Scientific Series

In the usual operational picture, operations are represented by boxes having inputs and outputs. Further, we usually consider the causally simple case where the inputs are prior to the outputs for each such operation. In this talk (motivated by an attempt to formulate an operational probabilistic field theory) I will consider what I call the "causally complex" situation. Operations are represented by circles. These circles have wires going in and out. Each such wire can represent an input and an output. Further, each operation will have a causal diagram associated with it. The causal structure can be more complicated than the simple case. These circles can be joined together to create new operations. I will discuss conditions on these causally complex operations so that we have positivity (probabilities are non-negative) and causality (to be understood in a time symmetric manner). I will also discuss how these properties compose when we join causally complex operations. Causally complex operations are related to objects in the causaloid formalism as well as to quantum combs.

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Zoom link https://pitp.zoom.us/j/99425886198?pwd=ODR0VVFzQUJHeER4OVJ2cEo3cVdDQT09

A high specific abundance of millisecond radio pulsars has been observed in globular clusters (GCs), motivating theoretical studies of the formation and evolution of these sources through stellar evolution coupled to stellar dynamics. In this talk, I will first demonstrate how we model millisecond pulsars in GCs using realistic cluster simulations. I will show the importance of electron-capture supernovae for neutron star retention, and how millisecond pulsar formation is greatly enhanced through dynamical interaction processes. I will also present some latest results on isolated millisecond pulsars, which are especially intriguing given the fact that millisecond pulsars are descendants of binary star systems. I will demonstrate the potential formation channels of isolated millisecond pulsars, some of which may also link to the formation of magnetars and the newly discovered fast radio bursts in a GC.

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Zoom link https://pitp.zoom.us/j/97622593487?pwd=SHNoM1o3T1JjWVROTkJoZ0NWYmdyQT09

It is well-known that one-dimensional systems at finite temperature, such as the classical Ising model, cannot spontaneously break a discrete symmetry due to the proliferation of domain walls. The validity of this statement rests on a few assumptions, including the spatial locality of interactions. In a situation where a one-dimensional system exists as a defect in a critical, higher-dimensional bulk system, the coupling between defect and bulk can induce an effective long-range interaction on the defect. It is thus natural to ask if long-range order can be stabilized on a defect in a critical bulk, which amounts to asking whether domain walls on the defect are relevant or not in the renormalization group sense. I will explore this question in the context of Ising conformal field theory in two and higher dimensions in the presence of a localized symmetry-breaking field. With both perturbative techniques and numerical conformal bootstrap, I will provide evidence that indeed the defect domain wall must be relevant when 2 < d < 4. For the bootstrap calculations, it is essential to include “endpoint” primary fields of the defect, which lead to a rigorous and powerful way to input bulk data. I will additionally give tight estimates of a number of other quantities, including  scaling dimensions of defect operators and the defect entropy, and I will conclude with a discussion of future directions.

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Zoom link https://pitp.zoom.us/j/92671628591?pwd=WjNma3VEV2M4T011dFlLMzM2ZUJiUT09

Costello, Witten and Yamazaki proposed a 4d Chern-Simons theory as a unified way to engineer integrable models. In the presence of 'Disorder' defects (for non-ultralocal 2d theories), this correspondence has been established only classically. As a first quantum check, I will derive the matching of 1-loop divergences between the 4d and 2d theories. My assumptions are general and seem to isolate sigma-models among the 2d theories. (Based on 2309.16753)

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Zoom link https://pitp.zoom.us/j/99362983669?pwd=NE1uQ3FmWityQ1R0NUVnZkRPZTRUdz09

Significant effort has been devoted to searching for new fundamental forces of nature. At short length scales (below approximately 10 nm), many of the strongest experimental constraints come from neutron scattering from individual nuclei in gases. The leading experiments at longer length scales instead measure forces between macroscopic test masses. I will present a proposal that combines these two approaches: scattering neutrons off of a target that has spatial structure at nanoscopic length scales. Such structures will give a coherent enhancement to small-angle scattering, where the new force is most significant. This can considerably improve the sensitivity of neutron scattering experiments for new forces in the 0.1 - 100 nm range. I will discuss the backgrounds due to Standard Model interactions and a variety of potential target structures that could be used, estimating the resulting sensitivities. I will show that, using only one day of beam time at a modern neutron scattering facility, our proposal has the potential to detect new forces as much as four orders of magnitude beyond current laboratory constraints at the appropriate length scales.

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Zoom link https://pitp.zoom.us/j/98201041537?pwd=MS9weFpNcHVFTVIwMTVoYmpxeTd6Zz09

The deep learning architecture associated with ChatGPT and related generative AI products is known as transformers. Initially applied to Natural Language Processing, transformers and the self-attention mechanism they exploit have gained widespread interest across the natural sciences. We will present the mathematics underlying the attention mechanism and describe the basic transformer architecture. We will then describe applications to time series and imaging data in astronomy and discuss possible foundation models.

