I’m not an expert on quantum computers by any means, but I do like to blog about things I know a little about. So bear with me.

One of the funnest seminars I ever attended wasn’t even in my major. I’m a biologist by trade, but occasionally in grad school I’d wonder over to the physics department to eat their pizza and have a gander at the science they were touting (they occasionally came for our pizza too so fair’s fair). On one occasion, the CEO of a quantum computer (QC) company (formerly a professor of physics) came by to talk about the exciting new happenings in QC and give us some history.

At that point I had only a surface understanding of QC, and so I had a lot of fun learning how his QC worked and the challenges he’d overcome. He also had a great sense of humor and a fun presentation. At the end he took lots of questions, and so I was also impressed by how he didn’t really sugarcoat or overhype anything. He was very open and honest that the field still had work to do and that working QCs weren’t just around the corner, whereas reading news articles you’d think they were 5 years away at most.

When it came time for my question, I asked the only thing that I, a biologist could think of: “when will we have logical qubits?”

To back up a big, just as classical computers store information in bits, QCs store information in qubits. Classical bits are binary, existing as either 1 or 0. Qubits are quantum mechanical in nature, and they exist as a superposition of states so that every time you measure them they have a probability of being either 1 or 0. This superposition is why QCs can do all those amazing things that people talk about like breaking encryption or what have you.

But this superposition is also highly unstable. Any interaction with the outside world at all will destroy the superposition, rendering it useless. This has long been the bane of QC companies and researchers, how do you make a qubit that doesn’t fall apart before you can usefully use it? When the superposition falls apart, it leads to an error, and error correction in QC is all the tricks and ways that researchers are trying to either keep the superposition stable or rebuild it if it fails.

The holy grail of error correction is the “logical qubit.” A single qubit can of course fall apart at any time so it is a poor store of information. But what if many qubits could be networked together in some way, such that if one fails the others will correct it back to its previous information value. Together, all these qubits would allow the information to be held indefinitely, even if the superposition in 1 or many of the qubits fails individually. And so together these qubits would act as a single “logical qubit,” that is a stable qubit that perfectly holds information, as opposed to the normal qubits that fall apart when you look at them funny.

It has been theorized that a thousand or more qubits will be needed to make a logical qubit, and that the technology for networking qubits to create logical qubits is still not fully formed. So when I asked the seminar speaker how far off logical qubits were, he humbly said that they may not even be possible. From his research, quantum computers may be useful but their utility and longevity will always be undercut by the fragility of the qubit superposition.

I was kind of stunned because in my readings on the field it was taken as writ that as soon as you can produce 1 qubit, you can scale up and produce thousands. Once you produce thousands, you now have logical qubits which will make all our QC programs work perfectly.

What’s interesting is that IBM is now saying it will make a QC with over 1000 qubits, which is around about the number is supposed to be needed to make 1 logical qubit. Yet by in large I haven’t seen much talk about having our first crack at producing a logical qubit.

So again I’d like to ask the question: how long until we have a logical qubit? If qubits will always be unstable superpositions, then I doubt a mass market consumer QC will ever be workable. And while the hype for logical qubits seemed ever-present when they were still a far-off dream, it seems to have subsided as they get closer to being tested for validity. I wonder if they were always nothing by hype.