“Quantum” Will Never Make Sense

Brian N. Siegelwax
3 min readNov 10, 2021

Even Though It Otherwise Makes Sense

I have an active poll on LinkedIn and Twitter asking the question, “classical simulation of quantum systems IS / IS NOT quantum computing.” I have lamented in the past about the lack of standardization of quantum computing terminology, but we need to address the fact that even the term “quantum computing” is not standardized.

What is “quantum computing?”

To me, a computer scientist, as well as to most poll respondents thus far, “quantum computing” is the act of using quantum computers. Maybe you include quantum computing simulation in that, maybe you don’t. Maybe you include on-paper algorithms intended for quantum computers, and maybe you don’t. But, the key correlation with the term “quantum computing” is “quantum computer.” How can you possibly do quantum computing without quantum computers, right?

Now, strap on a seat belt. This road gets bumpy.

So, what is “quantum computing?”

I don’t want to make a sweeping generalization here, but to at least some physicists “quantum computing” is a hardware-agnostic approach. If I calculate the ground state Hamiltonian of a Hydrogen molecule with a pencil and paper, that’s “quantum computing.” And if I design a full adder quantum circuit and perform basic arithmetic on a quantum processor, that’s classical computing.

Those are two greatly different definitions right there. So, how is the quantum computing industry supposed to develop, when we can’t even agree on what “quantum computing” is?

Also, what is “quantum simulation?”

When many people think of “quantum simulation,” they think about the various simulators you can run circuits on instead of quantum processors. These simulators run on classical computers. But, that’s not “quantum simulation.” That’s better characterized as “quantum computer simulation” or “quantum computing simulation,” but, of course, there’s no standard definition. “Quantum simulation” is better defined as the simulation of quantum systems, which is not the same as the simulation of quantum computers, but feel free to argue with me. Since neither one of us will be right, we can…