Is it a quantum computer?

What makes quantum, quantum is just a matter of taste

German Chancellor Dr. Angela Merkel calls it a “miracle of technology.” The device, an IBM Q System One, was recently unveiled as the most powerful quantum computer in Europe. But what does it mean for it to be “the most powerful” — heck, what does it mean for it to be a quantum computer?

What is called a quantum computer today seems to be a matter of taste.

Consider the fact that my smartphone works by flipping bits encoded in transistors that are only a few dozen atoms thick. There are about a billion of them inside my phone, which could not be made without understanding quantum mechanics. But my smartphone is not a quantum computer — it’s a digital computer. This is because it encodes bits — digital, classical data — and not qubits, the building blocks of quantum data.

But, wait. It’s not that simple. I can indeed encode qubits quite faithfully with the transistors in my smartphone. My smartphone could quite comfortably emulate a perfect quantum computer, provided it had less than 30 qubits or so. When I hijack all the computing resources in my phone to do this, it really is a 30-qubit quantum computer.

OK, but you could argue in that scenario the qubits aren’t physical. When I googled “what is the most powerful quantum computer,” the top hit was a press release from Honeywell claiming that their trapped ion quantum computer was the most powerful (in mid-2020). This device encodes one qubit of information onto each individual atom (I guess 6 of them from the limited information) that is isolated in their chip. In such a device, you can make a correspondence between the information you want to encode and the information need to describe each atom. Thus, qubits are physical.

But… not quite. Once we have large, reliable quantum computers, the qubits will not be encoded onto individual atoms. Error-correcting codes will be used to encode the qubits virtually to protect them from physical errors. This is part of the reason why simply counting qubits in a device doesn’t reveal its true utility, and why Honeywell could claim their device was “more powerful” than competing devices that had 10 times as many qubits. Hell, I could pick up a pile of dirt and claim it’s a quantum computer since each dirt atom encodes quantum data. The problem is, it’s not reliable data. My 30-qubit smartphone reliably encodes quantum data. In fact, by some accepted measures it is the most powerful quantum computer!

Honeywell quotes the “quantum volume” of their devices, which was 64 in June 2020 but is now up to 1024 using a 10-qubit chip. The quantum volume measures the difficulty a digital computer would face in emulating a given quantum device. It is 2 to the power of the number of qubits (provided that number of instructions could be carried out without error). Honeywell has 10 qubits that can carry out 10 complex instructions without error, and 2¹⁰ = 1024. My smartphone can carry out instructions perfectly forever and so it has a quantum volume of 2³⁰ or about a billion. (Not surprising because it has 8 billion bytes of RAM and it takes about 8 bytes to represent a qubit of data.) So. Much. Quantum.

Alright, I admit I’m being pedantic. The real reason I wouldn’t really call my smartphone a quantum computer is because it is limited to 30 qubits and there is no way the technology can scale significantly beyond that. Whereas there is hope for Honeywell, IBM, Google, and many others that the devices they are building can scale the number of qubits up and the errors down indefinitely.

Maybe they shouldn’t call their devices quantum computers today, though — at least not until they outperform my smartphone. The question then is when will that happen? The nice thing — if you really think this is a problem — is that quantum computing is divergent technology. Far enough into the future, there will be no ambiguity about whether a computer device is really a quantum computer because there will not be enough atoms in the universe to construct smartphone memory capable of encoding the quantum data the new device can carry. So, one solution is to just hold off on our bickering until that time.

Alas, so long as Twitter exists, the bickering will never end. So, I’ve created another solution. It’s called Not A Quantum Computer and it’s a web app where you can upload a photo of your candidate device and it will tell you whether or not you have a quantum computer on your hands. Congrats, IBM Q, you have a quantum computer!