How much non-Gaussianity does it take for fermionic systems to reproduce the random-matrix statistics associated with quantum chaos? scirate.com/arxiv/2610.05093 In our new work, “The magic cost of fermionic randomness”, with Chenfeng Cao and Yifan Tang, we show that even Haar-random mode mixing cannot overcome the limitations of Slater inputs: an extensive non-Gaussian input resource is necessary. For products of bounded-size blocks, this condition is also sufficient for anticoncentration. We connect the sampling statistics to preparation costs through a quantity accessible experimentally via two-copy number fluctuations. Fermionic randomness comes with a quantitative magic cost. This is now the last paper submitted for QIP. Warm thanks to @Physics_CC and @YifanTang1999 for the collaboration.

“Non-equilibrium dynamics revisited: There and back again”. A talk I will give at the Max Planck Institute for the Physics of Complex Systems as part of #QMBSOE26 on quantum many-body systems out of equilibrium: Relaxation, thermalization, and ergodicity breaking. I will distinguish between a “forward direction” and a “backward direction.” In the forward direction, I will discuss how thermalization can be derived from microscopic non-equilibrium dynamics using a new notion of trembling-hand typicality. In the backward direction, I will ask—and challenge—what can actually be inferred about non-equilibrium dynamics from observational data. Thanks to Lennart Dabelow, Michael Kastner and Peter Reimann for the kind invitation.

There has been extensive discussion about how many qubits are required for a fault-tolerant #quantumcomputer capable of demonstrating quantum utility. However, we should not overlook the fact that, for now, available quantum computers are not fully error-corrected. How far can we go under these conditions? We present a comprehensive analysis of how deep noisy quantum circuits can be for a broad class of physically reasonable noise channels. The key takeaway—spoiler alert—is that such circuits are logarithmically deep, in a precise average-case sense. The glass may be seen as half full or half empty: the achievable circuit depth ultimately depends on the exact average gate fidelities. Nonetheless, it is important to recognize that these circuits will effectively be noise-induced shallow circuits, and they will not exhibit barren plateaus during training. I am delighted to share this work, authored together with the wonderful team of @QuAntonioMele, @acl_angrisani, @soumik375, @SumeetKhatri6, @dsfranca, and @quekpottheories, now published in @NaturePhysics. nature.com/articles/s41567-0…

How compactness curbs entanglement growth in bosonic systems This work addresses a puzzling phenomenon in the study of entanglement in continuous quantum field descriptions. scirate.com/arxiv/2603.16775 #Zeromodes, understood here as degrees of freedom with vanishing confining frequency, play a central role in the #nonequilibrium dynamics of bosonic systems. In Gaussian models, however, they lead to an unbounded, logarithmic growth of entanglement entropy. In this work, we show that this divergence is not an intrinsic property of zero modes themselves, but arises specifically for non-compact zero modes. Their non-compact configuration space allows unbounded spreading in position space, while their continuous spectra enable indefinite dephasing in momentum space. By contrast, compact zero modes in #compact bosonic systems behave fundamentally differently: Spreading and dephasing are eventually halted, so that compactness caps the #entanglement entropy at a finite value, making its dynamical role most transparent in the presence of a zero mode. We demonstrate this mechanism in a minimal setting by comparing two coupled harmonic oscillators with two coupled quantum rotors. We then show that the same physics persists in many-body systems by contrasting an N-site compact rotor chain with the non-compact harmonic chain. Finally, we relate these insights to #ultracoldatom realizations of compact quantum field theories. In particular, we clarify when a compact free-boson (Tomonaga-Luttinger liquid) description is required and when the commonly used non-compact massless Klein-Gordon model breaks down. Even when the initial state is accurately captured by a non-compact Gaussian description, compactness ultimately governs the late-time quench dynamics, curbing entanglement growth rather than allowing a dynamical divergence. Warm thanks to Stefan Aimet, Philipp Schmoll, Jörg Schmiedmayer, and Spyros Sotiriadis for the wonderful collaboration. I find it particularly exciting when theoretical considerations and experimental expertise are coming together.

