Qubit quality and control

35:14>> Um so so we're going to we're going to think about three criteria >> about what makes a good quantum computer. Okay. cubit quality, cubit control, and the scalability and the economics of the thing. >> All right, so let's get into what each of these things mean. Yep. >> So cubit quality. All right, so you've made a cubit. >> Question is, how hard is it to keep isolated? >> Um, how long does it last? If your cubit doesn't last very long and you need more time to poke it around with your quantum circuit to get an answer and by the time the circuit has ended, your cubid has failed. Well, you're kind of done,

35:54right? Yes. So, the gates that you saw in the those rotations, those rotations need to happen much faster than the cubit >> can forget what it is. the entanglement states that we had seen in the previous Yeah. Um the idea again here is is uh efficiency and and uh data fidelity. >> That's exactly right. Yeah. It's the the cubic quality is is effectively how efficient is the is the circuit that you're going to implement in relation to um how long your cubit can last. Right? If for example like for example if you take a a bunch of transistors and you're trying to do and gates on them, right? If the transistor forgets if it's a zero and a one before you can even implement

36:34the ANDgate, then the thing you're going to get out of the ANDgate is not really going to be the end of the two. It's going to be some random zero or one, right? I' I'd like to actually do computation. And so the transistor better be stable. Well, in the quantum sense, it's very similar. The decoherence time is what we call it. Um, of how long the cubit can remember itself >> and not get messed around with all of the outside noise. um that needs to be much larger than the gate time, >> right? Um and that brings us to another question which is how does it compare to your like what's the noise sources that has to do with the cubic quality? What are the noise sources and can you get rid of them? >> Exactly. Exactly. So that's cubic quality. Then there is cubit control which is can you initialize, manipulate

37:16and read out the state without your control rack um setting on fire. >> Right? Initialize meaning like state preparation. >> Mh. So, you know, preparing all of your cubits in the zero state. Yeah. That's something NMR couldn't do. Yeah. Which is why Denzo wrote that thing in the first place. >> The pure initial state point. >> Exactly. >> Where we see the blue and the left where it's like we all have we have to start everything at >> Yeah. >> a a known base. >> Yeah. Like it better like I better know the states of my system before I start doing quantum logic. Right. Otherwise, like what what are we doing? Um >> and then can I manipulate them with my gates? And then can I read it out? Can I measure what the thing actually is?

37:56Right? Um, now when it comes to manipulation and gate fidelity, this middle part right here, um, here's where the noise directly translates into error rates because the the question is, um, can you reliably hit, let's say, two cubit gates, two cubic gates where you take one cubit and another cubid, and let's say you want to entangle them or you want to rotate one based on the based on the, um, state of the other. Like if the other one is near a one, then I rotate. If not, then that's called a controlled knot. >> Can I just take a moment here because I think this is a really key idea to make sure that people are are getting right. Right. So, you know, we talked about earlier when we have these >> cubic gates that there's two um there's two states.

38:37>> One of them is let's again to simplify it north northern north pore south pole and then the other one is like where in that north or northern >> where in the longitude you are east west how far we can even just call it that. Let's just call it that. north and east. And so part of it is like north south can be uh just a zero or one. It can just be a functional answer that >> we can then move on in whatever our process is. >> But when you just talked about for example when we start having >> two uh like two uh of these states, two of these cubits interacting with each other. >> Yeah. >> Um the phase is maybe what matters more. The east west is maybe what matters more

39:18than the north south. >> Yeah. I mean sometimes well it depends on the gate really sometimes the but but I think what your point being that like both of these directions matter >> right right in terms of what the next >> one >> does or does not do >> and the the ability for those two things to interact >> successfully in this quantum state >> um is meaningful like that's this orange band >> that's the computation part >> that okay and I just wanted to >> that's that's exactly right and the the point that I'm trying to make over here is that when we do these gates, >> they're not going to be 100%. It's not like classical computing where the transistor, if I wanted to flip that transistor, unless like a cosmic ray

39:59comes in or some nonsense or unless I'm operating the computer at like 200° C, >> if I want to flip the bit from a zero to a one or back back and forth, that thing is going to flip, right? It's extremely reliable because it depends on like bulk electronics. Here it's depending on single cubits, right? These are very finicky things. >> And so there's a chance that you try to flip something or you try to rotate something and it just doesn't work >> because of whatever reason >> for sure. >> Right? You need to bring that chance down. You need to be able to do this >> reliably. This is the cubic control

40:41point which is the criterion number two because this is a highly um volatile system >> or or a highly um >> uh maybe volatile is not the right word but >> no volatile yeah I would say >> and so but this is and I'm trying to nail down this point which is by nature it's not like transistors no where you're always going to get a zero or you're always going to get a one based on some macro >> and if you want to flip it it's going to flip >> flip because of that the cubic control really does is like a very important in this computation layer >> uh especially >> is like a lot of there's a lot of meat there >> to actually nail down. >> Yeah. And and you better be good at it.

41:21>> And you better be good at it. >> Yeah. And you know you're not going to get perfect. >> You're never going to get perfect because quantum mechanics is a probabilistic thing. You're living not at absolute zero blah blah blah. So one of the tricks that people use is um they use