In solids, the interactions between electrons and atoms conspire to produce the material's properties: how well it conducts electricity, how magnetic it is, and so forth. When an external voltage is applied, the result is a number of particle-like quantum excitations. The problem is that these excitations come in groups, the result of complex interactions in the material. However, just as it is sometimes useful to have access to single electrons or photons, researchers would like to make and manipulate single excitations.

A new experiment has managed to do so, using an idea from earlier theoretical calculations. By crafting the variation in strength of the voltage they sent through the material, J. Dubois and colleagues produced single quantum excitations. They named these particle-like objects "levitons" for physicist Leonid S. Levitov, who predicted their existence in a set of papers with various collaborators. The discovery opens up an entirely new subfield of experiments involving quantum excitations.

When you pluck a guitar string, both the string and the guitar body vibrate at many different frequencies simultaneously, creating the fundamental tone and its overtones. The combination of frequencies is what makes a guitar distinct from another instrument. That idea is analogous to quantum excitations in most materials: if you apply an external voltage (pluck the string), it produces fluctuations that behave like particles (the vibrating string) but also affect the bulk of the material (the guitar body). The particular excitations depend on the material's properties and the temperature; lower temperatures reduce the possibility of random fluctuations dominating the excitations.

For most purposes, we don't care about the noise of random fluctuations. However, quantum excitations are particle-like, which is why they are often known as quasiparticles. They have mass and electric charge, which means they interact with each other and with the electrons that gave them birth.

In some circumstances, physicists would like to have a single quasiparticle that is not coupled strongly to the material, something that behaves more like a free particle. After all, free particles can be more easily controlled, and multiple free particles can be entangled with each other, which is the basis for quantum computing and networking.

Solitary quasiparticle excitations were predicted in a series of papers by Levitov and colleagues in publications that started in 1996. To make them in practice requires a voltage pulse with a particular time dependence. The resultant quasiparticle—a leviton—behaves like a non-dispersive wave called a soliton. The most famous type of soliton is a tsunami, which wreaks destruction because it does not disperse its energy as it travels across the ocean. Other solitons exist in optical communications and narrow water channels.

While levitons are not literally solitons, they share many characteristics, including their independence from the underlying medium. They also have sharp peaks in their energy spectrum, which is one of their particle-like aspects. The excitations have specific widths, controlled by the duration of the original voltage pulse. They may also have a range of electric charges (both positive and negative), including fractions of the fundamental charge carried by electrons. (The study of quasiparticles carrying fractional charges was awarded the Nobel Prize in 1998.)

The newly published paper describes the first experimental realization of levitons in a semiconductor. Besides actually producing them, the researchers needed to demonstrate that they had made something compatible with theory, which required scattering signals off the quasiparticles. (Yes, you can scatter "real" particles off quasiparticles. Quantum excitations are real.) The behavior of levitons is distinct from that of other excitations arising from electrons, so the results were unambiguous.

Unlike other quasiparticles, levitons don't require delicate nanofabrication techniques, just cold temperatures and more-or-less ordinary semiconductors. This is exciting, as it potentially inaugurates a whole new regime for experiments on quantum excitations. With single-quasiparticle states to work with, the potential for leviton entanglement and quantum logic experiments is huge.

Nature, 2013. DOI: 10.1038/nature12713 (About DOIs).