News and Views (7&8)

© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. 1 An exact many‐body solution for Fermi polarons by Hui Hu (PRL) Physicists at the Swinburne University of Technology in Melbourne, Australia, have proposed an exact and solvable model for Fermi polarons that is experimentally realizable in current cold-atom laboratories. The resulting exact many-body solution allowed them to rigorously prove all exact and universal quasiparticle features of Fermi polarons. The behavior of an impurity immersed in a many-body background—i.e., polaron physics—is a long-standing problem in condensed matter physics and many-body physics. The first study involving polaron physics can be traced back to Lev Davidovich Landau in 1933, who proposed one of the most fundamentally important concepts in modern physics: “quasiparticles,” a situation where a part of the impurity may still behave like a free particle under the interaction with the many-body background, as measured by quasiparticle residue, which is positive but smaller than unity. Over the past two decades, there have been numerous efforts from the ultracold atom community to quantitatively understand polaron physics. The unique advantage the ultracold atom community provides is the possibility for unprecedented controllability. The interaction between the impurity and the many-body background can be tuned very precisely. The mass of the impurity can be changed. One can also choose the many-body background to be either fermionic or bosonic, leading to the so-called Fermi polaron or Bose polaron problems. A few salient quasiparticle features of polarons have now been predicted by approximate diagrammatic theories, including the ground-state attractive polaron, the excited repulsive polaron with a finite lifetime, and the dark continuum and the molecule-hole continuum, which separate the attractive and repulsive polaron branches. Experimental observations of these salient quasiparticle properties are important, providing a stringent test of the many-body theories of both Fermi and Bose polarons. In this respect, an exact solution of the polaron problem that exactly establishes these salient quasiparticle features would be of great interest. However, in quantum many-body physics, exact solutions are rare, especially in dimensions higher than one. In a recent work [1, 2] contributed by researchers at Swinburne University of Technology, Australia, an exact and solvable model of Fermi polarons is surprisingly provided in the heavy impurity limit and is solved using a novel functional determinant approach. The model, which describes an impurity immersed in a BCS Fermi superfluid, is feasible to experimentally realize, by using, for instance, heavy 133Cs atoms (as impurities) in a BCS Fermi superfluid of 6Li atoms. The key ingredient of their exact and solvable model is the superfluid pairing gap of the many-body background, which strongly suppresses the multiple particlehole excitations in the background BCS superfluid, as the pair-breaking costs energy. This avoids the famous “Anderson’s orthogonality catastrophe,” which would completely destroy the quasiparticle residue of a heavy impurity in a non-interacting Fermi sea due to infinitely many particle-hole excitations. As a result, all the universal salient features of a polaron are revealed via an inprinciple exact calculation, as shown in Fig. 1. Open Access AAPPS Bulletin


An exact many-body solution for Fermi polarons by Hui Hu (PRL)
Physicists at the Swinburne University of Technology in Melbourne, Australia, have proposed an exact and solvable model for Fermi polarons that is experimentally realizable in current cold-atom laboratories. The resulting exact many-body solution allowed them to rigorously prove all exact and universal quasiparticle features of Fermi polarons. The behavior of an impurity immersed in a many-body background-i.e., polaron physics-is a long-standing problem in condensed matter physics and many-body physics. The first study involving polaron physics can be traced back to Lev Davidovich Landau in 1933, who proposed one of the most fundamentally important concepts in modern physics: "quasiparticles, " a situation where a part of the impurity may still behave like a free particle under the interaction with the many-body background, as measured by quasiparticle residue, which is positive but smaller than unity.
Over the past two decades, there have been numerous efforts from the ultracold atom community to quantitatively understand polaron physics. The unique advantage the ultracold atom community provides is the possibility for unprecedented controllability. The interaction between the impurity and the many-body background can be tuned very precisely. The mass of the impurity can be changed. One can also choose the many-body background to be either fermionic or bosonic, leading to the so-called Fermi polaron or Bose polaron problems. A few salient quasiparticle features of polarons have now been predicted by approximate diagrammatic theories, including the ground-state attractive polaron, the excited repulsive polaron with a finite lifetime, and the dark continuum and the molecule-hole continuum, which separate the attractive and repulsive polaron branches. Experimental observations of these salient quasiparticle properties are important, providing a stringent test of the many-body theories of both Fermi and Bose polarons.
In this respect, an exact solution of the polaron problem that exactly establishes these salient quasiparticle features would be of great interest. However, in quantum many-body physics, exact solutions are rare, especially in dimensions higher than one. In a recent work [1,2] contributed by researchers at Swinburne University of Technology, Australia, an exact and solvable model of Fermi polarons is surprisingly provided in the heavy impurity limit and is solved using a novel functional determinant approach. The model, which describes an impurity immersed in a BCS Fermi superfluid, is feasible to experimentally realize, by using, for instance, heavy 133 Cs atoms (as impurities) in a BCS Fermi superfluid of 6 Li atoms.
The key ingredient of their exact and solvable model is the superfluid pairing gap of the many-body background, which strongly suppresses the multiple particlehole excitations in the background BCS superfluid, as the pair-breaking costs energy. This avoids the famous "Anderson's orthogonality catastrophe, " which would completely destroy the quasiparticle residue of a heavy impurity in a non-interacting Fermi sea due to infinitely many particle-hole excitations. As a result, all the universal salient features of a polaron are revealed via an inprinciple exact calculation, as shown in Fig. 1. This interesting work builds on their earlier work on a novel "crossover" polaron problem [3], which addresses the behavior of an impurity immersed in a strongly interacting Fermi superfluid, connecting between a noninteracting Fermi gas (i.e., the Fermi polaron problem) and a weakly interacting molecular condensate (the Bose polaron problem).

