What makes quantum computing so challenging?

Real quantum systems are subject to a lot of noise, and the hard thing about quantum engineering is making devices that preserve the probability amplitudes. The low temperatures, a few thousandths of a degree above absolute zero, are all about removing noise, but Google’s device is still really noisy. What they measure is almost entirely a random signal with a small deviation, where the small deviation is coming from the quantum mechanics.

Based on Google’s estimate in their Nature paper, a classical supercomputer would need 10,000 years to complete what the quantum computer did, but then IBM says it would only need a couple of days using a different method. Could you explain this discrepancy?

IBM said they have an algorithm that could be faster than the 10,000 years that Google stated and that was because they realized that it is just possible to store that state of 254 qubits on the hard drives of the Oak Ridge supercomputer, the largest in the world, operating for two days.

Does IBM’s conjecture take away from the overall significance of what Google did?

I don’t think it changes the fact that this demonstration is showing a clear separation in how hard it is to perform this calculation in a classical computer versus a quantum device. It’s absolutely true that people can come up with different ways of calculating things, and the performance of our classical supercomputers and algorithms will continue to improve. IBM is absolutely right to point out this discrepancy and also to make the larger point that the quantum supremacy demonstration is not really useful, so we should continue to wait for devices that can run quantum algorithms with known applications. It’s also important for IBM to run the simulation to see if it really does take two days because sometimes running things on supercomputers is not as obvious as in a theorist’s head. Google posted the output from their quantum calculations, so then we can check to see if they really are measuring the quantum effects they believe. Ultimately, I think this demonstration will go down in history as a landmark achievement. Although there are other quantum devices—or materials for that matter—that are hard to simulate classically, this is the first device matching that description that is an engineered, fully programmable quantum computer. That is an important distinction since there is a natural blueprint for how one “scales” the system into larger devices that can run more complex calculations. With a quantum computer, adding just one qubit doubles the computational capacity, so things can move quickly now.

What comes next?