Image obtained with the LKB's quantum gas microscope. Each red dot corresponds to the fluorescence signal from a single lithium atom frozen in an optical lattice.
Image obtained with the LKB's quantum gas microscope. Each red dot corresponds to the fluorescence signal from a single lithium atom frozen in an optical lattice. © Joris Verstraten et al., 2024.
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How can a quantum wave packet be observed in situ?

Résultat scientifique

Researchers have succeeded in directly observing the free expansion of a single atom's quantum wave packet in continuous space. Using a new imaging technique that combines optical trapping and quantum gas microscopy, they have tracked the spread of the probability density associated with an atom released from its trap with unprecedented precision.

Références

In Situ Imaging of a Single-Atom Wave Packet in Continuous Space, Joris Verstraten et al., Phys. Rev. Lett. 134, 083403– Publié le  28 Février 2025
Doi :10.1103/PhysRevLett.134.083403
Open-access archive : arXiv  
 

Since the work of Louis de Broglie in 1924, the notion of wave-particle duality has been at the heart of quantum mechanics. When an atom is confined in a trap, its wave function is localized as a particle would be, but as soon as it is released (by extinction of the trap), it gradually spreads out in space, according to an evolution predicted by Schrödinger's famous equation. Directly observing the dynamics of such an object in free space is a major experimental challenge, one that would provide access to the fundamental properties of many quantum systems. Until now, experiments on quantum gases with resolution at the level of the individual atom - a technique known as quantum gas microscopy - have been limited to ensembles of atoms evolving in a discrete structure made up of spatially periodic traps, and have not allowed the study of continuous quantum systems.

Ces recherches ont été menées dans les laboratoires suivants
 

  • Laboratoire Kastler Brossel (LKB, CNRS/Collège de France/ENS-PSL/Sorbonne Université)
  • Laboratoire collision agrégats réactivité (LCAR, CNRS / Université de Toulouse)

A recent study opens the way to quantum gas microscopy in continuous space. In their experiment, carried out at Laboratoire Kastler Brossel, the researchers first prepared wave packets of individual lithium atoms confined in the potential wells of an optical lattice. Once released in a plane where they could move freely, these atoms were recaptured using a projection method in a deep optical lattice, enabling precise reconstruction of their displacement. By repeating this experiment on a large number of atoms, the researchers were able to reconstruct the spatio-temporal evolution of the wave function with unequalled fidelity. This study establishes a protocol for projecting out an atom from continuous space to the nearest lattice site in a controlled manner, with a fidelity greater than 99%, and lays the foundations for quantum gas microscopy in continuous space. The imaging method developed here can thus be compared to a “CCD sensor” for atomic wave functions.

Beyond this experimental validation, the new method for imaging individual atoms developed by the research group paves the way to the study of more complex quantum systems in continuous space. This imaging method is already being used by the study's researchers to explore strongly correlated fermion systems at the microscopic scale, which are difficult to tackle theoretically. These results are published in the Physical Review Letters.

Figure (a). Image obtenue avec le microscope à gaz quantique du LKB. Chaque point lumineux rouge correspond au signal de fluorescence d’un seul atome de lithium figé dans un réseau optique. En haut à droite de (a), zoom sur un atome individuel. La taille observée est déterminée par les caractéristiques de l'objectif de microscope utilisé et la pixellisation est due à la caméra de détection. En bas à droite de (a), zoom sur une région de l’image où l’on a reconstruit les positions des sites du réseau optique (points blancs). (b). Protocole expérimental utilisé pour la mesure de l’expansion libre du paquet d’onde d’atomes individuels. Après expansion, le réseau optique est allumé brusquement, menant à la projection des atomes depuis l’espace continu sur les sites du réseau. À chaque réalisation expérimentale, l’atome se localise sur un site différent, mais la densité de probabilité |ψ(r,t)|2 peut néanmoins être obtenue en moyennant les positions obtenues sur de nombreuses répétitions. La méthode d’imagerie développée ici (Microscopie de gaz quantique en espace continu), peut être comparée à un « capteur CCD » pour des fonctions d’onde atomiques.
Figure (a). Image obtained with the LKB's quantum gas microscope. Each red dot corresponds to the fluorescence signal from a single lithium atom frozen in an optical lattice. Top right of (a), zoom in on an individual atom. The observed size is determined by the characteristics of the microscope objective used, and the pixelation is due to the detection camera. Bottom right of (a), zoom on a region of the image where the positions of the optical lattice sites have been reconstructed (white dots). (b). Experimental protocol used to measure the free expansion of the wave packet of individual atoms. After expansion, the optical grating is switched on abruptly, leading to the projection of atoms from continuous space onto the grating sites. At each experimental realization, the atom localizes on a different site, but the probability density |ψ(r,t)|2 can nevertheless be obtained by averaging the positions obtained over many repetitions. The imaging method developed here (Continuous Space Quantum Gas Microscopy), can be compared to a “CCD sensor” for atomic wave functions. Copyright : Joris Verstraten et al., 2024.


 

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Tarik Yefsah
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