September 2018 - ITP-quantumseminare

visitor talk today 2:00 - 2:30 pm: Valerio Scarani
by Joe Renes 28 Sep '18

28 Sep '18

Hi all, Valerio Scarani is visiting us today from CQT/NUS in Singapore. His group works on device independent quantum protocols as well as theoretical quantum optics and thermodynamics, and he'll tell us a bit about what he's been working on lately. Current plan is for 2pm unless there is a preference for a different time; the place is not fixed yet, but I'd assume E41.1 or something on the K floor will work. Best, -joe

1 1

today's meeting: HIT F31.2 @ 15:45
by Joe Renes 24 Sep '18

24 Sep '18

Hi all, today Vilasini is visiting from York and will tell us about causal structures and Tsallis entropies. See the title and abstract below. Note that we're starting a little later today and in a different room than usual. Best, -joe Title: Tsallis entropies and causal structures Abstract: Distinguishing between classical and non-classical correlations that could arise from a given causal structure is a central problem in the study of causality which has applications in device independent quantum cryptography. The entropy vector method is often employed for tackling this problem and so far, the Shannon entropy has been used in this method for analysing causal structures, but it has been found to have limitations in certifying non-classicality for certain causal structures. A natural question that arises is whether using other entropic measures can avoid these limitations. We discuss the use of Tsallis entropies for this task; at first they appear to be a good candidate for the task as they satisfy certain desirable properties for the entropy vector method and one might expect them to provide such advantages for other causal structures as well. However, in the presence of a causal structure, we find that the Tsallis entropies do not satisfy the simple conditional independence constraint that the Shannon entropy does. We derive the corresponding causal constraints satisfied by the Tsallis entropies for some specific cases, conjecture the general causal condition for these entropies and provide various evidence supporting this conjecture. Even if we assume that these causal constraints hold, the computational task of projecting to the marginal polytope to obtain new entropic inequalities becomes too time consuming for relatively small causal structures such as the bipartite Bell scenario. We then analyse the post-selected bipartite Bell scenario with 2 inputs and 3 outputs per party, predicting the existence of a family of non-local correlations whose non-locality may be undetectable by both Shannon and Tsallis entropic inequalities. Contrary to expectation, our work indicates that Tsallis entropies may not be useful for analysing causal structures with present techniques and computational power.

1 0

meeting today E41.1 @ 3:30 pm
by Joe Renes 17 Sep '18

17 Sep '18

Hi all, Today we'll have a brief meeting to start off the semester. I'll tell you about an uncertainty relation I'd like to prove; it involves min entropies and could be applied to quantum error correction. Mainly we need to determine if 3:30 Monday's is a good time for the meeting this semester. Best, -joe

1 0

talk today by Karl Mayer: HIT E41.1 @ 2pm
by Joe Renes 14 Sep '18

14 Sep '18

Hi all, today we have a visitor, Karl Mayer, from Manny Knill's group at the University of Colorado. Karl will give a ~1/2 hour talk about his research on tomography at 2pm in E41.1. See below for the title and abstract. Best, -joe Paper: https://arxiv.org/abs/1805.04491 Title: Quantum Process Fidelity Bounds from Sets of Input States Abstract: We investigate the problem of bounding the quantum process fidelity given bounds on the fidelities between target states and the action of a process on a set of pure input states. We formulate the problem as a semidefinite program and prove convexity of the minimum process fidelity as a function of the errors on the output states. We characterize the conditions required to uniquely determine a process in the case of no errors, and derive a lower bound on its fidelity in the limit of small errors for any set of input states satisfying these conditions. We then consider sets of input states whose one-dimensional projectors form a symmetric positive operator-valued measure (POVM). We prove that for such sets the minimum fidelity is bounded by a linear function of the average output state error. The minimal non-orthogonal symmetric POVM contains d+1 states, where d is the Hilbert space dimension. Our bounds applied to these states provide an efficient method for estimating the process fidelity without the use of full process tomography.

1 0

no meeting today --- holiday
by Joe Renes 10 Sep '18

10 Sep '18

Hi all, no meeting today due to the holiday: https://www.knabenschiessen.ch/ -joe

1 0

talk by Tony Short: Thursday 11:00 HIT K52
by Joe Renes 05 Sep '18

05 Sep '18

Hi all, we'll have a special talk tomorrow by Tony Short from Bristol. It's at 11 in K52; here's the title and abstract: Title: Indefinite causal order via post-selection Abstract: We consider how post-selection can be used to generate indefinite causal structures, which cannot be explained by any fixed (or random) temporal ordering, and the connection to process matrices and backwards in time signalling. Based on arXiv:1701.08638 and arXiv:1708.00669. Best, -joe

1 0

today's talk K52 @ 15:30
by Joe Renes 03 Sep '18

03 Sep '18

Hi all, today we'll hear from Luca Mondada on his semester project with Raban. Note that we're in K52 today, as there's a small conference on topological order taking place on the ground floor. Best, -joe

1 0