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  • today, I'm gonna talk about laser cooling.

  • Laser cooling seems to be a contradiction in terms, because I remember seeing James Bond splayed out on a table and a big laser coming along, apparently chopping him in half or threatening him to on.

  • That's the image that most people have a laser.

  • You shine it on yourself.

  • He goes into you on.

  • Maybe does you damage.

  • Now you can cool a gas of atoms of gas, of sodium atoms or rubidium atoms from very high temperatures down to temperatures less than a millionth of a degree.

  • Kelvin Absolute on the guys downstairs.

  • Peter Kroger.

  • It'll just under my feet have laser beams running along there that can do this.

  • So this is an optical table and one of our laser cooling labs cool with him.

  • Here, Gas of atoms consists of particles.

  • We shall whizzing around running this way and that way and going in.

  • I can't do this, and some are going slowly because they're old like me, and I think I really fast and there's a whole collection of these all over the place.

  • But for laser cooling, obviously what you need is a laser or actually, several lasers, but not so many.

  • So this is one of them.

  • This is a commercial device that produces about a what of laser light in this nice red color that you see everywhere.

  • This motion, this jiggling around motion of atoms is what we call heat in a gas.

  • It is the kinetic energy, the energy of motion of these particle whizzing around that corresponds to the heat and gives them a temperature.

  • So if you had a gas of atom, say letters, take our sodium atoms or rubidium atoms and put them in the container to call them.

  • We've got to get rid of all this jiggly motion jiggling around in every single direction.

  • I'm doing this just to wind you up, Brady, because you can't follow me.

  • All right.

  • Now then, you send in a laser beam to this atom.

  • Now, the laser beam is rather like a sound wave on.

  • If I'm running towards this sound wave, the frequency goes up as you hear when ambulances or trains go past on the gay.

  • Indeed, Ardito and it changes frequency.

  • So if you're running towards it, you get a higher frequency mirrors here that only used to steer the beam along different paths.

  • Sometimes we need to split up being into several different paths.

  • Then we have these little cubes here.

  • There's one thing about atoms which is different from me is that atoms can only hear a one frequency or absorb light at one frequency.

  • It's a so I've only got is which work at one frequency and I can.

  • If I hear that frequency, then I will be affected by.

  • So if I'm running towards a laser beam or sound wave, I will only hear it if I actually match the frequency as I hear it, Thio, get everything here and thio by the equipment.

  • And someone takes a few months, of course, but to actually, once you have everything in place and to set it on the table, that's the question of the week, maybe for an experienced person.

  • So in our experiment we set our laser beam, which is equivalent emitting a sound wave on.

  • If I'm running towards it, I will only hear that sound if I'm running at a particular speed which matches the speed which is given out when it's big Doppler shift.

  • If I'm running away, I won't hear it because I'm here sound wave which is shifted away from it.

  • If I'm running upwards or that I can't run upwards or forwards or backwards, I will be completely unaffected by it.

  • So these atoms, if they move towards the laser beam if they exactly matched the frequency when it's Doppler shifted that's emitted by this, then they will absorb a photo on.

  • They'll absorb a photonic, go to an excited state on there will be a judging motion is there a photon comes in and baths me Because I get a slight recoil a momentum change as this particle of light comes and hits me So I slow down a little bit.

  • I don't feel that I want to move so fast on.

  • That means that if I keep the same frequency there, it will go straight through me because I'm off the right frequency.

  • So what I have to do is change the frequency of the laser bit by bit by bit.

  • You all you see all of this in red and the this red color is 6 671 nano meters in wavelength.

  • Um and you can imagine that that you need not only the 671 under meters but also 671.5 or something like that.

  • So each time of photon comes and baths me I slow down a bit Then I change the frequency Another photon comes in and baths me I slow down a bit They changed the freaks and it hits me hits me again And it says so I'm waiting through tree Cool they call it Optical molasses is because this is light coming out on is acting like treacle or molasses of the Americans say so I slowed down and after 10,000 of these baths into my side, I've slowed down so much that I'm hardly moving that way at all And I've lost all speed in that direction.

  • Okay, so once we've prepared all this light in the way we want actually have these two these three components here, which are in coupling devices to put the light from these mirrors into optical fibers and those fibers you see here, these blue things and they actually transport the light all the way over to this other table so that we can treat our laser system in our laser cooling separately, that just dealing with things moving that way.

  • In reality, they're going to be moving up and down.

  • They're going to be moving this way or that way.

  • So we set up lasers in this direction coming in, coming in, lasers this way and this way.

  • Coming in.

  • So in this central region, there is a whole how lot of photons coming in all directions on it slows down all components of the velocity of the particles.

  • Alvar jiggly motion in every single direction gets loader on.

  • They all grind to a halt.

  • We shine laser beams that come from the other table through these fibers.

  • Here, these fiber end so out of the light comes out of these fibers and these locations here.

  • So we have this direction, this direction and then from the bottom.

  • So we have three orthogonal directions from which the light comes here, here and here.

  • And then we have mirrors that simply retro reflect the light beams into themselves.

  • So basically, we have pairs of beams, for example, the one from the bottom gets reflected by this mirror from the top, and then they light beams meet, and we have to adjust these things, and that's what these little screws are for.

  • We confined to in these laser beams to exactly meet each other in the middle of this vacuum chamber.

  • And so instead of having things zapping around a great speed, they're old men crawling around, and so they can hardly move it all.

  • And that means that if they're hardly moving, the heat is reduced enormously.

  • They've lost a huge amount of energy, and they're coming to a very low temperature.

  • You can get down to temperatures, which are much, much smaller than anything you can imagine down to a millionth of a degree.

  • Kelvin, just measured from the distribution of speeds of these gas atoms, big advantages once the atoms don't move anymore, and that is basically what cooling means.

  • You stop all or essentially all of the motion of the of the atoms.

  • You can control them much better, so it's really like if you want to study animals, you put them in the cage and you you watch them very carefully and that we can do here with our with our atoms.

  • We bring them essentially to zero temperature.

  • We slow them down so that we can trap them in a trap so we can use magnetic fields again, laser light of slightly different type or electric fields, and we can confine them to it to a very small room.

  • And then we can.

  • We can study them sometimes even one by one, and sometimes want to look at the collective behavior of all these gases.

  • When they, for example, formed the famous Bose Einstein condensates well, the original point is that there was a prediction made by Bows and Einstein way back in the twenties, saying that if you got gas atoms of things like sodium or rubidium because they're a particular type of particle, they could all go in the same quantum mechanical state.

  • And that's the thing which people subsequently have been known to do because of the development of this technique of laser cooling that's called Bose Einstein condensation.

  • It's a so you're making a funny sort of rain that the particles all dripping out into a single quantum state.

  • You're getting a new state of matter.

  • So this is love this because it's pushing the boundaries of physics into regions which haven't bean ex roll before.

  • On the sooner you get to that stage you begin to be.

  • I think that nature is whispering to you.

  • Some deepen strange truth.

  • Besides, it's if you get to the stage where you actually discover something before anybody else, it is the second most enjoyable thing that I know.

today, I'm gonna talk about laser cooling.

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脈衝光冷卻 - 六十個符號 (Laser Cooling - Sixty Symbols)

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    林宜悉 發佈於 2021 年 01 月 14 日
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