Scientists at the Laboratoire Charles Fabry (LCF) in Palaiseau and the University of Lille have for the very first time performed a direct measurement of a Van der Waals force – the weak intermolecular force that causes, in some cases when there isn’t a strong force present, to attract and “stick” to one another. The Van der Waals force is what actually most of the time keeps gas molecules together, as well as liquids and some solids, and allows them to travel in bulk as a fluid. Some animals like geckos use Van der Waals forces to climb just about any surface.

This fantastic achievement was made after the researchers trapped two Rydberg atoms with a laser and then measured the force as a function of the distance separating them. Previously, indirect measurements of Van der Waals force were made with varying degree of accuracy. Examples include analysing the net forces experienced by macroscopic bodies or using spectroscopy to work out the long-range behaviour of the force between two atoms in a diatomic molecule.

“What we have done here, for the first time to our knowledge, is to measure directly the Van der Waals interaction between two single atoms that are located at a controlled distance, chosen by the experimenter,” says Thierry Lahaye, who is part of the LCF team.



Direct measurement of Van der Waals forces has eluded scientists up until now, however, since atoms are very spaced apart by very short distances, making it difficult to measure the distance between them. The French researchers had to use Rydberg atoms to solve one part of the problem. These atoms are marge larger than most of the others, and thus have a large relative distance between them. Also, these atoms have one electron in a highly excited state. This means that they have a very large instantaneous dipole moment – and therefore should have very strong Van der Waals interactions over relatively long distances.

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To measure the Van der Waals between the two Rydberg atoms, the researchers first trapped each individual atom with tightly focused laser beams. Another laser beam fired at a specific wavelength was the shone on the atoms, which caused them to oscillate. By measuring these oscillations, the team worked out the Van der Waals force between the two Rydberg atoms. Moreover, by adjusting the trapping laser beam, the scientists were able to adjust the distance between the two atoms. As the researchers changed the distance R between the atoms, the force varied as 1/R6– exactly as expected for the Van der Waals force.

The French researchers work will most likely have more significant impact in practical applications, then in the actual milestone put in place. With their direct measurement of the Van der Waals, the researchers show that the quantum evolution of the state of the two interacting Rydberg atoms was fully coherent,  identical to that of a quantum-logic gate operating on two quantum bits (qubits).

“This will allow us to engineer small quantum systems of increasing size, from two to hopefully a few tens of Rydberg atoms, over which we have full control of the interactions,” explains Lahaye.

Another step forward towards developing the first quantum information devices. ZME folks, are you ready for the age of quantum computers? We’re really stoked about this; share your thoughts in the comment section below.

The experiment is described in Physical Review Letters. [source Physics World]