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Thanks a lot to Gianmichele Blasi and Géraldine Haack from Uni Geneva, Yuri Minoguchi and @entangledanarchist.bsky.social from Vienna for the amazing collaboration where we combined tools from quantum transport, condensed matter and thermodynamics to arrive at this cool result!

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This phenomenon is rooted in the spectral rigidity also encountered in random matrix theory, and ubiquitous in fermionic systems. Thus, rigidity of the resulting time signal in this clock is remarkably robust: thermal noise, disorder, and finite size effects only perturbatively affect the scaling.

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Therefore, when two excitations (ticks) are too close to each other, the following excitation arrives a bit later, correcting for the previous error. This leads to an exponentially reduced variance growth compared to independent errors.

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For independent errors, the timing variance usually grows linearly in time. Here, the fermionic excitations are intrinsically correlated due to the Pauli statistics. This prevents excitations from clustering too closely together but also from being too far apart.

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Nicolai Friis @improbabilitydrive.bsky.social · 20/08/2025

From one tick to the next, small timing uncertainties in clocks usually add up because of the independence of errors. Using fermions tunneling through a quantum wire as clock ticks, we found a way to build a clock where errors cancel out rather than accumulate. arxiv.org/abs/2601.10785

Chain of quantum two level systems. Excitations traveling from left to right, each time marking a tick when they leave on the right.

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Glad someone took the time to debunk the (obviously nonsensical) claim that Bell inequalities could be violated w/o entanglement. It is astonishing (to say the least) that this claim was made by experienced authors & that it passed peer review in the first place, does not reflect well on the journal

arxiv.org

Bell Inequality Violations Without Entanglement? It's Just Postselection

Recently Wang et al. have reported a violation of a Bell inequality without entanglement [arXiv:2507.07756]. We show that their result is an artifact of postselection. It is well known that postselect...

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Henrik Wilming @arrr.de · 23/07/2025

A fun project with Seok Hyung, Jeongrak ( @perp-waterfall.bsky.social ), @nellynghy.bsky.social , and Paul Boes. We dusted off some old notes because there seemed to be renewed interest in what differentiates thermal operations from Gibbs-preserving maps. arxiv.org/abs/2507.16637

arxiv.org

Thermal operations from informational equilibrium

Thermal operations are quantum channels that have taken a prominent role in deriving fundamental thermodynamic limitations in quantum systems. We show that these channels are uniquely characterized by...

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Florian Meier @mathsmire.bsky.social · 15/07/2025

time-reversal with indefinite causal order

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Florian Meier @mathsmire.bsky.social · 13/06/2025

Congrats to Jake and all the collaborators for this fantastic work. They take the well studied problem of cooling quantum systems and use cool mathematics from group and graph theory to systematically characterize and solve that problem.

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Florian Meier @mathsmire.bsky.social · 11/06/2025

In that sense, I'd put thermo on a level above, a meta-theory.

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Florian Meier @mathsmire.bsky.social · 11/06/2025

Good question! Well, I think it would indeed be more appropriate to put thermo on a different level of the hierarchy. Thermo is based on very general principles and those are usually fulfilled by microscopic theories which is why we see emergent thermodynamic phenomena in QTFs or also GR.

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Thanks and congrats of course too 😀 Will be a volume with exciting theory

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The beautiful artistic illustration in the first post is by Alexander Rommel (www.aerroscape.de), copyright by Alexander Rommel / TU Wien. I also thank my fantastic collaborators from @tuwien.at, @iqoqi-vienna.bsky.social, Chalmers and Malta from the @aspects-quantum.bsky.social consortium!

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What this project also shows in my opinion is how important it is to create an informal, inviting and friendly discussion culture in science where also junior people happily express their opinions and thoughts; and how such an environment can seed exciting scientific projects. go.nature.com/4jWxUm7

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Here, you can see an animation of the excitation travelling around a ring of 50 sites (y-axis = occupation probability, x-axis = where).

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For example with quantum clocks it is possible to circumvent such limitations. The ring-clock does this by counting cycles completed by a single quantum particle traveling a ring. Since coherent transport is dissipation-free, this clock breaks the linear entropy-precision bound exponentially.

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Generally, the more accurate a clock is, the more entropy it produces. For a wide range of classical stochastic clocks this is proved by the so-called "Thermodynamic Uncertainty Relation", which provides a linear relationship between clock precision and entropy.

