June 2025 - ITP-quantumseminare - lists.phys.ethz.ch

This week's QIT seminars
by Ladina Hausmann 30 Jun '25

30 Jun '25

Hi all, This week we will have two seminars: (1) On Tuesday, our visitor Riccardo Castellano will talk about “The entropic coherence is a necessary resource for non-energy preserving gates”. The talk will take place on Tuesday at 12:30 in HIT E41.1. (2) On Thursday, our other visitor Wolfgang Wieland will talk about ”Luminosity Bound of Gravitational Waves by Non-Perturbative Quantum Effects of Gravity”. The talk will take place at 11:00 in HIT E41.1. See below for the abstracts. Best, Ladina *** Title: The entropic coherence is a necessary resource for non-energy preserving gates Abstract: Under the energy conservation law, we consider the task of implementing non-energy preserving gates (NEPG) on a finite-dimensional system S via an interaction with an external battery B providing or absorbing the required energy. We prove that the entropic coherence (closely related to the relative entropy of coherence) is a resource for this task, and find a lower bound on its minimum amount that has to be present in the battery to be able to implement NEPG with fixed precision. Assuming that the dimensionality of the battery scales polynomially with the precision, we strengthen the latter bound by a factor of 2. An immediate corollary is that any finite-dimensional battery can not exceed a certain minimal error in the gate implementation task. Moreover, under assumptions on the density of energy levels in the battery Hamiltonian, our main results imply additional lower bounds on the minimal amount of energy and quantum Fisher information required to implement the gate. We show that these bounds can be stronger than the universal bounds previously established in the literature. ************* Title: Luminosity Bound of Gravitational Waves by Non-Perturbative Quantum Effects of Gravity Abstract: This talk presents recent progress on a non-perturbative quantisation of gravitational subsystems on a light cone. Starting from the covariant phase space for the γ-Palatini--Holst action, we identify an auxiliary conformal field theory (CFT), which carries a representation of the constraint algebra of general relativity on a null surface. In the model, the radiative data, which is encoded into the shear of each null generator, is mapped into an auxiliary current algebra on each light ray. We study the resulting quantum theory for both bosonic and fermionic representations. In the fermionic representation, the central charge on each null ray is positive, for bosons it is negative. To avoid non-unitary representations, the central charge must be positive. I explain how this requirement alters the spectrum of the radiated power. In this way, we obtain a bound on the radiated power (Bondi flux) of gravitational waves in asymptotically flat spacetimes. The talk is based in part on arXiv:2402.12578, arXiv:2401.17491, arXiv:2104.05803, arXiv:2504.10802.

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Thursday's QIT seminar
by Ladina Hausmann 23 Jun '25

