Erik Gauger Room: Higgs Centre Seminar Room

Adam Burgess Room: Higgs Centre Seminar Room

Andrew Hallam Room: Higgs Centre Seminar Room

Tyler Chauvy Room: Higgs Centre Seminar Room

Brendon Lovett: "Modelling open quantum systems that are strongly coupled to their environment with tensor networks" Room: Higgs Centre Seminar Room In order to model realistic quantum devices it is necessary to simulate quantum systems strongly coupled to their environment. To date, most understanding of open quantum systems is restricted either to weak system-bath couplings, or to special cases where specific numerical techniques become effective. I will present a general and yet exact numerical method to calculate the dynamics of an open quantum system strongly coupled to an environment. The method involves expressing the equations of motion for such an open quantum system as a tensor network [1]. In fact, the structure of this network is such that the environment parts can be pre-contracted efficiently, independent of the system Hamiltonian [2]. This so-called process tensor matrix product operator (PT-MPO) approach enables the calculation of dynamics for different time-dependent system Hamiltonians and allows the efficient calculation of any system multi-time correlation function. I will show that such an approach enables us to optimize a control pulse for creating excitons in quantum dots [3], before going on to show how to combine it with standard tensor network methods to treat a chain of coupled systems interacting with independent environments [4]. I will also discuss how the non-additive properties of multiple environments are exposed using this technique [5]. I will go on to present work on using PT-MPO to model two-dimensional electronic spectroscopy, which is a technique used to measure coherent energy transport in molecular systems [6.7]. Finally, I will discuss recent work on extending our methods to systems with larger Hilbert spaces [8]. Our codes are publicly available [9] and we would be happy to support colleagues who would like to try them out on other problems. [1] A. Strathearn, P. Kirton, D. Kilda, J. Keeling, and B. W. Lovett. Efficient non-Markovian quantum dynamics using time-evolving matrix product operators, Nat. Commun. 9 3322 (2018). [2] Exploiting the causal tensor network structure of quantum processes to efficiently simulate non-Markovian path integrals, M. R. Jørgensen and F. A. Pollock, Phys. Rev. Lett. 123 240602 (2019). [3] G. E. Fux, E. Butler, P. R. Eastham, B. W. Lovett and J. Keeling, Efficient exploration of Hamiltonian parameter space for optimal control of non-Markovian open quantum systems Phys. Rev. Lett. 126 200401 (2021). [4] Thermalization of a spin chain strongly coupled to its environment G. E. Fux, D. Kilda, B. W. Lovett and J. Keeling, Phys. Rev. Research 5 033078 (2023). [5] Exact dynamics of non-additive environments in non-Markovian open quantum systems, D. Gribben, D. M. Rouse, J. Iles-Smith, A. Strathearn, H. Maguire, P. Kirton, A. Nazir, E. M. Gauger and B. W. Lovett, Physical Review X Quantum 3 010321 (2022). [6] Process tensor approaches to modeling two-dimensional spectroscopy, R. de Wit, J. Keeling, B. W. Lovett and A. W. Chin, Phys. Rev. Research 7 013209 (2025). [7] Extracting Coupling-Mode Spectral Densities with Two-Dimensional Electronic Spectroscopy, R. de Wit, J. Keeling, B. W. Lovett and A. W. Chin, arXiv:2503:21685, J. Phys. Chem. Lett. 16 6594 (2025). [8] Efficient construction of time-invariant process tensors for simulating high-dimensional non-Markovian open quantum systems, É. Cochin, J. Keeling, B. W. Lovett and A. W. Chin arXiv:2603.06840 (2026). [9] OQuPy - Open Quantum Systems in Python, oqupy.readthedocs.io

Luke Smith Room: Higgs Centre Seminar Room

Pablo Poggi: "Dynamical sensitivity in qubit systems: from quantum control to quantum chaos" Room: Higgs Centre Seminar Room In this talk, I will present a framework to study sensitivity to a given class of perturbations in multiqubit systems with and without control. I will start by introducing the notion of universal robustness for general quantum systems and its connection to unitary designs and robust quantum control. I will then focus on multiqubit systems and the study of sensitivity to classes of perturbations, e.g., one-body, two-body, etc. I will show that the degree of robustness to a multi-body perturbation is determined by the operator-growth properties of the time-evolution operator; in particular, persistent and extensive spreading of U(t) in the Pauli basis is required to achieve robustness against any single-body error. We prove that there is no time-independent Hamiltonian that is universally robust, but also that there exist N-qubit Hamiltonians that are robust against any single- or two-body perturbation. By leveraging the concept of k-uniform states, we show explicitly how these Hamiltonians can be constructed and discuss their application to dynamical decoupling protocols requiring no active control. We further discuss how this dynamical sensitivity is related to a generalized version of the spectral form factor.

