July 2026 - ITP-quantumseminare - lists.phys.ethz.ch

Next weeks QIT seminars
by Ladina Hausmann 30 Jul '26

30 Jul '26

Hi all, Next week we will have two seminars: (1) On Monday Matthias Lotze will talk about “Second order decoupling”. The talk will take place at 15:00 in HIT E41.1. (2) On Wednesday our visitor Samuel Fedida will talk about “A groupoidal approach to quantum reference frames”. The talk will take place at 11:00 in HIT K5. See below for the abstracts. Best, Ladina %%%%%%%%%%%%%%%%% Title: Second order decoupling Abstract: If a quantum system A evolves separately from an environment E, the correlation between them generally decreases. The Decoupling Theorem gives a sufficient criterion for A to become (almost) independent of E. The Decoupling Theorem is limited in that it only applies to one state and one evolution at a time: given a quantum state and a channel, it yields the existence of a unitary that achieves decoupling. Using a version of Lévy's lemma for the unitary group, we find a single unitary that yields decoupling for all channels of a family exponentially large in the dimension of system A. We call this technique second order decoupling, as it relies on controlling not only the average but also the fluctuations of the decoupling function via its Lipschitz constant. We introduce the technique and illustrate it by answering the following question: How many POVMs must a family of measurements contain in order to distinguish quantum states as well as the set of all measurements does? %%%%%%%% Title: A groupoidal approach to quantum reference frames Abstract: We propose a groupoid-based formulation of operational quantum reference frames and apply it to develop relational quantum field theory on curved spacetimes. The starting point is the observation that the usual group-based QRF formalism, while natural on homogeneous spaces of locally compact groups, is not directly suited to generic curved Lorentzian backgrounds, where global symmetry groups are typically absent or too small. The notion of local symmetry should instead be considered within the quantum reference frames formalism. We formulate a notion of quantum reference frame for a continuous groupoid in terms of a Hilbert bundle, a fiber-wise ultraweakly continuous unitary groupoid representation, and a fiber-wise covariant family of positive operator-valued measures. This yields a groupoid relativization map and relational observables. We show that a localization limit recovers the ordinary non-relational description. We prove that the construction reduces to the standard operational QRF formalism for locally compact groups and torsors for suitable groupoids. We review relational quantum field theory in Minkowski spacetime. We then argue that, for curved spacetimes, the correct replacement for the Poincaré group is the Poincaré groupoid of the spacetime, i.e. the gauge groupoid of its orthonormal frame bundle. We also indicate how the framework extends further to internal gauge symmetry and relational quantum gauge field theory via Atiyah groupoids. These results suggest that groupoids provide the natural kinematical language for operational quantum reference frames and relational quantum field theory beyond the homogeneous space setting. This work is a collaboration with Alberto Ibort and Arnau Mas-Dorca.

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No QIT seminar next week
by Ladina Hausmann 23 Jul '26

23 Jul '26

Hi all, There is no QIT seminar planned for next week. Best, Ladina

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Next week's QIT seminar
by Ladina Hausmann 16 Jul '26

16 Jul '26

Hi all, Next week, our visitor Bruno de S.L Torres will talk about “Quantum effects on local observers in an exactly solvable model of quantum gravity”. See below for the abstract. The talk will take place on Thursday at 14:00 in HIT E41.1. Best, Ladina %%%%%%%%%% Title: Quantum effects on local observers in an exactly solvable model of quantum gravity Abstract: Recent developments in quantum gravity have emphasized the role of an "observer" when making sense of quasi-local quantum-gravitational effects. In this talk, we will describe an explicit model of a gravitationally dressed observer in Jackiw-Teitelboim (JT) gravity, an exactly solvable model of gravity in two spacetime dimensions where quantum effects far beyond the semiclassical regime can be studied in detail. We will show how properly accounting for gravitational dressing in JT gravity affects the experience of a local probe crossing the horizon of a black hole, and also study nonperturbative effects on the thermal atmosphere outside the black hole in this model. In particular, we will see that quantum-gravitational effects can allow a particle detector to sense the location of a black hole horizon locally, unlike in semiclassical gravity, and also that nonperturbative effects lead to an explicitly finite entanglement entropy for matter fields in the exterior of a black hole. Based on arxiv:2507.20983 with Thomas Mertens and Thomas Tappeiner, and arxiv:2607.03344 with Victor Franken and Thomas Mertens.

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Next week's QIT seminars
by Ladina Hausmann 09 Jul '26

09 Jul '26

Hi all, Next week we will have two seminars: (1) Finn Holler will talk about “Device Independence Without Nonlocality: An Introduction to Computational Self-Testing”. The talk will take place on Monday at 15:00 in HIT E41.1. (2) Guangzhao Yang will talk about “Geometric Mechanics of Perspective-Neutral Reference Frames”. The talk will take place on Friday at 15:00 in HIT E41.1. See below for the abstracts. Best, Ladina %%%%%%%%%%%%% Title: Device Independence Without Nonlocality: An Introduction to Computational Self-Testing Abstract: How can a classical machine gain command of a quantum system? Building on Bell's theorem from 1964, this question has been extensively studied in quantum foundations, complexity theory, and cryptography. Many approaches rely on demonstrations of quantum nonlocality between spacelike-separated devices, making loophole-free implementations experimentally demanding. Recently, an alternative paradigm has emerged: replacing information-theoretic security with cryptographic assumptions, which removes the need for spacelike separation. In this talk, I will introduce the key ideas underpinning classical control of quantum systems in the nonlocal setting. I will then show how these ideas can be transferred to the cryptographic setting through a compilation procedure, yielding a modular approach to computational self-testing, i.e. certifying the quantum behavior of a black-box device under computational assumptions. Title: Geometric Mechanics of Perspective-Neutral Reference Frames Abstract: In the perspective-neutral approach to quantum reference frames, trivialization maps play a crucial role in transforming canonical variables from the neutral perspective to an internal physical perspective. This thesis re-examines this construction from the viewpoint of geometric mechanics, revealing several mathematical and conceptual subtleties concerning symmetry reduction and the construction of trivialization maps. This thesis resolves these difficulties using geometric mechanics at the classical level, by formulating physical perspectives as local chart descriptions while keeping the underlying geometry fixed. To distinguish physical reference systems from their chart descriptions, we introduce the notions of reference and reference chart as separate concepts, and we generalise these definitions to the setting of classical field theory. Within this framework, passive trivialized charts on the full phase space induce an atlas on the reduced phase space, thereby encoding all physical perspectives. Transformations between them become chart transitions that keep the underlying physical configuration unchanged. Using the theory of Hamiltonian G-systems, we show that Lie algebra representations on the full phase space naturally encode operational freedoms such as scale, and that these representations descend consistently to the reduced phase space. The resulting framework offers a consistent geometric description of changes of inertial origin while preserving the underlying symmetry level set. Explicitly, we construct a family of origin‑changing transformations that encodes all consistent chart transitions on the reduced phase space, thereby exhausting all admissible changes of inertial origin compatible with symmetry reduction. Finally, we discuss how a perspective-neutral phase space may be reconstructed from a collection of internal physical perspectives using theorems on symplectic fibrations, together with the topological obstructions that arise in the non-compact setting.

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No QIT seminar next week
by Ladina Hausmann 05 Jul '26

05 Jul '26

Hi all, There is no QIT seminar planned for next week. Best, Ladina

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