Registrations and the call for abstracts are now open for the 24th AIP Congress
Register now and submit abstracts at the official Congress website: https:// aip-congr ess. org. au/ index. html. Register to attend our AIP Congress in Adelaide this December and you will be able to watch two Nobel Prize winners-Professors Donna Strickland and Kip Thornegive plenary talks. Full registration for the Congress also includes the Welcome Reception, catered poster sessions, and the Congress dinner.

Intense lasers and gravitational waves
Professor Donna Strickland (University of Waterloo) was jointly awarded the 2018 Nobel Prize in Physics with Professor Gérard Mourou for developing chirped pulse amplification, a method of creating very intense, ultrashort laser pulses without destroying the amplifying material. Modern applications of this technology in medicine and industry include its use in laser eye surgery and in the machining of small glass parts employed in mobile phones. At

More info on the Congress
This year's Congress will be held at the Adelaide Convention Centre from 11-16 December.
It will be co-locating with the Australian and New Zealand Conference on Optics and Photonics (ANZCOP) and the 7th International Workshop on Speciality Optical Fibres (WSOF).
Submission of abstracts is open until 29 July, and registrations are open until 2 December.
Please visit the Congress website for more information. It is updated regularly.
Photo credit: University of Waterloo (Prof Strickland) and Caltech (Prof Thorne).

In memory of Prof. Bernard Bigot by Weiping Liu
With great sadness, I report that Prof. Bernard Bigot, director of the International Thermonuclear Experimental Reactor (ITER) Organization passed away on May 14, 2022. A great and creative researcher and leader for more than 40 years in the fields of science and nuclear energy, he made great contributions to ITER in the past 7 years through his effective promotion of ITER. His unexpected passing is a great loss to the international scientific community. Prof. Bigot's outstanding service to ITER and the French Alternative Energies and Atomic Energy Commission (CEA) management ensured both ITER and the CEA projects' respective successes.
Since March 2015, Dr. Bigot, working together with ITER international team members, strove to overcome great difficulties. The ITER project transformed from a prior state that was marked with complex inconsistencies and cultural differences as well as financial challenges, to a project with well-managed multinational manufacturing and design capabilities, and progress in construction. With his wisdom, personal inferences, and great management and negotiation skills, the world's most complicated R&D team was able to work together, even during the pandemic period, with an understanding and collaborative in-house culture. With great interest, I visited the ITER headquarters in Cadarache, France, in 2019 and had a memorable talk with Dr. Bigot. He highly valued Chinese contributions and guided me to the huge assembly site, where a construction team from the China National Nuclear Cooperation (CNNC) did a great job in assembling the center parts of the ITER test bench.
I first became acquainted with Prof. Bigot during his visit in 2014 to CIAE, and later on, I had a number of meetings with him and established a great personal friendship. We had a nice exchange between CEA and CIAE, and we felt similarly and passionately about multiple research areas in nuclear science, spanning from basic research in the Grand Accélérateur National d'Ions Lourds (GANIL) and other CEA research institutes to applied research such as the reprocessing of nuclear waste. Those fields are becoming fruitful and fast-developing areas of research, thanks to Prof. Bigot's extensive efforts.
Prof. Bigot had a long history in research and management and was awarded a number of high-level positions in research and management. When he chaired CEA, Prof. Bigot pushed for sustainable development and international collaboration of national research organizations with research centers in broad research fields, not only in nuclear science but also in clean energy and advanced materials. His contributions helped France to become the most advanced country with electricity powered by nuclear energy, and he collaborated extensively with scientists from the Asia-Pacific region. Due in part to his great efforts, Japan, China, Korea, and other countries and regions established wide networks of collaboration in nuclear fuel cycle technology, fusion research, and nuclear basic research and education. Prof. Bigot's contributions are recognized by many awards; he was a holder of the Japanese Order of the Rising Sun and the recipient of the China Friendship Award.
I miss Prof. Bigot due to his unparalleled vision of the future, his great kindness to collaborators, and his rich multicultural background. In the current challenging world, those great advantages are surely critical factors in order to establish mutual understanding and collaboration with partners, internationally. All of us will remember his life-long desire to create a cleaner world. As Prof. Bigot himself stated, "I've always been concerned with energy issues. Energy has long been the driver of social and economic development. Yet 80 percent of the energy consumed in the world comes from fossil fuels, and we all know that this resource will not last forever. With fusion energy, we hold in our hands the promise of a clean energy resource for millions of years. Harnessing hydrogen fusion is an opportunity we cannot miss. " We will deeply miss Prof. Bernard Bigot as he was a talented leader in physics, a kind friend, and a great educator. In honor of his memory, we will work hard to continue to support scientific research and to promote collaboration between EU, France, Asia, and China, including fusion and nuclear science studies at ITER and with other nuclear research devices.

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