iiinova — where your digital self keeps evolving

Alexander Jahn @physicistalex.bsky.social · 01/10/2026

Exactly, the legendary chalk that's nowadays made solely in Korea! After years of office whiteboards, I feel like I'm going back to an old artisanal craft...

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Alexander Jahn @physicistalex.bsky.social · 01/10/2026

With a beautiful view and a full box of hagoromo in my new office, I'm off to a perfect start at APCTP in Pohang, South Korea!

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Alexander Jahn @physicistalex.bsky.social · 13/08/2026

Politics in Berlin is such a mess that one could call it a... mission impossible?

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Alexander Jahn @physicistalex.bsky.social · 12/08/2026

The resolution is to assume "black hole complementarity": Rather than Hawking modes being entangled with the interior, both live in the *same* Hilbert space!

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Alexander Jahn @physicistalex.bsky.social · 12/08/2026

Adding gravity to quantum degrees of freedom affects their spatial independence: A qubit at point X no longer perfectly commutes with a qubit at a different point Y. We previously suggested that this is described by the quantum information notion of overlapping qubits: arxiv.org/abs/2304.02673

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Alexander Jahn @physicistalex.bsky.social · 12/08/2026

Three years ago, we proposed "overlapping qubits" as a description of information encoding in quantum gravity. In our new paper with Charles Cao's group at @virginiatech.bsky.social, we finally apply overlapping qubits to evaporating black holes! arxiv.org/abs/2608.10085

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Alexander Jahn @physicistalex.bsky.social · 26/07/2026

Big news: I'll be moving to APCTP in Pohang, South Korea, starting a new group in October! I'll be hiring two postdocs on a range of topics from high-energy to quantum information theory, collaborating across the Asia-Pacific region and beyond. Deadline Aug 12. More: www.apctp.org/bbs/board.ph...

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Alexander Jahn @physicistalex.bsky.social · 13/04/2026

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.

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Alexander Jahn @physicistalex.bsky.social · 25/03/2026

Why does this charging station remind me of a certain American president?

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Alexander Jahn @physicistalex.bsky.social · 16/03/2026

Berlin's pubs are already adjusting to new geopolitical realities: "Beer now cheaper than gasoline - Don't drive away, drink and stay!"

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Alexander Jahn @physicistalex.bsky.social · 07/10/2025

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!

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Alexander Jahn @physicistalex.bsky.social · 25/08/2025

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 · 25/08/2025

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 · 25/08/2025

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 · 25/08/2025

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 · 25/08/2025

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 · 13/08/2025

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 · 11/08/2025

We look at different error models - erasures, depolarizing, and biased-Pauli - and find *thresholds* for all of them: If you scale up the code, errors below a certain rate get suppressed arbitrarily well!

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Alexander Jahn @physicistalex.bsky.social · 11/08/2025

These codes should be easier to run on a quantum device, but are they actually any good? That's what we find out in our paper! Turns out that they form *subsystem codes* in which your bulk states can be (partially) gauge-fixed, protecting the remaining qubits against errors.

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Alexander Jahn @physicistalex.bsky.social · 01/08/2025

Even before being replaced by AI, the jobs of American comedians have apparently been made obsolete.

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Alexander Jahn @physicistalex.bsky.social · 24/07/2025

However, there's a caveat: You can build an MQA with any aperiodic sequence. But these will usually not correspond to consistent layers in a hyperbolic tiling - they don't have a proper "dual" bulk geometry. And indeed, we find that they don't produce CFT-like features!

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Alexander Jahn @physicistalex.bsky.social · 24/07/2025

One ansatz class that 𝘥𝘰𝘦𝘴 lead to CFT-like phases - we call it MQA=multiscale quasicrystal ansatz - superimposes the symmetries of all bulk layers of a hyperbolic tiling. The resulting disorder appears to preserve critical properties when applied to a non-disordered system.

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Alexander Jahn @physicistalex.bsky.social · 24/07/2025

Some background: When discretizing hyperbolic bulk spaces (e.g. by tensor networks), one gets critical boundary states with peculiar 𝘲𝘶𝘢𝘴𝘪𝘱𝘦𝘳𝘪𝘰𝘥𝘪𝘤 symmetries, a property first described properly in arxiv.org/abs/1805.02665:

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Alexander Jahn @physicistalex.bsky.social · 24/07/2025

The first paper with my PhD student Dimitris Saraidaris is now published in Quantum! We explore the symmetries of discrete-holographic models and find that they generically produce critical phases in spin systems - but only when the symmetries describe a "dual" bulk geometry!

Building a critical Hamiltonian from a Multi-scale Quasicrystal Ansatz (MQA).

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Alexander Jahn @physicistalex.bsky.social · 27/06/2025

Very proud of my PhD student Lev Shaposhik for winning a poster prize at QIQG25 at @perimeterinstitute.ca! He's the main driver behind our work on von Neumann algebras in infinite tensor networks from April, see thread below.

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Alexander Jahn @physicistalex.bsky.social · 25/06/2025

POV: You think you're attending a quantum gravity conference, but it's just a Lenny Susskind birthday party in disguise.

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Alexander Jahn @physicistalex.bsky.social · 23/06/2025

At Perimeter Institute in Waterloo for QIQG25 this week! Lenny Susskind started his opening talk on de Sitter holography in classic Lenny Susskind style: "Hello children."

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Alexander Jahn @physicistalex.bsky.social · 11/05/2025

Prepared a papal dish this Sunday.

