Computers operate by manipulating sequences of elementary objects, called bits, which can take one of two values, zero or one. By contrast, the equivalent quantum object, a qubit, can be in any superposition of these two values. This is a bizarre consequence of an aspect of quantum mechanics that allows a quantum system to be in two different states at once. For instance, a quantum particle can find itself in two positions, or have two velocities, at the same time. This is the origin of the famous paradox of Schrödinger’s cat, which can be simultaneously alive and dead!
Even stranger are the properties of pairs of quantum particles, such as the light produced by an optical parametric amplifier, illustrated in Figure 20. This device emits photons in pairs, for instance a blue and a red photon, or two green photons. Photons, the elementary particles of light, are characterized by a polarization, which can be thought of as an arrow pointing up or down along some direction. If a pair of green photons—the two bright green spots on the picture—is emitted by that optical parametric amplifier, it is fundamentally impossible to know with certainly the polarization of either one. However, if we determine that one of the photons has “up” polarization, we can be sure that the other has “down’’ polarization. Pairs of quantum particles with this type of correlated properties are said to be entangled. They are so intermixed that there is no way to describe either separately.
Entangled pairs of particles lead to apparently paradoxical and counterintuitive effects, such as the “spooky action at a distance” that so disturbed Einstein. The distinction in the behavior of pairs of classical and quantum particles was quantified in a mathematical form called Bell’s inequalities, which are satisfied classically but violated in the quantum world. This violation has been unambiguously demonstrated in a series of beautiful atomic physics experiments.
At the fundamental level, these recent advances have shed light on the mind-bending transition from the microscopic to the quantum world. On the practical side, they are opening up the way to a technological revolution: quantum information technology. They have led to the first demonstrations of quantum teleportation, which allows a quantum system to be exactly reproduced at a distant location. Another application, quantum cryptography, allows the development of unbreakable protocols to secretly exchange information. And someday, quantum computers, which are based on qubits instead of bits, could implement the quantum algorithms being developed for tackling problems that are unsolvable using classical computers.