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.
鈥榮 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. 鈥淭hat makes the light relatively easy to see,鈥� says Regan. 鈥淵our eye is nearly single-photon sensitive.鈥� But it would make a poor reading lamp, he jokes.
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Quantum conundrum
It is bright enough, though, to shed light on one of the fundamental incompatibilities in physics 鈥� 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 鈥� whether you travel backwards or forwards in time, there should not be more disorder. 鈥淚t鈥檚 not clear how we get from quantum mechanical laws, which describe electrons in perpetual orbits around nuclei, to the thermodynamic laws that says it鈥檚 going to be tough to get that wine stain out of your carpet,鈥� says Regan.
The carbon nanotube filament could help. 鈥淚t is large enough that the statistical assumptions of thermodynamics should apply,鈥� he says, 鈥渂ut it鈥檚 also small enough that one can consider it as a molecular, or quantum, mechanical system.鈥�
Perfect black body
Using it, the team will investigate 鈥� 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鈥檚 work underpinned the development of quantum mechanics later in the century.
His law assumes that thermal radiation released from a black body 鈥� a perfect absorber and radiator of energy 鈥� 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 鈥� the photons it releases may be less random that those from larger filaments.
鈥淨uantum 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,鈥� says Regan. 鈥淲e don鈥檛 have a theory for the intermediate regime between these two limits, so that鈥檚 where we鈥檙e doing the experiment.鈥�
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