Search Talks

Primordial black holes (PBHs), if they exist, may shed light on long-standing questions on the nature
of dark matter and mechanism driving cosmic inflation. If we associate their origin to the presence of enhanced primordial scalar fluctuations generated during inflation, the underlying dynamics that populates these objects can also provide distinctive sources of gravitational waves (GWs), potentially detectable with current or forthcoming GW experiments. Therefore, their population, along with the associated GW signal offer promising opportunities to shed light on the nature of inflation, by opening up a unique window to its dynamics at scales inaccessible by conventional CMB probes. In this talk, I will review some of the compelling inflationary scenarios able to trigger the formation of such objects in the post-inflationary universe by enhancing the amplitude of the primordial scalar perturbations. In this context, I will discuss single and multi-field realizations with a focus on theoretical aspects of model building, discussing common themes shared among models in conjunction with their distinctive phenomenological implications in the form of a primordial GW background.

Zoom Link: https://pitp.zoom.us/j/98857635213?pwd=UGZoMXY4TnJBSmc1RlUvbGprSWlRQT09

Universal properties of two-dimensional conformal interfaces are encoded by the flux of energy transmitted and reflected during a scattering process.

In this talk, I will develop a method that allows me to extend previous results based on thin-brane holographic models to smooth domain-wall solutions of 3-dimensional gravity.

As an application, I will compute the transmission coefficient of a Janus interface in terms of its deformation parameter.

Zoom link:  https://pitp.zoom.us/j/98684574364?pwd=WGdrQXhRcHRJZUZMYmNObUVZT1ZCZz09

Fermi liquids are gapless quantum many-body states of fermions, which describes electrons in the normal state of most metals at low temperature. Despite its long history of study, there has been renewed interest in understanding the stability of Fermi liquid from the perspectives of emergent symmetry and quantum anomaly. In this talk, I will introduce the concept of Fermi surface anomaly and propose a possible scheme to classify it. The classification scheme is based on viewing the Fermi surface as the boundary of a Chern insulator in the phase space, with an unusual dimension counting arising from the non-commutative phase space geometry. This enables us to extend the notion of Fermi surface anomaly to the non-perturbative cases and discuss symmetric mass generation on the Fermi surface when the anomaly is canceled. I will provide examples of lattice models that demonstrate Fermi surface symmetric mass generation and make connections to the recent progress in understanding the pseudo-gap transition in cuprate materials.

Zoom link: https://pitp.zoom.us/j/97223165997?pwd=SkhJZEt1ejhQRm0yK2tKS3NhM2o2Zz09

Quantum field theories in two dimensions (2d) with an underlying Bondi-van der Burg-Metzner-Sachs (BMS) symmetry augmented by u(1) currents are expected to holographically capture features of charged versions of cosmological solutions in asymptotically flat 3d spacetimes called Flat Space Cosmologies (FSCs). I will present a study of the modular properties of these field theories and the corresponding partition function. Furthermore, I will derive the density of (primary) states and find the entropy and asymptotic values of the structure constants exploiting the modular properties of the partition function and the torus one-point function. The expression for the asymptotic structure constants shows shifts in the weights and one of the central terms and an extra phase compared to earlier results in the literature for BMS invariant theories without u(1) currents present. The field theory results for the structure constants can be reproduced holographically by a bulk computation involving a scalar probe in the background of a charged FSC.

Zoom Link: https://pitp.zoom.us/j/99205444635?pwd=Tk02UlgvcjJCU3JSWWphY1JQSlhFQT09

Tensor Processing Units are application specific integrated circuits (ASICs) built by Google to run large-scale machine learning (ML) workloads (e.g. AlphaFold). They excel at matrix multiplications, and hence can be repurposed for applications beyond ML. In this talk I will explain how TPUs can be leveraged to run large-scale density matrix renormalization group (DMRG) calculations at unprecedented size and accuracy. DMRG is a powerful tensor network algorithm originally applied to computing ground-states and low-lying excited states of strongly correlated, low-dimensional quantum systems. For certain systems, like one-dimensional gapped or quantum critical Hamiltonians, or small, strongly correlated molecules, it has today become the gold standard method for computing e.g. ground-state properties. Using a TPUv3-pod, we ran large-scale DMRG simulations for a system of 100 spinless fermions, and optimized matrix product state wave functions with a bond dimension of more than 65000 (a parameter space with more than 600 billion parameters). Our results clearly indicate that hardware accelerator platforms like Google's latest TPU versions or NVIDIAs DGX systems are ideally suited to scale tensor network algorithms to sizes that are beyond capabilities of traditional HPC architectures.

