The Sign Problem of Fermions: Computational Challenges and Emerging Approaches

Apr 20, 2026, 8:45 AM → Apr 24, 2026, 7:00 PM Europe/Rome

Aula Renzo Leonardi (ECT*)

Giovanni Garberoglio, Hongwei Xiong, Michele Casula, Simone Taioli, Tommaso Morresi

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The Fermion Sign Problem (FSP) poses a significant challenge in computational quantum many-body physics, hindering the application of Quantum Monte Carlo (QMC) methods to Fermionic systems. This issue is due to the antisymmetric nature of Fermionic wavefunctions, which results in an exponential decay of the signal-to-noise ratio as system size and inverse temperature increase, leading to numerical instabilities in large-scale simulations. Despite extensive research over the years, a universal solution to this problem remains elusive, positioning it as a critical barrier for achieving accurate first-principles simulations in Fermi liquids, quantum chemistry, nuclear matter and lattice QCD. However, recent innovative strategies have shown promise in reducing the impact of the FSP within QMC simulations, as evidenced by comparisons with benchmark calculations for liquid 3He and warm dense matter simulations relevant to modeling novel inertial fusion technologies. This workshop aims to address these advancements and will focus on discussing recent breakthroughs in QMC methodologies, theoretical treatments of the sign problem, and effective mitigation strategies. Key topics will also encompass algorithmic innovations, mathematical frameworks, and practical applications in fields such as nuclear physics, condensed matter physics, quantum chemistry and warm dense matter, promoting collaboration and knowledge sharing among participants.

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Supporting project and institution

 

Mon, April 20

1 Registration

2 Welcome ECT* Director and organizers

3 Wick’s Route to Fermionic and Quantum Spin Simulations

Speaker: Riccardo Rossi

10:30 AM COFFEE BREAK

4 The Challenge of Working with Divergent Series

Speaker: Boris Svistunov

Svistunov.pdf

5 Cluster algorithms, a strategy to lower the scaling of the variance

Speaker: Roland Assaraf

cluster_sign_problem (1).pdf

12:45 PM LUNCH

6 The notorious past of the sign problem

Speaker: David Ceperley

7 Imaginary-Time Path-Integral Perspective on Warm Dense Matter: From Theory to Experiment

Speaker: Maximilian Boehme

4:00 PM COFFEE BREAK

8 Accelerated free energy estimation and reweighting estimators for density response in ab initio path integral Monte Carlo

Speaker: Pontus Svensson

ect_star_2026_PRESENTATION_Svensson.pdf

Tue, April 21

9 The nature of the sign problem in the FCIQMC method: some numerical results based on the Hubbard model

Speaker: Ali Alavi

10 On the accuracy of diffusion Monte Carlo-based estimates of interaction energies

Speaker: Andrea Zen

AZen_Trento.pptx

10:30 AM COFFEE BREAK

11 Fermion Sign Problem from the Perspective of Lee-Yang Phase Transition Theory

Speaker: Xinzheng Li

12 New Methods for Overcoming the Fermion Sign Problem

Speaker: Yunuo Xiong

12:30 PM LUNCH

13 Complex phase diagram

Speaker: Qi-Jun Ye

14 One Basis to Rule Them All Bosons and Fermions from Unsymmetrized Hyperspherical Harmonics

Speaker: Mario Gattobigio

3:45 PM COFFEE BREAK

15 Physics-informed Neural Networks and quantum mechanical many-body problems: applications to quantum dot systems

Speaker: Morten Hjorth-Jensen

ECTworkshopMHJ (1).pdf

16 Sign problems and sign blessings in quantum simulations

Speaker: Alessandro Roggero

Wed, April 22

17 Advancing time-dependent density functional theory for applications at elevated pressure and temperature conditions

Speaker: Zhandos Moldabekov

Trento_ECT_2026.pptx.pdf

18 A deep dive into ab initio path integral Monte Carlo simulations of warm dense matter using fictitious identical particles

Speaker: Tobias Dornheim

10:45 AM COFFEE BREAK

19 Lattice Regularized Diffusion Monte Carlo: an effective way to deal with the Fermionic sign problem

I will review the Lattice Regularized Diffusion Monte Carlo (LRDMC) algorithm and show how it can be effectively used to deal with the Fermionic sign problem. The LRDMC allows one to simulate an effective Hamiltonian that can be tailored to cross the nodal surface in a controlled way with an adjustable parameter. Extrapolating the energies as a function of this parameter seems to be a promising root for approaching the Fermionic ground state.

Speaker: Michele Casula

20 Hydrodynamic Backflow for Easing the Fermion Sign in Finite-Temperature Electron Path Integral Simulations

Speaker: Ingvars Vitenburgs

Presentation.pptx

12:45 PM LUNCH

21 Guided tour in Trento’s city center

7:30 PM DINNER

Thu, April 23

22 One-particle spectral function of the warm dense electrons from Path-integral Monte Carlo simulations

Speaker: Paul Hamann

23 Learning the Fermion sign structure in path-integral Monte Carlo

Speaker: Jarvist Moore Frost

2026-04-23_JMFrost_Trento-QMC.pdf

10:30 AM COFFEE BREAK

24 Approximating Many-Electron Wave Functions using Deep Neural Networks

Speaker: Matthew Foulkes

talk_Trento_Foulkes.pdf

12:00 PM LUNCH

25 Speeding up PIMD and PIMC Simulations of Indistinguishable Particles

Speaker: Barak Hirshberg

BH_Trento_April_Susan.pptx

26 Fermionic trial wave functions for extended systems

Speaker: Markus Holzmann

talk.pdf

3:30 PM COFFEE BREAK

27 Hamiltonian Monte Carlo enhanced by Exact Diagonalization

Speaker: Johann Ostmeyer

talk_ECT-Trento-26.pdf

28 Aperitivo and free discussion

Fri, April 24

29 Interplay of Gauss Law and the Fermion Sign Problem in Quantum Link Models with Dynamical Matter

Speaker: Emilie Huffman

10:30 AM COFFEE BREAK

30 New opportunities in quantum simulation with Rydberg atoms

Speaker: Lode Pollet

12:00 PM LUNCH