【文章內(nèi)容簡介】 World Wide Web. So there39。s all sorts of things that the world has changed that we don39。t really think of. I think I once figured out that in an average hospital there39。s probably more transistors than there are stars in the Milky Way galaxy. So we don39。t notice how often this es about. So this all came about thanks to semiconductor and solidstate physics which was enabled due to quantum mechanics.How many people don39。t know quantum mechanics here? Great. That will change by the end of the talk. I have like about 40 minutes to teach you all quantum mechanics which leaves me with a problem. What am I going to do with the other 35? [laughter]. It has a reputation for being weird and inprehensible. The ideas are certainly weird, but they39。re no weirder than saying that we live in a sea of invisible electromagnetic waves, only a small slice we can actually detect. We never think about that unless you can39。t get five bars or your phone or your laptop. Then you notice that the sea is missing. So let39。s  let me boil it down to understand how quantum mechanics leads to something like a laser. Let me boil it down to three things like in the ic book, three suspensions of disbelief, three things that you would have to buy into. That light has both a wave and particle like property associated with the term photons, matter has both particle and wavelength properties, and of everything, light and matter has an intrinsic spin. These are  this is all  if you buy that, that39。s  this is not, this has nothing to do with, you know, wave indeterminacy or the measurement problem or Schrodinger39。s cat. This is  I39。m taking what I describe a working man39。s view of quantum mechanics. This is the stuff that we experimentalists make use of when designing semiconductor devices. So let39。s look at the first principle. Light has both a wave and particle like property, photon. A manifestation of that is that when you shine light on a metal, if light was just a continuous series of waves like washing up on a beach, the waves might eventually push some pebbles up the slope of the beach. But really what the light is posed of is a series of  is a machine gun bullet spray. And by changing the frequency of the light, is the waves would just  the frequency would just be the spacing between the crests, and that would just determine the rate at which the pebbles are advanced. But the frequency actually controls the energy of the bullets and so by increasing the frequency we can have bullets that can promote the electrons out of the material. This is called the photoelectric effect. There are of course practical applications to this that we all know, right, that you can get [laughter] photon in this way. Matter has both partially and wave like properties. One of the most striking examples of this is interference. That39。s the hallmark of wave phenomena. You have a wave striking a surface and it gets reflected. But maybe part of it passes through and reflects off the bottom and if these two waves are in phase they would add up coherently, we would get a very strong signal. But if they e in out of phase here, then they would add up destructively and cancel out. And so when you look at an oil slick on a wet driveway, the oil slick floats on the water and create a free floating  freestanding film. And if it39。s not too thick, some of the light can be reflected from the top or passed through and e out because the oil slick is not a uniform thickness. Some regions might have a thickness such that some colors add up coherently. We39。ll see the red light out of the white light that39。s striking the surface whereas the other wavelengths interfere destructively and cancel and then we might get a blue light over here and so on. And this interference pattern is a hallmark of the wave phenomena. If I pass a laser light through a screen, I can get an interference pattern. By choosing the grid of the screen, I can get this nice circular with dot pattern. But if I send electrons on a crystal, electrons have a wavelike nature that39。s associated with their momentum. We don39。t notice our wavelike aspect because we39。re made of lots of atoms. So we have even moving very slowly, we have an enormous momentum pared to an electron. The bigger the momentum, the smaller the wavelength. If I walk across the room, the momentum of my matter wave is a trillionth, trillionth smaller than the nucleus of an atom. It39。s impossible to detect. There39。s no way to see such a thing. For electron inside an atom, its wavelength is about the size of the atom. It39。s impossible to ignore. And that39。s why this wavelike nature only became evident when studying the properties of atoms in detail. Back in the 1920s. You send  you choose the momentum of the electrons correctly and they scatter off the atoms in a crystal and well, if the room were darker you could actually see this a little bit better, but you see the same type of ring pattern with the spots that I saw  this is for laser light, and these are for electrons. So you have the same interference pattern for matter or light. And then the last part is that everything light and matter has an intrinsic spin. Of course those who look at the old  those old pulp magazines know that every month had an application of spin. But I39。m actually more talking about the spin say like of a twirling ballerina. Even that is a bit of a misnomer. It39。s not as if the electron were actually rotating like a top but this is still the phrase that39。s used. This rotation is also associated with a magnetic field that has both the North and South Pole, and we know the North and South Pole the magnetic fields flow out of the north into the south. And so that means that electrons can have magnetic fields that can point in two directions. It can either have a North Pole down here or it can have a North Pole here. Because the charge of the elec