Classical computers of the type we use every day are based on bits, which can be in one of two states (0 or 1). In contrast, quantum computers use qubits, which can also be in superpositions of states. These computers have the potential to perform some calculations much faster than conventional computers. It has been postulated that quantum computers could even solve problems that classical computers cannot solve within thousands of years. This concept is called “quantum supremacy”.
John M. Martinis, Google AI Quantum, Mountain View, and University of California, Santa Barbara, both CA, USA, and colleagues claim that they have found the first computation that can be performed practically only on a quantum processor, i.e., the first example of quantum supremacy. The team designed a computational problem that is particularly well suited to quantum computing and used a new quantum processor, called Sycamore, to tackle it. The processor consists of 53 functioning qubits in a two-dimensional array, each of which can be coupled to its neighbors to form two-qubit pairs.
The problem chosen by the team is sampling the output of a pseudo-random quantum circuit (in principle, a random number generator with different probabilities for different outcomes) and computing the underlying probability distribution. According to the team, their quantum processor takes about 200 seconds to sample such a quantum circuit a million times. They estimated how long it would take a classical computer to perform the same calculation, using test calculations on the Summit supercomputer developed by IBM and on Google’s own computer clusters as a basis for their extrapolation. The researchers claim that an equivalent sampling on such currently available supercomputers would take approximately 10,000 years. These numbers would confirm the desired quantum supremacy breakthrough.
Researchers at IBM are skeptical of this claim . They acknowledge that Google’s experiment is an excellent demonstration of the progress in quantum computing. However, according to them, the team at Google has not compared their quantum device to the most efficient way to perform the calculation on a classical computer system. The team at IBM estimates that, when optimized, the simulation could take only 2.5 days on a conventional computer instead of 10,000 years—but this has not been tested in practice so far.
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