23 Jun '25

Hi all, This week Mattia Chiurco will talk about “Optimizing Quantum Time Dynamics with Classical Support”. See below for the abstract. The talk will take place on Thursday at 11:00 in HIT E 41.1. Best, Ladina **** Title: Optimizing Quantum Time Dynamics with Classical Support Abstract: Quantum time evolution is fundamental for simulating many-body systems in quantum chemistry and condensed matter physics. Trotter–Suzuki decomposition approximates the full evolution operator with deep circuits that often exceed near-term hardware coherence times. Variational quantum time evolution (VarQTE) leverages McLachlan’s variational principle to derive an ordinary differential equation for a parameterized circuit that approximates evolution on a given input state using much shallower circuits. However, computing the quantum geometric tensor and the energy gradient at each time step to solve this ODE incurs considerable measurement overhead, scaling poorly with the number of variational parameters. In this work, we introduce and benchmark three classical methods to alleviate this bottleneck in a hybrid quantum-classical pipeline by performing the majority of QGT and gradient calculations classically and focusing quantum resources on classically intractable subroutines, such as state sampling. First, we employ lightcone truncation, simulating only the relevant causal subcircuits so that each variational quantity depends on just a small subset of qubits, enabling efficient evaluation for shallow-depth ansätze on the order of a hundred qubits. Next, we apply a Pauli propagation algorithm to extend our reach to deeper circuits at the cost of increased computational complexity. Finally, we implement a lightcone‐aware overlap procedure within the matrix product state framework to evaluate the QGT and the energy gradient by restricting tensor contractions to the causal subspace, enabling simulation of systems with lightcone sizes beyond the reach of statevector methods. To assess the expressive power of VarQTE, we compare to second order Suzuki–Trotter dynamics across transverse-field Ising and isotropic Heisenberg Hamiltonians on one- and two-dimensional lattices. We quantify compressibility factors and demonstrate that a moderate number of variational layers can effectively emulate numerous Trotter steps with comparable accuracy. We also investigate adaptive ansatz expansion protocols, including layer freezing, and identify trade-offs between parameter growth and lightcone spread. Furthermore, we benchmark VarQTE against approximate quantum compiling (AQC). The comparison reveals that VarQTE matches the performance of AQC for 1D systems. However, for higher dimensional systems the integration of AQC becomes increasingly complex, and VarQTE becomes the more amenable routine. Finally, we demonstrate that our VarQTE pipeline can be used to generate a subspace expansion protocol which in turn can be integrated into a spectral estimation application via sample-based Krylov quantum diagonalization (SKQD) on 12- and 52-qubit Kagome lattices. Our results chart a pathway toward efficient variational dynamics and spectral estimation on near-term quantum computers platforms

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Quantum Computing summer school, 21-25 July 2025 at UZH
by Lidia del Rio 17 Jun '25

17 Jun '25

Dear all, It is a pleasure to share the news of our incoming *summer school on quantum computing*, to take place at UZH, *21-25 July 2025*. The *deadline for registration is the 30th of June*. We'd appreciate it if you could announce the school with your students (undergrads, masters and PhD). *Website and registration:* https://qc.squids.ch/ In 2025, we celebrate the 100th anniversary of the discovery of quantum mechanics, with the UNESCO International Year of Quantum Science and Technology. In fact, Schrödinger’s equation was discovered in 1925 at the University of Zurich! As part of a year of festivities, the Digital Society Initiative of UZH and the Institute of Physics of UZH are happy to host a 5-day summer school on quantum computing! The summer school will run take place 21-25 July 2025 in the City Campus of UZH. It is aimed at bachelors, masters and PhD students from the fields of physics, mathematics and computer science, who are interested in quantum computing and would like to learn a solid base on basic topics and current research approaches. Our lecturers are world-leading researchers and will cover subjects spanning from “what is a qubit?” to quantum algorithms, quantum complexity theory, quantum games, error correction, quantum cryptography and quantum chemistry. Lectures: - *Quantum Algorithms*, by *Ronald de Wolf*, CWI & University of Amsterdam, The Netherlands - *Quantum Complexity*, by *Tony Metger*, ETH Zurich, Switzerland - *Fault-tolerant Quantum Computing*, by *Manuel Rispler*, FZ Jülich, Germany - *Quantum Cryptography*, by *Ramona Wolf*, University of Siegen, Germany - *Quantum Games*, by *Evert van Niewenburg*, University of Leiden, The Netherlands - *Quantum Chemistry*, by *Freek Witteveen*, University of Copenhagen, Denmark Best wishes, Lídia del Rio, Nuriya Nurgalieva and Titus Neupert -- ......... Dr. Lídia del Rio Essentia Foundation Research Fellow for Quantum Information Theory Institute of Physics, University of Zurich Office hours: I am currently on partial sick leave. Please allow several days for email responses. For urgent matters, please contact one of my coworkers. I will be on maternity leave from July-October 2025. As such, I am unable to take on new commitments for 2025.

