A research team that includes scientists from Google AI Quantum; NASA Ames Research Center; University of California, Santa Barbara; Oak Ridge National Laboratory; and other institutions in the U.S. and Europe has demonstrated that a quantum computer can outperform a classical computer at certain tasks, in a feat known as quantum supremacy. The quantum computer, built by Google and named Sycamore, consists of 53 qubits. The classical computer competing with Sycamore was Summit from Oak Ridge National Laboratory (ORNL). Summit is a powerful machine with more than 4600 compute nodes. Google’s quantum supreme cryostat with Sycamore inside. Courtesy of Erik Lucero/Google. Both systems performed a task known as random circuit sampling, designed specifically to measure the performance of quantum devices such as Sycamore. The simulations took 200 seconds on the quantum computer. After running the same simulations on Summit, the team extrapolated that the calculations would have taken the classical computer more than 10,000 years to complete with current state-of-the-art algorithms. “It is likely that the classical simulation time, currently estimated at 10,000 years, will be reduced by improved classical hardware and algorithms, but, since we are currently 1.5 trillion times faster, we feel comfortable laying claim to this achievement,” said Brooks Foxen, a graduate student researcher in Google scientist John Martinis’ group. Not only was Sycamore faster than its classical counterpart, but it was also approximately 10 million times more energy efficient. To test the quantum computer’s ability to hold and rapidly manipulate a vast amount of complex, unstructured data, the researchers used a sequence of operations that produced a complicated superposition state that when measured, returns bit string (a quantum circuit’s output) with a probability determined by the specific sequence of operations used to prepare that particular superposition. The exercise for Sycamore was to verify that the circuit’s output corresponded to the sequence used to prepare the state. Sycamore sampled the quantum circuit a million times in just a few minutes, exploring all possibilities before the system could lose its quantum coherence. The Sycamore quantum processor. Courtesy of Erik Lucero/Google. “We performed a fixed set of operations that entangles 53 qubits into a complex superposition state,” said Ben Chiaro, a graduate student researcher in Martinis’ group. “This superposition state encodes the probability distribution. For the quantum computer, preparing this superposition state is accomplished by applying a sequence of tens of control pulses to each qubit in a matter of microseconds. We can prepare and then sample from this distribution by measuring the qubits a million times in 200 seconds.” The researchers also estimated the performance of individual components to accurately predict the performance of the entire Sycamore device, demonstrating that quantum information behaves consistently as it is scaled up. The team used a method called cross-entropy benchmarking to compare the quantum circuit’s output to its corresponding ideal probability, computed via simulation on a classical computer, to ascertain that the quantum computer was working correctly. The team’s results could provide a proof of concept for quantum supremacy and establish a baseline comparison of time-to-solution and energy consumption. While the experiment was chosen as a proof-of-concept for the computer, the research has resulted in a certified random number generator, a tool that could be useful in a variety of fields. Random numbers can ensure that encrypted keys cannot be guessed, or that a sample from a larger population is truly representative, leading to more robust machine learning applications. The speed with which the quantum circuit can produce its randomized bit string is so great that there is no time to analyze and “cheat” the system. The research was published in Nature (https://doi.org/10.1038/s41586-019-1666-5). Members of the Martinis lab explain their quantum supremacy feat — a milestone in quantum computing research that opens up new possibilities for this technology. Courtesy of Google AI Quantum/University of California, Santa Barbara.