Exchange-only qubits

2:29:52>> So Danchenzo came up in 2000 in 2000 with this nature paper along with a bunch of his colleagues with the exchangeonly cubit. >> Mhm. >> Okay. [clears throat] This is a cubit where both single cubit gates and two cubit gates are mediated only by the exchange interaction. >> Meaning we now no longer need two separate basically vehicles for interacting with the or controlling the system that are almost related but independent variables that when you come to like the experimental apparatus >> like need to be controlled. >> Yeah. And maybe it's better, maybe it's better if there's just only one thing we

2:30:33do, which is just exchange. It's all DC, right? There's no there's no microwave signal that's going in. It's only one thing that we got to get really really good at, >> right? Which is ideal. >> Yeah. The caveat though >> is that now >> your cubit is not a single electron. You need three electrons in entangled states >> and then you can now have a singlet and a triplet be your zero and one. Before it's like this was a zero and one. Now it's like this is your zero and then another set of spins is your one. >> Okay. >> Okay. So the caveat is you've made your problem a bit bigger. >> Mhm. Right now, instead of a single

2:31:13electron, you're worried about three electrons at a time >> because that's what's required to give you a >> That's the trade-off. >> That's the trade-off. Um, and this is where the exchange only interaction comes in and this exchange only cubit. Three electrons now. >> Yeah. >> Okay. the the first two whether the first two are in a singlet or a triplet state these two different spin states that we don't have to worry about for this episode that's going to be your zero and one [clears throat] okay and um the third electron is there to give you full cubit control on the block sphere meaning like you know for for a full

2:31:54cubit I need to be able to rotate on two axes right I need to access this and I need to access every longitude and every latitude and so the exchange on the First two is going to let you rotate on the north south. >> Yeah. Yep. >> And the exchange on the second two, two and three, is going to let you rotate somewhere near the equator. Not actually at the equator, >> but somewhere off the equator, just based on how the um the algebra of the space works out. >> This is interesting. And so, so you it requires three electrons >> now to basically define the two states of zero and one. Yep. based on how the first two of those three interact versus

2:32:35the second two of those three interact. Yeah. And so we're basically deriving two states from a threebody system. >> Yes. Now with the threebody system, right, there's actually one there there's um an another advantage here. So the first advantage I've already told you which is that um everything is DC. All you have to do is lower the barriers in between them. So everything is DC control. Yeah. Yeah. >> So you don't have to worry about like AC >> oscillating electromagnetic fields and all that other kind of crap going in. Um the other thing is that because this is um there's three the algebra also works out such that you are insensitive to global magnetic fields. If there's a giant magnetic field

2:33:16>> that's going through this >> um you don't care because all of these spins are rotating in the same way. It's kind of like remember in um Interstellar that scene where they tried to dock with a rotating >> like spaceship. It's like tar we need to we need to match the rotation and then like tar makes the thing spin in a certain way and um whoever well who's the actor? >> Matthew McC. >> Matthew McConna is like he's like he's like pushing his head in the other direction to counteract the G forces and they're spinning in such a way that they match the rotation and then they can dock. Similarly in this case like the magnetic field a big magnetic field is going to come in as long as it's the same across all three spins everything

2:33:57is going to rotate in the same way and your quantum information is going to be preserved and now because because we have that third body in the system it's partly is why like the that that enables um >> it's the system is large enough locally >> that a larger external factor is going to impact but the system still has enough >> yeah there's like there's there's like an algebra in here that is going to just be invariant to all of the big rotations, right? Crucially, if there's local magnetic fields, that's still a problem, right? Like if the third one is rotating >> in a different way than the first two, >> still a problem. >> That's still a problem. But a global magnetic field, like the Earth's big magnetic field, you don't care about it

2:34:37because it's mostly >> going to be the same across these three, like a few tens of nanometers. >> Um, so it's like gauge hacking. I call it gauge hacking. >> Um, because there's like a weird gauge theory that happens here. Ah, gauge theory made it in. >> Exactly. So, so that's kind of cool, right? >> Okay. >> Now, why are people initially excited about just spins in general? Okay. When Denzo came out in um 2000 with this paper or in 1998 with the first paper, people were excited because silicon is nice and you can do this in silicon. This was a proposal to make a quantum computer in silicon, >> right? which means I can now leverage

2:35:18all of the silicon manufacturing that humanity has gotten really really good at. >> Right? >> This is a single silicon wafer. It's a piece of I don't know 99.999999% pure silicon that then you you you plug through an ASML um EUV lithography machine and then you print chips. This is where your GPUs come from. This is where the chips in your laptop come from. Everything y >> is made out of silicon in today's economy >> and and so the idea is this methodology of spin cubit like of sorry of what Denenzo had come up with at the time the substrate you could build it on top of

2:35:59did not require lasers did not require barerium it did not require uh what was the other one um uh the y the the >> um neutral atoms >> yes yes it didn't utum did not require any of these things. >> Tantelum like all of that >> it's just silicon. >> It's just we can and the part of the point here is our entire uh current >> economy. >> Uh yes uh manufacturing base as it relates to computing systems are well suited for scaling silicon. That's convenient. >> This was in 2000 which wasn't even at the point where this is that big yet.

2:36:40>> Yeah. But nowadays it's even more clear or more robust. >> Yeah. >> So in terms of scalability, this seems like >> already like there's there's there's ways that this could work. Right. >> Now, why have people been skeptical though? >> Yeah. >> Right. Cuz clearly that guy at Alice and Bob didn't even know about spin cubits. And even the big higherups who know about spin cubits, they're always like, "Yeah, but probably not." M >> there's there there has been good reason for skepticism. >> Okay. >> Okay. Um for one, the fabrication requirement here is difficult because you need to you need to use one of these major fabs, right? That has this like so so um