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Alexander Jahn @physicistalex.bsky.social · 5d

This week, we're in beautiful Kraków for a conference on tensor networks and all their applications. My PhD students Dimitris and Lev already gave amazing talks about discrete-holographic boundary symmetries and von Neumann algebras in holographic codes!

Alexander Jahn @physicistalex.bsky.social · 19d

Or dare we say... Engineering? 😬

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Alexander Jahn @physicistalex.bsky.social · 29d

You can tell that the #QIP2026 deadline has not yet passed, since @zoltanzimboras.bsky.social has not given word on his submission yet.

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Reposted by Alexander Jahn

Sean Carroll @seanmcarroll.bsky.social · Sep 5

Postdoc job! I expect to have an opening at Johns Hopkins for a postdoctoral researcher working somewhere in the broad realms of physics, philosophy, and complexity. Apply at Academic Jobs Online:

academicjobsonline.org/ajo/jobs/30496

Johns Hopkins University, Physics and Astronomy

Job #AJO30496, Postdoctoral Fellow in Foundations of Physics, Complexity, and Emergence, Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, US

academicjobsonline.org

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Alexander Jahn @physicistalex.bsky.social · Aug 31

Thanks Zoltan! You should petition the museum to add some hyperbolic tilings as well, there's plenty of material in our papers. 😁

Alexander Jahn @physicistalex.bsky.social · Aug 29

It would be a lost opportunity if they didn't call it the Ministry of Magic (state distillation).

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Reposted by Alexander Jahn

Francesco Arzani @frarzani.bsky.social · Aug 26

Looking for a postdoc to work on bosonic quantum error correction!
Join me and the QAT team at ENS & INRIA Paris — flexible start date.
Details here 👉 recrutement.inria.fr/public/class... or feel free to reach out!

Post-Doctoral Research Visit F/M Senior postdoctoral researcher in bosonic quantum error correction

Offre d'emploi Inria

recrutement.inria.fr

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Alexander Jahn @physicistalex.bsky.social · Aug 25

For more details, you'll have to read our paper! As always, many thanks for the support of Berlin Quantum for our work at @freieuniversitaet.bsky.social.
arxiv.org/abs/2103.02634

Emergent statistical mechanics from properties of disordered random matrix product states

The study of generic properties of quantum states has led to an abundance of insightful results. A meaningful set of states that can be efficiently prepared in experiments are ground states of gapped ...

arxiv.org

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Alexander Jahn @physicistalex.bsky.social · Aug 25

This suggests a deep relationship between equilibration strength and entanglement phases in many-body quantum systems! The main idea: More entanglement = stronger equilibration.

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Alexander Jahn @physicistalex.bsky.social · Aug 25

For the condensed-matter theorists among you, our work also leads to an interesting conjecture: RTNs on different geometries describe different phases of entanglement scaling. We show that D_eff follows a sharp hierarchy between area- and volume-law phases.

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Alexander Jahn @physicistalex.bsky.social · Aug 25

This means that random tensor networks know a lot more about holographic dynamics than we expected, and may be able to hold more insights into (holographic) quantum gravity.

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Alexander Jahn @physicistalex.bsky.social · Aug 25

And surprisingly, the result matches gravitational degree-of-freedom counting in holography: If we "fuse" tensors together, i.e., replace part of the bulk geometry by a "black hole", D_eff always *increases*. Just as in gravity, where a black hole is the highest-entropy state!

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Alexander Jahn @physicistalex.bsky.social · Aug 25

This brings us to holography: For holographic RTNs, we can now compute the minimum effective dimension D_eff that describes late-time dynamics! From the geometry and bond dimension of the RTN alone, we can determine how complex its dynamics must be.

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Alexander Jahn @physicistalex.bsky.social · Aug 25

Now here's the kicker: For random ensembles, we can strictly lower-bound D_eff *without knowing H*! In a sense, the randomness cancels out its exact eigenstate structure. This is a trick we learned from Haferkamp et al., who used it on random MPS:
arxiv.org/abs/2103.02634

Emergent statistical mechanics from properties of disordered random matrix product states

The study of generic properties of quantum states has led to an abundance of insightful results. A meaningful set of states that can be efficiently prepared in experiments are ground states of gapped ...

arxiv.org

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Alexander Jahn @physicistalex.bsky.social · Aug 25

The key quantity to describe the strength of equilibration is the "effective dimension" D_eff, which basically counts how many (energy) states are needed to describe late-time dynamics.

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Alexander Jahn @physicistalex.bsky.social · Aug 25

Here's how it works: In a quantum system, expectation values of observables fluctuate. At late times, even a pure state will *equilibrate*, meaning that local expectation values will fluctuate within a fixed window. This happens for all Hamiltonians H with "non-degenerate gaps".

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Alexander Jahn @physicistalex.bsky.social · Aug 25

In our paper, we bring in ideas from quantum statistical mechanics to show that the opposite is true: Thanks to the randomness in RTNs, we can probe late-time dynamics without knowing the explicit Hamiltonian! The key concept that enables this is called *equilibration*.

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Alexander Jahn @physicistalex.bsky.social · Aug 25

That makes choosing a Hamiltonian that performs time evolution on the boundary difficult: Any choice, e.g. motivated from AdS/CFT arguments, would time-evolve different RTN samples differently. Thus, it seemed that randomness made time evolution impossible to describe!

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Alexander Jahn @physicistalex.bsky.social · Aug 25

This sparked hundreds of follow-up papers - many of which refined the original proposal - but there was one limitation: Random tensor networks (RTNs) produce an *ensemble* of states, with every random sample looking quite different locally.

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Alexander Jahn @physicistalex.bsky.social · Aug 25

Some background: In a seminal paper from 2016, Hayden et al. showed that tensor networks with locally random tensors, if put on a hyperbolic geometry, reproduce quantum states that very closely resemble boundary states of the AdS/CFT duality.
arxiv.org/abs/1601.01694

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Alexander Jahn @physicistalex.bsky.social · Aug 25

Very happy to have this paper with @jenseisert.bsky.social and his PhD student Shozab Qasim out on the @arxiv.bsky.social!

It achieves something that, until recently, I thought to be impossible: To use random tensor networks to study holographic *dynamics*.

Jens Eisert @jenseisert.bsky.social · Aug 25

Random #tensornetworks provide a powerful framework for probing and understanding complex quantum systems, especially in regimes where conventional tools fail. Here, we rigorously investigate dynamical properties of holographic toy models.

scirate.com/arxiv/2508.1...

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Alexander Jahn @physicistalex.bsky.social · Aug 16

They've obviously been best friends for years, I don't know why this is so hard for the media to acknowledge.

The Bulwark @thebulwark.com · Aug 15

Trump claps for Putin as he arrives in Alaska.

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Reposted by Alexander Jahn

Matt Green @mattgreencomedy.com · Aug 16

At least Chamberlain got a piece of paper

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Alexander Jahn @physicistalex.bsky.social · Aug 13

Back from an exciting week visiting the great @zoltanzimboras.bsky.social in Budapest!

As you can see, I was also very busy pensively staring at Platonic solids at the Hungarian National Museum.

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Alexander Jahn @physicistalex.bsky.social · Aug 11

A big thanks to my amazing collaborators from TU Delft, U Queensland, and Okinawa's OIST!

And of course, we're always grateful for the local support from FU Berlin @freieuniversitaet.bsky.social and Berlin Quantum.

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