Alexander Jahn

Junior research group leader in Berlin. Working in the borderlands of quantum information, condensed matter physics, and string theory.

[Alexander Jahn @PhysicistAlex]

Stock-based net worth is such a strange concept, since Elon actually selling many shares would quickly cause the
rest to lose value.

Someone should go full resource theory on this and compute the "extractable net worth" this corresponds to.

[The New York Times @nytimes]

Breaking News: Elon Musk became the world’s first trillionaire as SpaceX shares soared above $150 on its first day of trading. https://t.co/bq5ARi7rgB

[Alexander Jahn @PhysicistAlex]

@jenseisert That outfit looks familiar... You don't happen to have a grumpy neighbor named Paschulke?

[Alexander Jahn @PhysicistAlex]

Ironically this is the exact opposite of the current situation, in which one is effectively stuck *inside* the US while applying for a Green Card on a temporary visa.

[Alexander Jahn @PhysicistAlex]

This - perhaps intentionally - would make tenure-track positions in the US extremely unattractive to foreigners.

In order to get a permanent *visa*, a foreigner on such a position would have to leave the US, thus forfeiting the position.

[Homeland Security @DHSgov]

An alien who is in the U.S. temporarily and wants a Green Card must return to their home country to apply.

This policy allows our immigration system to function as the law intended instead of incentivizing loopholes.

The era of abusing our nation’s immigration system is over.

[Alexander Jahn @PhysicistAlex]

Such a European response: "Oh no, we get so many proposals for amazing science! How can we possibly handle the workload this is causing us?"

Shouldn't we be positive about lots of young researchers having great ideas they want to realize at EU universities?

[European Research Council (ERC) @ERC_Research]

The number of grant applications is rising sharply. Our capacity for their evaluation isn’t.

ERC President Maria Leptin explains why stricter resubmission limits are being introduced for 2027 calls and what this mean for applicants.
https://t.co/YavrW4P8FJ

[Alexander Jahn @PhysicistAlex]

Gave a talk at the University of Vienna today, in a room once used by the great logician Kurt Gödel!

However, my treatment of the subject - holographic fault tolerance - remains incomplete.

[Alexander Jahn @PhysicistAlex]

Berlin's pubs are already adjusting to new geopolitical realities:

"Beer now cheaper than gasoline -
Don't drive away, drink and stay!"

[Alexander Jahn @PhysicistAlex]

@pepel_klaasa They couldn't even spell Johns Hopkins properly.

[Alexander Jahn @PhysicistAlex]

Korean sandwich engineering has remarkably tight margins.

[Alexander Jahn @PhysicistAlex]

In Kyoto this week to present our joint work at the "Extreme Universe 2025" conference. @ExUniverseja

A welcome opportunity to visit this beautiful city again!

[Alexander Jahn @PhysicistAlex]

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

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

[ExtremeUniverse @ExUniverseja]

学変A「極限宇宙」主催のExU2025国際研究会も、残り二日になりました。
講演ビデオは以下のサイトから閲覧できますので、ご活用ください。
https://t.co/oZKHUestGK

[Alexander Jahn @PhysicistAlex]

@QuantumMemeing You'll know when she ghosts you after you mention your algorithm idea relies on Grover speedups.

[Alexander Jahn @PhysicistAlex]

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]

For more details, you'll have to read our paper! As always, many thanks for the support of @qt__berlin for our work at @PhysikFUBerlin.
https://t.co/pyeBo9b9SN

[Alexander Jahn @PhysicistAlex]

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

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

[Jens Eisert @jenseisert]

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.

https://t.co/rcy1CJUVnI

In detail, recent years have enjoyed substantial progress in capturing properties of complex quantum systems by means of #randomtensornetworks (RTNs), which form ensembles of quantum states that depend only on the tensor network geometry and bond dimensions. Of particular interest are RTNs on hyperbolic geometries, with local tensors typically chosen from the unitary Haar measure, that model critical boundary states of holographic #bulkboundary dualities.

In this work, we elevate static pictures of ensemble averages to a dynamical one, to show that RTN states exhibit equilibration of time-averaged operator expectation values under a highly generic class of Hamiltonians with non-degenerate spectra. We prove that RTN states generally equilibrate at large bond dimension and also in the scaling limit for three classes of geometries: Those of #matrixproductstates, regular #hyperbolictilings, and single "black hole" tensors. Furthermore, we prove a hierarchy of #equilibration between finite-dimensional instances of these classes for bulk and boundary states with small entanglement.

This suggests an equivalent hierarchy between corresponding many-body phases, and reproduces a holographic degree-of-freedom counting for the effective dimension of each system. These results demonstrate that RTN techniques can probe aspects of late-time dynamics of quantum many-body phases and suggest a new approach to describing aspects of holographic dualities using techniques from statistical mechanics.

Warm thanks to Shozab Qasim and @PhysicistAlex for the fruitful collaboration. And thanks to our funders, specifically at the @dfg_public (#CRC183 and #FOR2724), and the @ERC_Research.

[Alexander Jahn @PhysicistAlex]

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