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ASPECTS @aspects-quantum.bsky.social · 28/04/2025

What started as informal discussions on the Danube beach in Vienna during QTD2023 has been published at last. Trying to come up with an experimentally feasible quantum clock we came across a rich model – the ring clock – that overturned a common wisdom in the community. doi.org/10.1038/s415...

Copyright by Alexander Rommel / TU Wien.

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Don't forget to submit your abstract for our conference in Dublin, August 2025! www.aspects-quantum.com/flqt2025

aspects-quantum.com

FLQT 2025 — ASPECTS

Fundamental Limits of Quantum Technologies conference, Dublin, 25-29 August 2025

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Mark Mitchison @mitchison.bsky.social · 07/04/2025

We are excited for Fundamental Limits of Quantum Technologies conference, Dublin, 25-29 August. Only three weeks left until the abstract submission deadline, don't delay! All details at www.aspects-quantum.com/flqt2025

aspects-quantum.com

FLQT 2025 — ASPECTS

Fundamental Limits of Quantum Technologies conference, Dublin, 25-29 August 2025

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Jake Xuereb @curlyqubit.bsky.social · 31/03/2025

Sms alot of people wanna go to Benasque QI than one can fit in the beautiful tiny village. Having organised a bunch of stuff in Malta, I feel the urge to organise a straggler event on the island of Gozo. If interested, fill this form so I can gauge interest - share! forms.gle/azwrJe8AidrS...

forms.gle

Gozo QI - Expression of Interest

I really like no timetable free discussion workshops. It seems like the Benasque Quantum Information Workshop has gotten too many applicants to cater for the demand. Having organised a couple confer...

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Mark Mitchison @mitchison.bsky.social · 25/03/2025

We’re organising a conference in Dublin! Fundamental Limits of Quantum Technologies 2025 — everything from error mitigation on NISQ devices, through control constraints on complex quantum systems, to quantum thermodynamics and energetics, and much more! Abstract submission and registration open! 👇

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Philip Taranto 🍋 @tarantophilip.eurosky.social · 14/03/2025

Wrote up a little tutorial + review on higher-order quantum operations. If you love quantum channels, you can think of these as linear maps that act on channels themselves in manner fully consistent with quantum theory! I hope it helps future students / researchers! 🤗 arxiv.org/abs/2503.09693

arxiv.org

Higher-Order Quantum Operations

An operational description of quantum phenomena concerns developing models that describe experimentally observed behaviour. $\textit{Higher-order quantum operations}\unicode{x2014}$quantum operations ...

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Mark Mitchison @mitchison.bsky.social · 04/02/2025

Latest preprint from the ASPECTS consortium! arxiv.org/abs/2502.00096 Experiments by the amazing @naresgroup.bsky.social and theory by @mathsmire.bsky.social of @quitphysics.info We measured the entropy cost of generating and recording ticks in a microscopic clock 1/6 🧵👇

Depiction of the quantum-classical transition in a microscopic clock. A quantum clock transitions between three discrete states, driven by thermal noise. These transitions are amplified by a sensor into a classical signal, which can be shared among classical observers as an objective record of the passage of time.

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Wow, so beautiful. And very cool thesis cover!

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... and here are the pendula themselves

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... and here the pendulum center of mass

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Haha, it seems I can not upload Gifs properly 😅 Let's hope this works: here is the 'coolest' animation ...

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a) Ignorance about microscopic degrees of freedom and b) a past hypothesis saying you start in a low entropy state. Combined, the assumptions lead to entropy increase over time. For the pendulum, a) is given by the compass needle only seeing the 'center of mass' and b) by the initial configuration.

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In our work, we asked whether you can derive a similar statement about entropy, that is, in which sense does entropy in an isolated quantum system increase over time, even when the underlying evolution is unitary. The answer lies with two key assumptions:

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A more quantum version of this insight has been first formalized by Reimann's doi.org/10.1103/Phys..., after which there have been many amazing works exploring how expectation values can equilibrate in quantum systems, even when the underlying evolution is fully unitary and thus reversible.

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If we now imagine that in the top right of the setup, you have a compass needle pointing towards the center of mass of the collection of pendulum masses (say the masses are magnetized), the compass needle will have a 'quite random' behavior, as the animations show, but it stays somewhat centered.

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Think of a collection of mathematical pendula, all swinging with a different frequency. The time-evolution of this system in the idealized setting without friction is fully reversible.

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