https://arstechnica.com/science/2019/09/paper-leaks-showing-a-quantum-computer-doing-something-a-supercomputer-cant/

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'd 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 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.