March 2021 - ITP-quantumseminare

Tomorrow's QIT Seminar
by Joe Renes 29 Mar '21

29 Mar '21

Hi all, Tomorrow Mischa will tell us about one of his recent projects, entitled "How to tell the time with a clock that's always off." See below for the abstract. We'll start at 2pm in zoom: https://ethz.zoom.us/j/362994444. Best, Joe Abstract:Conventional wisdom suggests that in order for a clock to be able to tell the time, the clock cannot be a static device, since if it were, the clock at different times would be indistinguishable. In other words, the clock must be switched on. This widely held belief is indeed true for all classical clocks. We show however, that somewhat counter-intuitively, a quantum clock can be proven to measure to arbitrary high precision how much time has passed between two events, even though it has always been off during and before these events took place. Our results use counterfactual reasoning to show that the mere fact that a clock could have been switched on allows us to predict time even if it was never switched on in the past, present, or future. Our protocols provide fresh insight into an over 2000 year old debate regarding the nature of time | lending support for the substantival theory of time.

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tomorrow's QIT Seminar
by Joe Renes 22 Mar '21

22 Mar '21

Hi all, Tomorrow our new postdoc Ramona Wolf will tell us about her research, in particular "Fusion categories and lattice models". See below for the abstract. We start at 2pm on Zoom: https://ethz.zoom.us/j/362994444 Best, Joe Abstract: While fusion categories are already an interesting subject from a purely mathematical perspective, they can furthermore be used in physics to study conformal field theories via 1D and 2D lattice models. In my talk I will explain the basic theory of fusion categories and explain their connection to conformal field theory. I will also discuss how to construct a lattice model from a fusion category (and the context in which this problem arises) via the Levin-Wen stringnet model.

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Tomorrow's QIT Seminar
by Joe Renes 15 Mar '21

15 Mar '21

Hi all, Tomorrow Julian Arnold will tell us about his master's thesis project, on "Entropy production in ticking clocks". See below for the abstract. We start at 2pm in zoom: https://ethz.zoom.us/j/362994444 Best, Joe Abstract: The concept of time and its role in physics, in particular quantum theory, remains an active field of research. The act of timekeeping is linked to the usage of clocks as reference systems. Thus, many open questions about time can be addressed through the study of these devices. Ticking clocks provide information about the flow of time in terms of discrete ``ticking'' events. In this thesis, we studied autonomous ticking clocks based on a model derived from axiomatic principles [M. P. Woods, Quantum 5, 381 (2021)]. We tried to answer the question whether the theory of quantum information imposes any constraints on the relation between the entropy production per tick of a ticking clock and its accuracy. Here, the entropy production serves as a measure for the exchanged information between a ticking clock and its outside. We find that there exist quantum ticking clocks with coherent internal dynamics that approach infinite accuracy at zero entropy production per tick. In contrast, ticking clocks whose state remains incoherent at all times must produce a minimal amount of entropy in each tick that increases with increasing accuracy. For such clocks the entropy production per tick acts as a fundamental resource for measuring time.

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Causality seminar by Tom Purves, Tony Short (Bristol) Monday 4 pm CET
by Dr. Ralph Silva 12 Mar '21

12 Mar '21

Hi all Next Monday (March 15) at 4 pm we welcome Tom Purves and Tony Short from the University of Bristol who will speak to us about their recent important result, *"Quantum theory cannot violate a causal inequality"*, https://arxiv.org/abs/2101.09107. The talk will take place in the usual QIT seminar zoom room, https://ethz.zoom.us/j/362994444, at 16:00 Zürich time (3 pm U.K. time). *Abstract:* *Within quantum theory, we can create superpositions of different causal orders of events, and observe interference between them. This raises the question of whether quantum theory can produce results that would be impossible to replicate with any classical causal model, thereby violating a causal inequality. This would be a temporal analogue of Bell inequality violation, which proves that no local hidden variable model can replicate quantum results. However, unlike the case of non-locality, we show that quantum experiments can be simulated by a classical causal model, and therefore cannot violate a causal inequality.* Regards Ralph

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Tomorrow's QIT Seminar
by Joe Renes 08 Mar '21

08 Mar '21

Hi all, Tomorrow our new postdoc Esteban Castro will tell us about his research, specifically "Regular representation approach to quantum reference frame transformations". See below for the abstract. We start at 2pm on zoom: https://ethz.zoom.us/j/362994444 Best, Joe ------ Recent works on quantum reference frames outline a promising research direction to study space and time from an inherently quantum point of view. In this talk, I will summarise some important aspects of these works and present a new method for quantum reference frame transformations, which complements previous approaches. As an application of the method, I will discuss quantum reference frame transformations for the (centrally extended) Galilei group.

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tomorrow's group meeting 2pm zoom
by Joe Renes 01 Mar '21

01 Mar '21

Hi all, Tomorrow Anian Altherr will present his master's thesis, entitled "Quantum metrology for non-Markovian processes". See below for the abstract. We start at 2pm on zoom: https://ethz.zoom.us/j/362994444 Best, Joe --------------------- Quantum metrology is concerned with finding precision bounds in parameter estimation of a given sample. Whereas classical strategies allow the precision to scale at most linearly with the number of resources used, quantum strategies can achieve a quadratic scaling. Noise, however, in general reduces the quadratic scaling to a linear scaling. Most work so far has dealt with assessing Markovian noise, but in the future, assessing time-correlated noise, e.g. in deep quantum circuits, will be of increasing interest. A general framework to assess such non-Markovian noise has been missing. In this work, we use quantum combs to model non-Markovian noise and express its quantum Fisher information in terms of a semidefinite program and the conditional min-entropy. We apply the semidefinite program to collision models, which serve as an instance of non-Markovian noise. Numerics show that the non-Markovian setting allows for more precise estimators than the Markovian setting. Finally, we employ variational circuits to find an explicit metrological setup that achieves the maximal precision.

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