The International School of Physics “Enrico Fermi” in Varenna: mixtures of people and ideas

Since its foundation in 1953, the International School of Physics “Enrico Fermi” has been the crucible of extraordinary ideas and of cutting-edge scientific perspectives. These were carried on with enthusiasm by outstanding scientists who initially as students and later as lecturers and Directors have fostered some of the most terrific advancements of modern physics, often certified by the Nobel Prize in Physics.

The Courses linked to the revolution produced by the laser have certainly been significant for the development of science and technology. Pioneers such as Isidor Rabi and Charles Townes contributed significantly to this history. Isidor Rabi, one of the most prominent and influential American physicists, ran a School in 1955 shortly before the beginning of the laser era. The School was dedicated to Enrico Fermi, with a special speech of Rabi, and there was also a lecturer in molecular structure named Townes. Charles Townes returned to Varenna as a lecturer in 1960 for a School on masers directed by Adriano Gozzini, the father of atomic physics in Italy. Three years later, in 1963, Townes directed the first School of quantum electronics, a field of research related to quantum optics and laser applications. In 1964, Charles Townes received the Nobel Prize in Physics for his contributions to the development of the maser and the laser. His research had an enormous impact on laser technology and applications in a wide range of fields.

The Varenna Schools organized in the following decades recognized the importance of lasers in different contexts, both as a spectroscopy instrument and, more recently, as the essential tool for the production and manipulation of ultracold atomic matter. For years, researchers in this field have constituted a family of people interacting with each other, mixing ideas, teaching as inspiration. In this respect, Varenna Schools represented key moments and meeting occasions to create and consolidate such a scientific community.

From the pioneering laser cooling techniques, reported in the 1991 edition Laser Manipulation of Atoms, and the first achievements and preliminary studies on Bose-Einstein condensation in Atomic Gases and Ultra-cold Fermi Gases, in the 1998 and 2006 Schools, to the demonstration of advanced studies on Quantum Matter at Ultralow Temperatures in the 2014 edition, Varenna Schools have always accompanied the field, fixing the main results and providing the participants with prestigious lecturers.

The 1998 Varenna School on Bose-Einstein condensation (BEC) took place just three years after the experimental realization of BEC and it played a crucial role in emphasizing the important perspectives in the emerging field of quantum-degenerate atomic gases. It is still regarded today as a historical School for ultracold quantum gases and the proceedings volume containing the contributions from the lecturers still represents the main reference for students who approach this field of research. This event provided an opportunity to celebrate the achievements of the three 1997 Nobel Laureates in laser cooling techniques, Claude Cohen-Tannoudji, William D. Phillips, and Steven Chu, and it also set the foundation for the recognition of the three 2001 Nobel Laureates for their work on achieving Bose-Einstein condensates: Eric A. Cornell, Wolfgang Ketterle, and Carl E. Wieman. Figure 1 shows a group photo with most of the lecturers of the 1998 School: among them, we can count six Nobel Prize winners.

In this photo, Lev P. Pitaevskii, scientific father and continuous source of inspiration for the whole community working on the theoretical aspects of Bose-Einstein condensation and superfluidity (sadly passed away in 2022, only a few weeks after the School on quantum mixtures), is also portrayed.

The discussions that took place in Varenna in 1998 opened prospects for producing atomic mixtures, which subsequently led to the first works on mixtures of fermions and bosons.

The School organized in 2006 addressed the new achievements on quantum-degenerate Fermi gases, and it was also the occasion to commemorate the 80th anniversary of the discovery of Fermi statistics. A key scientist in this field was Deborah (Debbie) S. Jin (sadly passed away in 2016), who played a crucial role in the study of quantum degeneracy of fermionic potassium, with the first realization of an ultracold Fermi gas in 1999 and pioneering investigations of its properties. This School marked a new direction in the field, showcasing the remarkable contributions of eminent scientists, in the production of ultracold, strongly correlated Fermi matter.

The 2014 School “Quantum Matter at Ultralow Temperatures” covered a range of different subjects, reflecting how far the frontiers of laser-based manipulation of atomic gases had expanded: quantum simulation with optical lattices, artificial gauge fields, topological quantum matter, long-range interacting systems, strongly correlated few- and many-body systems, quantum mixtures with coherent coupling between their components.

Given the recent proliferation of experiments involving different kinds of atoms in various configurations, which drastically increases the richness of the system and the physics that can be explored, the topic of the 2022 Varenna School focused on Quantum Mixtures of Ultra-cold Atoms.

International speakers addressed different types of quantum mixtures, from multi-component spin mixtures to mixtures of different atomic species, providing theoretical or experimental approaches for studying such powerful and promising platforms, characterized by ultimate control and able to show unprecedented features related to the interplay between two or more superfluid systems.

The lectures provided a broad overview on the physics of atomic mixtures from the few-body regime, with the study of Feshbach resonances and the impurity problem, to the many-body regime, with the observation of new types of solitons and vortices and the realization of superfluid ferromagnetic systems. Several bosonic and fermionic mixtures showing complementary features were studied in different configurations, in harmonic, flat or lattice potentials, or in the presence of coherent coupling. Different kinds of interactions were explored, from short-range forces treatable within mean-field and beyond-mean-field schemes in spin mixtures, to long-range in dipolar gases and in hybrid systems made of charged ions immersed in a neutral gas.

We would like to express our sincere gratitude to all the participants that contributed to the stimulating scientific discussions in a truly friendly atmosphere. Special thanks go to the Italian Physical Society for organizing the event with the precious support of Barbara Alzani, Ramona Brigatti and Angela Di Giuseppe from the editorial office.

Rudolf Grimm, Massimo Inguscio, Giacomo Lamporesi and Sandro Stringari