February 2021 - ITP-quantumseminare

Tomorrow's QIT Seminar
by Joe Renes 22 Feb '21

22 Feb '21

Hi all, Tomorrow Elias Huber will present his semester project, entitled "Maxwell's Demon". See the abstract below. Usual time and place, 2pm at https://ethz.zoom.us/j/362994444. Best, Joe Abstract: Maxwell's demon has been proclaimed exorcised more than once but continues to fascinate in many ways. In this talk I will first give an introduction to the theory of phenomenological thermodynamics presented by Dr. Philipp Kammerlander in his PhD thesis [https://doi.org/10.3929/ethz-b-000413414] I then talk about the role of phenomenological thermodynamics in the discussion of Maxwell’s demon and go on to give a detailed description of the Szilard engine. This description will be based on Landauer’s principle and in agreement with the postulates of thermodynamics. Finally, we will look at another thought experiment of an automated (Szilard engine-like) demon and talk about the importance of fluctuations.

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tomorrow's QIT Seminar
by Joe Renes 15 Feb '21

15 Feb '21

Hi all, Tomorrow Roman Wixinger will present his semester project “Uncomputation and Entanglement in High-level Quantum Programming Languages”, conducted under the supervision of Prof. Christian Mendl at TU München. See below for the abstract. We start as usual at 2pm on zoom: https://ethz.zoom.us/j/362994444. Best, Joe Quantum programming languages traditionally focus on the hardware level and are therefore not really good at representing the intentions of the programmer. Explicit formulation of uncomputation, which is essential for the safe and efficient use of the qubits, makes the code unnecessarily complex. In recent work, Vechev et al. (2020) introduced Silq, a high-level language that allows for safe, automatic uncomputation just using its type system. This feature makes the code significantly shorter and more intuitive. The type system can also ensure that any program that compiles is physical. In this project, we compared Silq’s solution of handling uncomputation with other approaches and give an overview of the features of quantum languages. We have also tried to understand whether a qubit can be safely discarded by directly looking at the entanglement.

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today's QIT Seminar
by Joe Renes 09 Feb '21

09 Feb '21

Hi all, Apologies for the late email. This afternoon Diane Saint Aubin will tell us about her semester project, entitled "Lower and upper bounds on quantum key distribution protocols". See below for the abstract. We'll start as usual at 2pm in/at https://ethz.zoom.us/j/362994444. Best, Joe Abstract: The 3-state quantum key distribution protocol has two entanglement-based descriptions, raising the question of which one is best used to compute the key rate. We show that the descriptions are equivalent, and that symmetrising the state used in the protocol can give a lower bound on the secret key rate. In addition, we provide an upper bound for the BB84 protocol using the intrinsic information which just outperforms a recent bound by Xing Wang. For the device independent protocol based on the CHSH game, thresholds on the key rate are found depending on the observed parameters. We find an upper bound for protocols using one way-classical communication, which is much tighter than the previous two-way classical communication bound for low values of the Bell violation.

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tomorrow's QIT Seminar
by Joe Renes 01 Feb '21

01 Feb '21

Hi all, Tomorrow our new postdoc Christopher Chubb will tell us about his research, in particular his latest paper "General tensor network decoding of 2D Pauli codes". See below for the abstract, or the full paper at https://arxiv.org/abs/2101.04125. We start at 2pm in zoom: https://ethz.zoom.us/j/362994444. Best, Joe Abstract: In this work we develop a general tensor network decoder for 2D codes. Specifically, we propose a decoder which approximates maximally likelihood decoding for 2D stabiliser and subsystem codes subject to Pauli noise. For a code consisting of $n$ qubits our decoder has a runtime of $O(n\log n+n\chi^3)$, where $\chi$ is an approximation parameter. We numerically demonstrate the power of this decoder by studying four classes of codes under three noise models, namely regular surface codes, irregular surface codes, subsystem surface codes and colour codes, under bit-flip, phase-flip and depolarising noise. We show that the thresholds yielded by our decoder are state-of-the-art, and numerically consistent with optimal thresholds where available, suggesting that the tensor network decoder well approximates optimal decoding in all these cases. Novel to our decoder is an efficient and effective approximate contraction scheme for arbitrary 2D tensor networks, which may be of independent interest.

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