Electron Hole, Meet Your Fractional Cousin

Researchers have designed a nano-electronic circuit that can tease into existence a strange new kind of quantum “particle.” Its existence confirms decades of speculation about the behavior of electronic circuits in very low temperatures and high magnetic fields—and opens the door for possible applications in next-generation quantum computers.

However, this quasiparticle carries only a fraction of an electron’s charge. It is, to be clear, not substantively an actual single particle but rather more likely an ensemble of electrons acting collectively in certain extreme quantum environments. The excitation does, in other significant ways, behave like a particle. Much like an “electron hole” in conventional semiconductors, this “anyon” acts as if it’s its own discrete entity with its own characteristic mass, charge and spin.

And, unlike the +1 charge of an electron hole, these newly studied anyons (whose name Nobel laureate Frank Wilczek jokingly coined after their seemingly “anything goes” nature) carry just one-third of the electron’s charge.

James Nakamura, postdoctoral researcher in the lab of Michael Manfra at Purdue University, said the quantum trajectories of the anyon are also curious. Its paths through the test circuit interact with other anyons—and indeed even with other quantum incarnations of itself moving through other elements of the circuit—and form interference patterns.

These interference patterns are analogous, Nakamura said, to the wavy patterns of ripples in a conventional laser interferometer. Except, instead of the patterns of light and darkness on a screen that a laser interferometer produces, this interferometer tracks anomalous shifts in conductance as parameters like gate voltage and magnetic field strength are slowly varied.

The circuit—cooled to 10 thousandths of a degree above absolute zero (10 milli-Kelvin) and immersed in a powerful magnetic field of 9 Tesla—exhibits discrete jumps in its conductance. Manfra, Nakamura, and co-authors infer from these observations the presence of the longhypothesized anyon.

The finding recalls Robert Millikan’s 1909 oil drop experiments that measured the electron’s fundamental charge. Only this time, the Manfra group discovered a quantum of charge that is only 33 percent of that contained by the seemingly indivisible electron.

The group, which published its finings in a recent issue of the journal Nature Physics, not only adduce the existence of these 1/3-charged anyons but they also track how the anyons evolve as they move through the interferometer.

“Quantum mechanical phase is a very subtle thing,” Nakamura said. “But there is a way you can see phases, and that’s through interference measurements… Electrons, since they’re quantum mechanical, have a phase. Also these quasiparticles have a phase. And that’s what we’re studying.”

Nakamura says the group’s experiment and these fractionally charged anyons may not have immediate applications for any quantum technologies yet devised. However, he said, slightly weaker magnetic fields with slightly different conditions are also expected to produce an anyon with 1/4 of an electron’s charge.

This quasiparticle has already been discussed as a possible fault-tolerant qubit for an advanced “topological” quantum computer that codes its quantum information in the anyon’s changing state as it interacts with itself and other anyons moving through a circuit.

But all of that would depend on future experiments that begin by first doing what Manfra, Nakamura, an co-authors did for the 1/3 anyon: observing the quasiparticle and proving that you can track it through the circuits of a nano-sized interferometer. Then it would be possible to discover what a universe composed of fractional charges can cook up.

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