Let’s try to shed some light onto the matter of quantum information by defining “information” as something that is encoded in the state of a physical system. As we have discussed, information may be encoded in the voltage on a line, a specific polarization of light, the side of a coin facing up, or vibrations in the air that we decode as a spoken word. In the case of the quantum world, John Bell showed that quantum information is encoded in nonlocal correlations between the different parts of a physical system. As we know by now—and probably the reason why this is all so confusing—is that these correlations have no classical counterpart. We can thus expect that quantum information will demonstrate many unusual properties.
The first difference is that while classical information is independent of its physical representation, quantum information cannot be read without disturbing the physical process that encodes it. For example, the number “9” in binary is “1001” and could be transmitted through a wire encoded as the series of voltages “5 V, 0 V, 0 V, 5 V.” It could be measured with an oscilloscope, and then written on the back of a napkin as “9”, “nine”, “nueve”, “1001”, and so on without changing the actual information that is encoded. These are only representations of the number “9.” and measuring it with an oscilloscope or writing it down on a piece of paper does not prevent a computer down the line from correctly decoding the number “9.” In contrast, it is impossible to write down on paper the previously unknown information contained in the polarization of an entangled-photon pair.
The second difference stems from Heisenberg’s Uncertainty Principle. We know that measuring any property of a quantum system introduces a disturbance, making it impossible to copy quantum information with perfect fidelity (this is known as the quantum no-cloning principle). If we could make a perfect copy of a quantum state, we could measure a property of the copy without disturbing the original and we could thus defeat Heisenberg’s Uncertainty Principle. On the other hand, nothing prevents us from making as many perfect copies of classical information. For example, our DVD of Back to the Future contains the same exact bits as every other DVD printed from the same master. In addition, the witty plot and outcome don’t vary depending on who watches the movie, or whether it has been watched before.†††
These properties make quantum information sound like an unnecessarily difficult way of encoding information that we, as well as our computers and telecommunication networks, handle very well with classical information. However, the very peculiar nature of quantum physics enables many tricks that are simply impossible with classical communications and computing. For example, the fact that measuring a quantum system causes it to collapse into a certain state has been used to build secure communication channels that show with certainty if someone has eavesdropped.
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