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Région Ile de France
29/01/2025

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Production and stabilization of a spin mixture of ultracold dipolar Bose gases

Researchers of the BEC team from the Laboratoire Kastler Brossel, Collège de France, published an article in the Physical Review Journals.

The authors :

  • Maxime LECOMTE
  • Alexandre JOURNEAUX
  • Julie VESCHAMBRE
  • Jean DALIBARD
  • Raphael LOPES

Binary dipolar mixtures hold great potential for realizing exotic quantum phases, including supersolids—states of matter that combine crystalline order with superfluidity. In this work, we demonstrate the creation and stabilization of a binary mixture of ultracold dysprosium atoms in different internal states, with a spectacular suppression of inelastic dipolar relaxation by two orders of magnitude compared to the Wigner threshold law. This enhanced stability enabled the first estimation of the mixture’s elastic scattering properties. These results pave the way for probing supersolidity and other strongly correlated phases in binary dipolar gases.

© Raphael Lopes, LKB, Collège de France

Schematic representation of the experimental protocol.Left: two orizontal laser beams (Raman 1 and Raman 2) induce a Raman transition between nearest Zeeman sublevels. The vertical laser beam induces a spin-dependent light shift, allowing us to selectively couple the two lowest-energy Zeeman sublevels. Right panel: absorption images of Bose-Einstein condensates in different internal states, captured after time-of-flight (TOF) expansion in the presence of a magnetic field gradient. The rightmost absorption image corresponds to a BEC preparation in |−7⟩ with purity >95%. Dashed lines serve as guides to the eye for the spatial position of atoms in states |−8⟩, |−7⟩, and |−6⟩.

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29/01/2025

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Tunable Generation of Spatial Entanglement in Nonlinear Waveguide Arrays

Researchers of the QITE Photonics team from the MPQ laboratory, recently published an article in the Physical Review Journals.

The authors :

  • A. RAYMOND
  • A. ZECCHETTO
  • J. PALOMO
  • M. MORASSI
  • A. LEMAÎTRE
  • F. RAINERI
  • M.I. AMANTI
  • S. DUCCI
  • F. BABOUX

High-dimensional entangled states of light provide novel possibilities for quantum information, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the spatial degree of freedom stands out as particularly suited for on-chip integration. Traditional photonic circuits generate and manipulate photons in a step-by-step fashion, using a sequence of discrete optical components. By contrast, nonlinear waveguide systems offer a promising alternative where photons can be generated and interfere in a continuous manner, unveiling novel capabilities within a reduced footprint.

Here we exploit this concept to implement a compact and versatile source of spatially entangled states of light in AlGaAs nonlinear waveguides arrays. A classical pump beam injected into one or several waveguides generates photon pairs at telecom wavelength by spontaneous parametric down-conversion (SPDC), thanks to the strong second-order nonlinearity of the material (Fig. 1). These photon pairs then continuously tunnel from one waveguide to the other during their propagation, implementing random quantum walks. Compared to previous studies, the walkers are here generated directly inside the device, and the generation can take place at any position along the propagation axis. Besides a gain of integration, this configuration allows for a progressive buildup of spatial entanglement, due to the interference between quantum walks initiated at all possible longitudinal positions. We use a multi-pump configuration to engineer the output quantum state and implement various types of spatial correlations, violating a nonclassicality criterion by several tens of standard deviations [1]. Combined with the ability to engineer at will the device geometry, this novel approach opens the way to simulate in a controlled environment physical problems that are otherwise difficult to access in condensed-matter systems, such as the Anderson localization of multiparticle states or the topological protection of entanglement.

© F. Baboux, MPQ, Université Paris Cité

(a) Principle of spatial entanglement engineering by cascaded quantum walks in a nonlinear waveguide array.

(b) SEM image of a fabricated AlGaAs nonlinear waveguide array.

(c) Example of measured spatially correlated state and

(d) anticorrelated state.

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27/01/2025

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Loss features in ultracold 162 Dy gases: Two- versus three-body processes

Researchers of the BEC team from the Laboratoire Kastler Brossel, Collège de France, published an article in the Physical Review Journals.