As academics, we all receive a steady stream of pro bono requests. Personally, I write a lot more than ten recommendation letters each month, referee journal submissions, and—somewhat more tediously—prepare external assessment letters for hiring committees. I am, of course, committed to contributing to the profession alongside my primary responsibilities: conducting extensive research, teaching, and managing other academic duties. I see myself as a mentor and try to support colleagues and students whenever I can. And I am happy to do so. Yet, I sometimes find myself pausing. When I respond to substantial requests from hiring committees only to receive out-of-office replies lasting weeks, I cannot help but wonder: Wait, why is this request considered so legitimate and urgent on my end? Many academic publishers generate enormous revenues—some in the billions annually. Publishing is undoubtedly important. But are we allocating our publicly funded time and effort wisely? This leads to broader questions. Should we reconsider the structure of our refereeing system and the expectation that so much academic labor be provided pro bono? Should referees be compensated for their work? Why are these responsibilities so often taken for granted? Should external review letters for hiring decisions be paid? And if such labor remains unpaid, why is it acceptable that those requesting it are unreachable? I admit and confess that I do not have clear answers. I am deeply committed to science and perhaps overly motivated when it comes to contributing to the academic community. Still, I am uncertain about whether our current system is well designed. More fundamentally, should we rethink the incentive structures that shape academic life? And to what extent might academia be allowing itself to be exploited? Should we rethink to some extent how academia is being done? Comments most welcome, I have no good answers.

Do Bose-Hubbard type systems have a speed of sound for particle propagation? scirate.com/arxiv/2601.00111 Yes. Indeed, it has long been an open question whether interacting bosonic systems also feature finite speeds of sound in information and particle propagation—a question that was only recently resolved. Standard Lieb-Robinson bounds do not apply here, as the operators involved are not bounded. For some bosonic systems, one can even design contrived counterexamples that violate relativity on the level of the model. This work proves a strikingly simple bound of this kind for particle propagation—shown in literally a few elementary, yet not straightforward, lines—for generalized Bose–Hubbard models defined on general lattices. It demonstrates that appropriately locally perturbed stationary states exhibit a finite speed of sound in particle-number propagation. In fact, I completed a first version of this argument in 2021, and in the meantime a number of beautiful publications have appeared that largely settle the question. However, the argument presented here is so simple and physically insightful that I decided to make it public nevertheless. It is also expected to serve as a starting point for new endeavours on this old, long-standing, and fascinating problem in quantum many-body physics. On a higher level, what I like about academia is that one can work on the hot topics the quantum industry is also working on, or can try to push quantum technologies forward. And then one has the liberty to think about genuinely interesting problems that have no application whatsoever.

This is a manuscript I am particularly happy with. It concerns topics I have already been working on for my PhD, namely good old entanglement theory, going back to the work of Bennett and others. Yet it still has a fresh flavour, relating the subject to modern developments in quantum computing. I also like its “rebellious” aspect, since it points out that the operations commonly applied in mainstream entanglement theory are inefficient and require superpolynomial resources. If one reasonably restricts to efficient operations, the theory is actually altered quite drastically — which surprised us. I am glad to see this work in press in @NaturePhysics. nature.com/articles/s41567-0… Thanks to the wonderful dream team of @lorenzo_leone_, @jacoporz, and Sofiène Jerbi for making this possible. A nice press release of the @FU_Berlin can be found here. fu-berlin.de/en/presse/infor… And we thank the funders at the @bmftr_bund (@QuantenTech), @Berlin_Quantum, @MunichQuantum, @dfg_public, @QuantERA_EU, @QuantumFlagship and the @ERC_Research. We also sincerely thank the editor for the usually professional handling and the referees for the scrutinizing and careful reports.

