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A new state of matter shows up when a dilute sample of atoms is cooled below a critical ultralow temperature. Then occurs a phenomenon known as the Bose-Einstein Condensation; this phase transition comes along with the building up of a macroscopic material system whose properties are non-classical, ie are dictated by the laws of quantum theory. We perform experiments to study these quantum gases with a special focus on their magnetic properties.
Achieving ultralow temperatures
The interaction of laser light with properly chosen atoms in gas phase results in cooling them down to temperatures incredibly close to absolute zero (typically below 100 microKelvin). Since this cooling process is performed under vacuum on dilute samples, the atom cloud remains gaseous. Combining laser beams with magnetic fields, the atoms can be trapped. At this stage, one gets a remarkable system made of atoms whose velocities are on the order of cm/s, which are much less than those of corresponding gases at room temperature (on the order of some 100 m/s). Besides, these atoms are pinned within a volume smaller than or on the order of 1 mm3. As a result, the atom properties can be scrutinized with unprecedented precision. The performances of atom clocks have been upgraded for t(e beNefht of time definition with applications to GPS and to network synchronization. As well, the precision of inertial sensors based on atomic physics experiments nowadays overcomes that of classical instruments.
During the last century, the quest for low temperatures always came along with scientific breakthroughs among which super-fluidity and super- conductivity are the most famous. In the field of atom, molecular and optical physics, it became possible in the late 90’s to lower the temperature of cold trapped atoms by at least three orders of magnitude (down to the 100 nano-Kelvin range) through evaporative cooling schemes (selective elimination of the hottest atoms). When the temperature goes down below the critical temperature for the Bose-Einstein transition, something weird occurs. The sample is no more a gas with moderate thermal agitation; it becomes a macroscopic quantum object as all the atoms shrink into the quantum state of lowest energy. The resulting “super-atom”, known as a condensate, then forms a kind of atom laser. Will this new coherent atomic source give rise to as many applications as its optical counterpart, the laser, is to date an open question.
Exploring quantum magnetism
Our team has constructed an experimental setup to generate Bose-Einstein condensates (BECs) made of Chromium atoms. These atoms bear unusual properties due to their exceptionally high magnetic dipole moment. By transferring the chromium BECs into optical lattices, we create and study artificial systems of perfect purity and valuable tunability. Indeed, we can change almost at will their temperature, density, interactions, confining potential strength and shape, etc. Such systems mimic complex systems at the heart of modern condensed matter physics most notably in relation with magnetic properties. Ultra-cold atom physics is growing as a fascinating interdisciplinary domain. We are currently building a new experimental setup dedicated to the production of quantum gases made of strontium atoms. Our aim is to create and study new exotic magnetic materials.
B. Naylor et al., Phys. Rev. Lett., 115, 243008 (2015).
A. de Paz et al., Phys. Rev. Lett., 111, 185305 (2013).
Fig 1: Partial view of the Cr-BEC setup; the green laser beams create the optical lattices that trap the atoms.
Fig 2: Formation of the chromium BEC by forced evaporation in an optical trap.
Bruno Laburthe-Tolra, Olivier Gorceix