Introduction to Quantum Computing Systems／量子コンピュータシステム入門

Earlier in the course we saw the DiVincenzo Criteria. Which tell us about the technology that you build a quantum computer out of. It tells us what kind of characteristics that technology has to have, but it doesn’t tell us very much about how to actually build a large scale quantum computer. So today I am here with Professor Kae Nemoto from Japan’s National Institute of Informatics. Kae, welcome. Thank you. So, Kae and her group are experts in designing large scale quantum computers. She has been doing this for more than 15 years and I am proud to say that I was actually part of her group not long ago. Yes, yes, that’s right.

So tell me about the design process for a quantum computer. How do we actually design one? So you started with a rather difficult question to answer. You know there are lots of elements we have to think about to design a quantum computer. To start with, we need to have a module or a device to create a system. So we have to divide two concepts now here. So you think about photons or ions or something like this. These are candidates for qubits, but we need to have some kind of device which should be a fundamental building block. Then we can have a system created out of them. So where shall we start with? That’s great.

So, we’ve already met actually the basic idea of using photons and ions as qubits and from this point forward in the course, we are going to be talking about the devices themselves. So let’s talk first about the large scale architecture of this system. So a classical computer has a CPU and memory and they are separate and you load data from the memory into the CPU, compute on it and then store the data back into the memory. Will quantum computes have that same kind of structure? In some sense, it is quite the same, but there are fundamental differences, and that comes from errors. So in quantum computers, errors are treated differently.

Any large scale quantum computational system requires a fault-tolerant implementation, that of course requires quantum error correction. So architecture already has quantum error correction system included. So that is a big difference from classical computer. And 90% of time we are doing error correction. So in this sense we do not have a space dedicated for pure memory. So the memory in the system in a quantum computer is actually sort of active. It is sort of constantly performing this error correction. Is that right? Exactly, exactly. So we have to correct errors all the time, because there is no system we found stable enough to just store information for quite a long time. So we always need to actively error correct data. I see.

So does that error correction make the system larger or slower? Yes. Unfortunately that’s true, but it is not – while the good news is it is not slow enough to be problematic. So that means we have to spend a lot of resource time wise or temporal wise, but still quantum computer can be faster than classical computer. So even though we have to spend a lot of resource to correct data, still we can have speed up using quantum computer. So, what about the classical control of all of this. So, you have got a quantum computer and the quantum computer itself is great, but all of the control of it is classical. Correct? Exactly. So, quantum computer is interesting.

The quantum part is the most difficult of course part, but is a very small part in the sense in carrying only the data in there. So, that is important, but it is sort of if you think about technological layers, then lot of things belong to classical computation because you have to decode errors that is also classical computer’s responsibility and also then feedback the control. So you have to have very quick read out and computation and feedback. So, a classical computer also has to be developed to control the quantum computer. So this feedback cycle you are talking about, that is to run the error correction. Is that correct? Yes. Okay.

So that all has to be in real time because the qubits themselves are changing in real time and the system is all evolving very rapidly. Exactly! That means that process has to be quicker than clock frequencies in quantum computer. Do you think that is possible? Is it going to take new advances in classical systems to control the quantum systems or do we already have the technology available? At the moment we found it is possible to do it in the current computer we have. So, at the moment it is okay. But we have to consider. Once we created a quantum computer, it would be faster. So in that sense, we also have to have a faster classical computer to catch up.