Periodic Reporting for period 1 - CriLiN (An Atomic Quantum Simulator with long-range, multi-body interactions)
Okres sprawozdawczy: 2020-09-01 do 2022-08-31
Ultracold atoms have emerged as ideal building blocks for quantum simulations of paradigmatic condensed matter phenomena. In particular, there is a growing interest in simulators built with unequal-mass fermions and long-range interactions, as they are expected to greatly enhance the observability of elusive regimes of quantum matter, primarily in the context of unconventional superfluidity and quantum magnetism. Any experimental insight on this subject would impact material sciences and potentially lead to breakthrough technological advances affecting society as a whole, e.g. novel superconductors.
With CriLiN, I aimed at realizing a novel quantum simulator based on fermionic 6Li and 53Cr atoms. This peculiar combination of laser-cooled atomic species features a mass ratio M/m=8.8. Theory predictions show that the underlying few-body physics of such a Fermi mixture displays exceptional properties, which are expected to foster the observability of elusive superfluid regimes as well as itinerant ferromagnetism on a many-body level.
Before the start of CriLiN, only three Fermi mixture experiments existed worldwide, none of which possessed a suitable mass ratio for the task at hand. This project was highly ambitious and consisted in three non-trivial steps: the realization of a novel quantum degenerate Li-Cr mixture, the investigation of Li-Cr collisional properties in the ultracold regime, and the study of the mixture stability and few-body physics in the vicinity of a magnetic Fano-Feshbach resonance. On top of the lack of information about the inter-species collisional properties, typical to new double-species experiments, trapping and cooling of fermionic Cr represented a challenge of its own. Only moderate quantum degeneracy with a small Fermi gas of Cr had been previously shown in the literature.
We developed an experimental apparatus able to successfully overcome those challenges and we provided theoretical understanding of our atomic system which will be of reference to others. Our work resulted in the only existing machine able to produce strongly-interacting Li-Cr Fermi mixtures and showed the full potential of this system for future many-body physics studies.
With CriLiN, I aimed at realizing a novel quantum simulator based on fermionic 6Li and 53Cr atoms. This peculiar combination of laser-cooled atomic species features a mass ratio M/m=8.8. Theory predictions show that the underlying few-body physics of such a Fermi mixture displays exceptional properties, which are expected to foster the observability of elusive superfluid regimes as well as itinerant ferromagnetism on a many-body level.
Before the start of CriLiN, only three Fermi mixture experiments existed worldwide, none of which possessed a suitable mass ratio for the task at hand. This project was highly ambitious and consisted in three non-trivial steps: the realization of a novel quantum degenerate Li-Cr mixture, the investigation of Li-Cr collisional properties in the ultracold regime, and the study of the mixture stability and few-body physics in the vicinity of a magnetic Fano-Feshbach resonance. On top of the lack of information about the inter-species collisional properties, typical to new double-species experiments, trapping and cooling of fermionic Cr represented a challenge of its own. Only moderate quantum degeneracy with a small Fermi gas of Cr had been previously shown in the literature.
We developed an experimental apparatus able to successfully overcome those challenges and we provided theoretical understanding of our atomic system which will be of reference to others. Our work resulted in the only existing machine able to produce strongly-interacting Li-Cr Fermi mixtures and showed the full potential of this system for future many-body physics studies.
CriLiN was articulated in three main concurrent objectives:
1) Investigation of inter-species Feshbach resonances.
As soon as Li and Cr were brought to the ultracold regime, we focused onto Feshbach spectroscopy [1]. We investigated several spin combinations and performed extensive magnetic field scans unveiling more than 50 inter-species resonances. Thanks to our collaboration with theorist Prof. Andrea Simoni, we assigned a complete set of quantum numbers to each observed resonance and developed an accurate quantum collisional model. The Li-Cr system features a rich but non-chaotic Feshbach spectrum with both s-wave and p-wave well-resolved resonances with negligible two-body losses in several spin states and magnetic fields between 0G and 1500G. We concentrated our attention to s-wave resonances with largest magnetic field width (0.5 G) above 1.4 kG in lowest lying Zeeman levels, which we thoroughly characterized and used to demonstrate resonant tuning of the elastic interactions. We showed that the Feshbach spectrum is suitable for many-body physics studies. On top of that also a few intra-species Cr resonances were observed, setting the basis for future investigation of resonantly interacting homonuclear mixtures of Cr53.
2) Realization of a degenerate 6Li-53Cr Fermi mixture.
Before CriLiN, we had already shown double magneto-optical-trap (MOT) of Li and Cr atoms. In order to reach double degeneracy, we improved the Cr MOT number by two orders of magnitude, and devised an efficient way to transfer Cr atoms directly from the MOT to a bichromatic optical dipole trap, together with Li. Forced evaporation of Li and simultaneous sympathetic cooling of Cr was sufficient to reach the ultracold uK regime, and allowed us to work in parallel on objective 1). This proved crucial, since the understanding of collisional properties informed us on the best strategy to reach quantum degeneracy. In fact, we exploited sympathetic cooling enhanced by a narrow Feshbach resonance to achieve our goal (paper in preparation). Deeper degeneracy was obtained by changing trap geometry from single to cross-beam trap. We simultaneously realize large, degenerate and spin-polarixed samples of 200k Li and 100k Cr with normalized temperature of 0.25 with a 13s overall duty cycle [2]. This performance shows that this mixture is suitable for the practical realization of a quantum simulator. Details on the experimental apparatus and the experimental sequence can be found in [3].
3) Few-body interactions and mixture stability.
