Dec 18, 1995 NIST team demonstrates a quantum logic gate with a single trapped ion

Deutsch had described what a quantum computer would be; the decade that followed showed what one might do. Researchers found the first problems on which it would outpace conventional machines, and in 1994 Peter Shor showed that it could factor large numbers efficiently, enough to break much of the encryption that secures the internet. Yet not even the smallest component of such a device had been built.

Like any other computer, a quantum one would be assembled from logic gates, and a particular kind is indispensable: the controlled-NOT, in which the state of one qubit decides whether another flips. The difficulty is performing it without disturbing either of them, since even slight contact with the surroundings destroys a quantum state.

In May 1995 Ignacio Cirac and Peter Zoller proposed a method: trap charged atoms, or ions, in a vacuum, cool them nearly to rest, and manipulate them with laser light. Seven months later a NIST group in Boulder, Colorado, led by Christopher Monroe and David Wineland, demonstrated a version of the scheme on a single beryllium ion. One qubit was stored in the ion's internal state, the other in its vibration within the trap, and a sequence of pulses performed the controlled-NOT between them.

Combined with simple operations on individual qubits, already within reach, the controlled-NOT suffices for any quantum computation. The experiment had therefore supplied the essential ingredient of the machine Deutsch had described, and every quantum processor built since rests on the same step. In 2012 Wineland shared the Nobel Prize in Physics; the committee credited his work with "the very first steps towards building a new type of super-fast computer, based on quantum physics."

We demonstrate the operation of a two-bit "controlled-NOT" quantum logic gate, which, in conjunction with simple single-bit operations, forms a universal quantum logic gate for quantum computation.