arXiv subjects

Alessio Ciamei

Publications and source records attributed to Alessio Ciamei.

10 recordsLinked to original sources

arXiv2026

Anomalous diffusion and localization in a disorder-free atomic mixture

The concept of random walk, in which particles or waves undergo multiple collisions with the microscopic constituents of a surrounding medium, is central to understanding diffusive transport across many research areas. However, this paradigm may break down in complex systems, where quantum interference and memory effects render the particle propagation anomalous, often fostering localization. Here we report on the observation of such anomalous dynamics in a minimal setting: an ultracold mass-imbalanced mixture of two fermionic gases in three dimensions. We release light impurities into a gas of heavier atoms and follow their evolution across different collisional regimes. Under strong interspecies interactions, by lowering the temperature we unveil a crossover from normal diffusion to subdiffusion. Simultaneously, a localized fraction of the light gas emerges, displaying no discernible dynamics over hundreds of collisions. Our findings, incompatible with the conventional Fermi-liquid picture, are instead captured by a model of an atom propagating through a (quasi-)static disordered landscape of point-like scatterers. These results highlight the key role of quantum interference in our mixture, which emerges as a versatile platform for exploring disorder-free localization phenomena.

cond-mat.quant-gas↗

arXiv2025

Ultracold high-spin $Σ$-state polar molecules for new physics searches

We propose high-spin $Σ$-state polar molecules assembled from ultracold atoms to probe charge-parity violating physics beyond the Standard Model. We identify YbCr as a prime candidate to search for the electric dipole moment of the electron. We show that the combination of relativistic ytterbium and high-spin chromium, amenable to magneto-association, leads to molecules with easy-to-polarize parity doublets and large intramolecular electric fields. Based on \textit{ab initio} results for molecular constants, we predict a sensitivity of $δd_{\textrm{e}}= ( 6 \times 10^{-31} / \sqrt{n_{\mathrm{day}}})\,e\,\mathrm{cm}$ via standard spin-precession measurements, we assess the experimental feasibility, and discuss potential extensions to more advanced quantum control as well as searches of the nuclear magnetic quadrupole moment. This work paves the way to next-generation searches for new physics with ultracold molecules in both the leptonic and hadronic sectors.

cond-mat.quant-gas↗

arXiv2024

Ultracold LiCr: a new pathway to quantum gases of paramagnetic polar molecules

Quantum gases of doubly-polar molecules represent appealing frameworks for a variety of cross-disciplinary applications, encompassing quantum simulation and computation, controlled quantum chemistry and precision measurements. Through a joint experimental and theoretical study, here we explore a novel class of ultracold paramagnetic polar molecules combining lithium alkali and chromium transition metal elements. Focusing on the specific bosonic isotopologue $^{6}$Li$^{53}$Cr, leveraging on the Fermi statistics of the parent atomic mixture and on suitable Feshbach resonances recently discovered, we produce up to $50\times10^3$ ultracold LiCr molecules at peak phase-space densities exceeding 0.1, prepared within the least-bound rotationless level of the LiCr electronic $sextet$ ground state $X^6Σ^+$. We thoroughly characterize the molecular gas, demonstrating the paramagnetic nature of LiCr dimers and the precise control of their quantum state. We investigate their stability against inelastic processes and identify a parameter region where pure LiCr samples exhibit lifetimes exceeding 0.2 s. Parallel to this, we employ state-of-the-art quantum-chemical calculations to predict the properties of LiCr ground and excited electronic states. We identify efficient paths to coherently transfer weakly-bound LiCr dimers to their absolute ground state, to deliver ultracold gases of doubly-polar molecules with significant electric (3.3 D) and magnetic ($5\,μ_\text{B}$) dipole moments.

cond-mat.quant-gas↗

arXiv2020

Realization of a Cold Mixture of Fermionic Chromium and Lithium Atoms

We report on the production of a novel cold mixture of fermionic $^{53}$Cr and $^{6}$Li atoms delivered by two Zeeman-slowed atomic beams and collected within a magneto-optical trap (MOT). For lithium, we obtain clouds of up to $4 \,10^8$ atoms at temperatures of about $500\,μ$K. A gray optical molasses stage allows us to decrease the gas temperature down to $45(5)\,μ$K. For chromium, we obtain MOTs comprising up to $1.5\, 10^6$ atoms. The availability of magnetically trappable metastable $D$-states, from which $P$-state atoms can radiatively decay onto, enables to accumulate into the MOT quadrupole samples of up to $10^7$ $^{53}$Cr atoms. After repumping $D$-state atoms back into the cooling cycle, a final cooling stage decreases the chromium temperature down to $145(5)\,μ$K. While the presence of a lithium MOT decreases the lifetime of magnetically trapped $^{53}$Cr atoms, we obtain, within a 5 seconds duty cycle, samples of about $4\, 10^6$ chromium and $1.5\,10^8$ lithium atoms. Our work provides a crucial step towards the production of degenerate Cr-Li Fermi mixtures.

cond-mat.quant-gas↗

arXiv2019

Realization of a high power optical trapping setup free from thermal lensing effects

Transmission of high power laser beams through partially absorbing materials modifies the light propagation via a thermally-induced effect known as thermal lensing. This may cause changes in the beam waist position and degrade the beam quality. Here we characterize the effect of thermal lensing associated with the different elements typically employed in an optical trapping setup for cold atoms experiments. We find that the only relevant thermal lens is represented by the $TeO_2$ crystal of the acousto-optic modulator exploited to adjust the laser power on the atomic sample. We then devise a simple and totally passive scheme that enables to realize an inexpensive optical trapping apparatus essentially free from thermal lensing effects.

