November 2022 - ITP-quantumseminare

Tomorrow's QIT Seminar 2pm in HIT E 41.1 & zoom
by Ladina Hausmann 28 Nov '22

28 Nov '22

Hi all, Tomorrow our visitor Patrik Potts will tell us about his work on 'Optical coherent feedback control of a mechanical oscillator'. See below for the abstract. The talk will take place at 2pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444. Best, Ladina Optical coherent feedback control of a mechanical oscillator We present the theoretical description and experimental realization of an optical coherent feedback platform to control the motional state of a nanomechanical membrane in an optical cavity. The coherent feedback loop consists of a light field interacting twice with a mechanical oscillator in different cavity modes. Tuning the optical phase and delay of the feedback loop allows us to control the motional state of the mechanical oscillator, its frequency shift and damping rate, which we use to cool the membrane close to the ground state. In the optimal cooling conditions, we derive an expression for the minimal number of phonons and show that this new technique enables ground state cooling. Experimentally, we show that we can cool the membrane to a state with 4.89 ± 0.14 phonons (480 μK) in a 20 K environment. This lies below the theoretical limit of dynamical backaction cooling in the unresolved sideband regime. The described feedback scheme is very versatile and could be implemented in various optomechanical systems.

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Tomorrow's QIT Seminar 2pm in HIT E 41.1 & zoom
by Ladina Hausmann 20 Nov '22

20 Nov '22

Hi all, Tomorrow Vilasini will tell us about her work on 'Embedding cyclic causal structures in acyclic spacetimes: no-go results for process matrices'. See below for the abstract. The talk will take place at 2pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444. Best, Ladina %%%%%%%%% Title: Embedding cyclic causal structures in acyclic spacetimes: no-go results for process matrices V. Vilasini and Renato Renner, based on https://arxiv.org/abs/2203.11245 Abstract: Causality can be defined in terms of a space-time structure or based on information-theoretic structures, which correspond to different notions of causation. The process matrix framework describes quantum indefinite causal structures in the information-theoretic sense, but the physicality of such processes remains an open question. At the same time, there are several experiments in Minkowski spacetime (which gives a definite spacetime notion of causality) that claim to implement indefinite information-theoretic causal structures, suggesting an apparent tension between these notions. To address this, we develop a general framework that disentangles these two notions and characterises their relationship in scenarios where quantum systems are not necessarily localised in spacetime. Formulating (possibly cyclic) quantum causal structures in terms of a composition of quantum maps through feedback loops, we proceed to describe their embedding in acyclic background spacetimes. Then relativistic causality takes the form of a compatibility condition between the information-theoretic and spacetime causal order relations. Connecting the process matrix formalism to cyclic causal structures with a focus on causal relations that can be inferred operationally through agents’ interventions, we derive a number of no-go results for physical realisations of process matrices in a spacetime. In particular, this reveals that it is impossible to physically realise indefinite causal order processes with spacetime localised systems. Further, we show that any realisation of an indefinite causal order process respecting relativistic causality in a background spacetime ultimately admits a fine-grained description in terms of a definite acyclic information-theoretic causal structure that is consistent with the light-cone structure of the spacetime. This resolves the apparent tension between the two causality notions. Finally, we discuss the operational meaning of indefinite causal structures in light of our results.

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No QIT seminar today
by Joe Renes 15 Nov '22

15 Nov '22

Hi all, Apologies for the late notice, but there will be no QIT Seminar today. Best, Joe

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Tomorrow's QIT Seminar 2pm in HIT E 41.1 & zoom
by Hausmann Ladina 07 Nov '22

07 Nov '22

Hi all, Tomorrow Lukas Brenner will tell us about their Semesterthesis with Philipp Kammerlander entitled 'Comparing Entropies via Erasure Processes on Multiple-Conserved-Quantity-Memories'. See below for the abstract. The talk will take place at 2pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444. Best, Ladina %%%%%%% Abstract: The relevance of memories in the context of thermodynamics was illustrated by their appearance in Szilard’s engine as a solution to Maxwell’s demon paradox. We formulize the notion of memories in the language of the Kammerlander framework, which provides the possibility to talk about thermodynamics of single and multiple conserved quantities. Memories are introduced as a family of thermodynamic systems which allow for generalized erasure processes i.e. resetting a memory to a reference state is achievable outside the realm of energy conservation. Such processes can be exploited in order to compare entropies induced by different conserved quantities. We demonstrate that a universal ratio of entropies is established when relating optimal erasure processes of memories.

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