# Nikolay Prokofiev & Guido Pupillo

## Biography - Nikolay Prokof'ev

### Physics Department, University of Massachusetts, Amherst, United States & USIAS Fellow at the Institute of Supramolecular Science and Engineering (ISIS), University of Strasbourg and CNRS

Nikolay Prokof’ev is a Professor of Physics at the University of Massachusetts, Amherst, USA. He graduated from the Moscow Engineering Physics Institute in 1982, and received his PhD from the Kurchatov Institute, Moscow, in 1987, working on the tunneling motion of heavy particles in metals. He remained employed by the Kurchatov Institute as a researcher and then senior researcher until 1999. He spent two years (1992-1994) at the University of British Columbia, Vancouver (Canada), as part of the Natural Sciences and Engineering Research Council of Canada (NSERC) International Fellowship programme to explore mechanisms of decoherence in magnetic systems and developed (with Philip C. E. Stamp) the concept of the spin-bath environment. At the same time his research interests shifted towards strongly correlated systems with disorder, including low dimensional setups. In 1999 he moved to Amherst.

Currently his main research focus is on properties of interacting bosonic, electronic, and spin systems, superfluidity/superconductivity, critical phenomena, and numerical methods for their accurate description using field-theoretical tools. Nikolay Prokof’ev is a co-inventor (with Boris Svistunov and Igor Tupitsyn) of two widely used techniques, Worm Algorithm and diagrammatic Monte Carlo, that led to significant progress in the field. He is the recipient of two Kurchatov prizes (1987, 1998), the Samuel F. Conti Fellowship and the outstanding research awards from UMass, Amherst (2007, 2009), and was elected Fellow of the American Physical Society in 2007. In 2012-2017 he served as Division Associate Editor for Physical Review Letters.

## Biography - Guido Pupillo

### Institute of Supramolecular Science and Engineering (ISIS), University of Strasbourg and CNRS

Guido Pupillo is Distinguished Professor (PRCE) at the University of Strasbourg and Director of the “Laboratory of Quantum Physics” at the Institute of Supramolecular Science and Engineering (ISIS), where he is involved in the development of teaching and research programmes in quantum science and technology. He obtained a Master in Physics at the University of Bologna (Italy) in 2001 and a PhD in Physics in 2005 at the University of Maryland (USA). His research, conducted at the National Institute of Standards and Technology, focused on many-body properties of cold bosonic atoms in low dimensions. Until 2011 he was scientist and then senior scientist at the University of Innsbruck and the Austrian Academy of Sciences, where his research interests mainly focused on few- and many-body physics with cold polar molecules, Rydberg atoms and dipolar quantum gases. In 2011 he moved to Strasbourg.

Currently his main research focus is on the properties of interacting bosonic and fermionic systems with long-range interactions, applications of cavity quantum electro-dynamics to problems of transport in disordered media. He is recipient of several awards, including the 2012 ERC Starting Grant and the 2013 Guy Ourisson Prize. Since 2019 he is senior member of the Institut universitaire de France (IUF).

## Project - Diagrammatic Monte Carlo for effective field theories and dipole fermions

### 01/12/2019 - 31/12/2021

The so-called “sign problem” is one of the major unsolved problems in the physics of many-particle systems, common to fields as diverse as condensed matter physics, nuclear physics, theoretical chemistry, and materials science, to name a few. Its appearance is related to how fermionic particles, such as electrons, exchange places due to “Pauli exclusion principle”. It invariably arises in numerical calculations of the properties of quantum mechanical systems with a large number of strongly interacting fermions, causing an exponential increase of the computing time with the number of particles. A polynomial time solution to the sign problem using so-called quantum Monte-Carlo methods would be highly desired since it would potentially open the way to unbiased and numerically exact methods to simulate correlated quantum systems. In turn, this would be of invaluable help, for example, in electronic structure calculations of complex molecules and materials, in finding the mechanism for high-temperature superconductivity or in determining the properties of dense nuclear matter and quark matter.

Recently enormous progress has been made in developing the Diagrammatic Monte Carlo (DiagMC) method for studies of strongly correlated quantum systems. DiagMC takes full advantage of the fact that any interacting many-particle system can be formally described by the series of so-called Feynman diagrammes. In turn, these can in principle be computed up to very high order using stochastic sampling techniques, thus dramatically improving, if not solving, the sign problem in numerical calculations. The unique aspect whereby DiagMC deviates from standard approaches is that, instead of simulating finite ensembles of particles, it directly addresses the thermodynamic limit of the system. Computational complexity is then unrelated to the system size and, under certain conditions, is only polynomial in the inverse accuracy bound. Other advantages include a wide range of potential applications and the possibility of ”absorbing'' known analytical results, i.e. the simulation starts where existing analytics stop, and the efficiency of one depends on the other. DiagMC was pioneered by Professor Nikolay Prokof’ev and he is currently leading efforts for developing the method and its applications for various many-body problems. It appears that DiagMC has real potential for radically advancing our ability to make reliable predictions for two important cases, effective field theories and fermionic systems with dipole interactions. This collaborative project with Professor Guido Pupillo aims to develop the method for these purposes.

## Post-doc biography - Guido Masella

### Institute of Supramolecular Science and Engineering (ISIS), University of Strasbourg & CNRS

Guido Masella graduated in physics from the University of Pisa (Italy) in 2015. In 2016, he worked as a trainee for the Italian National Research Council - National Institute of Optics (CNR-INO) on an experiment involving ultra-cold gases of strongly dipolar atoms. He moved to Strasbourg in early 2017 to join, as a PhD student, the group of Professor Guido Pupillo at the Institute of Supramolecular Science and Engineering (ISIS). His work focused on investigating, with different numerical techniques, exotic phenomena in systems of interest for experiments with cold atomic gases. He obtained his PhD in December 2019, and remained in Strasbourg to work as a post-doc on the USIAS project.

Guido Masella’s main research interests are the study and development of numerical methods for accurate descriptions of interacting systems of fermionic and bosonic particles. To that end, he principally concentrates on the development of new numerical techniques that can deliver the properties of the abovementioned systems with high precision.

## Links

- Webpage of Nikolay Prokof'ev
- Webpage of Guido Pupillo
- Webpage of Nikolay Prokof'ev's team