If successful scientific theories can be thought of as cures for stubborn problems, quantum physics was the wonder drug of the 20^th century. It successfully explained phenomena such as radioactivity and antimatter, and no other theory can match its description of how light and particles behave on small scales.

But it can also be mind-bending. Quantum objects can exist in multiple states and places at the same time, requiring a mastery of statistics to describe them. Rife with uncertainty and riddled with paradoxes, the theory has been criticised for casting doubt on the notion of an objective reality - a concept many physicists, including Albert Einstein, have found hard to swallow.

Quantum computers are another long-term goal. Because quantum particles can exist in multiple states at the same time, they could be used to carry out many calculations at once, factoring a 300-digit number in just seconds compared to the years required by conventional computers.

But to maintain their multi-state nature, particles must remain isolated long enough to carry out the calculations - a very challenging condition. Nonetheless, some progress has been made in this area. A trio of electrons, the building blocks of classical computers, were entangled in a semiconductor in 2003, and the first quantum calculation was made with a single calcium ion in 2002. In October 2004, the first quantum memory component was built from a string of caesium atoms.

But particles of matter interact so easily with others that their quantum states are preserved for very short times - just billionths of a second. Photons, on the other hand, maintain their states about a million times longer because they are less prone to interact with each other. But they are also hard to store, as they travel, literally, at the speed of light.

Quantum physics is usually thought to act on light and particles smaller than molecules. Some researchers believe there must be some cut-off point where classical physics takes over, such as the point where the weak pull of gravity overwhelms other forces (in fact, gravity's effect on neutrons was recently measured). But macroscopic objects can obey quantum rules if they don't get entangled.

Certainly, harnessing troops of atoms or photons that follow quantum laws holds great technological promise. Recent work cooling atoms to near absolute zero have produced new forms of matter called Bose-Einstein and fermionic condensates. These have been used to create laser beams made of atoms that etch precise patterns on surfaces, and might one day lead to superconductors that work at room temperature.

All of these hopes suggest that, as queasy as quantum can be, it remains likely to be the most powerful scientific cure-all for years to come.

source :newscientist