I was disappointed to see optical quantum computers described as “exponentially faster than ordinary computers” in your article. Despite frequent misuse in the lay press, “exponentially” does not mean “a whole bunch.” It refers to a specific mathematical functional relationship, not merely a comparison of two numbers. The article doesn’t describe any such function. Even to posit an exponential relationship, we’d need an independent variable, of which ordinary and photon computer speeds are functions, in order to compare them.

Dick Dunn
Hygiene, Colo.

The amount of time it takes an ordinary computer to find prime factors of a number grows exponentially with that number’s number of digits. For a quantum computer, it grows only as a power of the number of digits. That makes quantum computers exponentially faster, quite literally. —Davide Castelvecchi