Physicists control quantum tunneling with light for the first time

April 5, 2012

Scientists at the Cavendish Laboratory in Cambridge have used light to help push electrons through a classically impenetrable barrier. While quantum tunnelling is at the heart of the peculiar wave nature of particles, this is the first time that it has been controlled by light. Their research is published today, 05 April, in the journal Science.

Particles cannot normally pass through walls, but if they are small enough says that it can happen. This occurs during the production of and in many as well as in scanning tunnelling microscopes.

According to team leader, Professor Jeremy Baumberg, "the trick to telling electrons how to pass through walls, is to now marry them with light".

This marriage is fated because the light is in the form of cavity photons, packets of light trapped to bounce back and forth between mirrors which sandwich the electrons oscillating through their wall.

Research scientist Peter Cristofolini added: "The offspring of this marriage are actually new indivisible particles, made of both light and matter, which disappear through the slab-like walls of semiconductor at will."

One of the features of these new particles, which the team christened 'dipolaritons', is that they are stretched out in a specific direction rather like a bar magnet. And just like magnets, they feel extremely strong forces between each other.

Such strongly interacting particles are behind a whole slew of recent interest from semiconductor physicists who are trying to make condensates, the equivalent of superconductors and that travel without loss, in semiconductors.

Being in two places at once, these new electronic particles hold the promise of transferring ideas from into practical devices, using quantum mechanics visible to the eye.

Explore further: Quantum electronics: Two photons and chips

More information: The paper, 'Controlling quantum tunnelling with light' is scheduled to be published on Thursday, 05 April in the journal Science.

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not rated yet Apr 05, 2012
Ok, this article has me completely baffled.

An electron absorbing a photon moves to a higher energy state. Yes?

In a higher energy state, they're seeing electrons pass through walls that can prevent the passage of electrons in lower energy states.

Where is the evidence for a new state of matter, again? A "marrying" of a photon and an electron to create a new particle with different properties than, say, a high-energy electron?

You lost me.
3 / 5 (2) Apr 05, 2012
When the photon hits the surface of material it will excite its electrons under formation of surface plasmons. These plasmons do behave like the photons, but they're a much heavier - they gain their mass from electrons in similar way, on which the Higgs mechanism is based, so they cannot propagate to very large distance. But this distance may be still larger, than the distance, at which the electrons are able to tunnel trough material under normal circumstances.

Note that the electrons are tunneling, so that the material must form an insulator. In the material with free electrons like the metals the surface plasmons aren't required to ionize the underlying material too much. At the case of insulators the formation of plasmon is always connected with separation of electrons from atom nuclei, which introduces a polarization of material and such a plasmon is called the polariton. At the case of thin layers the pairs of plasmons are formed at both sides and they're called a dipolaritons.
1 / 5 (2) Apr 05, 2012
The surface plasmons are no mystery. The surface of atoms covered with electrons can be modeled with mesh covered with thin elastic layer of fluid with strong surface tension, like the mercury. Such a model can for example explain the mechanism of reactions in organic chemistry in elegant way.

The impact of photons makes a ripples of plasma at the surface of electrons, so they're called a plasmons. For example, the specific absorption of photons under formation of plasmons is responsible for fancy color of metallic copper and gold. The polaritons are analogous to solitons of these surface ripples, i.e. the wave packets, in the formation of which the subsurface interactions are involved.
1 / 5 (2) Apr 05, 2012
I agree that the article does not provide a clear explanation. Such interesting and potentially useful work justifies far more details than this short article provides.

Two items attract my interest. The dipolaritons are stretched out in a specific direction, like bar magnets, and, like magnets, they feel extremely strong forces between each other. This implies that some correlated or coherent behavior is at work, although the article does not state how such behavior might produce this interesting result.

Second, the dipolaritons are created by "packets of light which sandwich the electrons oscillating through" the walls.

Taken together, this implies that the coherent and presumably directional sandwich regime drives the electrons, superseding the troublesome behavior of electrons that would otherwise prevent tunneling if the electrons were left to their own devices. The two photons provide adult supervision to the oscillating electron, lighting the tunnel somehow.

