Can a laser be used as an electrical conductor between two points that are not in physical contact? If so, is there a distance limitation?
Strictly speaking, no, a laser cannot be used as a conductor under any circumstances.
A laser is a highly concentrated, synchronized ('coherent') beam of electromagnetic radiation, light. It is indeed electromagnetic in nature, but what makes electromagnetic radiation unique is that it requires no medium and carries no substance. A beam of light carries electric and magnetic fields, and as a result, energy and momentum, but it carries no electric *charge*. Electrical conduction is the net motion of electric charge, and it cannot take place without the motion of matter on some scale. A pipe blowing electrically charged styrofoam pellets carries a current. A chunk of metal is capable of carrying a current, since its electrons are very free to move around. But light cannot.
There are, however, interesting cases when a laser can be used to make conductive some substance it just happens to be traveling through. For instance, if there's some sort of powder suspended in air, a laser can be used to vaporize and 'ionize' the powder along a thin cylinder traversed by the laser light. The ions and electrons produced can then conduct electric current. Alternatively, an extraordinarily strong laser could theoretically have such a high amplitude that its electric field component was strong enough to tear electrons off atoms and directly ionize a completely transparent substance such as the air. This would similarly produce a conductive cylinder, although I'm not sure if it's really practical to make such a powerful laser. My quick and dirty estimate is that it would need an intensity of approximately 3*1015 Watts/square meter.
As far as distance limitations go, there are really no theoretical limitations, only practical ones. A narrow beam of laser light gradually spreads out due to wave diffraction, and if it's ionizing some substance, it will gradually lose its intensity.
Micah Brodsky, Undergraduate Student, University of Washington
A laser beam, by itself, cannot conduct electricity because it contains no charge carriers such as electrons to produce a current flow. However, if a laser beam of sufficient power density is transmitted through a gas, such as our atmosphere, a small amount of ionization of the gas molecules will occur. This ionization provides a source of charged particles that can serve as charge carriers. If there is a difference in potential, or voltage, between two different points in the beam, a current will flow.
This effect has already been used for some interesting experiments, such as firing a pulsed laser into a charged storm cloud to initiate a lightning strike back down the light beam.
Two laser beams have also been successfully used to provide a current path for a kind of optical (no wire) TASER. The two beams take the place of the usual two wires. When they are shone on the body of the person to be stunned, a high-voltage pulse is injected using the two ionized beams, resulting in a non-lethal, but debilitating shock.
There are of course distance limitations presented by circumstances and optical properties. In the TASER example, the laser power must be kept low enough so that it will not cause other types of damage such as burning of clothes, skin or eyes. Also, the power supply must be kept small enough to be portable.
The lightning experiment has a much greater reach, but ultimate limitations such as optical scattering in the atmosphere by dust, precipitation or even the gas molecules themselves will cause the beam to diminish over distance. The natural divergence of the beam itself will also limit the power density, and hence the ionization level.
Some of these limitations in optics are the same as have to be dealt with when using a laser as a weapon to shoot down a plane of missile. Of course they are not intending the beam to conduct electricity, but it could, for instance, cause lightning to strike an aircraft by providing a slightly conducting path between the ground and an aircraft flying near a charged cloud.
Scott Wilber, President, ComScire - Quantum World Corporation