Quantum computers theoretically can prove more powerful than any supercomputer, and now scientists calculate just what quantum computers need to attain such "quantum supremacy," and whether or not Google achieved it with their claims last year.

Superposition lets one qubit perform two calculations at once, and if two qubits are linked through a quantum effect known as entanglement, they can help perform 2^2 or four calculations simultaneously; three qubits, 2^3 or eight calculations; and so on. In principle, a quantum computer with 300 qubits could perform more calculations in an instant than there are atoms in the visible universe.

It remains controversial how many qubits are needed to achieve quantum supremacy over standard computers. Last year, Google claimed to achieve quantum supremacy with just 53 qubits [ieee.org], performing a calculation in 200 seconds that the company estimated would take the world's most powerful supercomputer 10,000 years, but IBM researchers argued in a blog post [ibm.com] “that an ideal simulation of the same task can be performed on a classical system in 2.5 days and with far greater fidelity.”

To see what quantum supremacy might actually demand, researchers analyzed three different ways quantum circuits that might solve problems conventional computers theoretically find intractable. Instantaneous Quantum Polynomial-Time (IQP) circuits are an especially simple way to connect qubits into quantum circuits. Quantum Approximate Optimization Algorithm (QAOA) circuits are more advanced, using qubits to find good solutions to optimization problems. Finally, boson sampling circuits [ieee.org] use photons instead of qubits, analyzing the paths such photons take after interacting with one another.

Assuming these quantum circuits were competing against supercomputers capable of up to a quintillion (1018) floating-point operations per second (FLOPS), the researchers calculated that quantum supremacy could be reached with 208 qubits with IQP circuits, 420 qubits with QAOA circuits and 98 photons with boson sampling circuits.

How Many Qubits Are Needed For Quantum Supremacy? [ieee.org]

[Journal Reference]: Quantum Journal [quantum-journal.org]