ITP-quantumseminare May 2023

itp-quantumseminare@lists.phys.ethz.ch

1 participants
7 discussions

Tomorrow's QIT Seminar at 4pm in HIT E 41.1
by Ladina Hausmann 29 May '23

29 May '23

Hi all, Tomorrow Anqi Gong will tell us about her semester thesis with Joe entitled "Improved Logical Error Rate via List Decoding of Quantum Polar Codes". See below for the abstract. The talk will take place at 4pm in HIT E 41.1. Best, Ladina Title: Improved Logical Error Rate via List Decoding of Quantum Polar Codes Abstract: The successive cancellation list decoder (SCL) is an efficient decoder for classical polar codes with low decoding error, approximating the maximum likelihood decoder (MLD) for small list sizes. Here we adapt the SCL to the task of decoding quantum polar codes and show that it inherits the high performance and low complexity of the classical case, and can approximate the quantum MLD for certain channels. We apply SCL decoding to a novel version of quantum polar codes based on the polarization weight (PW) method, which entirely avoids the need for small amounts of entanglement assistance apparent in previous quantum polar code constructions. When used to find the precise error pattern, the quantum SCL decoder (SCL-E) shows competitive performance with surface codes of similar size and low-density parity check codes of similar size and rate. The SCL decoder may instead be used to approximate the probability of each equivalence class of errors, and then choose the most likely class. We benchmark this class-oriented decoder (SCL-C) against the SCL-E decoder and find a noticeable improvement in the logical error rate. This improvement stems from the fact that the contributions from just the low-weight errors give a reasonable approximation to the error class probabilities. Both SCL-E and SCL-C maintain the complexity O(LN logN) of SCL for code size N and list size L. We also show that the list decoder can be used to gain insight into the weight distribution of the codes and how this impacts the effect of degenerate errors.

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Tomorrow's QIT Seminar at 4pm in HIT E 41.1
by Ladina Hausmann 22 May '23

22 May '23

Hi all, Tomorrow Marc Sanz Drudis will tell us about his Masterthesis at IBM. See below for the abstract. The talk will take place at 4pm in HIT E 41.1. Best, Ladina Abstract: In this study, we investigate the feasibility of Gibbs state preparation through the application of Variational Quantum Imaginary Time Evolution, in conjunction with Hamiltonian Learning techniques for state validation. The significance of Gibbs states in characterizing equilibrium conditions for intricate stochastic systems, coupled with the inherent challenges in generating such states using classical computing resources, renders this issue a compelling subject for exploration within the realm of quantum computing. The critical analysis of the adherence of realistic state preparations to the prerequisites of various Hamiltonian Learning techniques constitutes a vital step in developing a practical protocol for the preparation of Gibbs states on quantum computing platforms. By examining the compatibility of these two methodologies, we lay the groundwork for the advancement of scalable Gibbs state preparation and verification on quantum computers.

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Special Seminar next week on 17th of May at 4pm
by Ladina Hausmann 17 May '23

17 May '23

Dear all, Hope you are doing well. This is to announce a special seminar next week, as well as two visitors to our group. Maximilian Lock and Veronika Baumann from The Institute for Quantum Optics and Quantum Information (IQOQI), Vienna will be visiting us between the 15th to the 19th of May. We will have a talk by Maximilian Lock at 4pm on Wednesday the 17th of May, which can be attended in-person at HIT E41.1 as well as online via the zoom link: https://ethz.zoom.us/j/64206497803. The title and abstract for the talk are given below. Title: Quantum measurements and equilibration: modelling the emergence of objectivity via entropy maximisation Abstract: Textbook quantum physics features two types of dynamics: reversible unitary dynamics and irreversible measurements. The latter stands in conflict with the laws of thermodynamics and has evoked debate on what actually constitutes a measurement. With the help of modern quantum statistical mechanics, we take a first step in formalising the hypothesis that quantum measurements are instead driven by the natural tendency of closed systems to maximise entropy, a notion that we call the Measurement-Equilibration Hypothesis. In this paradigm, we investigate how objective measurement outcomes can emerge within an purely unitary framework, and find that: (i) the interactions used in standard measurement models fail to spontaneously feature emergent objectivity and (ii) while ideal projective measurements are impossible, we can (for a given form of interaction) approximate them exponentially well as we collect more physical systems together into an "observer system". We thus lay the groundwork for self-contained models of quantum measurement, and conclude by proposing some improvements to our simple scheme. Hope to see you there! Best regards, Vilasini

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Tomorrow's QIT Seminar at 4pm in HIT E 41.1
by Ladina Hausmann 15 May '23