Zoom link:  https://pitp.zoom.us/j/99337818378?pwd=SGZvdFFValJQaDNMQ0U1YnJ6NU1FQT09

Soon after the discovery of high temperature superconductivity in the cuprates, Anderson proposed a connection to quantum spin liquids. But observations since then have shown that the low temperature phase diagram is dominated by conventional states, with a competition between superconductivity and charge-ordered states which break translational symmetry. We employ the "pseudogap metal" phase, found at intermediate temperatures and low hole doping, as the parent to the phases found at lower temperatures. The pseudogap metal is described as a fractionalized phase of a single-band model, with small pocket Fermi surfaces of electron-like quasiparticles whose enclosed area is not equal to the free electron value, and an underlying pi-flux spin liquid with an emergent SU(2) gauge field. This pi-flux spin liquid is now known to be unstable to confinement at sufficiently low energies. We develop a theory of the different routes to confinement of the pi-flux spin liquid, and show that d-wave superconductivity, antiferromagnetism, and charge order are natural outcomes. We are argue that this theory provides routes to resolving a number of open puzzles on the cuprate phase diagram.

As a side result, at half-filling, we propose a deconfined quantum critical point between an antiferromagnet and a d-wave superconductor described by a conformal gauge theory of 2 flavors of massless Dirac fermions and 2 flavors of complex scalars coupled as fundamentals to a SU(2) gauge field.

This talk is based on Maine Christos, Zhu-Xi Luo, Henry Shackleton, Ya-Hui Zhang, Mathias S. Scheurer, and S. S., arXiv:2302.07885

Zoom link:  https://pitp.zoom.us/j/97370076705?pwd=Q1MwQmNaSFkxaWFEdUl5NFZDS0E4Zz09

Schrodinger's thought experiment famously illustrates the dramatic effect of measuring a quantum state. The resulting wave function collapse is often thought to make states more classical and familiar. However, in this colloquium, we explore how measurements can be used as a chisel to efficiently build exotic forms of quantum entanglement. We focus on topological states of matter, whose quasiparticles exhibit generalized 'anyonic' exchange statistics with potential relevance to quantum computation. We use these ideas to experimentally realize the first controlled realization of non-Abelian anyons, which can remember the sequence in which they are exchanged. The smoking gun signature of this experiment is inspired by the coat of arms of the House of Borromeo.

Zoom Link: https://pitp.zoom.us/j/98167813390?pwd=aG5vcklVZzBWT1BRSjI4RVRtbDhBUT09

We investigate the evolution of quantum information under Pauli measurement circuits. We focus on the case of one- and two-dimensional systems, which are relevant to the recently introduced Floquet topological codes. We define local reversibility in context of measurement circuits, which allows us to treat finite depth measurement circuits on a similar footing to finite depth unitary circuits. In contrast to the unitary case, a finite depth locally reversible measurement sequence can implement a translation in one dimension. A locally reversible measurement sequence in two dimensions may also induce a flow of logical information along the boundary. We introduce "measurement quantum cellular automata" which unifies these ideas and define an index in one dimension to characterize the flow of logical operators. We find a Z_2 bulk invariant for Floquet topological codes which indicates an obstruction to having a trivial boundary. We prove that the Hastings-Haah honeycomb code belong to a class with such obstruction, which means that any boundary must have either non-local dynamics, period doubled, or admits boundary flow of quantum information.

Zoom Link: https://pitp.zoom.us/j/96083249406?pwd=MnhYbTEyU05ybVdyUlE3UGZrdEhPdz09

We construct two-dimensional quantum states associated to four-dimensional linearized rotating self-dual black holes in (2,2) signature Klein space. The states are comprised of global conformal primaries circulating on the celestial torus, the Kleinian analog of the celestial sphere. By introducing a generalized tower of Goldstone operators we identify the states as coherent exponentiations carrying an infinite tower of w1+inf charges or soft hair. We relate our results to recent approaches to black hole scattering, including a connection to Wilson lines, S-matrix results, and celestial holography in curved backgrounds.

Zoom link:  https://pitp.zoom.us/j/94163164577?pwd=RHFVZU5XUEN0T3c3Zm1VR3VhZnNsZz09

Despite first observing cosmic rays with energies above an EeV (10^18 eV) in the 1960s, the source of these particles remains an open question. Modern observatories, in particular the Pierre Auger Observatory and Telescope Array, have firmly established that the cosmic ray spectrum continues up to ~10^20.3 eV and have significantly advanced our understanding of these particles. However, limited statistics, uncertainties in particle physics, and significant deflections in the Galactic magnetic field have made progress towards discovering their astrophysical source extremely challenging. One key astrophysical input needed to understand ultrahigh energy cosmic ray data is the distribution of their sources, or the source evolution. In this talk, I will focus on multimessenger observations which have the potential to pin down the source evolution for the very first time.

Zoom Link:  https://pitp.zoom.us/j/94054513261?pwd=aHRNWE04VDI2cDA2RmRYQmtNRnd3dz09

Cosmology presents some puzzling aspects: the cosmological constant, the nature of the big bang, the source of inflation, and perhaps the tension in Hubble constant values. One normally uses standard effective field theory to study cosmology. But we know that such an approach fails in black holes due to the information paradox. We will draw on lessons on black holes to see how quantum gravity effects can modify evolution on macroscopic scales, and that such effects may be important is resolving puzzles in cosmology.

Zoom link:  https://pitp.zoom.us/j/98085421952?pwd=NFBCWXNBNkhrTVpaRkRlOWRtSkczdz09