The authors :

  • Maxime LECOMTE
  • Alexandre JOURNEAUX
  • Loan RENAUD
  • Jean DALIBARD
  • Raphael LOPES

Dipolar gases like erbium and dysprosium have a dense spectrum of resonant loss features associated with their strong anisotropic interaction potential. These resonances display various behaviors with density and temperature, implying diverse microscopic properties. Here we quantitatively investigate the low-field (𝐵<6G) loss features in ultracold thermal samples of 162 Dy . The atoms are spin polarized in their lowest internal state so that pure two-body losses due to spin relaxation are forbidden. However, our analysis reveals that some resonances lead to a two-body-like decay law, while others show the expected three-body decay. We present microscopic one-step and two-step models for these losses, investigate their temperature dependence, and detect a feature compatible with a 𝑑-wave Fano-Feshbach resonance that has not been observed before. We also report the variation of the scattering length around these resonances, inferred from the time-of-flight expansion of the condensate.

© Raphael Lopes, LKB, Collège de France

Schematic representation of the two possible processes behind three-body losses in dipolar gases.

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20/01/2025

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Vanishing bulk heat flow in the nu=0 quantum Hall ferromagnet in monolayer graphene

Researchers of SPEC, in collaboration with teams of the C2N (Palaiseau, France) and NIMS (Tsukuba, Japan) as well as the startup CRYOHEMT (Orsay, France) recently published an article in Nature physics.

The authors :

  • Raphaëlle DELAGRANGE
  • Manjari GARG
  • Gaëlle LE BRETON
  • Aifei ZHANG
  • Quan DONG
  • Yong JIN
  • Kenji WATANABE
  • Takashi TANIGUCHI
  • Preden ROULLEAU
  • Olivier MAILLET
  • Patrice ROCHE
  • François PARMENTIER

Under high perpendicular magnetic field and at low temperatures, graphene develops an insulating state at the charge neutrality point. This state, dubbed nu=0, is due to the interplay between electronic interactions and the four-fold spin and valley degeneracies in the flat band formed by the n=0 Landau level. Determining the ground state of nu=0, including its spin and valley polarization, has been a theoretical and experimental undertaking for almost two decades. Here, we present experiments probing the bulk thermal transport properties of monolayer graphene at nu=0, which directly probe its ground state and collective excitations. We observe a vanishing bulk thermal transport, in contradiction with the expected ground state, predicted to have a finite thermal conductance even at very low temperature. Our result highlight the need for further investigations on the nature of nu=0.

© François Parmentier – CNRS

The four possible ground states of ν=0, shown as two spins (red and blue arrows) distributed on the honeycomb lattice of graphene.

Yellow: antiferromagnetic phase: the two opposite pins reside on a separate sublattice.

Purple: ferromagnetic phase: the two spins are aligned, each on its sublattice.

Orange: Kekule distortion phase: the two opposite spins live on a superposition of the two sublattices.

Cyan: sublattice polarized phase: the two opposite spins live on the same sublattice.

The central cartoon depicts the principle of the experiment, where heat is carried for a hot electrode (red) to a cold one (purple) across ν=0. Only the antiferromagnetic and Kekule distortion phase are thermal conductors at low temperature.

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16/01/2025

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Differential polarizability of the strontium intercombination transition at 1064.7 nm

The “Relaxation dynamics in quantum gases” team from the Charles Fabry Laboratory recently published an article in the Physical Review Journals.

The authors :

  • Romaric JOURNET
  • Félix FAISANT
  • Sanghyeop LEE
  • Marc CHENEAU

We measure the scalar, vector, and tensor components of the differential dynamic polarizability of the strontium intercombination transition at 1064.7 nm. We compare the experimental values with the theoretical prediction based on the most recently published spectroscopic data, and find a very good agreement. We also identify a close-to-circular “magic” polarization where the differential polarizability strictly vanishes, and precisely determine its ellipticity. Our work opens new perspectives for laser cooling optically trapped strontium atoms and provides a benchmark for atomic models in the near-infrared spectral range.

© Journet et al.,Phys. Rev. A 110, 032819 (2024)

Differential Stark shift of the 1S0 – 3P1 , m = −1 strontium transition as we vary the ellipticity of the Stark beam polarization. The direction of the atomic dipole (x) is fixed by the external magnetic field B and is parallel to the Stark beam propagation axis. The ellipticity of the Stark beam polarization u is controlled by the orientation of a quarter-wave plate. The differential Stark shift is found to be zero for a magic ellipticity Cmagic = +0.847 ± 0.023 (red dots), corresponding to the latitude +57.9◦ on the Poincaré sphere (red circle).

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