In this series, Life's Little Mysteries explains complex subjects in exactly 200 words.
Ordinary computers manipulate "bits" of information, which, like light switches, can be in one of two states (represented by 1 or 0). Quantum computers manipulate "qubits": units of information stored in subatomic particles, which, by the bizarre laws of quantum mechanics, may be in states |1> or |0>, or any "superposition" (linear combination) of the two. As long as the qubit is left unmeasured, it embodies both states at once; measuring it "collapses" it from the superposition to one of its terms. Now, suppose a quantum computer has two qubits. If they were bits, they could be in only one of four possible states (00,01,10,11). A pair of qubits also has four states (|00>,|01>,|01>,|11>), but it can also exist in any combination of all four. As you increase the number of qubits in the system, you exponentially increase the amount of information they can collectively store. Thus, one can theoretically work with myriad information simultaneously by performing mathematical operations on a system of unmeasured qubits (instead of probing one bit at a time), potentially reducing computing times for complex problems from years to seconds. The difficult task is to efficiently retrieve information stored in qubits — and physicists aren't there yet.