Quantum processors can surpass classical computing without error correction

American scientists have shown a quantum processor that can surpass classical computing without error correction. An IBM 127 qubit processor prepares and measures highly entangled quantum state expectations (estimated average results from repeated experiments) beyond the capabilities of current best classical computational methods. This demonstration suggests that quantum computers may be able to be used for specific calculations in the near future, without the need for fault tolerance (i.e., avoiding or quickly correcting errors when running quantum computers to keep them under control), which may be many years away. The study was published June 14 in Nature.

A key goal of quantum computing is to perform specific tasks efficiently beyond the possibilities of classical computing. To achieve this, many practical challenges will need to be addressed, such as keeping error rates low and penetrating quantum “noise” (interference from underlying systems or the environment) while increasing the scale of quantum computers. Errors and noise can reduce or eliminate the benefits of quantum computing over classical computing. With current technology, fault tolerance is still out of reach. While existing quantum processors have been able to surpass classical computers on some specific but artificial problems, there is still debate about whether current or near-future noisy quantum computers are good enough to perform useful (e.g. for research purposes) quantum computing.

Microsoft Thomas S. Youngseok Kim, Abhinav Kadala of the J. Watson Research Center and colleagues have shown that their quantum chip can reliably generate, manipulate, and measure quantum states that are so complex that classical approximation methods cannot reliably estimate their properties. This demonstration suggests that quantum machines, even without error correction, may already be able to help solve specific problems that classical computers can’t do (such as studying physical models). The experiments reported by the authors were based on a 127-qubit processor, running 60 layers of circuits deep and about 2800 two-qubit gates (quantum versions of classical computer logic gates). This quantum circuit produces large, highly entangled quantum states that are too demanding to be reliably reproduced by numerical approximations on classical computers. The study shows that the quantum computer can accurately estimate the nature of these states by measuring the expected value. Manufacturing and measuring these gigantic states without too many computationally debilitating errors is achieved by high-quality and noise-compensating post-analytical processing methods for manufacturing chips.

“This fundamental quantum advantage is scale, not speed—127 qubits encode a huge state space, not as much memory as a classical computer.” In a news and opinion piece published at the same time, Goran Wendin and Jonas Bylander of Chalmers University of Technology in Sweden write. (Source: China Science News, Jinnan)

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