In the shadow of the Hadamard test: Using the garbage state for good and further modifications I am excited to see this improvement of the important primitive of the Hadamard test in #quantumalgorithms in press in the @PhysRevLett. We combine the usually unused output with techniques of shadow estimation. journals.aps.org/prl/abstrac… The #Hadamardtest is naturally suited for the intermediate regime between the current era of noisy quantum devices and complete fault tolerance. Its applications use measurements of the auxiliary qubit to extract information, but disregard the system register completely. Separate advances in classical representations of quantum states via classical shadows allow the implementation of even global classical shadows with shallow circuits. This work combines the Hadamard test on a single auxiliary readout qubit with #classicalshadows on the remaining n-qubit work register. We argue that this combination inherits the best of both worlds and discuss statistical phase estimation as a vignette application. There, we can use the Hadamard test to estimate eigenvalues on the auxiliary qubit, while classical shadows on the remaining n qubits provide access to additional features such as, (i) fidelity with certain pure quantum states, (ii) the initial state's energy and (iii) how pure and how close the initial state is to an eigenstate of the Hamiltonian. Finally, we also discuss how anti-controlled unitaries can further augment this framework. Warm thanks to the dream team of @PaulFaehrmann and @RichardKueng for this - once again - wonderful collaboration.

Warm congratulations to John Clarke, Michel Devoret, and John M. Martinis for the much deserved #NobelPrize in physics. Their work was revolutionary to make #superconducting #quantumcomputers work.

Computational relative entropy We take steps towards a computationally efficient quantum information theory. scirate.com/arxiv/2509.20472 Our capacity to process information depends on the computational power at our disposal. Information theory captures our ability to distinguish states or communicate messages when it is unconstrained with unrivaled elegance. For computationally bounded observers the situation is quite different. They can, for example, be fooled to believe that distributions are more random than they actually are. In our work, we go beyond the prevailing single-shot approach and take a new direction in computational #quantuminformationtheory that captures the essence of complexity-constrained information theory while retaining the look and feel of the unbounded asymptotic theory. As our foundational quantity, we define the #computationalrelativeentropy as the optimal error exponent in asymmetric hypothesis testing when restricted to polynomially many copies and quantum gates, defined in a mathematically rigorous way. Building on this foundation, we prove a computational analogue of #SteinsLemma, establish computational versions of fundamental inequalities like Pinsker's bound, and demonstrate a computational smoothing property showing that computationally indistinguishable states yield equivalent information measures. We derive a computational entropy that operationally characterizes optimal compression rates for quantum states under computational limitations and show that our quantities apply to computational entanglement theory, proving a computational version of the #Rainsbound. Our framework reveals striking separations between computational and unbounded information measures, including quantum-classical gaps that arise from cryptographic assumptions, demonstrating that computational constraints fundamentally alter the information-theoretic landscape and open new research directions at the intersection of quantum information, complexity theory, and cryptography. Warm thanks to the dream team of @jj_xyz, @non_unitary, @jacoporz, @lorenzo_leone_, and @SofieneJerbi for the fruitful collaboration.

