Periodic Reporting for period 1 - 2DDip (Two-dimensional Dipolar Quantum Gases: Fluctuations and Orders.)
Période du rapport: 2022-02-01 au 2024-07-31
The main objective of 2DDip is to study the special ordering features of two-dimensional (2D) quantum gases made of dipolar particles, namely dysprosium magnetic atoms, in experiment.
Ordering in 2D is fundamentally different from what we know in 3D: Thermal fluctuations preclude conventional long-range order. An unconventional ordering mechanism based on topological defects may occur. The resulting order is topological, remains fluctuating, and intimately depends on the interparticle interactions. Many systems, including frustrated magnets, superconductors, colloids, and quantum gases, exhibit such orders. Their topological origin confers them exotic properties, of high technological interest, yet still partly enigmatic.
In ultracold Bose gases of magnetic atoms, short-range and long-range anisotropic dipolar interactions compete. Recently, such gases were observed to stabilize despite attractive mean interactions, through the effect of quantum fluctuations. This fueled the discovery of several new phases of matter in such 3D gases, a prominent example being supersolids, which show simultaneous superfluid and crystal orders (see recent review published during the first year of 2Ddip [ROPP 86, 026401 (2022)]. With key actors in the field, we here review the seminal achievements in quantum gas experiments with magnetic atoms over the last 20 years).
Building on my expertise in both fields, I propose in 2DDip to expand the rich physics of magnetic-gas orders to the exotic 2D universe. Due to the unique variety of orders featured in magnetic quantum gases –superfluid, crystal, plus their anisotropic features (orientational orders)– and their remarkable intertwining, an unprecedented playground for 2D topologically driven ordering will be at hand. By exploring the phase diagram, I will aim to bring a deep understanding of ordering and reveal new quantum phases.
Ordering in 2D is fundamentally different from what we know in 3D: Thermal fluctuations preclude conventional long-range order. An unconventional ordering mechanism based on topological defects may occur. The resulting order is topological, remains fluctuating, and intimately depends on the interparticle interactions. Many systems, including frustrated magnets, superconductors, colloids, and quantum gases, exhibit such orders. Their topological origin confers them exotic properties, of high technological interest, yet still partly enigmatic.
In ultracold Bose gases of magnetic atoms, short-range and long-range anisotropic dipolar interactions compete. Recently, such gases were observed to stabilize despite attractive mean interactions, through the effect of quantum fluctuations. This fueled the discovery of several new phases of matter in such 3D gases, a prominent example being supersolids, which show simultaneous superfluid and crystal orders (see recent review published during the first year of 2Ddip [ROPP 86, 026401 (2022)]. With key actors in the field, we here review the seminal achievements in quantum gas experiments with magnetic atoms over the last 20 years).
Building on my expertise in both fields, I propose in 2DDip to expand the rich physics of magnetic-gas orders to the exotic 2D universe. Due to the unique variety of orders featured in magnetic quantum gases –superfluid, crystal, plus their anisotropic features (orientational orders)– and their remarkable intertwining, an unprecedented playground for 2D topologically driven ordering will be at hand. By exploring the phase diagram, I will aim to bring a deep understanding of ordering and reveal new quantum phases.
During the first two years of the project, my team and I designed, built, and developed a new setup required to tackle the objectives of 2DDip. This new setup now routinely produces degenerate quantum gases of dysprosium atoms in tailorable trap geometries and is soon to be further upgraded to access the quasi-2D regime.
More precisely, during the first year of 2DDip, we designed and implemented a new and high-performance cold dysprosium source based on an innovative scheme. The source has been fully operational since the end of 2022 and its novelty is detailed in the team's first experimental publication (PRA 108, 023719 (2023)).
In the second year of 2DDip, we continue to develop our experimental platform towards the desired tunability and control. We implemented a 3D optical trapping potential based on two crossed laser beams of 1064 nm wavelength with tunable shape. In this trap, we implemented an evaporative cooling scheme which led to the achievement of quantum degeneracy of the 3D Bose gas of dysprosium atoms within 1.5 years after the start of the project. Under optimised conditions, we were able to produce Bose-Einstein condensates of 150,000 atoms with more than 75% condensed fraction, which is amongst the best performance in the state of the art.