Alassandro Civolani: "Quantum Molecular Vibrational Dynamics with Tensor Network Techniques" Room: Higgs Centre Seminar Room Accurate simulation of molecular vibrational quantum dynamics towards rigorously solving the multidimensional Schrödinger equation with direct grid-based methods is severely limited by the required storage space and computational effort, which scale exponentially with the number of coupled degrees of freedom. In this talk, I will show how to use tensor networks to solve partial differential equations, focusing on how to adapt these techniques for simulating complex vibrational dynamics. By treating the function over the grid as a tensor network, this approach provides data compression. We will first benchmark this methodology by calculating the infrared spectrum of water and formaldehyde under the second-order vibrational perturbation approximation. I will then show how to extend the method to larger molecules while including the exact potential energy surface.

Jake Iles-Smith: "Non-Equilibrium Open Quantum Systems: From Quantum Optics to Quantum Thermodynamics" Room: Higgs Centre Seminar Room Standard descriptions of open quantum systems rely heavily on weak-coupling, Markovian, and slow-driving approximations. In quantum optics, the quantum regression theorem relates single-time Liouvillian evolution to multi-time correlation functions, governing key observables such as emission spectra and photon statistics. In quantum thermodynamics, weak-coupling master equations similarly constrain the analysis of energy fluctuations to low-order work moments. However, quantum systems routinely exist far beyond these limits—where strong system-environment coupling, structured reservoirs (such as vibrational modes in solid-state emitters), and rapid driving induce memory effects that invalidate standard frameworks. In this talk, I will present two powerful, non-perturbative approaches designed to overcome these limitations across quantum optics and quantum thermodynamics. First, I will introduce filter theory (the sensor method), which incorporates auxiliary detector degrees of freedom to directly compute multi-time correlation functions and optical spectra, completely bypassing the quantum regression theorem. Second, I will present a unified process-tensor framework that maps non-Markovian dynamics onto generalised multi-segment time contours. By integrating physical time evolution with an auxiliary counting field $\chi$ along a single generalised-time axis, this tensor-network methodology enables the numerically exact evaluation of full work counting statistics across arbitrary coupling strengths and driving protocols. Finally, I will demonstrate how these complementary techniques reveal rich non-equilibrium physics at the intersection of optics and thermodynamics. From resolving fine-grained dressed-state interference in optical emission spectra to exposing microscopic quantum control features in full work distributions that are completely masked by low-order moments, these approaches establish a versatile toolkit for probing thermodynamics and photonics in modern quantum technologies.

Rishi Sundar Room: Higgs Centre Seminar Room

Callum Duncan: "Building Quantum-Ready Scientific Computing with Tensor Networks" Room: Higgs Centre Seminar Room A key challenge for quantum technologies is identifying applications where quantum algorithms can ultimately deliver transformative advantages. At Aegiq, we are pursuing a quantum-ready strategy: developing computational workflows that create value today using tensor network methods while maintaining a clear pathway towards future quantum implementations. In this talk, I will present our tensor-network library for partial differential equations, covering applications in computational fluid dynamics and plasma dynamics. I will discuss recent developments in mesh generation and solver technology, and share benchmark results on textbook CFD problems that probe the strengths and limitations of tensor-network approaches. The talk will explore both the practical engineering opportunities and the broader connection between tensor-based scientific computing and the future quantum computing ecosystem.

Dean Johnstone Room: Higgs Centre Seminar Room

Jonathan Keeling: "Multimode cavity QED: Spin glass, associative memory & beyond" Room: Higgs Centre Seminar Room

Aleksandra Ziolkowska Room: Higgs Centre Seminar Room

Sion Meredith: "Efficient optimisation of multi-parameter quantum control protocols for strongly-coupled systems" Room: Higgs Centre Seminar Room Achieving high-fidelity control in the presence of strong non-Markovian noise is critical for the optimization of emergent solid-state quantum devices. We present a highly efficient optimization framework that combines automatic differentiation with the non-Markovian uniTEMPO algorithm, enabling direct gradient-based optimization of complex objective functions. We apply this method to semiconductor quantum dots, optimizing multi-pulse excitation schemes—specifically Swing-UP of a Quantum EmmiteR (SUPER) and Floquet-engineered Two-Photon Excitation (FTPE)—for single- and bi-exciton generation. Our approach yields high preparation fidelities within experimentally accessible parameter regimes. By integrating adiabatic rapid passage (ARP), we systematically enhance both SUPER and FTPE, demonstrating that these optimized protocols consistently outperform standard resonant $\pi$-pulses and two-photon excitation. Notably, this performance gap widens at elevated temperatures, establishing the superior thermal robustness of our optimized multi-pulse strategies for real-world quantum hardware.

Anish Chakraborty Room: Higgs Centre Seminar Room

Kian Damezin Room: Higgs Centre Seminar Room