Localized statistics decoding for quantum low-density parity-check codes nature.com/articles/s41467-0… Quantum low-density parity-check (#qLDPC) codes are a promising candidate for #faulttolerant #quantumcomputing moving much to the attention recently. Indeed, such codes have highly attractive features and considerably reduce overhead compared to the surface code. For this reason, much research effort has been invested into finding such good codes. However, actual fault-tolerant quantum computing is possible only if such codes are accompanied with highly efficient classical decoding algorithms. Indeed, the lack of a practical #decoding algorithm remains a barrier to their implementation. In this work, we introduce #localizedstatisticsdecoding, a reliability-guided inversion decoder that is highly parallelizable and applicable to arbitrary quantum low-density parity-check codes. Our approach employs a parallel matrix factorization strategy, which we call on-the-fly elimination, to identify, validate, and solve local decoding regions on the decoding graph. Through numerical simulations, we show that localized statistics decoding matches the performance of state-of-the-art decoders while reducing the runtime complexity for operation in the sub-threshold regime. At the same time, we augment our findings with rigorous statements and assessments involving percolation theory. Importantly, our decoder is more amenable to implementation on specialized hardware, positioning it as a promising candidate for decoding real-time syndromes from experiments. I am excited to see this work out in print in @NatureComms. Warm thanks to the dream team of @timohillmann, Lucas Berent, @armanda_oq, @rbrtwll and @QuantumGizmos for this wonderful collaboration on one of the important topics in #quantumerrorcorrection. And thanks to our funders, in particular the @bmftr_bund (@QuantenTech), the @dfg_public, @Berlin_Quantum, @ML4Q_cluster, the @MunichQuantum (for which this is yet another Berlin-Munich joint publication), the @QuantumFlagship under @Millenion_Q (for which this is again a Berlin-Munich joint publication) and the @ERC_Research.

Where do thermal states in nature come from? This question, which already occupied the minds of the founders of quantum mechanics, has kept me thinking for many years. In this work, we present technical progress on the issue: we show that if initial states are slightly perturbed—and in any case, one cannot know them with arbitrary precision—then local thermalization can be rigorously proven, offering a novel perspective on the concept of typicality. nature.com/articles/s42005-0… Warm thanks to Christian Bertoni, Clara Wassner, and @Quantum_GiaGuar for the wonderfully productive and fun collaboration.

A wonderful article in @physorg_com about our work nature.com/articles/s41567-0… on probing a quantum many-body reading of #Landauer’s principle in #quantumthermodynamics with Stefan Aimet, Mohammad Amin Tajik, Gabrielle Tournaire, Philipp Schüttelkopf, João Sabino, Spyros Sotiriadis, Giacomo Guarnieri, and Jörg Schmiedmayer - by Ingrid Fadelli. phys.org/news/2025-06-approa…

I am extremely delighted to see our joint theoretical-experimental work probing notions of Landauer's principle in the many-body regime published in @NaturePhysics. Landauer's principle represents one of the deepest connections between information theory and thermodynamics, linking the entropy change of a system during a process to the average energy dissipated to its environment. nature.com/articles/s41567-0… In this work, we explore new ramifications of Landauer’s principle in the quantum many-body regime using a quantum field simulator based on ultracold Bose gases. Employing a dynamical tomographic reconstruction scheme, we track the temporal evolution of the quantum field following a global mass quench from a massive to a massless Klein–Gordon model. We analyze the thermodynamic and information-theoretic contributions to generalized entropy production for various system–environment partitions of the composite system. While Landauer's theorem is, by definition, a theoretical result that does not require experimental verification, we uncover a number of additional details enriching our understanding of this connection. Our results not only confirm quantum field theoretical calculations—interpreted through a semiclassical quasiparticle picture—but also demonstrate the capability of ultracold atom-based quantum field simulators to experimentally investigate aspects of quantum thermodynamics. Warm thanks to the team of @AimetStefan, Amin Tajik, Gabrielle Tournaire, Philipp Schüttelkopf, João Sabino, @SpyrosSoti27402, Giacomo Guarnieri, Jörg Schmiedmayer for this wonderful collaboration, involving the @FU_Berlin, the @HZBde, the TU Vienna, @Caltech, the @UBC, the @TUofCrete, and the University of Pavia. And thanks to our funders, specifically the @dfg_public (@QuantumThermo), the @FQXi, the @bmftr_bund, @Berlin_Quantum, the @MunichQuantum, and the @ERC_Research.

Experimentally probing Landauer's principle in the quantum many-body regime scirate.com/arxiv/2407.21690 Landauer's principle states that to erase information, one needs to deposit energy in an environment. We explore Landauer's principle for quantum fields in theory and experiment.