Scientific Series

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

We explore how measurements and unitary feedback can generate distinct phases of matter from a given resource state, with a specific focus on the toric code in two dimensions. First, we map random Pauli measurements on the toric code to a classical loop model with crossings, and we show how measurement-induced entanglement exactly maps to watermelon correlators of the loop model.  Then, we consider measuring all but a 1d boundary of qubits, and we map this setup to hybrid circuits in 1+1 dimensions.  In particular, we find that varying the probabilities of different Pauli measurements can drive phase transitions in the unmeasured boundary between phases with different orders and entanglement scaling, corresponding to short and long loop phases in the classical model.  Finally, by utilizing single-site boundary unitaries conditioned on the bulk measurement outcomes, we generate mixed state ordered phases and transitions that can be experimentally diagnosed with linear observables. Our findings showcase the potential of measurement-based quantum computing setups in producing and manipulating phases of matter.

Zoom Link: https://pitp.zoom.us/j/99159680593?pwd=V29wRit6T3NlSjZGTDEvTnRFcTlrUT09

EMRIs are one of the primary targets of spaceborne gravitational wave (GW) detectors and will be ideal GW sources for testing fundamental laws of gravity. In a generic non-Kerr spacetime,  the EMRI system  is non-integrable due to the lack of the Carter constant. As a result,  chaos along with resonance islands arise in these systems leaving a non-Kerr signature in the EMRI waveform as proposed in many previous studies. In this work, we systematically analyze the dynamics of an EMRI system near orbital resonances and we have derived an effective resonant Hamiltonian that describes the dynamics of the resonant degree of freedom  with the action-angle formalism. We have two major findings: (1) the chaotic orbits in general produce unique commensurate jumps in actions and (2) the EMRI orbits driven by radiation-reaction in general do not cross the resonance islands.

Zoom Link: https://pitp.zoom.us/j/95975225333?pwd=V05NZzQ3cE9neUZpN3RIOCt0UE5mZz09

I will describe a correspondence between hyperkähler/quaternion-Kähler geometry and two dimensional chiral algebras that arises from twistor theory. As an application, I will explain how to use correlators of these chiral algebras to compute gravitational scattering amplitudes in four dimensional general relativity.

Zoom Link: https://pitp.zoom.us/j/98706048161?pwd=a3k1U3l2UXJlUjlqUElkOEZhZGwwUT09

We develop the bulk geometric description of correlation functions of operators whose scaling dimensions are of order the central charge in AdS/CFT. We follow a bottom-up approach, discussing solutions to Einstein gravity that are closely related to familiar black holes in AdS. In order to reproduce the correct dependence of a conformal correlation function on the location of operator insertions, we must introduce a novel Gibbons-Hawking-York boundary term associated with the stretched horizon of each black hole. We discuss the bulk dual of two point functions in CFT’s living in arbitrary dimensions. Specializing to AdS3 allows us to discuss higher point functions, where we find that the dual geometries are sometimes multi-boundary wormholes, whose holographic interpretation has been the focus of much recent activity.

Zoom Link: TBD

Getting the strongest physics conclusions from collider particle physics experiments regarding the (in)consistency of the Standard Model with actual measurements requires Effective Field Theory techniques. This approach (known as the SMEFT) has been rapidly advanced in recent years, leading to new analyses of the data being executed by Atlas and CMS. The current state of the theoretical art is not precise enough as such EXP studies are continued into the future, as the measurements continue to become more precise. We need to be able to calculate more precisely in the SMEFT to keep up. After an intro to this area of research, I will discuss some recent calculations that have pushed things to the two loop level in precision for higgs production/decay in the SMEFT, and how thinking geometrically (in terms of field space connections and the resulting Higgs geometries) in EFT is the key to keep advancing the theoretical state of the art.

Zoom link:  https://pitp.zoom.us/j/99931154202?pwd=SUMzK2JIS0prNk5KaGZWakphckZhdz09

I will discuss a recently observed connection between Gaussian thermal partition functions in d=2L+1 dimensions and L-loop conformal graphs in D=4. While the origin of such a connection is still unclear, I will argue that it offers some interesting perspectives in quantum field theory and in mathematics.

Zoom link:  https://pitp.zoom.us/j/98795029694?pwd=aGdSdWtDQ1VqbzFvSWE4SkM5c3lJUT09

Gravitational scattering provides valuable insight into classical dynamics and is likely of fundamental importance in quantum gravity.  However, a complete framework for gravitational scattering does not yet exist.  The lack of a clear framework hinders progress; for example, different groups have reported different results for the loss of angular momentum in post-Minkowskian scattering.  In this talk I will report on recent work 2303.17124 with Compere and Wei constructing a general framework for classical gravitational scattering of finite-sized massive bodies in four spacetime dimensions.  We formulate assumptions and definitions such that the five asymptotic regions (past/future timelike/null infinity and spatial infinity) share a single Bondi-Metzner-Sachs (BMS) group of symmetries and associated charges and derive global conservation laws stating that the total change in charge is balanced by the corresponding radiative flux.  Our assumptions are compatible with all known properties of scattering spacetimes, including certain logarithmic corrections that invalidate common falloff assumptions.  Among the new implications are rigorous definitions for quantities like initial/final spin, scattering angle, and impact parameter in multi-body spacetimes, without the use of any preferred background structure.  We show that spin is supertranslation-invariant, while impact parameter is not.  To complement these derivations I will emphasize a helpful "puzzle piece" diagram that faithfully represents all five asymptotic regions, illustrating their roles in the scattering problem.

Zoom link:  https://pitp.zoom.us/j/94358515412?pwd=cUFocVNjQ1pqUmp4MDN0RmRLbjE0QT09