A machine consisting of nearly 100 quantum circuit elements can compute the solution to a classic problem in mathematics, but is it a quantum computer?
What is a quantum computer? We could say it’s a machine that calculates solutions to problems using quantum components. But this definition is incomplete; after all, an abacus is made of quantum elements (atoms) and can do arithmetic. Rather, when physicists envision a quantum computer that, for example, factors large numbers in the blink of an eye, they are imagining a machine whose inner workings harness two purely quantum phenomena: the ability to prepare an object in a superposition of states, such as an electron spin that points both “up” and “down,” and entanglement, in which the quantum states of two objects are intertwined, even at a distance. A report in Physical Review Letters from Zhengbing Bian of D-Wave Systems in Canada and colleagues brings this question of what constitutes a quantum computer front and center. They compute the solution to a problem in graph theory on a machine consisting of 84 logical elements designed to function as quantum bits (or qubits) —a large number of qubits compared to other prototype quantum computers. But the report is certain to meet with skepticism: many more tests would be needed to conclude that the logical elements are functioning as qubits and that the device is a real quantum computer.