Enthusiasm for quantum computing comes and goes — this year, it’s way up. Stocks of the most prominent names, IonQ, D-Wave Quantum, and Rigetti Computing, have, on average, doubled in price.

But, the fundamental challenge for all quantum companies hasn’t changed in two decades. They are all trying to get to “scale,” and it’s not clear when, if ever, any of them will.

I’m emphasizing this point because the scaling issue is fundamental to all computing; without scaling, nothing is meaningful.

Scaling means that as a computer gets more complex — basically, bigger — the amount of work the computer can do increases in proportion. The main example is the integrated circuit, which became more and more powerful over six decades, increasing dramatically what could be done, to the point that you now have a supercomputer in your pocket with today’s phones.

Quantum is not there yet. All the companies have shown interesting machines, but none have proven they can scale those machines the way the traditional semiconductor scaled.