So far, we have explored the fundamentals of quantum physics, perhaps paying a bit too much attention to its mind-boggling philosophical implications. In the real world of academic and industrial physics however, quantum mechanics is applied very successfully to the solution of physics problems and in the development of electronic devices without the need for a philosophical interpretation.
When pushed for a philosophical stance, professional physicists often fall back on the expression “shut up and calculate,” which is often attributed to Nobel Prize recipient Richard Feynman. This is because, despite all the business about Heisenberg’s Uncertainty, wave–particle duality, quantum entanglement, and the counterintuitive nature of quantum behavior, quantum physics nevertheless makes exact predictions that are useful for the development of real-world applications. Semiconductors, superconductors, lasers, magnetic resonance imaging machines, and other many of our modern everyday devices involve an understanding and application of the principles of quantum physics. Each of these has matured into its own field.
In this final section, we will distance ourselves from philosophy, and instead take a brief look at the way in which physicists are beginning to harness quantum weirdness to develop new technologies that will take us from the information age into the quantum information age.
Currently, the basic unit of digital information is the bit, which can have a value of either zero or one. In a computer, these digital values may be represented by two different voltages. For example, in a TTL circuit, “0” = 0 V and “1” = 5 V. When communicating through a fiber-optic cable, the bit’s value could be encoded through two different light polarizations. For example, “0” = vertical polarization and “1” = horizontal polarization.
These orthogonal polarizations could be easily analyzed through a special type of beam splitter that separates a beam of light depending on its polarization. As shown in Figure 150, a two-channel beam splitter or polarizing beam splitter (usually called a PBS for short) transmits all photons with vertical polarization, but reflects all those with horizontal polarization. However, a photon polarized at any intermediate angle between horizontal and vertical will be in quantum superposition. Each such photon may be detected exiting one or the other port, with probability dependent on the polarization angle.
Figure 150 A PBS transmits all photons with vertical polarization, but reflects all those with horizontal polarization. (a) Randomly polarized light is divided into two equal beams, one vertically polarized and one horizontally polarized. Digital information can be encoded on a light beam by changing its polarization, such that a PBS can decode vertical polarization as a digital “0” (b), and horizontal polarization as a “1” (c). (d) However, a photon polarized at an intermediate angle will be in quantum superposition. Each such photon may be detected exiting one or the other port, with probability dependent on the polarization angle. Each photon in quantum superposition can then be thought of as a quantum bit or qubit.
So long as it is not detected, a photon in a superposition of states could thus be considered to be encoding a bit in both the vertical and horizontal polarizations at the same time. This is the basic unit of data in the quantum world, and is known as a qubit (for quantum bit).
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