Talks

In quantum metrology, one of the major applications of quantum technologies, the ultimate precision of estimating an unknown parameter is often stated in terms of the Cramér-Rao bound. Yet, the latter is no longer guaranteed to carry an operational meaning in the regime where few measurement samples are obtained. We instead propose to quantify the quality of a metrology protocol by the probability of obtaining an estimate with a given accuracy. This approach, which we refer to as probably approximately correct (PAC) metrology, ensures operational significance in the finite-sample regime. The accuracy guarantees hold for any value of the unknown parameter, unlike the Cramér-Rao bound which assumes it is approximately known. We establish a strong connection to multi-hypothesis testing with quantum states, which allows us to derive an analogue of the Cramér-Rao bound which contains explicit corrections relevant to the finite-sample regime. We further study the asymptotic behavior of the success probability of the estimation procedure for many copies of the state and apply our framework to the example task of phase estimation with an ensemble of spin-1/2 particles. Overall, our operational approach allows the study of quantum metrology in the finite-sample regime and opens up a plethora of new avenues for research at the interface of quantum information theory and quantum metrology. TL;DR: In this talk, I will motivate why the Cramér-Rao bound might not always be the tool of choice to quantify the ultimate precision attainable in a quantum metrology task and give a (hopefully) intuitive introduction of how we propose to instead quantify it in a way that is valid in the single- and few-shot settings. We will together unearth a strong connection to quantum multi-hypothesis testing and conclude that there are many exiting and fundamental open questions in single-shot metrology!

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Zoom link https://pitp.zoom.us/j/92247273192?pwd=ZkprOFZ0eEdQYjJDY1hneFNLckFDZz09

In the last years, asymptotic symmetries have regained a lot of interest, and various extensions of the well known BMS group have been considered in the literature. Many charges associated to the diffeomorphisms of the sphere (superboosts and superrotations) have been proposed, but it has not been clear if these charges can be derived from a symplectic potential that is covariant and stationary, i.e satisfying the Wald-Zoupas usual requirements. In this talk I will consider a new asymptotic symmetry group, which is a one dimensional extension of the generalized-BMS group, and construct a stationary symplectic potential, covariant with respect to these symmetries, by adding corner terms to the usual Einstein-Hilbert symplectic potential. Then, we will recover the charges introduced by Compère, Fiorucci and Ruzziconi for superboosts and superrotations. In order to ensure covariance, we will need to introduce an edge mode which has already appeared in the literature, the supertranslation field. I will also explain that its introduction as a corner term can lead us to construct a local (asymptotic) notion of energy for the gravitational waves, providing a physical interpretation of the new charges.

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Zoom link https://pitp.zoom.us/j/97926664729?pwd=VzV2VmQ4eVlzcFdaZkNBNnpqRkMvUT09

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