The new carbon nanotube filament is 100,000 times narrower than a standard tungsten filament (Image: SAKKI/Rex Features)

The smallest ever incandescent lamp, made using a single carbon nanotube, has been created by physicists in the US. At 1.4 micrometres long and just 13 nanometres wide, the filament is invisible to the naked eye until it is switched on.

Chris Regan‘s team at the University of California, Los Angeles attached a palladium and gold electrode to each end of the carbon nanotube, which spans a tiny hole in a silicon chip and is held in a vacuum.

When electricity runs along the nanotube it heats up and begins to glow, releasing millions of photons every second, of which a few thousand reach the eye. “That makes the light relatively easy to see,” says Regan. “Your eye is nearly single-photon sensitive.” But it would make a poor reading lamp, he jokes.


Quantum conundrum

It is bright enough, though, to shed light on one of the fundamental incompatibilities in physics – the mismatch between thermodynamics and quantum mechanics.

The second law of thermodynamics says entropy, or disorder, increases with time, but on the quantum scale time things are not so directional – whether you travel backwards or forwards in time, there should not be more disorder. “It’s not clear how we get from quantum mechanical laws, which describe electrons in perpetual orbits around nuclei, to the thermodynamic laws that says it’s going to be tough to get that wine stain out of your carpet,” says Regan.

The carbon nanotube filament could help. “It is large enough that the statistical assumptions of thermodynamics should apply,” he says, “but it’s also small enough that one can consider it as a molecular, or quantum, mechanical system.”

Perfect black body

Using it, the team will investigate Planck’s black-body-radiation law – a century-old theory that predicted how much light would be emitted from a source by assuming energy was released in discrete packets, or quanta. Planck’s work underpinned the development of quantum mechanics later in the century.

His law assumes that thermal radiation released from a black body – a perfect absorber and radiator of energy – will be as disordered, or as random, as possible. For instance, a hot incandescent lamp gives off many photons of different colours which together make up make white light. But because the nanotube filament can be considered a quantum mechanical system, Regan thinks it may not obey that law – the photons it releases may be less random that those from larger filaments.

“Quantum mechanics is the right theory to use in systems with very few particles, and thermodynamics is the right theory to use in systems with very many particles,” says Regan. “We don’t have a theory for the intermediate regime between these two limits, so that’s where we’re doing the experiment.”

Journal reference: Physical Review Letters, in press