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Nobel Prize in Physics: Claude Cohen Tannoudji with Steven Chu and William D. Phillips in 1997

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Source: La lettre, no. 35, December 2012

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Genetics of the Bacterial Cell

1Spectroscopy, the observation of the line spectra emitted by an environment, is therefore an important way of identifying this environment’s components. It is in fact the essential source of information at our disposal in astrophysics, on distant objects like planets, stars and galaxies. Over the last few decades, the focus has shifted to another aspect of atom-light interactions. Light can be used to manipulate atoms and to control their polarization, their position, and their speed. This possibility is the result of conservation laws. When an atom absorbs a photon, it acquires some of this photon’s physical properties, like its energy, its linear momentum and its angular momentum. By suitably choosing the characteristics of a beam resonantly exciting a set of atoms, we can modify these atoms’ properties.

2A first law of conservation is the conservation of the angular momentum. This value characterizes the state of rotation of an object around its axis, similar to the rotation of a spinning top. A circularly polarized light is constituted of photons which all have a certain angular momentum. When we make atoms resonantly absorb these photons, the photons’ angular momentum is transferred to the atoms and it is thereby possible almost to completely polarise atoms, to ensure that all the atoms turn in the same direction around parallel axes.

3I myself wrote a PhD thesis on the subject, which I defended in 1962. This research enabled me to further develop the quantum theory of optical pumping and to predict and observe new effects like atoms’ energy levels being shifted by light. A second law of conservation is that of the linear momentum.

4In the same way that a canon firing a shell moves backwards, an atom which emits a photon moves backwards with a ­linear momentum equal to that of the photon emitted. Likewise, ­during absorption, an atom absorbing a photon moves backwards in the direction of the photon absorbed. With the develop­ment of intense laser sources with great spectral purity thus emerged new research domains like laser cooling which allows for an atom’s speed to be reduced using a laser beam, and for the dispersion of the speeds around their mean value to be reduced. Radiative forces can also be used, depending on the position, and restoring forces obtained which allow for the movement of the atom to be confined to a region in space. The movement of atoms can thus be manipulated ad libitum. Here are a few orders of magnitude: with these methods temperatures of the order of 10-6 Kelvin are commonly reached when the room temperature is of the order of 300 Kelvin. At these temperatures the atoms move at speeds of no more than a few millimetres per second, whereas at room temperature their speed is of the order of a kilometre per second.

5Cold atoms moving much slower can be observed for much longer periods of time. As in physics measurement precision is much greater the longer the observation time, laser cooling affords spectacular improvement of measurement precision, particularly for atomic clocks. For example, it is now possible to produce atomic clocks which would have less than a second’s lag after a billion years. The fundamental theories of physics, like general relativity, can also be tested with far greater precision. Other important research domains have also opened up thanks to cold atoms. They are linked to the fact that the Broglie wavelength of a corpuscle, which characterizes its wave nature, is inversely proportional to its speed. The lower this speed, which is enabled by the cooling of the laser, the greater Broglie’s wavelength, and the easier it becomes to demonstrate the wave nature of atoms. We can thus now produce matter waves associated with cold atoms and perform with them all the well-known experiments in optics.

This is the “optical pumping” imagined
and accomplished by Alfred Kastler and
Jean Brossel in the early 1950s. This method allowed for numerous fundamental discoveries and earned Alfred Kastler the 1966
Nobel Prize in Physics.

6Finally, if one is able to cool trapped atoms very effectively with a sufficiently high density, one can observe a condensation phenomenon resulting from Bose’s quantum statistics and predicted by Einstein at the beginning of the last century: all the atoms cluster together on the same quantum level of the trap con­fining them, thereby forming a macroscopic quantum system. The demonstration in 1995 of these “Bose Einstein condensates” was rewarded by the 2001 Nobel Prize in Physics awarded to Éric Cornell, Carl Wieman and Wolfgang Ketterle. The interest of these condensates stems from the fact that all the physical parameters characterizing them can be controlled with great precision, including the sign and magnitude of the inter­actions between atoms, owing to resonating processes in the collisions between cold atoms which can, transitorily, associate to form molecules. We thus have macroscopic quantum systems which can serve as models to simulate and better understand more complex situations arising in other domains of science like superconductors or neutron stars. Almost three decades after their discovery, atoms cooled and trapped by laser beams therefore continue to allow for the birth of new research fields.

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« Nobel Prize in Physics: Claude Cohen Tannoudji with Steven Chu and William D. Phillips in 1997 », La lettre du Collège de France [En ligne], 7 | 2015, mis en ligne le 29 octobre 2015, consulté le 21 février 2017. URL :

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