Building a Practical Quantum Computer

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Building a Practical Quantum Computer   By Don Monroe

Communications of the ACM, July 2022, Vol. 65 No. 7, Pages 15-17   10.1145/3535191

Researchers have speculated about quantum computation for decades, but recent years have seen steady experimental advances, as well as theoretical proofs that it can efficiently do things that classical computing devices cannot. The field is attracting billions of dollars from governmental research agencies and technology giants, as well as startups. Conventional companies also are exploring the potential impact of quantum computing.

Despite this excitement, including successful sensing devices, quantum computing has not made practical contributions. Moreover, there is still no winner among very different schemes to physically implement quantum bits, or qubits. None of them is 'good enough' yet to achieve supercomputer-scale calculations, and they all face major barriers to low error rates and large device counts.

Even optimistically, it could take many years to realize large-scale, error-corrected quantum computing. In the interim, researchers and companies are seeking uses that can exploit the small, less-reliable systems that already are available.

Quantum Promises

The number of possible states that a computational system can represent doubles with each additional bit, but a "classical" system can only be in one state at a time. In principle, a set of qubits can explore many more combinations simultaneously, since each can be a mixture of 0 and 1.

Figure. IBM's Q System One quantum computer.

Exploiting this "superposition," however, requires qubit manipulations that end up, with very high probability, in a state that solves a target problem. Two such algorithms were devised by independent Bell Labs researchers in the 1990s. There have been few new proposals, but one of the early ones, Peter Shor's scheme to factor large numbers, has driven sustained concern in cryptography.

Such delicate manipulations are extremely difficult, however, even in the laboratory. The process fails unless the coherence between all the qubits is established and maintained precisely through the entire sequence of operations.

Candidate qubits—including atoms, ions, crystal defects, photons, and superconducting circuits—are sensitive to fluctuations in their environment, which force them prematurely into one or another classical state. Specialists in each field have been striving to reduce the error rates, which would allow more operations to be completed on more qubits before an error occurs.

A leading candidate is superconducting circuitry, which encodes the qubit in the collective motion of the electrons in a superconductor. These planar circuits can exploit scalable integrated-circuit manufacturing to make and interconnect two-dimensional arrays of qubits—currently scores of qubits demonstrated by behemoths Google and IBM, and dedicated company Rigetti.  ....