The Loss–DiVincenzo proposal

2:09:53review A's 50th anniversary and as part of the celebrations the journal um presented a collection of milestone papers 50 milestone papers this was one of them it was listed as a milestone paper quantum computation with quantum dots by Daniel Loss who was at UC Santa Barbara and Bassel and um Danchenzo who was at IBM and UC Santa Barbara. So in this paper loss and devenzo they laid out a proposal for quantum computation based on quantum dots. Quantum dots meaning little tiny like localities of charge that are in some kind of solid state device. Um and it was a detailed investigation into how you can do one

2:10:35cubit gates, two cubit gates with these quantum dots, single electrons that are localized in some solid state device in some like piece of metal. Okay. Now, the idea is you're going to use some piece of metal to confine the electrons into a geographically isolated place. This thing is going to have to be small, something like tens of nanometers for each electrons room, so to speak, because the electrons wave function is, you know, that big. You don't want to fit too many into. So, so this thing has to be really small. >> Um, it's going to be confined in the Z direction by the material itself. And I

2:11:15think we've got a um a thing that shows that. >> So in the you know you can imagine you have a silicon substrate like some kind of chip >> and you do something to the silicon such that all of the electrons want to live in a plane. So you get a two-dimensional electron gas a two is what they call it in the industry. Um this can be either because you know you do silicon and then you put some silicon oxide and then and then you put some more silicon or you put silicon and then germanmanium and then you put some more silicon. Some kind of heterosructure that makes the electrons want to live >> in a plane. >> Everyone wants to live on the second floor of the apartment. Not the third, fourth and fifth, not the first, but for

2:11:56whatever reason we all want >> for whatever reason >> we all want to live on the second floor. >> We all want to live on the second floor. Now crucially that means that one dimension of confinement has already been taken care of because of the material >> it's at at the architecture level we've already confined the system to some variable we know >> yeah right what that means is I only need to confine it now in these two dimensions into this spot and this spot and this spot and this spot like kind of like a chess checkerboard >> as opposed to when we talked about trapped ions and neutral atoms they're in a 3D threedimensional space. So the complexity of maintaining, you know, X,

2:12:36Y, and Z, pun intended, uh is significantly more difficult. >> Exactly. Specifically with trapped ions, right? Cuz they're kind of related to electrons. Electrons have charge just like trapped ions. And one of the fundamental um theorems that you learn about in undergrad electro magnetism is this idea that a static electric field cannot confine you in three dimensions. That's why the trapped ions needed that rotating saddle. >> But here I've already got 1D confinement. And so in order to confine in 2D, I can use static electric field, >> right? I don't need to >> oscillate stuff in order to localize a charge here and here. >> We don't need a harmonic oscillator.

2:13:17>> No, >> that's quite nice. >> That's quite nice. No harmonic oscill Well, there's still going to be a harmonic oscillator, >> but like you know what I mean. >> I know what you mean. I know what you mean. Um, so what what we can do there and if you could bring up that >> 59 again. Yeah. Yeah. >> If you could bring that up. So so you you're confining it into 2D. >> Okay. >> And now what you can do is in the 2D you can you can create an egg carton potential landscape. Okay. For example, you can have an electrode like a little wire that goes up top here, pops down. On the left you see a electron microscope of all of these wires that are going down from all these places and that are going to plop down at a certain spot. I can have the wire go down,

2:13:58maintain that wire at plus 5 volts, have another wire right next to it that goes down, maintain that wire at negative 5 volts, right next to it, plus 5 volts, neg 5 volts or millolts or whatever. You know, the idea is not the scale. The idea is that the sign I can switch >> such that I create um a mountain and a valley and a mountain and a valley and all of the electrons are going to want to sit at the valley. >> Mhm. >> Mhm. And so now I've got a way to confine single electrons. >> Okay, that's the idea. >> Okay. >> Okay. So all of these electrons are now confined in these single electron wells is what we would call it >> because we've been able to confine

2:14:40everything to floor two >> and then we have these rooms >> that that we can basically decide which has a twotory >> on the second floor and we can just turn off the twotory second floor room or not. >> Or not. That's very good. Yeah. Um and yeah, it's like we can open the door to the room or not based on like the the voltage there >> that's coming in >> right the the cubid itself in the in the original proposal the law Steven proposal in that original proposal the cubid itself was the spins of individual electrons. So you can split them up using a magnetic field like if the spin is this way and then the one neighboring it is also spinning in one direction you can split them up with a magnetic field. Um and how do you talk to the cubits?

2:15:21How do you talk to these like LD cubits? you apply a microwave magnetic field that's going to switch them from one spin to another. And the you can make them talk to each other by simply lowering the voltage barrier between two adjacent spins. >> So you've got one electron, let's say in this egg >> um depression and then the one right next to it. >> There's a barrier in between that's keeping them separate. >> If you want them to talk to each other, just lower the barrier, >> right? There's a there's a little electrode that is maintaining that barrier. It goes negative 5 volts positive 5 vol 5 volt. Actually, it's opposite because electrons have negative charge, but whatever whatever that barrier is, just lower it and now the

2:16:03electrons mush together and they can talk to one another. >> Would it be like in those hotels where they have the two doors on each side of the hotel room and you open that door? >> Yeah, you just open that door >> to allow them. So, it's like the first level is like opening the door to get into the room. The second level which is creating the well and the second level is the door in between the two wells like the hotel room. Yeah. To allow them to communicate. >> Exactly. And that's called the exchange interaction. It was um it was first proposed by Heisenberg when he was trying to talk about um magnetic fields. But here's the idea with the exchange interaction and why that works so well. First, >> it is a DC signal. [clears throat]