Still focusing on high-field s-wave resonances, we measured the lifetime of our mixture as function of the detuning from the resonance pole for several spin-state combinations. Experimental data showed evidence for Feshbach LiCr molecule creation and loss. Thus, we exploited the very same resonances for demonstrating production of large samples of up to 60k bosonic LiCr dimers at 200nK via magneto-association (paper in preparation). We measured the magnetic moment of such a molecule, confirming the assignment of our theory collaborator.
[1] A. Ciamei et al., Phys. Rev. Lett. 129, 093402 (2022)
[2] A. Ciamei et al., preprint arXiv:2207.07579 (2022)
[3] S. Finelli, Master Thesis (University of Florence, 2022)
1) Investigation of inter-species Feshbach resonances.
As soon as Li and Cr were brought to the ultracold regime, we focused onto Feshbach spectroscopy [1]. We investigated several spin combinations and performed extensive magnetic field scans unveiling more than 50 inter-species resonances. Thanks to our collaboration with theorist Prof. Andrea Simoni, we assigned a complete set of quantum numbers to each observed resonance and developed an accurate quantum collisional model. The Li-Cr system features a rich but non-chaotic Feshbach spectrum with both s-wave and p-wave well-resolved resonances with negligible two-body losses in several spin states and magnetic fields between 0G and 1500G. We concentrated our attention to s-wave resonances with largest magnetic field width (0.5 G) above 1.4 kG in lowest lying Zeeman levels, which we thoroughly characterized and used to demonstrate resonant tuning of the elastic interactions. We showed that the Feshbach spectrum is suitable for many-body physics studies. On top of that also a few intra-species Cr resonances were observed, setting the basis for future investigation of resonantly interacting homonuclear mixtures of Cr53.
2) Realization of a degenerate 6Li-53Cr Fermi mixture.
Before CriLiN, we had already shown double magneto-optical-trap (MOT) of Li and Cr atoms. In order to reach double degeneracy, we improved the Cr MOT number by two orders of magnitude, and devised an efficient way to transfer Cr atoms directly from the MOT to a bichromatic optical dipole trap, together with Li. Forced evaporation of Li and simultaneous sympathetic cooling of Cr was sufficient to reach the ultracold uK regime, and allowed us to work in parallel on objective 1). This proved crucial, since the understanding of collisional properties informed us on the best strategy to reach quantum degeneracy. In fact, we exploited sympathetic cooling enhanced by a narrow Feshbach resonance to achieve our goal (paper in preparation). Deeper degeneracy was obtained by changing trap geometry from single to cross-beam trap. We simultaneously realize large, degenerate and spin-polarixed samples of 200k Li and 100k Cr with normalized temperature of 0.25 with a 13s overall duty cycle [2]. This performance shows that this mixture is suitable for the practical realization of a quantum simulator. Details on the experimental apparatus and the experimental sequence can be found in [3].
3) Few-body interactions and mixture stability.
Still focusing on high-field s-wave resonances, we measured the lifetime of our mixture as function of the detuning from the resonance pole for several spin-state combinations. Experimental data showed evidence for Feshbach LiCr molecule creation and loss. Thus, we exploited the very same resonances for demonstrating production of large samples of up to 60k bosonic LiCr dimers at 200nK via magneto-association (paper in preparation). We measured the magnetic moment of such a molecule, confirming the assignment of our theory collaborator.
[1] A. Ciamei et al., Phys. Rev. Lett. 129, 093402 (2022)
[2] A. Ciamei et al., preprint arXiv:2207.07579 (2022)
[3] S. Finelli, Master Thesis (University of Florence, 2022)
The MSCA CriLiN offered a unique opportunity to combine the ER’s experience on laser cooling and spectroscopy on non-alkali species with the host group’s know-how on Fermi mixtures and interactions.
The successful implementation of this action has resulted in:
1)Realization of a novel Fermi mixture together with theoretical understanding and modeling of the underlying two-body physics.
2)Preliminary investigation of mixture stability in preparation for many-body physics studies
3)Realization of large Fermi gases of Cr
4)Realization of high phase-space density gases of LiCr Feshbach molecules
Results 1) and 2) show that theoretical predictions on mass-imbalanced fermions will find their real-world application in an ultracold atomic mixture of Li and Cr, which was not known prior to the present experimental effort. This will have a great impact on the ultracold atom community, and beyond that, to the areas of strongly correlated systems and condensed matter physics. Result 3) represents a substantial technical improvement compared to previous experimental investigations, and it could be exploited to study few-body interactions and many-body physics in a dipolar Fermi Gas. Result 4) could be exploited for the creation of high-density ground state molecules with electric and magnetic dipole moments, of great interest to the ultracold molecule community.
The successful implementation of this action has resulted in:
1)Realization of a novel Fermi mixture together with theoretical understanding and modeling of the underlying two-body physics.
2)Preliminary investigation of mixture stability in preparation for many-body physics studies
3)Realization of large Fermi gases of Cr
4)Realization of high phase-space density gases of LiCr Feshbach molecules
Results 1) and 2) show that theoretical predictions on mass-imbalanced fermions will find their real-world application in an ultracold atomic mixture of Li and Cr, which was not known prior to the present experimental effort. This will have a great impact on the ultracold atom community, and beyond that, to the areas of strongly correlated systems and condensed matter physics. Result 3) represents a substantial technical improvement compared to previous experimental investigations, and it could be exploited to study few-body interactions and many-body physics in a dipolar Fermi Gas. Result 4) could be exploited for the creation of high-density ground state molecules with electric and magnetic dipole moments, of great interest to the ultracold molecule community.