In other words, he showed that an entanglement-free quantum circuit was easy to simulate on a classical computer. In a computational sense, the circuit wasn\u2019t intrinsically quantum. The collection of all such non-entangling circuits (or, equivalently, all arrangements of qubits that might come out of these non-entangling circuits) formed something of a classically simulable island in a vast quantum sea.<\/p>\n

In this sea were the states resulting from truly quantum circuits, the ones for which a classical simulation might take billions of years. For this reason, researchers came to regard entanglement not just as a quantum property, but as a quantum resource: It was what you needed to reach the uncharted depths, where powerful quantum algorithms like Shor\u2019s resided.<\/p>\n

Today, entanglement is still the most studied quantum resource. \u201cIf you ask 99 out of 100 physicists [what makes quantum circuits powerful], the first thing that comes to mind is entanglement,\u201d Fefferman said.<\/p>\n