We then gained control over the uniform bias magnetic field applied to the atoms, both in terms of its magnitude and direction, and observed the first signatures of interaction tuning. We have implemented a high-resolution imaging objective and have observed uniform superfluid and density-modulated states both in time of flight and in situ through this objective.
We are currently implementing a dedicated trap to reach the quasi-2D regime, a so-called 'accordion lattice trap', and a fully tailorable in-plane trap based on a digital micromirror device setup. Both setups have been designed and thoroughly tested. With the implementation of these traps, we expect to have a fully operational experimental setup to produce, address, and probe ultracold quasi-2D dipolar gases within the next few months. This will serve as the core for 2DDip investigations in the coming years.
On the other hand, we have also carried out preliminary theoretical work on several aspects related to the 2DDip core research, ranging from the exploration of the phase diagram, the investigation of the nature of the underlying phase transition, the role and interplay of topological defects in solid and superfluid orders, as well as the more practical effect of potential defects on the underlying physics.
Based on this preliminary work and experimental advances, we will now begin our central investigation into the special ordering features of quasi-2D quantum gases of dipolar atoms.
More precisely, during the first year of 2DDip, we designed and implemented a new and high-performance cold dysprosium source based on an innovative scheme. The source has been fully operational since the end of 2022 and its novelty is detailed in the team's first experimental publication (PRA 108, 023719 (2023)).
In the second year of 2DDip, we continue to develop our experimental platform towards the desired tunability and control. We implemented a 3D optical trapping potential based on two crossed laser beams of 1064 nm wavelength with tunable shape. In this trap, we implemented an evaporative cooling scheme which led to the achievement of quantum degeneracy of the 3D Bose gas of dysprosium atoms within 1.5 years after the start of the project. Under optimised conditions, we were able to produce Bose-Einstein condensates of 150,000 atoms with more than 75% condensed fraction, which is amongst the best performance in the state of the art.
We then gained control over the uniform bias magnetic field applied to the atoms, both in terms of its magnitude and direction, and observed the first signatures of interaction tuning. We have implemented a high-resolution imaging objective and have observed uniform superfluid and density-modulated states both in time of flight and in situ through this objective.
We are currently implementing a dedicated trap to reach the quasi-2D regime, a so-called 'accordion lattice trap', and a fully tailorable in-plane trap based on a digital micromirror device setup. Both setups have been designed and thoroughly tested. With the implementation of these traps, we expect to have a fully operational experimental setup to produce, address, and probe ultracold quasi-2D dipolar gases within the next few months. This will serve as the core for 2DDip investigations in the coming years.
On the other hand, we have also carried out preliminary theoretical work on several aspects related to the 2DDip core research, ranging from the exploration of the phase diagram, the investigation of the nature of the underlying phase transition, the role and interplay of topological defects in solid and superfluid orders, as well as the more practical effect of potential defects on the underlying physics.
Based on this preliminary work and experimental advances, we will now begin our central investigation into the special ordering features of quasi-2D quantum gases of dipolar atoms.
On the experimental side, our main achievement beyond the state of the art to date is the design and implementation of a new and high-performance cooling scheme for dysprosium atoms. This achievement is reported in our first experimental publication (PRA 108, 023719 (2023)) and has high potential to be adopted by several groups worldwide. We have already been contacted by several experts in the field to obtain more detailed information on the scheme. This could set a new standard for quantum gas experiments with magnetic atoms.
We have also developed a theoretical line of research that is preliminary to the experimental exploration of 2DDip and already represents significant achievements. This is the theoretical study of the properties of the continuous transition from superfluid to supersolid. In particular, we have shown discontinuities in the elementary excitation and response properties of the system (PRR 5, 033161 (2023)). This paves the way for further studies in 2D geometries and in experimentally relevant traps. We have also theoretically investigated the role of potential defects on the quantum gas density under the competition of contact and dipolar interactions close to experimental considerations (arXiv:2403.04719).
We have also developed a theoretical line of research that is preliminary to the experimental exploration of 2DDip and already represents significant achievements. This is the theoretical study of the properties of the continuous transition from superfluid to supersolid. In particular, we have shown discontinuities in the elementary excitation and response properties of the system (PRR 5, 033161 (2023)). This paves the way for further studies in 2D geometries and in experimentally relevant traps. We have also theoretically investigated the role of potential defects on the quantum gas density under the competition of contact and dipolar interactions close to experimental considerations (arXiv:2403.04719).