cond-mat.quant-gas↗

arXiv2018

The RbSr $^2Σ^+$ ground state investigated via spectroscopy of hot & ultracold molecules

We report on spectroscopic studies of hot and ultracold RbSr molecules, and combine the results in an analysis that allows us to fit a potential energy curve (PEC) for the X(1)$^2Σ^+$ ground state bridging the short-to-long-range domains. The ultracold RbSr molecules are created in a $μ$K sample of Rb and Sr atoms and probed by two-colour photoassociation spectroscopy. The data yield the long-range dispersion coefficients $C_6$ and $C_8$, along with the total number of supported bound levels. The hot RbSr molecules are created in a $1000 \,$K gas mixture of Rb and Sr in a heat-pipe oven and probed by thermoluminescence and laser-induced fluorescence spectroscopy. We compare the hot molecule data with spectra we simulated using previously published PECs determined by three different ab-initio theoretical methods. We identify several band heads corresponding to radiative decay from the B(2)$^2Σ^+$ state to the deepest bound levels of X(1)$^2Σ^+$. We determine a mass-scaled high-precision model for X(1)$^2Σ^+$ by fitting all data using a single fit procedure. The corresponding PEC is consistent with all data, thus spanning short-to-long internuclear distances and bridging an energy gap of about 75% of the potential well depth, still uncharted by any experiment. We benchmark ab-initio PECs against our results, and give the PEC fit parameters for both X(1)$^2Σ^+$ and B(2)$^2Σ^+$ states. As first outcomes of our analysis, we calculate the $s$-wave scattering properties for all stable isotopic combinations and corroborate the locations of Fano-Feshbach resonances between alkali Rb and closed-shell Sr atoms recently observed [Barbé et al., Nat. Phys., 2018, DOI:10.1038/s41567-018-0169-x]. These results should greatly contribute to the generation of ultracold alkali$-$alkaline-earth dimers, whose applications range from quantum simulation to quantum chemistry.

physics.atom-ph↗

arXiv2017

Observation of Feshbach resonances between alkali and closed-shell atoms

Magnetic Feshbach resonances are an invaluable tool for controlling ultracold atoms and molecules. They can be used to tune atomic interactions and have been used extensively to explore few- and many-body phenomena. They can also be used for magnetoassociation, in which pairs of atoms are converted into molecules by ramping an applied magnetic field across a resonance. Pairs of open-shell atoms, such as the alkalis, chromium, and some lanthanides, exhibit broad resonances because the corresponding molecule has multiple electronic states. However, molecules formed between alkali and closed-shell atoms have only one electronic state and no broad resonances. Narrow resonances have been predicted in such systems, but until now have eluded observation. Here we present the first observation of magnetic Feshbach resonances in a system containing a closed-shell atom, Sr, interacting with an alkali atom, Rb. These resonances pave the way to creating an ultracold gas of strongly polar, open-shell molecules, which will open up new possibilities for designing quantum many-body systems and for tests of fundamental symmetries.

physics.atom-ph↗

arXiv2017

Observation of Bose-enhanced photoassociation products

We produce ${^{84}\mathrm{Sr}_2}$ molecules using Bose-enhanced Raman photoassociation. We apply the stimulated Raman adiabatic passage (STIRAP) technique on a Bose-Einstein condensate (BEC) to produce more than $8 \times 10^3$ ultracold molecules. This chemical reaction is only made possible because of the Bose enhancement of the optical transition dipole moment between the initial atomic state and an intermediate molecular state. We study the effect of Bose enhancement by measuring the transition Rabi frequency in a BEC and by comparing it with measurements for two atoms in sites of a Mott insulator. By breaking the dimers' bond and directly observing the separated atoms, we measure the molecular inelastic collision rate parameters. We discuss the possibility of applying Bose-enhanced STIRAP to convert a BEC of atoms into a BEC of molecules, and argue that the required efficiency for STIRAP is within experimental reach.

cond-mat.quant-gas↗

arXiv2017

Efficient production of long-lived ultracold Sr$_2$ molecules

We associate Sr atom pairs on sites of a Mott insulator optically and coherently into weakly-bound ground-state molecules, achieving an efficiency above 80\%. This efficiency is 2.5 times higher than in our previous work [S. Stellmer, B. Pasquiou, R. Grimm, and F. Schreck, Phys. Rev. Lett. 109, 115302 (2012)] and obtained through two improvements. First, the lifetime of the molecules is increased beyond one minute by using an optical lattice wavelength that is further detuned from molecular transitions. Second, we compensate undesired dynamic light shifts that occur during the stimulated Raman adiabatic passage (STIRAP) used for molecule association. We also characterize and model STIRAP, providing insights into its limitations. Our work shows that significant molecule association efficiencies can be achieved even for atomic species or mixtures that lack Feshbach resonances suitable for magnetoassociation.

cond-mat.quant-gas↗

arXiv2014

Fermionic statistics suppresses Fano resonances

Fano resonances and bound states with energy in the continuum are ubiquitous phenomena in different areas of physics. Observations, however, have been limited so far to single-particle processes. In this work we experimentally investigate the multi-particle case and observe Fano interference in a non-interacting two-particle Fano-Anderson model by considering propagation of two-photon states in engineered photonic lattices. We demonstrate that the quantum statistics of the particles, either bosonic or fermionic, strongly affects the decay process. Remarkably, we find that the Fano resonance, when two discrete levels are coupled to a continuum, is suppressed in the fermionic case.

quant-ph↗