1 / 5 (2) Apr 05, 2012
My main interest is superconductivity, which arises, I believe, from synchronized oscillations of electrons. I have made many PhysOrg posts on this subject, usually citing work done by Art Winfree circa 1967 on coupled oscillators in biology (Malaysian fireflies, gaits of a horse, and so on).

The foregoing article on Cavendish and dipolaritons attracted my interest because it provides some support--sketchy and indirect, admittedly--for my theory. As the article suggests, in the penultimate paragraph, "semiconductor physicists" are trying to make condensates that "travel without a loss" in semiconductors, producing a result that is "equivalent" to "superconductors and superfluids." The results are similar. The coherent behavior of these dipolaritons, a form of synchronized behavior, may be the key.

Art Winfree's work on coupled oscillators is now orthodox in math and biology, but my proposed application of his work to physics is highly unorthodox. So proceed with caution...
3 / 5 (2) Apr 06, 2012
Here is the abstract from the paper.
Tunneling of electrons through a potential barrier is fundamental to chemical reactions, electronic transport in semiconductors and superconductors, magnetism, and devices such as THz-oscillators. While typically controlled by electric fields, a completely different approach is to bind electrons into bosonic quasiparticles with a photonic component. Quasiparticles made of such light-matter microcavity polaritons have recently been demonstrated to Bose-condense into superfluids, whereas spatially separated Coulomb-bound electrons and holes possess strong dipole interactions. Using tunneling polaritons, we connect these two realms, producing bosonic quasiparticles with static dipole moments. Our resulting three-state system yields dark polaritons analogous to those in atomic systems or optical waveguides offering new possibilities for electromagnetically induced transparency, room-temperature condensation, and adiabatic photonic to electronic transfer
1 / 5 (1) Apr 06, 2012
As I wrote already here, the plasmons are surface waves of electrons and the polaritons are solitons of these surface waves, which do propagate like particles. At the water surface a two kind of solitons exist: the solitons of transverse waves formed above the water surface, so-called the Russel's solitons which are analogy of photons in the vacuum. But at the water surface another kind of solitons exists too: the so-called Falaco solitons, which correspond the neutrinos. Being solitons of the underwater longitudinal waves, they don't cary energy observable at the water surface - on the contrary, the water surface appears surprisingly still at the place, where the Falaco soliton is spreading. We could call them a dark solitons. The water surface composed of colliding particles is rather poor model of vacuum behaviour, but the surface of electron fluid, where the repulsive forces of many particles overlap heavily at distance can serve as a much better analogy of vacuum behaviour.
1 / 5 (1) Apr 06, 2012
The memo is, at the surface of electron fluid the plasmon ripples can be formed. When these ripples involve the interactions of underlying atoms, these plasmons waves are changing into soliton particles, called the polaritons. These polaritons can be of two kinds, after then: so called bright-state polaritons, corresponding the Russel's solitons or particles of matter in the vacuum - and the dark-state polaritons, which do correspond the Falaco solitons and/or antiparticles.
1 / 5 (1) Apr 06, 2012
Due the high density of electron field the polaritons are moving a way slower speed, than the photons or neutrino in vacuum, so that their velocity can be observed and measured easily. It could help to solve some theorems of dense aether model, for example the prediction, that the photons are moving with slightly subluminal speed whereas the neutrinos are usually moving with slightly superluminal speed (because they're solitons of gravitational waves, which are highly superluminal in dense aether model). The observation of bright and dark polaritons at the surface of metals could help to resolve this question in cheap table-top experiment under the microscope.
Apr 07, 2012
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not rated yet Apr 07, 2012
BTW It's not first example of controlling of quantum tunnelling with light, rather the first example of quantum tunnelling of massive particles. The achieve the control of tunnelling of photons (laser induced transparency) in such way is way easier. http://prl.aps.or.../p5094_1
1 / 5 (1) Apr 09, 2012
This is not control of quantum tunneling of light. This is tunneling electrons. They are using light to control it.

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