Alexander Jahn
@physicistalex.bsky.social
Junior research group leader in Berlin. Working in the borderlands of quantum information, condensed matter physics, and string theory.
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!
October 7, 2025 at 3:26 PM
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!
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.
August 25, 2025 at 1:05 PM
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.
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!
August 25, 2025 at 1:05 PM
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!
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.
August 25, 2025 at 1:05 PM
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.
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".
August 25, 2025 at 1:05 PM
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".
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
arxiv.org/abs/1601.01694
August 25, 2025 at 1:05 PM
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
arxiv.org/abs/1601.01694
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.
As you can see, I was also very busy pensively staring at Platonic solids at the Hungarian National Museum.
August 13, 2025 at 9:13 PM
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.
As you can see, I was also very busy pensively staring at Platonic solids at the Hungarian National Museum.
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!
August 11, 2025 at 12:36 PM
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.
August 11, 2025 at 12:36 PM
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.
Even before being replaced by AI, the jobs of American comedians have apparently been made obsolete.
August 1, 2025 at 10:01 PM
Even before being replaced by AI, the jobs of American comedians have apparently been made obsolete.
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!
July 24, 2025 at 4:02 PM
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.
July 24, 2025 at 4:02 PM
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.
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:
July 24, 2025 at 4:02 PM
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!
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!
July 24, 2025 at 4:02 PM
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!
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!
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.
He's the main driver behind our work on von Neumann algebras in infinite tensor networks from April, see thread below.
June 27, 2025 at 10:40 PM
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.
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.
June 25, 2025 at 10:06 PM
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."
Lenny Susskind started his opening talk on de Sitter holography in classic Lenny Susskind style:
"Hello children."
June 23, 2025 at 8:53 PM
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."
Lenny Susskind started his opening talk on de Sitter holography in classic Lenny Susskind style:
"Hello children."
Prepared a papal dish this Sunday.
May 11, 2025 at 12:37 PM
Prepared a papal dish this Sunday.
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.
May 3, 2025 at 8:26 AM
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.
In the countryside of Kyotango, central Japan.
April 23, 2025 at 3:22 PM
In the countryside of Kyotango, central Japan.
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
arxiv.org/abs/2102.02619
April 23, 2025 at 11:09 AM
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
arxiv.org/abs/2102.02619
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!
April 15, 2025 at 11:51 AM
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!
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!
April 15, 2025 at 11:51 AM
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!
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.
April 15, 2025 at 11:51 AM
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.
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
Why is that surprising? Well...
arxiv.org/abs/2504.10386
April 15, 2025 at 11:51 AM
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
Why is that surprising? Well...
arxiv.org/abs/2504.10386