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This week's QIT seminar on Tuesday at 15:00 in HIT H 42
by Ladina Hausmann 16 Jun '25

16 Jun '25

Hi all, This week, Liuhang Ye will talk about “Geometric and Information-theoretic Aspects of the Black Hole Information Puzzle”, see below for the abstract. The seminar will take place on Tuesday in HIT H42 at 15:00. Best, Ladina *** Title: Geometric and Information-theoretic Aspects of the Black Hole Information Puzzle Abstract: Black holes provide a unique arena in which the principles of general relativity, quantum field theory and quantum information theory intersect. Hawking’s discovery that black holes radiate thermally, together with Bekenstein’s identification of horizon area with black hole entropy, gives rise to a profound tension between semi-classical gravity and unitarity of quantum theory - known as the black hole information puzzle. This thesis offers an in-depth investigation of the puzzle from both geometric and information-theoretic perspectives, aiming to clarify the logical structure underlying recent key developments. In this presentation, I will explain the background of the information puzzle, and then review the replica method for calculating entropies in QFT, for both fixed and dynamical geometries. In ordinary QFT the replica geometry factorizes into independent copies of the single-sheet geometry, whereas in gravity the Lewkowycz–Maldacena prescription reveals non-factorizing “replica wormhole” saddles that lead to the Island formula. We discuss the connection between wormholes and the notion of elusive references arising from the quantum de Finetti theorem, providing operational meaning of the Page curve. Finally, the interplay between information and geometry is explored, and we conjecture that the elusive reference encountered in the black hole context is a distinctive feature of dynamical gravity, rooted in the non-factorizable geometries that emerge in the gravitational path integral.

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Thursday's QIT seminar in HIT E41.1 at 11:00
by Ladina Hausmann 09 Jun '25

09 Jun '25

Hi all, In this week’s seminar, Alex Steinhauser will talk about “The entanglement power of 3-qubit unitaries”. The talk will take place on Thursday at 11:00 in HIT E 41.1. Best, Ladina **** Title: The entanglement power of 3-qubit unitaries Abstract: In this presentation, I explore how much entanglement can be generated by a 3-qubit unitary. Understanding a unitary’s capacity to produce entanglement is important for quantum computing, resource theories, and classical simulation. To analyze this, I use an entanglement measure based on quasiprobability simulation. The computation relies on the Cartan decomposition, which breaks down complex unitaries into simpler components. Building on known results for 2-qubit systems, I extend the approach to 3-qubit unitaries. This results in an explicit protocol and provides an achievability bound.

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This weeks QIT seminar on Thursday at 11:00 in HIT E41.1
by Ladina Hausmann 02 Jun '25

02 Jun '25

Hi all, This week, Alberto Spalvieri will talk about “Local Operations and Field Mediated Entanglement without a Local Tensor Product Structure”. See below for the abstract. The talk will take place on Thursday at 11:00 in HIT E41.1. Best, Ladina ******* Title: Local Operations and Field Mediated Entanglement without a Local Tensor Product Structure Abstract: Quantum Information Theory (QIT) has become a powerful framework for probing the foundations of quantum mechanics and has found increasing application across physics. In particular, applying QIT to gauge theories, such as electromagnetism and gravity, might offer a promising route toward understanding the quantum nature of spacetime and field interactions. However, such applications face conceptual challenges: gauge constraints prevent the physical Hilbert space from factorizing into a local tensor product structure, challenging the standard definitions of local operations and entanglement. This thesis addresses these issues using a two-dimensional lattice toy model, closely analogous to electromagnetism, that captures key features of infinite-dimensional gauge theories. Building on previous work, we define gauge-invariant local algebras and induce a physically meaningful decomposition of the Hilbert space. This enables us to formulate operationally consistent notions of locality and entanglement, even without a standard tensor product structure. As an application of our framework, we revisit field-mediated entanglement protocols proposed in recent tests of the quantum nature of gravity. We show that our generalized notions of local operations and entanglement extend the applicability of the LOCC theorem (Local Operations and Classical Communication) to gauge theories, demonstrating that such arguments remain valid even without a trivial local decomposition. Our results also offer a concrete operational understanding of entanglement generation in this context and clarify the role of field-matter interactions.

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