today i'm going to 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 and
apparently chopping him in half or
threatening him too and that's the image
that most people have a laser you shine
it on yourself heat goes into you and
maybe does you damage
now
you can cool a gas of atoms a gas of
sodium atoms or rubidium atoms from very
high temperatures down to temperatures
less than a millionth of a degree kelvin
absolute
and the guys downstairs
peter kroger et al just under my feet
have laser beams running along there
that can do this
so this is an optical table in one of
our laser cooling labs we call the
lithium here gases of atoms consists of
particles which are whizzing around
running this way and that way and going
i can't do this and some are going
slowly because they're old like me now
they're going really fast and there's a
whole collection of these all over the
place 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 it produces about a watt 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 particles whizzing
around that corresponds to the heat and
gives them a temperature
so if you had a gas of atoms say let us
take
sodium atoms or rubidium atoms and put
them in a container
to cool 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 and if i'm running towards
this sound wave the frequency goes up as
you hear when ambulances or trains go
past and they're they're going dee dee
dot and it changes frequency so if
you're running towards it you get a
higher frequency
mirrors here that that are only used to
steer the beam along different paths
sometimes we need to split up a beam
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 at one
frequency or absorb light at one
frequency it's as though i've only got
ears which work at one frequency and i
can if i hear that frequency then i'll
be affected by it so if i'm running
towards a laser beam or a sound wave i
will only hear it if i actually match
the frequency
as i hear it to
get everything here and to buy the
equipment and so on takes a few months
of course but
to actually once you have everything in
place and to set it on the table it's a
question of a week maybe for an
experienced person so in our experiment
we set up a laser beam which is
equivalent to emitting a sound wave and
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 it's given out when it's been
doppler shifted 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 down i can't run
upwards or forwards or backwards i would
be completely unaffected by it
so these atoms
if they move towards the laser beam if
they exactly match the frequency when
it's doppler shifted that's emitted by
this then they will absorb a photon
they'll absorb a photon go to an excited
state and there will be a judging motion
is there a photon comes in and baffles
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 and 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 the this red color is
671 nanometers in wavelength
um
and you can imagine that that you need
not only the 671 nanometers but also
671.0005 or something like that so each
time a photon comes and baffs me i slow
down a bit then i change the frequency
another photon comes in and baffles me i
slow down a bit i change the frequency
and it hits me it hits me again and it
says so i'm wading through tree cool
they call it optical molasses because
this is light coming out and it's acting
like tree cool or molasses as the
americans say so i slow 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 we 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
and 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
con
a whole lot of photons coming in all
directions and it slows down all the
components of the velocity of the
particles all the jiggly motion in every
single direction gets slowed down and
they all grind to a halt we shine laser
beams that come from the other table
through these fibers here these fiber
ends so out of the light comes out of
these fibers and in 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 the light
beams meet and they we have to adjust
these things and that's what these
little screws are for we can fine-tune
these laser beams to exactly meet each
other in the middle of this uh vacuum
chamber and so instead of having things
zapping around at great speed they're
old men crawling around so they can
hardly move at all
and that means that if they're hardly
moving
their heat is reduced enormously they've
lost a huge amount of energy
and they're coming to a very low
temperature and 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 that these gas atoms have the big
advantage is 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 a 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 in traps so we can use
magnetic fields
again laser light of slightly different
type or electric fields and we can
confine them to a very small room and
then we can we can study them sometimes
even one by one and sometimes we want to
look at the collective behavior of all
these gases when they for example
form the famous bose-einstein
condensates
well the original point is that there
was a prediction made by bose and
einstein way back in the 20s saying that
if you've 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 able to do because of the
development of this technique of laser
cooling what's that called
that's called bose einstein condensation
it's as though you're making a funny
sort of rain that the particles are all
dripping out into a single quantum state
and you're getting a new state of matter
so physicists love this because it's
pushing the boundaries of physics into
regions which haven't been explored
before and as soon as you get to that
stage you begin to be think that nature
is whispering to you some deep and
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