In last week's article I discussed the limitations of information transfer imposed by the speed of light in a vacuum, c. Circuits and electronic chips are rapidly decreasing in size. In fact, a company called Cambridge NanoTech and a team of scientists at the University of Delaware recently took another step toward quantum computing by inventing what's known as a "spin chip." Developments like these are helping to dispel myths that circuit size is the ultimate barrier to future computing. On the contrary, the speed of light appears to be the next obstacle to tackle.
Einstein theorized that information cannot propagate faster than the speed of light in a vacuum. While all sorts of exceptions and loopholes have been applied to the theory of relativity since Einstein's time, this upper limit of data transfer has proven exceptionally steadfast. Scientists have struggled to find ways to explain the physical phenomenon that they've dubbed "action at a distance," while still conforming to the rules set forth by the theory of relativity.
As the name implies, action at a distance signifies seemingly instantaneous actions across large distances. Gravity, for instance, was difficult to characterize because it appeared to act on bodies immediately, over distances that would imply faster than light speed. The traditional view of gravity asserted that it propagated instantaneously.
For example, if the earth's gravity could be controlled with a simple on and off switch, one could turn gravity off, then on again. Everything in earth's gravitational field would be weightless for exactly as long as the switch was turned off. Without getting into the particle nature of gravity and the hypothetical existence of the graviton, it suffices to say this traditional view was disproved. Even gravity must obey the universal speed limit of c.
The spin chip is interesting in that it takes advantage of the quantum-mechanical property of particles known as "spin." Just like we can characterize objects by color or shape, physicists can characterize particles by their spin; which can either spin "up" or spin "down." The scientists behind the spin chip utilized electron spin for its small size and manipulability, but other theorists have speculated about even more profound uses. Even those who are modestly computer-savvy know that computers operate on a binary basis. Everything, as a computer sees it, can be represented by either a 1 or a 0. In large quantities, those numbers can come to represent complex programs that perform complex tasks. Upon first realizing this fact, one might be tempted to juxtapose the binary system found in computer systems next to the binary nature of electron spin.
Indeed, the similarities between the two form the basis of quantum computing. Advantages of such technology over traditional silicon-based chips include decreased size, non-volatility of data, and reduced power consumption. It almost sounds too good to be true, but believe it or not, computing is headed in that direction.
Let's briefly consider another property of electron spin. The spin of one electron is physically coupled to the spin of another - something called "quantum entanglement." An electron in an entangled pair must have a spin opposite that of the other. Even more interesting, pairs can be separated by any distance, and they will still predictably and instantly interact with one another.
Two theories need to be introduced here to properly characterize what's going on. The first is the theory of local realism, which states that objects can be "directly influenced" by other objects in proximity.
The second is the no-communication theorem, which posits that systems that violate local realism still don't necessarily mean action at a distance.
Entanglement definitely violates local realism, but isn't quite action at a distance. The reasoning is rather metaphysical, but very true; should an entanglement experiment take place, the occurrence of such could not be confirmed until local data was exchanged and compared. In other words, scientists on both ends would need to meet and compare notes, and thus, useful information was not transferred.
Hence, entanglement is a potentially viable method of information transfer, but it must be coupled with a "classical information channel," such as a copper wire, fiber optic cable, or radio signal. As such, the speed of light in a vacuum still presents an upper limit to communications. We can, however, expect entanglement to permeate technology as quantum computers become the norm and replace silicon-based systems.
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