Scientists have developed a number of quantum computing systems that use ions or electrons as bits of data; mathematical “operations” can be performed on them with beams of light or electrical pulses. Until recently, however, these systems could only perform the specific tasks they were designed to do. But a group of NIST scientists have published a description of a quantum processor that can receive virtually any set of instructions and perform them on a set of inputs—in short, they’ve made the first programmable quantum processor.
In order to do general calculations, a computer must be able to perform an arbitrary number of unitary transformations, operations that change the state while preserving the structure of the system. Unlike a regular computer, a quantum computer stores information in “qubits,” or quantum bits. A regular bit may hold only one piece of data (0 or 1), but a qubit can hold a superposition of 0 and 1; it only adopts a definite value when measured.
That’s just one of a number of important differences between quantum and regular computers. Standard computers use logic gates, such as Boolean operators (and, or, not, and so on). Quantum computers use quantum gates, which are always reversible operators, unlike many logic gates.
Finally, qubits can undergo entanglement. “Entanglement” in quantum mechanics means the properties of the qubits can be dependent on each other, even if they are physically separated. For example, in a system where two qubits may only be 0 at the same time or 1 at the same time, measuring one of them as 0 will ensure that the other is 0 as well.
In the case of quantum information science, developing a system that can perform any arbitrary transformation on qubits is actually just as hard as it sounds. To get there, the NIST scientists created a system that has two quantum gates, one that deals with a single qubit at a time, and another entangled two-qubit gate. The physical incarnation of the system used two beryllium ions trapped in a finite region, with the gate operations, or transformations on the states of the ions, performed by stimulating them with ultraviolet lasers.