As I told you before, there is a distinction between a quantum computer and a computer based on flipping coins. This distinction is a phenomenon called entanglement. For example, two of four oracle types introduced in the previous lecture use this entangling operator CNOT. We cannot implement CNOT on the flipping coins. Honestly, it's not easy to implement even on quantum particles. But with small number of qubits, two, to be exact, we can do it. It's not much we can compute with two qubits but at least we can demonstrate the action of David Deutsch algorithm. So, the main idea here is that we are going to entangle qubits that are carried by different quantum properties of one particle, photon in our case. You remember that under some circumstances, photon can take two paths. This position of photon in interferometer can encode one qubit for us. Another useful characteristic of photon is its polarization. It can encode another qubit, the second qubit, and these two characteristics, position and polarization, we are able to entangle. So, this is it, a real quantum computer with two qubits. The main part of it is here. It is the Mach-Zehnder interferometer, which consists of two semi-transparent mirrors and two ordinary mirrors. The whole thing is settled on a heavy and flat cheap boards. The real interferometers there usually based on some surfaces made of cast iron because the Mach-Zehnder interferometer is a device very sensitive to the slightest vibrations and slightest deformations of these basement because it causes the change of path length inside the interferometer. Since we are doing everything ourselves, cheap boards works for us. It is heavy enough and it is flat enough. The source of photons is here. This simple laser pointer, red light, 650 nanometers and 100 milliwatts. To polarize the photon sphere, use this linear polarizer. We already made before in this course. The beam splitters here, implemented this beam splitting cubes, non polarizing and best performance on the red light. The oracle will be implemented as these two half wave plates, and we can see the interference picture there on the screen and the button of this interference will show us the result, we will read our result of our computation by this button. On this slide, you can see the top view of our quantum computer. So, the laser is here and it emits photons of this wavelengths and then a photon goes through this polarizer and reaches the beam splitter and you can see here in this picture, the position of semi-transparent surfaces of this beam splitters which are here. Now, if you don't want to buy this beam splitters, you can find one in old photo cameras. Use that to divide the beam of light which goes to the camera, to the film, a beam splitter here, and to the eye of a photographer. Now, if your old camera looks like this, so here's a small window for the light which goes to your eye, then probably there is no beam splitter. But if there's no such window, then probably the beam splitter must be there. Now, this beam splitters, they split the photon two ways and it is ready now to reflect from these ordinary mirrors. Now, these mirrors are not exactly ordinary. They must not have glass surface on them, because a mirror which has a glass surface like this has two reflection surfaces. One is glass and another is the mirror itself. This small delay, this small change of wavelength allows this photon to interfere and this interference can overshadow the interference from the different path in interferometer. So, we need mirrors without this glass here. This type of mirrors are usually used by dentists. For some reason, I suppose that, if the mirror with a glass breaks inside someone's mouth, it is [inaudible] if there is no glass. So, you can easily buy mirrors without the glass. You just need to pretend to be a dentist. We have now these two paths inside the interferometer. The photon reflects from here and from here and reaches the second beam splitter. Red goes here, this photon reflects, goes here and it goes through also and this photons goes through and reflects. Here, we can place the screens to see the interference of the photon around these two ways. Now then, the type of this interference will tell us what is inside of this interferometer. The most important thing here is these wave plates which will represent an oracle to us. Let's learn how wave plates broke. I already told you about crystals which have an optical axis, for example polarizers. From the previous week, we have optical axis and the photons polarize it along this axis, pass it through this crystal and the orthogonal protons. They were stopped by this crystal. Now, the wave plates are also crystals with optical axis. Again, photons polarize it along this axis, pass through this crystal without any trouble, and this axis is often marked as fast, you can see it here, and it forms the ordinary beam. Now, the photons that are polarized orthogonal to this fast axis, they also pass through wave plate, but they experience a little delay in this process. This is why these wave plates are often called the retarders from French "retarder. " Okay. Now, the delay which experiences a photon polarized orthogonal to the wave plates axis, depends on the thickness of the crystal. We can fit this thickness. So, the delay will be half wave length. In this case, this will be half wave plate and the photons polarize it orthogonal to the fast axis produce the extraordinary beam, not only on a halfway plate but in any wave plate. Now, let's see what happens to a photon which is polarized not orthogonally, not along this axis but something like this. So, this is our polarization of a photon. Since the polarization is a quantum mechanical characteristic, we can present it as two photons like this and this, and this photon goes freely through wave plate and this photon experience the delay and if you have a half wave plate, the delay is half of a wavelength. If you remember the graph of the sine function, you understand that half-wave, the delay of half length is the same thing as multiplication on minus one. So, this photon here reflects here and we have on the exit of the wave plate this photon reflected along this fast axis. So, this multiplication by minus one is exactly the same thing, which we have in our quantum oracle in the states like this.