentation introduces a field theoretical analysis of the effects of quantum and thermal fluctuations on the statistical mechanics of free-standing solid membranes. For zero temperature the interplay between phonon-phonon interactions and quantum fluctuations drives logarithmic renormalizations of elastic parameters. For small but nonzero temperatures, we use arguments of finite-size field theory to derive relations between the zero-point renormalizations and the low-temperature behavior of thermodynamic quantities such as the thermal expansion coefficient and the entropy. The analysis allows to revisit results obtained by different methods in earlier investigations.

Newton as Effective Field Theorist

• Patrick Shields (Notre Dame)

Abstract: In this talk, I will briefly discuss how Corollaries V and VI to the laws of motion in Newton's Principia can be thought of as defining EFTs from which we can recover Newton's laws using the coset construction. This is, in effect, reverse-engineering Newton's laws from the symmetry-breaking principles that result from them. I will conclude with some lessons from this about scientific understanding and how to think about what a physical theory is.

Looking into the early Universe with a solid-state device

• Yevheniia Cheipesh (Leiden U.)

Detecting relic neutrinos is a longstanding goal in fundamental physics. It is one of the predictions of the Standard Model that has not been yet confirmed. Additionally, it carries a photographic image of the early Universe, albeit from a much older epoch of neutrino decoupling. The conventional approach to this problem is to search for the peak in the spectrum of electrons emitted in a beta-decay. Experimentally, this goal is extremely challenging as the required energy resolution is defined by the tiny neutrino masses (~10 meV). While it seems possible to achieve a required energy resolution of the measuring apparatus, the intrinsic physics of the beta-emitting source imposes additional limitations. 

The current consensus is that sufficient statistics together with the clean spectrum could only be achieved if beta decayers are attached to a solid state substrate. If one wants to keep the energy resolution as high as the mass of the neutrino, one needs to account for the presence of the solid state environment. This opens a whole new field of research in surface physics, both experimental and theoretical. I will outline the main directions and questions to be addressed and their implications for the design of the full scale relic neutrino experiment.

Genetic optimization of quantum annealing

• Annarita Scocco (Scuola Superiore Meridionale)

Adiabatic quantum computation and quantum annealing exploit slow quantum evolutions to solve hard problems in different areas. Long-time dynamics are often infeasible and leave the system more prone to noise. Therefore we present a numerical approach based on genetic algorithms to speed up quantum annealing, optimizing the annealing schedules starting from the polynomial ansatz and exploiting shortcuts to adiabaticity. With this genetically optimized annealing schedules and/or optimal driving operators, we are able to perform quantum annealing in relatively short timescales and with higher fidelity compared to traditional approaches.

Electronic spectra of the pseudogap metal in the ancilla theory of the single band Hubbard model

• Alexander Nikolaenko (Harvard)

The diverse phenomena associated with high-temperature superconductivity in the cuprates present a long-standing theoretical challenge. Various emerging phases have been thoroughly studied experimentally, including angle-resolved photoemission (ARPES), scanning tunnelling microscopy (STM), transport and thermodynamic measurements, but a complete theoretical understanding is still lacking. Many theoretical models have been proposed to describe the pseudogap regime of the cuprate superconductors. Some of them assume that the pseudogap is a precursor to some ordered phase, such as a spin density wave (SDW), or a charge density wave (CDW), or a pair density wave (PDW). A different class of models assume that the pseudogap is a distinct phase of matter characterized by spin liquid physics, which likely undergoes a confinement crossover to a more conventional broken symmetry phase at low temperatures. Here, we will investigate a model in the latter class, which describes the pseudogap metal as a fractionalized Fermi liquid (FL*): a state which has electronic quasiparticles around a Luttinger-rule violating small Fermi surface along with neutral spinon excitations. We will show how using a recently introduced 'ancilla' theory of FL* phases in a single band model yields simple models which can be successfully compared to a wide range of ARPES experiments in Bi2212 and Bi2201 in both the nodal and anti-nodal regions of the Brillouin zone.

Apparently Superluminal Superfluids

• Ioanna Kourkoulou (Columbia)

The superfluid 4-velocity becomes space-like when approaching a vortex core, while still keeping enough distance to stay within the regime of validity of the EFT. The fluctuations around this background are stable. This is in contrast to the case of normal fluids, which develop instabilities around a spacelike background current.

