Mathematically, it's easy to demonstrate that a working general-purpose quantum computer can easily outperform classical computers on some problems. Demonstrating it with an actual quantum computer, however, has been another issue entirely. Most of the quantum computers we've made don't have enough qubits to handle the complex calculations where they would clearly outperform a traditional computer. And scaling up the number of qubits has been complicated by issues of noise, crosstalk, and the tendency of qubits to lose their entanglement with their neighbors. All of which raised questions as to whether the theoretical supremacy of quantum computing can actually make a difference in the real world.
Over the weekend, the Financial Times claimed that Google researchers had demonstrated "quantum supremacy" in a draft research paper that had briefly appeared on a NASA Web server before being pulled. But the details of what Google had achieved were left vague. In the interim Ars has acquired copies of the draft paper, and we can confirm the Financial Times' story. More importantly, we can now describe exactly what Google suggests it has achieved.
In essence, Google is sampling the behavior of a large group of entangled qubits—53 of them—to determine the statistics that describe a quantum system. This took roughly 30 seconds of qubit time, or about 10 minutes of time if you add in communications and control traffic. But determining those statistics—which one would do by solving the equations of quantum mechanics—simply isn't possible on the world's current fastest supercomputer.
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