In the computers of tomorrow, bits of information might be encoded in individual particles of light. But the simplest light-based quantum-computing scheme will work only if each photon is identical to all the others. Now, a team of physicists has found a way to squeeze nearly indistinguishable photons out of a tiny dot of solid material.
Given the choice, photons prefer to fall into the same quantum state, and this weird chumminess could someday drive ultrafast quantum computers in which individual photons race around optical circuits. Such computers would exploit the fact that, thanks to quantum mechanics, a photon can be polarized in two different directions at the same time or otherwise be forced into two different states at once. That means each photon could serve as a "qubit" that can be set to 0 and 1 at the same time. Such qubits would enable quantum computers to crunch many numbers at the same time instead of in sequence. But this scheme relies on having a source of photons with identical quantum properties--a stipulation that researchers thus far have been unable to satisfy.
But a tiny "quantum dot" of semiconductor can crank out cookie-cutter light particles, physicists Charles Santori, Yoshihisa Yamamoto, and colleagues at Stanford University report in the 10 October issue of Nature. They embedded dots of indium arsenide only a few nanometers wide in 5-micrometer-tall pillars made of alternating layers of galium arsenide and indium arsenide. The layers above and below a dot acted like mirrors for any light coming from it. When the researchers zapped a pillar with a laser, the light excited exactly one electron in the dot, causing the dot to emit one photon. Because the photon had to have just the right characteristics to bounce back and forth between the mirrors and emerge from the top of the pillar, every photon that came out had very nearly the same properties as the next one.
To prove this, the researchers shot two consecutive photons at opposite sides of a partially reflective mirror so that they arrived at the same time. One of the light particles went through the mirror while the other reflected off it, so that the two almost always emerged in the same direction. That is, they stuck together in the same quantum state, something that could happen only if they were nearly indistinguishable.
"This is one of the most important experimental developments in the field," says Ataç Imamoglu, a physicist at the University of California, Santa Barbara. Researchers must greatly reduce the remaining differences between subsequent photons before they can even think of constructing a light-based quantum computer, Imamoglu says, but the current result is a crucial first step in that direction.