Identifying non-abelian channels in exotic quantum Hall states

• Mitali Banjeree (EPFL)

Quantum Hall states – the progenitors of the growing family of topological insulators – are also a source of exotic quantum states. These states can be abelian (e.g., integers & fractions) or non-abelian (e.g., special fractions)., and thus may host electrons, fractionally charged quasiparticles, and neutral bosonic or Majorana (in general, para-fermionic) quasiparticles. Since the bulk is insulating (with the quasiparticles localized), counter-propagating gapless edge modes mirror the bulk’s topological order (due to ‘bulk-edge’ correspondence). The most theoretically studied non-abelian state has a filling 𝜈=5/2. The state supports charge- neutral quasiparticles accompanied by e/4 charges. This filling, however, permits different topological orders, which can be abelian or non-abelian. While numerical calculations favor the non-abelian Anti- Pfaffian (A-Pf) order, our recent thermal conductance measurements found the unexpected order, Particle-Hole Pfaffian (PH-Pf). Employing a novel interface method, where the bulk of the 𝜈=5/2 filling was interfaced with a bulk of integer filling 𝜈=2 or 𝜈=3, an isolated interface channel of 𝜈=1/2 emerged. Studying the latter via measuring heat flow, we re-verified the PH-Pf order of the 𝜈=5/2 state (and its non-abelian nature). Identifying the correct topological order is crucial in testing the numerical predictions. While such experiments are more complicated than the ubiquitous conductance measurements, their ‘power’ is already evident. Moreover, isolating the fractional channel can be most helpful in complex interference (braiding) experiments. Banerjee et al, Nature. 545, 7652, 75-79 (2017). Banerjee et al, Nature. 559, 7713, 205-210 (2018). Dutta et al., Science. 375, 6577, 193-197 (2021). Dutta et al., arXiv: 2109.11205, (accepted in Science).

Thermodynamics and conformal operators of the Sachdev-Ye-Kitaev model

• Grigory Tarnopolskiy (Carnegie Mellon)

Quantum mechanical models with random interactions have an infinite number of bilinear operators, the scaling dimensions of which can be computed explicitly in the large N limit. The lowest dimension operators play an important role in thermodynamical properties of these models and define the behavior of various correlation functions in the infrared limit. In this talk I’ll discuss effects of these operators on the SYK model free-energy and its numerical observation.

Plaquette-dimer liquid beyond renormalization

• Yizhi You (Oxford)

We consider close-packed tiling models of geometric objects -- a mixture of hardcore dimers and plaquettes -- as a generalisation of the familiar dimer models. Specifically, on an anisotropic cubic lattice, we demand that each site be covered by either a dimer on a z-link or a plaquettein the x-y plane. The space of such fully packed tilings has an extensive degeneracy. This maps onto a fracton-type `higher-rank electrostatics', which can exhibit a plaquette-dimer liquid and an ordered phase. We analyse this theory in detail, using height representations and T-duality to demonstrate that the concomitant phase transition occurs due to the proliferation of dipoles formed by defect pairs. The resultant critical theory can be considered as a fracton version of the Kosterlitz-Thouless transition. A significant new element is its UV-IR mixing, where the low energy behavior of the liquid phase and the transition out of it is dominated by local (short-wavelength) fluctuations, rendering the critical phenomenon beyond the renormalization group paradigm.

TBA

• Riccardo Rattazzi (EPFL)

First Principles Predictions for the Landau Parameter in Fermi Gases Near Unitary

• Ira Rothstein (Carnegie Mellon)

I will show how one can use the EFT for Fermi liquids to predict the (s-channel) Landau parameter in a systematic expansion, near the unitary limit, as a function of the scattering length and the contact parameter.

Thermalization and Chaos in 1+1d QFT

• Katz Emanuel (Boston U.)

We study aspects of chaos and thermodynamics at strong coupling in a scalar model in 1+1d QFT using numerical Hamiltonian truncation methods. We find that our eigenstate spectrum satisfies Wigner-Dyson statistics and that the coefficients describing eigenstates in our basis satisfy Random Matrix statistics, as expected in chaotic systems (even at weak coupling). We also find a few scar states, but only at weak coupling. We then use these chaotic states to compute the equation of state of the model, obtaining results consistent with CFT expectations at temperatures above the scale of relevant interactions. Finally, we test the Eigenstate Thermalization Hypothesis by computing the expectation value of local operators in eigenstates, and check that their behavior is consistent with thermal CFT values as we approach high temperatures.

