The leviton, a silent electron wave

These results were published in the October 31 edition of the journal Nature.

It is not easy to generate and transport an electrical charge through a conductor on the quantum scale. Attempts to do so are faced with one major difficulty. The charges are moving in a conductor that is full of electrons, and the introduction of an additional charge causes all the other charges to start moving. It is as though a single drop falling into an ocean could cause ripples the size of giant waves! In the quantum world, these waves of electrons manifest themselves in the form of fluctuating electrical currents. The end result is known as 'shot noise'[1].

The researchers were investigating a hypothesis proposed almost twenty years ago by the MIT theorist, Leonid Levitov. If a current pulse is applied to a conductor in a specific way, it may be possible to avoid generating any waves in the sea of electrons within the conductor. In order to fulfill the necessary conditions, the electrical charge must be a multiple of that of a single electron, and its distribution in the time domain must be Lorentzian.

Physicists from the CEA and the CNRS have succeeded in injecting such a pulse into a conductor. The pulse duration was no more than a few tens of picoseconds, achieved by using a system generating arbitrary signals every 40 picoseconds (10-12 s). The conductor and noise detector were designed by experts from the CEA Nanoelectronics Laboratory, a pioneer in noise measurement technology since the 1990s, in collaboration with the CNRS Laboratory for Photonics and Nanostructures, specialists in the development of high quality structural nanotechnologies since the mid-1980s.

The nanocircuit used included a quantum point contact intended to restrict the geometry of the conductor (nanowire). This contact consisted of two nano-electrodes mounted perpendicularly to the direction of charge flow and separated by just thirty nanometers. The measurement of the noise and its extinction was a considerable achievement as at no time did it exceed one femtoampere (10-15 ampere). The researchers were able to verify experimentally that only those electronic excitations satisfying the criteria specified by Leonid Levitov could extinguish the noise.

By analogy with solitons[2], solitary waves capable of propagating over very long distances without alteration, the researchers named these new fundamental excitations levitons, a contraction of Levitov and soliton.

This work opens up the possibility of new simple and reliable 'on demand' electron sources, capable of injecting signals consisting of just a few electrons into a conductor. In addition to its interest for quantum physicists, the generation of levitons is based on a remarkable property of wave modulation which may have applications in quantum information systems and other areas of physics.

This work has benefitted from European funding in the form of an European Research Council (ERC) grant awarded in 2008: Advanced Grant MeQuaNo (Mesoscopic quantum noise: From few electron statistics to shot noise based photon detection).

[1] Shot noise is the electronic analog of photon noise, and is associated with the particulate nature of these particles.

[2] Occurring in a number of physical phenomena, a soliton is a wave that propagates without deformation in a non-linear and dispersive medium. A tidal bore is an example of a soliton.