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Zoom link https://pitp.zoom.us/j/91226066758?pwd=TWZ5RVliMjVKYXdLcHdya09lNWZhQT09

Quantum state tomography (QST) is the art of reconstructing an unknown quantum state through measurements. It is a key primitive for developing quantum technologies. Neural network quantum state tomography (NNQST), which aims to reconstruct the quantum state via a neural network ansatz, is often implemented via a basis-dependent cross-entropy loss function. State-of-the-art implementations of NNQST are often restricted to characterizing a particular subclass of states, to avoid an exponential growth in the number of required measurement settings. In this talk, I will discuss an alternative neural-network-based QST protocol that uses shadow-estimated infidelity as the loss function, named “neural-shadow quantum state tomography” (NSQST). After introducing NNQST and the classical shadow formalism, I will present numerical results on the advantage of NSQST over NNQST at learning the relative phases, NSQST’s noise robustness, and NSQST’s advantage over direct shadow estimation. I will also briefly discuss the future prospects of the protocol with different variational ansatz and randomized measurements, as well as its experimental feasibility.

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Zoom link https://pitp.zoom.us/j/94167105773?pwd=TXR3TUtwNjV4VFB4SEpvTkhqd29SUT09

To develop a quantum gravity theory, it is fundamental to move away from the gauge-fixing approach and instead employ a gauge-free analysis. There is an increasing body of evidence suggesting that the symmetries employed for gauge-fixing might carry charge. Consequently, setting the associated fields to zero is a physical constraint on the system, which should be avoided. In this talk, we will examine a partial fixing of the Fefferman-Graham (FG) gauge, referred to as the Weyl-Fefferman-Graham (WFG) gauge, which restores boundary Weyl covariance. We will show that the diffeomorphism mapping WFG to FG can be charged and discuss how this relates to holography.

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Zoom link https://pitp.zoom.us/j/93359035909?pwd=aHo0TUFXekZaeCtrT2FzNDZKUW15Zz09

We investigate perturbations of the Schwarzschild geometry using a linearization of the Einstein vacuum equations within a Bondi-Sachs, or null cone, formalism. We develop a numerical method to calculate the quasinormal modes, and present results for the case ℓ= 2. The values obtained are different than those of a Schwarzschild black hole, and we interpret them as quasinormal modes of a Schwarzschild white hole.

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Zoom link https://pitp.zoom.us/j/93648313308?pwd=akZMZXFKejIwdFphOVU0ZjFpeW13dz09

We will present a teleportation protocol which uses the properties of superoscillatory wavefunctions. Then we will briefly introduce a different protocol, the port based teleportation. We will use it to study teleportation over infinitesimally small distances, where the vacuum of a quantum field serves as the source of entanglement. We find that the resulting motion is equivalent to a quantum teleportation-induced Brownian motion. Purifying the interactions, from measurements to unitary operations, leads to motion described by Schrodinger’s equation. Therefore, we find that the notions of teleportation and continuous quantum and classical motion are unified in this sense.

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Zoom link https://pitp.zoom.us/j/93504138534?pwd=VUFPSXpRd2g0TVNxVTNlY3Rxa1ZjUT09

Black hole thermodynamics plays a central role in the theoretical landscape, acting as both synthesizer and sieve for concepts in field theory, quantum gravity, information theory, and more. While its formulation and applications are well understood in asymptotically flat and anti-de Sitter spaces, significant difficulties arise when generalizing to cosmological settings. In this talk, I will discuss how the thermodynamic properties of black holes can be understood in such scenarios through the Euclidean path integral. I will also discuss black holes in a more generalized, semi-classical setting, and suggest a consistent framework for describing a wide variety of dynamical black hole models and ultracompact objects, with consequences for horizon formation, back-reaction, and the growth of astrophysical black holes.

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Zoom link https://pitp.zoom.us/j/97647270696?pwd=bXhFYWYyYUlrTEUyVXhtOUNDakhhQT09

Long-range entangled quantum matter encompasses a wealth of fascinating phenomena including fractionalization and criticality.  I will show how quantum dynamics involving measurements can both enable new kinds of long-range entangled states and facilitate their realization on quantum simulators.  In the first part, I will illustrate how competing measurements along with unitary time evolution can give rise to distinct universality classes of non-equilibrium criticality.  In the second part, I will show how measurements and unitary evolution conditioned on the measurement outcomes (“adaptive quantum circuits”) enable efficient preparation of long-range entangled matter.  Finally, I will demonstrate how environmental measurement (decoherence) can remarkably enrich quantum critical pure states, giving rise to renormalization group flows between quantum channels with important implications on the entanglement structure of the resulting critical mixed states.  

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Zoom link https://pitp.zoom.us/j/97087115629?pwd=NHdlWG9oM0xHUDIzU05sUWRKdWlFdz09