When an electron splits in two12 May 2015, by Lisa Zyga

An artistic illustration of electron fractionalization. Whenan electron travels along the outer 1D wire in aninterferometer, the Coulomb interaction between theouter and inner 1D wires produces two types ofexcitation pairs, as shown here: two pulses of the samesign (carrying a net charge) and two pulses of oppositesigns (which together are neutral). Because the twodifferent excitation pairs travel at different velocities, theoriginal electron eventually splits into two distinct chargepulses in the inner wire. Credit: Freulon, et al. ©2015Nature

(Phys.org)—As an elementary particle, the electroncannot be broken down into smaller particles, atleast as far as is currently known. However, in aphenomenon called electron fractionalization, incertain materials an electron can be broken downinto smaller "charge pulses," each of which carriesa fraction of the electron's charge. Althoughelectron fractionalization has many interestingimplications, its origins are not well understood.

Now in a new paper published in NatureCommunications, a team of physicists led byGwendal Fève at the Ecole Normale Supérieure inParis and the Laboratory for Photonics andNanostructures in Marcoussis have applied anexperiment typically used to study photons toinvestigate the underlying mechanisms of electronfractionalization. The method allows theresearchers to observe single-electronfractionalization on the picosecond scale.

"We have been able to visualize the splitting of an

electronic wavepacket into two fractionalizedpackets carrying half of the original electroncharge," Fève told Phys.org. "Electronfractionalization has been studied in previousworks, mainly during roughly the last five years.Our work is the first to combine single-electronresolution—which allows us to address thefractionalization process at the elementaryscale—with time resolution to directly visualize thefractionalization process."

The technique that the researchers used is calledthe Hong-Ou-Mandel experiment, which can beused to measure the degree of resemblancebetween two photons, or in this case electroncharge pulses, in an interferometer. Thisexperiment also requires a single-electron emitter,which some of the same researchers, along withmany others, have recently been developing.

The researchers first analyzed the propagation of asingle electron in the interferometer's outer one-dimensional wire, and then when that electronfractionalized, they could observe the interactionbetween its two charge pulses in the inner one-dimensional wire. As the researchers explain, whenthe original electron travels along the outer wire,Coulomb interactions (interactions betweencharged particles) between excitations in the outerand inner wires produce two types of excitationpairs: two pulses of the same sign (carrying a netcharge) and two pulses of opposite signs (whichtogether are neutral). The two different excitationpairs travel at different velocities, again due toCoulomb interactions, which causes the originalelectron to split into two distinct charge pulses.

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(a) An electron on the outer channel fractionalizes intotwo pulses. (b) A modified scanning electron microscopepicture of the sample. Credit: Freulon, et al. ©2015Nature

The experiment reveals that, when a single electronfractionalizes into two pulses, the final state cannotbe described as a single-particle state, but ratheras a collective state composed of severalexcitations. For this reason, the fractionalizationprocess destroys the original electron particle.Electron destruction can be measured by thedecoherence of the electron's wave packet.

Gaining a better understanding of electronfractionalization could have a variety of implicationsfor research in condensed matter physics, such ascontrolling single-electron currents in one-dimensional wires.

"There has been, during the past years, strongefforts to control and manipulate the propagation ofelectrons in electronic conductors," Fève said. "Itbears many analogies with the manipulations of thequantum states of photons performed in optics. Forsuch control, one-dimensional conductors areuseful, as they offer the possibility to guide theelectrons along a one-dimensional trajectory.However, Coulomb interactions between electronsare also very strong in one-dimensional wires, sostrong that electrons are destroyed: theyfractionalize. Understanding fractionalization isunderstanding the destruction mechanism of anelementary electron in a one-dimensional wire.Such understanding is very important if one wantsto control electronic currents at the elementaryscale of a single electron."

In the future, the researchers plan to performfurther experiments with the Hong-Ou-Mandel

interferometer in order to better understand whyfractionalization leads to electron destruction, andpossibly how to suppress fractionalization.

"The Hong-Ou-Mandel interferometer can be usedto picture the temporal extension (or shape) of theelectronic wavepackets, which is what we used tovisualize the fractionalization process," Fève said."It can also be used to capture the phaserelationship (or phase coherence) between twocomponents of the electronic wavepacket.

"This combined information fully defines the single-electron state, offering the possibility to visualizethe wavefunction of single electrons propagating ina one-dimensional conductor. This would firstprovide a complete understanding of thefractionalization mechanism and in particular how itleads to the decoherence of single-electron states.It would also offer the possibility to test if singleelectrons can be protected from this decoherenceinduced by Coulomb interaction. Can we suppress(or reduce) the fractionalization process byreducing the strength of the Coulomb interaction?We would then be able to engineer and visualizepure single-electron states, preserved fromCoulomb interaction.

"The next natural step is then to address few-particle states and electron entanglement inquantum conductors. Again, the question of thedestruction of such states by Coulomb interactioneffects will be a crucial one."

More information: V. Freulon, et al. "Hong-Ou-Mandel experiment for temporal investigation ofsingle-electron fractionalization." NatureCommunications. DOI: 10.1038/ncomms7854

© 2015 Phys.org

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APA citation: When an electron splits in two (2015, May 12) retrieved 13 May 2015 from http://phys.org/news/2015-05-electron.html

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