December 2021 - ITP-quantumseminare

Tuesday's QIT Seminar
by Joe Renes 20 Dec '21

20 Dec '21

Hi all, Tomorrow Oliver Knapp will tell us about his master thesis with Esteban, entitled "Relational quantities and subsystems in symmetry-constrained quantum theories". See below for the abstract. Join us at 2pm on Zoom: https://ethz.zoom.us/j/362994444. Best, Joe %%%%% Abstract: It is argued that only relational quantities, such as the distance between two objects, are physically meaningful. Consequently, unobservable quantities, such as the global position of a particle, should be considered as artifacts of our mathematical description. In this work, we review current approaches, based on coherent and incoherent group averaging, aimed at identifying and describing relational quantities, and discuss some difficulties arising within these approaches. In particular, we study subsystems defined w.r.t. reference frames and show that unexpected phenomena occur once we consider "unsharp" reference frames instead of ideal ones. Furthermore, we introduce a rigorous definition of relational subsystems based on group averaged operator algebras. However, we prove several theorems which suggest that group averaged operators cannot be used in quantum field theory to define a relational subsystem structure that is compatible with the causal structure of spacetime. We argue that such difficulties arise because approaches involving group averaging can only capture a subset of all relational quantities contained within a theory. Subsequently, we propose a general framework, based on probability theory, that allows for a rigorous description of all relational quantities. We demonstrate that within this framework many difficulties encountered when using group averaging can be overcome.

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Tomorrow's QIT Seminar
by Joe Renes 13 Dec '21

13 Dec '21

Hi all, Tomorrow Eliott Mamon will present his semester project with Victor, entitled "Exploring noncontextuality of prepare-and-measure scenarios with a numerical algorithm". See below for the abstract. Join us at 2pm on zoom: https://ethz.zoom.us/j/362994444 Best, Joe %%%%% Using a numerical implementation of the unit-separability algorithm proposed in [1], the question of existence of a noncontextual model is assessed for various single-qubit prepare-and-measure scenarios. Noncontextual scenarios that are not included in the Spekkens toy model scenario are pointed out, and a trend between contextuality and areas of randomly sampled planar scenarios is uncovered. (Non)contextuality is also assessed for scenarios involved in strategies of a two-agent encoding/decoding game, which exposes contextuality as necessary and sufficient for better-than-classical strategies at that game. [1] Gitton, V., & Woods, M. P. (2020). Solvable criterion for the contextuality of any prepare-and-measure scenario. arXiv preprint arXiv:2003.06426

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Tomorrow's QIT Seminar
by Joe Renes 06 Dec '21

06 Dec '21

Hi all, Tomorrow our guest Marco Tomamichel will tell us about some of his latest research, specifically "Sequential quantum hypothesis testing". The paper is available at https://arxiv.org/abs/2104.14706, and I have copied the abstract below. Join us at 2pm on zoom: https://ethz.zoom.us/j/362994444. Best, Joe %%%%% We consider sequential hypothesis testing between two quantum states using adaptive and non-adaptive strategies. In this setting, samples of an unknown state are requested sequentially and a decision to either continue or to accept one of the two hypotheses is made after each test. Under the constraint that the number of samples is bounded, either in expectation or with high probability, we exhibit adaptive strategies that minimize both types of misidentification errors. Namely, we show that these errors decrease exponentially (in the stopping time) with decay rates given by the measured relative entropies between the two states. Moreover, if we allow joint measurements on multiple samples, the rates are increased to the respective quantum relative entropies. We also fully characterize the achievable error exponents for non-adaptive strategies and provide numerical evidence showing that adaptive measurements are necessary to achieve our bounds under some additional assumptions.

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Special seminar talk tomorrow
by Joe Renes 01 Dec '21

01 Dec '21

>From Vilasini: Hi all, This week we have a visitor, Ämin Baumeler from IQOQI, Vienna who will be giving a seminar on “Logical limits on correlations and causal structures” at 4pm on Thursday, 2nd of December at HIT E 41.1. The abstract is as attached below. It would be a blackboard talk but I will aim to stream it live (and also record it) at the following zoom link, for those who would be interested in attending online. https://ethz.zoom.us/j/66298777504 Title: Logical limits on correlations and causal structures Abstract: The self-consistency principle stipulates that the only solutions to physical laws that occur locally are those that are self-consistent. In this talk, after a discussion of this principle, I present several consequences. First, I show that in a classical setting a violation of this principle not only encompasses what colloquially is known as the grandparent paradox, but also that this paradox is equivalent to the information paradox (with E. Tselentis, QPL 2020, 10.4204/EPTCS.340.1). Second, I present self-consistent multi-party correlations inspired by Bell, Ardehali, and Svetlichny that violate causal order to an unbounded degree (with A. S. Gilani and J. Rashid, arXiv:2004.12921 [quant-ph]). Finally, I discuss some recent characterizations of self-consistent and inconsistent causal structures (with E. Tselentis, work in progress). Best regards, Vilasini

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