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Quantum Cooling to (Near) Absolute Zero

how do you actually achieve temperatures

below liquid helium temperatures right

so this is a phenomenally fascinating

topic because the way you do it is you

use a machine that is looking like a

bunch of pipes and tubes and solder

joints and metal flanges but it's

actually entirely quantum mechanic what

it's based upon

is the idea of zero point motion okay

even at Absolute Zero things do not

stand still because of the Heisenberg

uncertainty principle okay if something

has a very well-defined position it

cannot have a very well-defined momentum

which means it must move so in the end

you get something that are somewhat

undefined in position and somewhat

undefined in momentum so essentially

they vibrate now how much they vibrate

depends on their mass so given a certain

zero-point energy a lighter particle

will vibrate more than a heavier packet

so add let's say absolute zero let's

take a helium-3 atom which is of course

25% lighter than a human at them the

unity atom will wiggle around by some

amount let's say do something like this

the helium 4 atom will wiggle as well

but a bit less okay because it's heavier

so now you ask yourself what is the

attractive force between let's say two

helium three atoms and the helium three

and a helium four the attractive force

between helium atoms is extremely weak

helium is a noble gas so in fact it

doesn't interact with anything it

doesn't ever make chemical compounds

with anything but when you put two

helium atoms very very close together

they actually slightly attract each

other very very very very slightly like

a van der Waal stir it's exactly a van

der Waals force but that that depends

very strongly in the distance so what

you find is that the attractive force

the attractive van der Waals force

between helium 3 and helium 4 is a

little bit stronger than that between

two

identical helium-3 atoms okay because

the hidden three can come a little bit

closer to the healing for this is not

recommended exactly so then what you do

to exploit this to go to very low

temperatures is the following take a

bucket and you fill it up with a mixture

of helium-3 and you look for and you

liquefy that mixture okay

what you would expect is that you have

the helium-3 floating on the top because

it's lighter right and then you'll have

the helium four at the bottom because of

the fact that the helium-3 is more

attracted to the helium four than it is

to another helium-3 in fact what you'll

find is that some helium-3 atoms will

spontaneously dilute themselves will go

and sit into the part where there is iam

four in fact you find that the

concentration is six point four percent

the equilibrium concentration so at

Absolute Zero there is six point four

percent helium-3 that spontaneously

dissolves into heating for the next

thing you do is that you shape it in the

form of a u-tube not as in the channel

YouTube but a tube in the shape of a u

like this okay so let's say you do this

and you fill this up with this mixture

of helium 3 and helium 4 so what you'll

find is that let's say on one side you

will have your pure helium-3 side and

then the rest is the helium 4 with a

little bit of helium-3 in it 6.4%

now the next thing you do is you connect

a pump to this YouTube so you connect

the pump that basically pumps away from

the right-hand side and reject to the

left-hand side the point here is that

helium 3 and helium 4 are like alcohol

and water like when you distill your own

vodka or some other liquid of your

preference effectively what you do you

heat up this this

start the material you have and the

alcohol has a lower boiling point than

the water so the alcohol vapors will

come out let's say you distilled booze

right and here you're doing the same

thing you're distilling helium-3

algorithm for helium-3 as a lower

boiling point than helium four so if the

temperatures are right what you'll find

is that there is only helium-3 that

comes out of here

you know four stays what it is and the

heat entry comes up just like alcohol

now as you extract helium-3 atoms from

the right hand side you are reducing the

concentration in this region right so

here you may have you know two or three

percent helium three only so what's

going to happen is that you have an

osmotic pressure that dries the helium-3

from this side on the left-hand side to

the right-hand side to balance out the

concentration but here you have the

interface with the pure helium-3 that

floats at the top so now if you're

taking away helium-3 from here you will

have new helium-3 atoms that can jump

into the helium two into the helium four

phase and as they do so they actually

absorb heat because the entropy is

increasing when you take atoms from a

pure phase into a dilute phase these

substance around here around this

interface is absorbing Heat but if you

connect something else for example a

spin cubed device to this part here so

you bolt something interesting some

interesting experiment to this YouTube

at this point it means that heat will be

taken away from your experimental

apparatus and dumped into this

circulating flow of helium-3

so um let's take example of the soup

we're talking about before so you've

made yourself a nice bowl of hot soup

but it's too hot

can't eat it yet so what do you do you

blow on it right when you blow on it

effectively you are trying to disturb

the equilibrium vapour concentration

above the soup right if the soup is

really at a hundred degrees

the water would like to have one

atmosphere of vapors above it of course

it's an air is not in a vacuum chamber

so that vapor would blow away but if you

actually blow on the soup you are you

know increasing the carrying away of the

disordered part of the liquid vapor

mixture right so the vapor is the

disordered part the entropic part and

the liquid is the more or the top so by

blowing on the soup you're doing like

what we're doing here you're taking away

the these or the parts of the system

like taking away that helium-3 from here

and then what's going to happen is that

some atoms from the ordered side which

is the pure liquid are going to jump

into the disorder phase just like water

molecule in your soup are going to

evaporate more quickly if you blow on it

and as you do so you're actually cooling

down the soup because you're taking away

energetic and disordered particles from

the liquid into a phase that is then

removed how come you get this in our

laboratory we get to about 20

millikelvin the record to the best of my

knowledge is 1.9 millikelvin one thing

you might find interesting is that these

techniques or the dilution refrigerator

technique that I just described is being

used in light and indeed to cool down a

gravitational wave antenna so what they

are doing there they are taking a

65-centimetre which means one and a half

ton in weight sphere of copper aluminium

alloy and with this technique they're

cooling it down to 20 millikelvin to

detect the change in the shape of this

sphere when a gravitational wave passes

by for example because of supernovae has

imploded so an object as big and as

massive as a 65-centimetre 1/2 ton

copper aluminium sphere will essentially

shrink and expand upon passing of a

gravitational wave by probably about 10

to the minus 20 meters which is less

than a billionth of the size of an atom

it's actually smaller than the size of a

nucleus yeah and yet you can so they

have these sensors which are of course

based on quantum mechanics that can

detect the displacement of about 10 to

the minus 21 meters so they got about

you know factor 10 extra range to tell

whether that ball which is being cooled

down by this method to 20 millikelvin is

actually expanding and shrinking because

of the passage of a gravitational wave

so this is one just one example of the

million things you can do with

low-temperature techniques why does it

need to be that cold oh because

otherwise you would have thermal

vibrations of its own that essentially

you know are much larger than the

vibrations due to the gravitational wave

passing by my advice sounds so simple I

mean just start making videos so why

don't we why does everyone find it so

difficult well the answer is you want to

make something that's good you want to

make something that's popular that

everyone likes you're worried that

what-have-you