Thirty years ago, the physicist David Pritchard, predicted the existence of a new quantum magic trick to make sometimes atoms invisible. With his colleagues, the Nobel laureate of physics Wolfgang Ketterle has just verified this prediction.
The revolution of quantum physics was initiated by Max Planck’s work on black body radiation, but what will really lead to the discoveries of Louis de Broglie, Warner Heisenberg and Erwin Schrödinger, from 1923 to 1926, it is the joint work of Niels Bohr and Albert Einstein on the structure of atoms and the way they absorb and emit light. They will lead Einstein to the discovery of the laser effect and the famous phenomenon of Bose-Einstein condensation which will be worth Wolfgang ketterle, Eric Cornell and Carl Wieman the Nobel Prize for physical from 2001.
A presentation of quantum physics from the history of its discovery. © CEA Research
Discreet energy levels with precise occupation and transition rules
Bohr and his followers, such as Arnold Sommerfeld, will show that the lines and spectral bands associated with atoms and molecules diatomic can be understood if the electrons in these atomic systems can exist only in discrete energy states. These states can be characterized by integers, and it is possible for electrons to switch from certain energy levels to others by emitting or absorbing photons. Not all quantum jumps are allowed, and there are so-called selection rules for this.
Finally, the study of these jumps shows, like the Nobel Prize in physics Wolfgang Pauli would understand, that electrons cannot pile up on a single energy level, or in other words that there seems to exist in nature an exclusion principle that does not allow two electrons in an atomic system to have the same quantum numbers integers associated with the same energy value for these electrons.
More recently, this principle is implicitly spoken of when discussing what has been called the Pauli blockade and which in fact illustrates this impossibility for electrons, or other analogous particles called fermions (e.g. example of protons, from neutrinos or some quarks), to occupy identical energy states (this only applies to indistinguishable particles of the same kind). The Pauli principle is closely related to the fact that fermions have a cinematic moment intrinsic that we call the spin, characterized by half-integers. An electron can then be seen partially as a sort of spinning magnet, but theanalogy cannot be pushed too far, in particular because the rotation of such a router should lead to speeds greater than that of light.
(This video is visible contrary to what one might think). To model the behavior of the electron in the atom, one, then three, then four quantum numbers were assigned to it. The fourth is spin, which makes the electron “the ultimate magnet” of matter. A spin measured with half-integers leads in quantum theory to the Pauli exclusion principle, responsible for what is called the Pauli blocking for quantum transitions. Without this effect, the electrons of the elements would all be on the same energy level and the chemistry would be much less complex. © Synchrotron SOLEIL
Pauli’s principle, a key to the behavior of matter
However, about thirty years ago, a physicist from the famous MIT (Massachusetts Institute of Technology), David Pritchard, predicted an astonishing consequence of the Pauli blocking phenomenon with populations of ultra-cold atoms behaving like fermions. At the time, he was Wolfgang Ketterle’s mentor when he came as a postdoctoral fellow at MIT. It is therefore perhaps no coincidence that today the Nobel Prize in Physics has just deposited on arXiv with his colleagues an article confirming Pritchard’s prediction.
It concerns a gas atoms of lithium laser cooled to the point of forming a dense grouping of these atoms at a temperature of 20 microkelvins, or about 1 / 100,000 of the temperature of interstellar space. ” This phenomenon had never been observed before, because people were not able to generate packages sufficiently cold and dense atoms Ketterle explains in an MIT statement.
When another laser beam falls on the collective of ultra-cold atoms, it passes through it as if it were almost completely transparent and in practice invisible. Calculations show that in fact it would be totally so if we could reach the absolute zero, which is of course impossible according to the laws of thermodynamics and even from the point of view of Quantum mechanics. Indeed, the Heisenberg inequalities forbid a particle to be completely still and without movement, however the temperature of a gas of particles is directly proportional to thekinetic energy average of a particle gas and therefore to their state of motion.
Normally, when we send photons on atoms, they are diffused like balls thrown on a “hard” sphere and it is for this reason (to put it simply and elementary) that the objects are visible. But when we cool a gas of atoms behaving like fermions, everything happens as if we had a large atom with quantum energy levels occupied not by electrons but with real atoms.
At very low temperature and high density, atoms tend more and more strongly to fully occupy the lower energy levels and they block each other in accordance with the Pauli exclusion principle. All the atoms can no longer transit from one energy level to another since they are generally occupied.
A beam of photons can therefore only act as if the majority of atoms are not there and they cannot therefore be absorbed by the atoms before they re-emit other photons in all directions as is ordinary the case according to the laws of quantum mechanics.
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