Putting “Quantumness” to the Test

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) [1]—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.