15 May '23

Hi all, Tomorrow Tenzan Araki will tell us about their Masterthesis at IBM entitled "Simulating open quantum systems via noise engineering on superconducting quantum computers". See below for the abstract. The talk will take place at 4pm in HIT E 41.1. Best, Ladina Title: Simulating open quantum systems via noise engineering on superconducting quantum computers Abstract: While quantum gates in a gate-based quantum computer are unitary, quantum computers can also be used to simulate open quantum systems, enabling the description of real-life quantum systems that interact with an environment. Despite the importance of understanding open quantum systems, their classical simulation is generally harder than that of closed quantum systems. Therefore, using quantum computers to simulate open quantum system dynamics is considered a possible use case to demonstrate practical quantum advantage. To this end, studies have commonly combined unitary quantum gates with non-unitary operations that utilize ancilla qubits and measurements to simulate the irreversible dissipative dynamics. However, the noise that is present in today’s quantum computers remains an obstacle that limits the fidelity of such simulation. Independently, a line of work known as "noise engineering" attempts to perform tasks on quantum systems through engineering the noisy environment coupled to them. Instead of attempting to protect qubits, this approach accepts the presence of noise and tries to utilize it as a resource. In this work, we simulate the dynamics of an open quantum system by an ancilla-free noise engineering approach. Instead of mimicking the environment with ancilla qubits, the noise spectral function acted on each qubit are controlled through fluctuations in the photon number of the readout resonator coupled to the qubit. An open quantum system is then simulated by Trotterization, similarly to how many closed quantum system dynamics are simulated, but with additional non-unitary gates corresponding to the dissipative channels that are controlled through noise engineering. In this talk, we introduce the theory behind this implementation using the dissipative Ising model as an example, and discuss progress in hardware demonstrations on the superconducting quantum devices provided by IBM Quantum.

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Tomorrow's QIT Seminar at 15:45 (!!) in HIT E 41.1
by Ladina Hausmann 08 May '23

08 May '23

Hi all, Tomorrow Lukas Schmitt will tell us about his Masterthesis with Vilasini entitled 'Operational causality and realizable processes in the Page-Wootters framework'. See below for the abstract. The talk will take place at 15:45 in HIT E 41.1. Best, Ladina Title: Operational causality and realizable processes in the Page-Wootters framework Abstract: Causality is a fundamental concept in science, but its definition varies across different disciplines. In quantum information theory, causality is associated with the flow of information, while in relativity, it is linked to spacetime geometry. Inspired by quantum gravity, the process matrix framework was proposed to describe indefinite causal structures in the information theoretic sense. However, the physical interpretation and realizability of general process matrices remain open questions. To address such questions, this work explores the set of process matrices realizable in the Page-Wootters (PW) framework by formulating operational notions of causality there. The PW framework is commonly used in quantum gravity models and describes quantum dynamics relative to a clock which is itself a quantum system. We investigate how operational causality notions such as signaling can be formalized in terms of PW history states and whether relativistic causality violations such as backwards in time signaling are possible in the PW framework. Our main contributions include defining signaling in PW states, developing a visual representation of PW states, finding constraint operators of PW states of causally indefinite processes, and characterizing the class of realizable processes in the PW framework under monotonically increasing clocks. Furthermore, we explore the consequences of backward ticking clocks and investigate their signaling structure. Non-causal processes such as the Lugano process can be realized in such a setting, but backward ticking clocks lead to normalization issues that challenge a physical interpretation.

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Visitor Talk at 4pm in HIT E 41.1
by Ladina Hausmann 03 May '23

03 May '23

Hi all, On Thursday at 4pm in HIT E 41.1 our visitor Stefan Ludescher will give a talk on 'Theory-independent randomness generation with spacetime symmetries’. See below for the abstract. Best, Ladina Title: Theory-independent randomness generation with spacetime symmetries Abstract: We introduce a class of semi-device-independent protocols based on the breaking of spacetime symmetries. In particular, we characterise how the response of physical systems to spatial rotations constrains the probabilities of events that may be observed: in our setup, the set of quantum correlations arises from rotational symmetry without assuming quantum physics. On a practical level, our results allow for the generation of secure random numbers without trusting the devices or assuming quantum theory. On a fundamental level, we open a theory-agnostic framework for probing the interplay between probabilities of events (as prevalent in quantum mechanics) and the properties of spacetime (as prevalent in relativity).

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This weeks QIT seminar at 16:00 in HIL E6 (different place!)
by Ladina Hausmann 01 May '23

01 May '23

Hi all, Tomorrow Carla Ferradini will tell us about her master thesis with V. Vilasini, Giulia Mazzola and Victor Gitton entitled 'Connecting Tensor Networks to Quantum Causal Models with applications to holography'. See below for the abstract. The talk will take place at 4pm in HIL E 9. Best, Ladina Title: Connecting Tensor Networks to Quantum Causal Models with applications to holography Abstract: Causal modelling aims to describe correlations arising from causal structures providing an information-theoretic notion of causation which is a priori independent on space-time. Conversely, in relativity and quantum field theory, the causal structure is considered to be an essential property of space-time, induced by well-defined light cones. In quantum gravity, space-time cannot be regarded as a fixed classical background and may itself be subject to quantum effects. Therefore, it was suggested that space-time might emerge from many-body quantum correlations. Tensor networks provide a framework to study this hypothesis and allow for a definition of quantum causal influence, which does not assume any pre-defined time slicing. In this work, we propose a connection between the frameworks of causal models and tensor networks which generalises the former and provides an operational meaning to the latter. Our approach is motivated by the similarity between the notions of causal influence in tensor networks and signalling in causal models. Firstly, we generalise quantum causal models in a tensor network-inspired manner and we provide a generalised definition of signalling and interventions. This allows us to define three notions of signalling currently used in literature and show their inequivalence. Then, we provide mappings from tensor networks to causal models and vice versa, showing explicitly a correspondence between signalling and causal influence. Our work lays the foundations for a connection between causal models and tensor networks which would enable an exchange of techniques between the different research fields and could serve as a starting point for novel studies on the emergence of space-time from operational properties of causal models.

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ITP-quantumseminare May 2023