By Tom Siegfried and Laura Sanders

Web edition: November 5, 2010
Print edition: November 20, 2010; Vol.178 #11 (p. 20)

Quantum weirdness

Some key concepts in quantum mechanics lead to rather startling results. In the quantum world, objects can be in two states at once and the outcomes of experiments can change depending on when, how and how often scientists make their measurements.

Double-slit experiment
An electron can be either a wave or a particle depending on the design of the experiment. If electrons pass through a single slit in a barrier and then strike a phosphorescent screen, they make patterns indicating the arrival of particles. But if two slits are available, an electron “wave” interferes with itself, producing the alternating bands of an interference pattern on the screen (bottom). This wave-particle duality is a fundamental feature of quantum physics and applies to all “particles” (including photons, particles of light) and even to atoms and molecules. Experiments have, for instance, shown the wavelike nature of fullerene molecules composed of as many as 70 carbon atoms.

Delayed-choice experiment
The delayed-choice experiment permits an observer to change the outcome of an event after it has already happened. View larger image

Quantum Zeno effect
The quantum Zeno effect gives truth to the adage that a watched pot never boils. Under some circumstances, repeatedly observing an unstable particle that would normally decay away quickly actually prevents it from decaying. The effect gets its name from the Greek philosopher Zeno, who held that an arrow in flight could not actually be moving because it seems to be standing still at each individual moment of observation. The quantum Zeno effect can be demonstrated with an apparatus that rotates the polarization of light. Polarized light waves oscillate in one plane only, such as up and down or side to side. View larger image

Illustrations: E. Feliciano

Quantum weirdness in action

Physicists can’t explain what lies behind weird quantum effects, such as the ability of particles to exist in two states at once and the mysterious connection between a pair of far apart particles. But that doesn’t stop researchers from taking advantage of the bizarre quantum properties.

Quantum cryptography
Quantum weirdness allows for the creation of eavesdropper-proof coded messages. In the most widely used setup, two partners (referred to as Alice and Bob) can create a secret coding key that they can later use to send secret messages. Though the concept works with just a stream of photons (shown below), quantumly linked photons, or entangled photons, can lend extra security. View larger image

Quantum teleportation
In quantum teleportation, the information stored in a quantum particle (typically a photon of light) is transferred from one location to another. In effect, that means that the information in one photon is destroyed while an identical photon, containing the same quantum information, appears in a new location. View larger image

Quantum computing
Like traditional computers, a quantum computer is made up of a network of logic gates (brown) that operate on information. Though current versions can perform only rudimentary operations, scientists hope future devices will be powerful alternatives for solving some types of problems.

Illustrations: T. Dubé