In 1994, a mathematician at AT&T Research named Peter Shor brought instant fame to "quantum computers" when he discovered that these hypothetical devices could quickly factor large numbers — and thus break much of modern cryptography. But a fundamental problem stood in the way of actually building quantum computers: the innate frailty of their physical components.
Unlike binary bits of information in ordinary computers, "qubits" consist of quantum particles that have some probability of being in each of two states, designated |0〉 and |1〉, at the same time. When qubits interact, their possible states become interdependent, each one's chances of |0〉 and |1〉 hinging on those of the other. The contingent possibilities proliferate as the qubits become more and more "entangled" with each operation. Sustaining and manipulating this exponentially growing number of simultaneous possibilities are what makes quantum computers so theoretically powerful.