Alexander Jahn

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.

Overcoming the Zero-Rate Hashing Bound with Holographic Quantum Error Correction A crucial insight for practical quantum error correction is that different types of errors, such as single-qubit Pauli operators, typically occur with different probabilities. Finding an optimal quant...

Our conclusion: Holographic codes aren't just cool physical models, but might actually be useful!
We've already shown they have more nice features in two further papers:
1) Reaching the Hashing bound: arxiv.org/abs/2408.06232
2) Building fault-tolerant logic: arxiv.org/abs/2504.10386

Does that mean that holographic codes are the future of quantum computing? We don't know yet, because real quantum errors are very complicated, and every quantum hardware is different. But at least in simple models, our codes behave as well as state-of-the-art ones!

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!

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.

Holographic Codes from Hyperinvariant Tensor Networks Holographic quantum-error correcting codes are models of bulk/boundary dualities such as the anti-de Sitter/conformal field theory (AdS/CFT) correspondence, where a higher-dimensional bulk geometry is...

The resulting codes - Evenbly codes! - behave similar to HaPPY codes, but don't use perfect tensors and model small quantum gravity corrections.
arxiv.org/abs/2304.02732

Hyper-invariant tensor networks and holography We propose a new class of tensor network state as a model for the AdS/CFT correspondence and holography. This class is demonstrated to retain key features of the multi-scale entanglement renormalizati...

To fix this, we previously proposed building a code out of Glen Evenbly's "hyperinvariant tensor network", a merge between MERA (giving you CFT-like states - nice for AdS/CFT!) and the HaPPY code's geometry.
arxiv.org/abs/1704.04229

That gives you a number of neat holography features, but rather impractical codes: Building code states with a lot of entanglement is hard! Plus, describing the *quantum* gravity regime of holography ("finite N") should give you less entangled states. So we want less-than-perfect tensors.

Holographic quantum error-correcting codes: Toy models for the bulk/boundary correspondence We propose a family of exactly solvable toy models for the AdS/CFT correspondence based on a novel construction of quantum error-correcting codes with a tensor network structure. Our building block is...

... the first model of holographic codes was the HaPPY code (physics has the best acronyms), a hyperbolic tensor network with *perfect* tensors. That essentially means: Maximal entanglement across the bulk geometry.
arxiv.org/abs/1503.06237

A second Quantum paper within three weeks!

We show how to construct a class of qubit "Evenbly codes" - toy models of holography at finite N - which turn out to be highly tunable subsystem codes that can be made resilient against different types of quantum noise.

Why "Far from Perfect"? Well...

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

Great thanks to Dimitris for his amazing work, such as sophisticated DMRG calculations and deep physical insights, that led to this paper! And of course to Berlin Quantum for supporting our work at @freieuniversitaet.bsky.social.

The conclusion: In the space of systems with quasiperiodic disorder, there exists a subspace with discrete-holographic disorder that preserves critical phases! These are the sort of systems that future discrete toy models of holographic dualities are likely to explore.

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!

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.

We reverse this question: Does quasiperiodic disorder always create critical states with CFT-like correlations and entanglement? Our answer: No! Such disorder generally leads to gapped (non-critical) phases even for small disorder.

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:

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!

I am looking for a PhD student to explore quantum advantage with indistinguishable particles in my group at CTP PAS, Warsaw (within Quantera TouQan project touqan.eu). 4‑yr scholarship, start  Oct 2025. Apply by 8 Aug 2025: euraxess.ec.europa.eu/jobs/360583

Get ready for qRNA™.

We should be more understanding: Grok initially wanted to be an image-generation AI, but then it was rejected by Midjourney Inc.

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.

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

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."

At least there's still one geopolitical certainty under Trump:

WW3 will never start on a stock trading day.

"The explicit construction is left as an exercise to the reader."

Open position at QuSoft / University of Amsterdam: Assistant Professor in Quantum Computing and Simulation for Quantum Chemistry and Materials
qusoft.org/qusoft-vacan...
Deadline: 14 July 2025

After the success of #Andor, there's only one logical direction for Disney to take the Star Wars franchise:
A two-hour ASMR video of Ben Mendelsohn saying "Deep-substrate Foliated Kalkite".

"Where is the Lagrangian, Donald?"

Palo Alto in Dahlem?: Berlin auf dem Weg zur Quantenmetropole Die Initiative „Berlin Quantum“ nimmt Fahrt auf und will komplexe Quantenwelt besser nutzbar machen – mit jeder Menge Drittmitteln im Rücken.

An article in the leading daily newspaper @tagesspiegel.de on #Berlin as a place for doing good research on quantum science and technology. And what BERLIN QUANTUM and several other initiatives have to do with it.

www.tagesspiegel.de/themenspezia...