[MUSIC] So this is, again, our quantum computer. Now, we have to agree about in it. And we decided to be the pulse and the parameter. So let it be 1 the left part and 0 at the right part And the polarization is the second photon. Let it be vertical polarization to be 1 and horizontal polarization to be 0. If polarizer here is arranged like this, then on the input at this point we have, This state. And after the first [INAUDIBLE] splitter, we'll have This state. And we are now ready to apply the quantum oracle because the one transforms are already applied by this polarizer and being splitted. So I told you that I'm going to implement the quantum oracle by this wave plates. And the wave plates I'm going to orient like this, orthogonal to the photon's polarization. So the action of a wave plate will be with be multiplication of this state, we have 1. Now, we have four different quantum oracles which in our interferometer. The first one is, you remember, identity, here, which means nothing happens to photons. And we don't place any of wave plates here. Now, the second is this, which applies X gate to the second photon independently of under radio of the first photon. Which means we have to multiply our state by minus one, On both ways. So the polarization must change independently of the path and we put wave plates on both paths. Well, the next situation is for f(x) = x, and we have to multiply the state by -1 only if vector X is 1. So it's the left path of the interferometer. Here, we have the first qubit equal to one, and we place the wave plate here. So for this qubit We have multiplication by -1 and for this, for this vector, we don't, and vice versa. For this case, we have to multiply the state by -1, the vector by -1 only if our first qubit is zero, which is the right path. So we place the wave plate on the right path. So with this wave plates, we can entangle the position of the photon with its polarization. And we can implement all four quantum operators, quantum oracles we need for solving the Deutsch's Problem. Now, imagine that we have some configuration of this wave plates here. And then someone comes and covers all this with some black cloth, so we can see what configuration we have. Can we still decide if the function implemented here is a constant or a bound set function? Yes, I told you about the interference so the photon goes too fast and it presents the interference button here and here also. Let's imagine this interference button, With no wave place inside the interferometer. It's going to be something like this. We have it's red Draw a maximus of interference picture so here both photons is their maximal places are here, which this photon comes, the points were the photon counts and counter face. Okay, we have this interference, and what if we put the wave plates on the both ways, on the both paths of the photon? Will the interference picture change? It will not because the indifference of the path doesn't change. The photon experiences a delay on both paths so this red and black spots will stay on the same place. And now, if we put a wave plate on one path and the other is free, then the points where the photons came both is their maximas. One photon still comes with its maxima, which goes free, and the second photon is delayed by half of its wavelength. So it comes here at it's minima. And on the red, occurence here, we will have on this case the black points. And instead of black points, we will have red, so the interference bottom will shift. And this is how we can distinguish the difference within the constant function and a balanced function. And this is what we have, the balanced function here. So the wave plate on one path, here on the right and here on the left path. And we see that the interference button is the same for both configurations. And for this type of configurations is either the wave plates on both ways or no wave plates. We have this shift of so we can see here, mark it with a pen, the shift and the band, and the black bands between these lighted bands. And when we change the configuration of the interferometer, these traces of now are on the dark places of the interference picture. Of course, if we [INAUDIBLE] our interferometer better, we would have not these bands but rings, as I draw in the light. And is a well tuned interferometer, these rings are much thicker. But this kind of trick is really hard to achieve on this interferometer, hand-made interferometer, hand-made on chip board. So still, we have implemented a real quantum algorithm on a real quantum computer. Lets go further.