Boundary Induced Symmetry Protected Topological Phases in 1-dimensional Superconductors

• Natan Andrei (Rutgers)

The nature of topological phases is typically determined by the bulk properties of the system. I will show in this talk that this is not always the case. I will begin by discussing one dimensional charge conserving superconductors. When open boundary conditions (OBC) are applied, the bulk superconducting instability determines the topological nature of a phase - Spin-triplet superconductor (STS) exhibits a topological phase which is protected by the Z2 spin flip symmetry, with two zero energy Majorana modes (ZEM) at each edge resulting in four fold topological degeneracy of the ground states with exponentially localized fractional spin Sz=+/-1/4 at each edge, while the spin-singlet superconductors (SSS) are topologically trivial with a unique ground state. I shall show however that more generally the topological nature of a phase depends not only on the bulk superconducting instability, but rather on the interplay between the bulk and the boundary. In particular I will show using Bethe Ansatz and bosonization that SSS can exhibit topological phase stabilized by a topological boundary fixed point when suitable twisted OBC are applied. I will show that a rich phase diagram emerges around the topological boundary fixed point exhibiting several regimes - topological and multiple mid-gap regimes. In the second part of my talk I will consider dynamical boundary conditions induced by coupling each edge to a Kondo impurity. This allows us to study the Kondo effect in the presence of strong correlations among electrons. I will show that depending on the relative strength of the Kondo coupling and the bulk interaction three regimes will emerge: the Renormalized Kondo regime, the unscreened local moment regime and the YSR (Shiba) regime where the impurity is locally screened by a bound state. The latter exists only in a narrow parameter regime, in contrast to the case of BCS supercondactors where it dominates. The full phase diagram of the system with two Kondo impurities, one at each edge, exhibits an emergent boundary supersymmetry (SUSY). If time permits I will briefly comment on the similarity of the boundary phase structure of XXZ spin chains and superconductors discussed above and show some DMRG results to compare with.

Quantum chaos in 2d gravity

• Alexander Altland (Koln U.)

The ergodic regime of many body quantum chaos defines the perhaps most universal phase of quantum mechanics. It is uniquely described by an effective zero-dimensional matrix theory, Efetov's supersymmetric nonlinear sigma model. To the best of our knowledge every chaotic quantum system maps onto that model in the long time limit. In this talk we apply this correspondence as a search principle in holography and two-dimensional quantum gravity. We will look at various effective models through this lens, among them the SYK model, JT gravity, and Kodaira-Spencer string field theory. Our main finding is that the latter is complete in that it is dual to the sigma model at large time scales. On this basis, it provides a full description of quantum chaos in the two dimensional bulk – from semiclassical gravitational fluctuations to deep black hole microstate quantization at the largest time scales.

Field theory of driven dissipative systems: symmetries, instantons and random perturbations

• Alex Kamenev (U. Minnesota)

I will present a quantum field theory of systems undergoing a Lindbladian evolution. A particular emphasis is on manifestations of underlying symmetries and their restoration via instanton trajectories. The latter run in a multidimensional phase-space, which is complexified and (Keldysh) doubled. I will also consider random dissipative systems and eigenvalue statistics of their stationary density matrices.

Quantum simulators: from the Fermi Hubbard model to quantum assisted NMR inference

• Eugene Demler (ETH)

I will discuss recent progress of the optical lattice emulators of the Fermi Hubbard model. The new feature of these experiments is availability of snapshots of many-body states with single particle resolution. I will review new insights from these experiments on the properties of doped Mott insulators, including demonstration of magnetically mediated pairing. I will also present the idea of using quantum simulators to perform inference of NMR spectra for biological molecules. I will review recent experimental realization of this algorithm on a quantum computer using trapped ions. Prospects for scaling this approach to solving practically relevant problems will be discussed.

Analogue simulators for fundamental physics: From quantum to classical

• Sebastian Erne (TU Wien)

Asymptotic construction of local conserved quantities: emergence of ergodicity and instability of Many Body Localization

• Polkovnikov Anatoli (Boston U.)

I will first discuss how one can understand and define quantum chaos through sensitivity of eigenstates encoded in the fidelity susceptibility and more generally in a quantum geometric tensor. I will argue that transition to chaos to integrability is highly nonperturbative characterized by emergence of exponentially long in the system size time scales. Then I will consider a specific setup of a strong impurity coupled to a bath and show how one can find approximate local integral of motion (LIOM) using Birkhoff construction and how this construction diverges leading to eventual decay of this LIOM. I will comment how these results imply that many body localization (MBL) is unstable and instead there is a slow transient glassy phase. At the end I will comment on the issues in both analytical and numerical analysis of the disordered systems, which previously lead to erroneous conclusions about stability of the localized phase in the thermodynamic limit.