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Alexander Jahn @physicistalex.bsky.social · 03/05/2025

The incoming chancellor Merz used to be one of Germany's most pro-US politicians. By embracing the AfD, a far-right competitor to Merz's conservative CDU, Trump's administration has alienated yet another ally.

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Alexander Jahn @physicistalex.bsky.social · 23/04/2025

In the countryside of Kyotango, central Japan.

Cherry blossoms at the top of rice field terraces.

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Alexander Jahn @physicistalex.bsky.social · 23/04/2025

Back in 2021, @jenseisert.bsky.social and I wrote a review on holographic tensor-network codes that just reached 100 citations! Glad to see this field being of interest to a growing crowd. Perhaps we'll soon need to write an updated version? arxiv.org/abs/2102.02619

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Alexander Jahn @physicistalex.bsky.social · 15/04/2025

Perhaps our coolest result is a new class of "black hole codes" with many logical qubits living on the horizon of a tensor network erasure. These, too, have universal fault-tolerant gates!

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Alexander Jahn @physicistalex.bsky.social · 15/04/2025

This leads to our result: "Heterogeneous holographic codes" are built from *two* types of elemental codes on a hyperbolic lattice. By choosing codes whose joint gateset is universal, we can run them layer-by-layer and preserve fault tolerance!

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Alexander Jahn @physicistalex.bsky.social · 15/04/2025

So what are fault-tolerant logical gates? Logical gates are composed of a series of physical gates. Unless those gates act *transversally*, they can spread small physical errors into bigger ones, overwhelming the code's error correction scheme.

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Alexander Jahn @physicistalex.bsky.social · 15/04/2025

We have another exciting paper coming out today: We show that - surprisingly - holographic codes can be used to run universal logical gates *fault-tolerantly*. Why is that surprising? Well... arxiv.org/abs/2504.10386

Different types of heterogeneous holographic codes

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Alexander Jahn @physicistalex.bsky.social · 13/04/2025

A benefit of visiting Japan is that you can watch peak Japanese cinema on their local Amazon, a wide range of classics such as: "Godzilla" (1954) "Mothra" (1961) "King Kong vs. Godzilla" (1962) "Mothra vs. Godzilla" (1964)

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Alexander Jahn @physicistalex.bsky.social · 10/04/2025

In Kyoto for a quantum error correction workshop, giving a talk on exciting upcoming work on holographic codes next week! (Didn't even use AI for this picture, this is how Japan looks like once you pass through immigration)

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Alexander Jahn @physicistalex.bsky.social · 07/04/2025

Cherry blossom at Freie Universität Berlin...

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Alexander Jahn @physicistalex.bsky.social · 02/04/2025

A rough way of explaining this is that type II algebras describe the EPR-like entanglement that "holds spacetime together" in a holographic bulk. In contrast, the entanglement of QFT subregions has an even more complicated *type III* structure.

A visualization of the EPR entanglement that leads to a type II von Neumann algebra for a subregion A.

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Alexander Jahn @physicistalex.bsky.social · 02/04/2025

However, we look at the *inductive limit* where these codes have infinitely many layers, and subregions are infinitely entangled with their exterior. In this limit, we show that they relate type I bulk algebras to *type II* boundary algebras!

Comparison of the unitary holographic encoding map in D. Harlow's arXiv:1607.03901 paper and the inductive map in our work.

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Alexander Jahn @physicistalex.bsky.social · 02/04/2025

We focus on codes with the property of *complementary recovery*: These holographic models neatly relate bulk and boundary bipartitions, and were shown to lead to simple "type I" von Neumann algebras by Harlow back in 2016. arxiv.org/abs/1607.03901

A visualization of complementary recovery for a holographic tensor network.

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Alexander Jahn @physicistalex.bsky.social · 01/04/2025

The Error Correction Zoo board right now.

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Alexander Jahn @physicistalex.bsky.social · 21/02/2025

Your favorite K-Drama just got cancelled? You feel like you're missing the "wow factor" of intense emotions and human struggle? Then you should read the Peer Review File of Microsoft's "Interferometric single-shot parity measurement in InAs–Al hybrid devices"! www.nature.com/articles/s41...

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Alexander Jahn @physicistalex.bsky.social · 19/02/2025

Met this visitor in front of our department this morning! Like many great thinkers he's rather quiet, but clearly has deep knowledge of low-temperature physics. I expect he'll stay for a few days.

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Alexander Jahn @physicistalex.bsky.social · 14/02/2025

Winter view from my balcony this morning. Sometimes I miss Southern California'a eternal summer, but I couldn't live long in a place without seasons.

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Alexander Jahn @physicistalex.bsky.social · 09/02/2025

Berlin winter can be gloomy sometimes, but this weekend it was beautiful outside.

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Alexander Jahn @physicistalex.bsky.social · 04/02/2025

I should have known that marginalizing probability distributions was woke nonsense!

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Alexander Jahn @physicistalex.bsky.social · 03/01/2025

We construct tensor-network versions of such maps and look at the overlapping qubits they induce. Even for expanding (de Sitter) space, we show that the overlap only produces small non-locality: So a constant-sized fundamental Hilbert space can even accommodate inflation!

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Alexander Jahn @physicistalex.bsky.social · 03/01/2025

Overlapping qubits appear when you cram more qubit operators into a Hilbert space than its dimension allows for. Telling whether your qubits are exact or overlapping can be surprisingly difficult, leading to the "Hilbert space dimension verification problem".

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