publications

2025

1. Beyond Markovian Dissipation at the Nanoscale

   Thibaut Lacroix

   Mar 2025

   Abs

   This book proposes innovative and timely modeling, as well as simulation strategies based on tensor networks, to tackle the difficult problem of describing the dynamics of open quantum systems at the molecular or nanometer scale beyond a Markovian treatment. Among the many insights it delivers, the work includes calculations of the dynamics of a quantum system coupled to a bosonic environment that can be potentially structured and/or possess spatial correlations. The relevance of these strategies is exemplified with the analysis of complex bio-inspired nanodevices. Researchers in the field will find here a clear and reliable contribution to the understanding of open quantum systems in a still little-explored regime where the reservoirs are no longer considered as simple baths but as sub-systems treated on an equal footing with the reduced system of interest. Moreover, the author discusses how to handle the situation of a system coupled to multiple baths. This is a very important and generic scenario, crucial, for instance, when discussing non-equilibrium steady states.

2024

1. Quantum

   From Non-Markovian Dissipation to Spatiotemporal Control of Quantum Nanodevices

   Thibaut Lacroix, Brendon W. Lovett, and Alex W. Chin

   Quantum, Apr 2024

   Abs DOI

   Nanodevices exploiting quantum effects are critically important elements of future quantum technologies (QT), but their real-world performance is strongly limited by decoherence arising from local ‘environmental’ interactions. Compounding this, as devices become more complex, i.e. contain multiple functional units, the ‘local’ environments begin to overlap, creating the possibility of environmentally mediated decoherence phenomena on new time-and-length scales. Such complex and inherently non-Markovian dynamics could present a challenge for scaling up QT, but – on the other hand – the ability of environments to transfer ‘signals’ and energy might also enable sophisticated spatiotemporal coordination of inter-component processes, as is suggested to happen in biological nanomachines, like enzymes and photosynthetic proteins. Exploiting numerically exact many body methods (tensor networks) we study a fully quantum model that allows us to explore how propagating environmental dynamics can instigate and direct the evolution of spatially remote, non-interacting quantum systems. We demonstrate how energy dissipated into the environment can be remotely harvested to create transient excited/reactive states, and also identify how reorganisation triggered by system excitation can qualitatively and reversibly alter the ‘downstream’ kinetics of a ‘functional’ quantum system. With access to complete system-environment wave functions, we elucidate the microscopic processes underlying these phenomena, providing new insight into how they could be exploited for energy efficient quantum devices.

2. J. Chem. Phys.

   MPSDynamics.jl: Tensor network simulations for finite-temperature (non-Markovian) open quantum system dynamics

   Thibaut Lacroix, Brieuc Le Dé, Angela Riva, and 2 more authors

   The Journal of Chemical Physics, Aug 2024

   Abs DOI Code

   The MPSDynamics.jl package provides an easy-to-use interface for performing open quantum systems simulations at zero and finite temperatures. The package has been developed with the aim of studying non-Markovian open system dynamics using the state-of-the-art numerically exact Thermalized-Time Evolving Density operator with Orthonormal Polynomials Algorithm based on environment chain mapping. The simulations rely on a tensor network representation of the quantum states as matrix product states (MPS) and tree tensor network states. Written in the Julia programming language, MPSDynamics.jl is a versatile open-source package providing a choice of several variants of the Time-Dependent Variational Principle method for time evolution (including novel bond-adaptive one-site algorithms). The package also provides strong support for the measurement of single and multi-site observables, as well as the storing and logging of data, which makes it a useful tool for the study of many-body physics. It currently handles long-range interactions, time-dependent Hamiltonians, multiple environments, bosonic and fermionic environments, and joint system–environment observables.

3. arXiv

   Connectivity matters: Impact of bath modes ordering and geometry in Open Quantum System simulation with Matrix Product States

   Thibaut Lacroix, Brendon W. Lovett, and Alex W. Chin

   Sep 2024

   arXiv:2409.04145 [cond-mat, physics:physics, physics:quant-ph]

   Abs DOI

   Being able to study the dynamics of quantum systems interacting with several environments is important in many settings ranging from quantum chemistry to quantum thermodynamics, through out-of-equilibrium systems. For such problems tensor network-based methods are state-of-the-art approaches to perform numerically exact simulations. However, to be used efficiently in this multi-environment non-perturbative context, these methods require a clever choice of the topology of the wave-function Ans}"atze. This is often done by analysing cross-correlations between different system and environment degrees of freedom. We show for canonical model Hamiltonians that simple orderings of bosonic environmental modes, which enable to write the joint {System + Environments} state as a matrix product state, reduce considerably the bond dimension required for convergence. These results suggest that complex correlation analyses in order to tweak tensor networks topology (e.g. entanglement renormalization) are usually not necessary and that tree tensor network states are sub-optimal compared to simple matrix product states in several applications.

4. arXiv

   Making Quantum Collision Models Exact

   Thibaut Lacroix, Dario Cilluffo, Susana F. Huelga, and 1 more author

   Nov 2024

   arXiv:2411.13166

   Abs DOI

   Quantum collision describe open quantum systems through repeated interactions with a coarse-grained environment. However, a complete certification of these models is lacking, as no complete error bounds on the simulation of system observables have been established. Here, we show that Markovian and non-Markovian collision models can be recovered analytically from chain mapping techniques starting from a general microscopic Hamiltonian. This derivation reveals a previously unidentified source of error – induced by an unfaithful sampling of the environment – in dynamics obtained with collision models that can become dominant for small but finite time-steps. With the complete characterization of this error, all collision models errors are now identified and quantified, which enables the promotion of collision models to the class of numerically exact methods. To confirm the predictions of our equivalence results, we implemented a non-Markovian collision model of the Spin Boson Model, and identified, as predicted, a regime in which the collision model is fundamentally inaccurate.

2023

1. BUP

   Du quantique au classique : Un modèle simple de décohérence

   Thibaut Lacroix

   Bulletin de l’Union des Physiciens, Oct 2023

   Abs

   Les objets macroscopiques (dont nous faisons partie) obéissent aux lois de la physique classique et sont pourtant composés de constituants obéissant aux lois de la physique quantique. Comment les propriétés quantiques présentes à l’échelle microscopique disparaissent-elles lors du passage à l’échelle macroscopique ? Dans cet article, nous présentons un modèle simple illustrant comment le couplage d’un système quantique à un environnement possédant un grand nombre de degrés de liberté engendre la perte de cohérence quantique du système.

2021

1. Phys. Rev. A

   Unveiling non-Markovian spacetime signaling in open quantum systems with long-range tensor network dynamics

   Thibaut Lacroix, Angus Dunnett, Dominic Gribben, and 2 more authors

   Physical Review A, Nov 2021

   DOI