- Physics world.
Hello and welcome to the Physics World Stories Podcast.
I'm Andre Gluster. And to take us on a journey
through the stories of laser cooling, here's our guest.
- My name's Chad Zel.
I'm a professor at Union College in Schenectady, New York.
And, uh, I'm also for my sins,
the chair of the department at the moment.
- The discovery of laser cooling has transformed the field
of atomic physics and led to a number of Nobel Prize wins
and real world applications through atomic clocks,
GPS technology, and now laying the foundations
for quantum computing.
In addition to his research in atomic, molecular
and optical physics, Chad has been an
accomplished science communicator.
He's a prolific blogger
and author of several books, including How
to Teach Quantum Physics to Your Dog.
Chad has been exploring the story so far of laser cooling
with his trilogy of features for physics world At the time
that this podcast goes live,
the first two articles are already available on physics
world.com, entitled Cold and Colder,
and the final installment called Coldest will be available
from early January, 2024.
- Yeah, laser cooling, right?
It, it sounds like something that is just, you know,
air conditioning the laser lab or, or something like that.
Uh, but in fact, it's,
it's a wonderfully counterintuitive area of physics,
which is you can make a gas of atoms cold
by shining laser light on it.
That seems like the
exact opposite of what you expect, right?
You think lasers shining on things.
You think things getting really hot, things exploding,
you know, the death star whatever.
Um, but in fact, uh,
what you can do is you can use carefully arrange laser beams
to slow down the motion of atoms
from something approximately the speed of sound, uh,
for a gas of atoms at room temperature, down to a speed
of a few centimeters per second of a,
a rapidly moving insect.
Um, and you can do that, uh,
by using forces exerted by laser light.
So just shining light on a gas
of atoms can make those atoms move slowly, slower.
Motion is equal to lower temperature,
and so lasers can cool a gas of atoms.
- You've done it now, 'cause you mentioned Star Wars,
but why is it then that if we fire a laser at
the death star, then it blows up?
I mean, maybe unsatisfactorily and it's able to be rebuilt
and everything, but why doesn't it just call it down?
- Uh, what happens if you're talking about something like a,
a macroscopic object, right?
And you shine light on it, it'll absorb
light over a very broad range of frequencies,
and it picks up the energy that was carried by the light,
and then that energy goes into vibrations of, of things.
It heats up the, the system
and it gets, gets transferred there.
Um, and so that, that energy just comes in and,
and doesn't, uh, go back out.
Um, in, in the same form.
It, it changes from energy carried by the light
to energy in the, the kinetic energy of, of the atoms
and molecules making a thing up.
And, you know, if you have enough,
you dump enough energy in, it'll,
it'll heat up significantly and, and blow up.
Um, if you're talking about, uh, atoms though,
atoms are very simple and they'll only absorb
and emit very particular frequencies of light,
and there's nowhere for that energy to go.
Once it's in the atom, right?
The atom, uh, has a photon of light come in,
it gets some amount of energy, it puts
that energy into the orbit of the electron
around the nucleus.
Uh, and then sometime later it will remit that, that light,
uh, at more or less the same frequency
that it, that it came in.
There's nowhere for that energy to go, generally speaking.
So, uh, whatever comes in, goes right back out, uh, in terms
of the internal states of the atom.
Now, the light can also affect the external states
of the atom, right?
So because the photon carries, uh, energy, the photon is,
you can think of it as a little bundle of some,
some very tiny amount of energy carried in the light
by Einstein's relativity.
That also means it has some momentum.
And when the atom absorbs the, the photon,
that momentum gets trans transferred to the atom as well.
So the energy mostly goes into, um, increasing the,
the energy of the electron and its orbit.
But this momentum goes into how the, the nucleus,
how the entire atom is moving.
Now, if an atom is sitting still
and absorbs a photon, it'll heat up exactly like you expect
to, because, you know, it's just sitting there.
It gets a kick from this, uh, photon coming in,
and that sends it off moving in some direction.
But if the atom is moving toward the laser,
when it absorbs the the photon, it will slow down.
It gets that same kick that momentum is transferred,
but that momentum acts to reduce
the speed at which the atom is moving, which, you know,
slows down its motion and slow equals cold.
The reason we want atoms to be moving more slowly is,
is the primary way we know about what's going on inside
of atoms is doing spectroscopy, looking at the colors
of the light that they absorb and emit,
and the colors of the light that they absorb
and emit are shifted by the doppler effect, right?
The doppler effect is this change in the frequency
of waves from a moving source
that's most familiar in the case of, of sound waves, right?
If you, if you watch, you know, you know, any,
any toddler can tell you, right?
The noise that a, that a race car makes, right?
Is that no noise as as it goes by.
That's the Doppler effect.
As the car is coming toward you, the engine sound is shifted
to a higher frequency.
And, uh, as it goes away from you, it's shifted
to a lower frequency,
and it changes very rapidly from one
to the other as it goes by.
Um, that doppler effect, uh, also happens with light.
So an atom that's moving at something like the speed
of sound has a fairly substantial Doppler effect, uh,
changing the frequency of the light that it absorbs,
and the light that it emits, which limits our ability to,
to study the properties of, of atoms.
If we can reduce that velocity,
if we can take it from the speed of sound down to, you know,
centimeter per second speeds,
then we can do incredibly precise spectroscopy of the,
the states of, of these atoms,
because they're moving
so slowly in the doppler shift is essentially eliminated.
Uh, this is most important in the case of atomic clocks, uh,
which are really in some sense light clocks, right?
Uh, the definition of a second is it's
9,192,631,770
oscillations of the microwaves that are absorbed
and emitted in making a transition between two states
and a cesium atom, right?
Our ability to measure that is constrained by
how fast those cesium atoms are moving.
So the very best atomic clocks made today use cesium atoms
that are laser cooled to a small fraction of a degree
above absolute zero speeds of centimeters per second.
At which point we can measure that, that frequency
with amazing precision, uh, the best, uh,
atomic clocks using laser cooled atoms are good to around,
uh, a second and a billion years, right?
If you had two of these clocks, uh,
two identical clocks running next
to each other would take a billion years, give
or take, for them to drift apart by, by one second.
Um, and we can do even better than that
with experimental clocks that are, that are good to,
you know, a second in more than the age of the universe.
Uh, so, uh,
and these, again, use these, these laser cooled atoms
to make these incredibly precise spectroscopic measurements.
- Okay? But why does that matter to people on the street?
- It turns out to matter enormously to people on the street
who want to know where they're going, uh,
because the, the basis of, uh, a lot
of modern navigation, right?
If you use one of those, the Google Maps
or Apple Maps, whatever, whatever app you use to navigate,
um, those are relying on the global positioning system,
which is a set
of atomic clocks on satellites up in space
that are broadcasting the time.
And your, your receiver, uh,
detects the time signal from several different satellites
and uses that to determine
how long it took the radio signal from the satellite to get
to you, which tells you your distance, the distance
between you and the satellite, um, which allows you
to determine your position on the, the surface of the earth.
If you know several of these, these travel times, um,
that allows you to determine your position, uh,
but it's, your position is only as good as the, the clocks
that you have, uh, and light travels.
Uh, it's the, the one case where, uh,
American units are superior,
light travels about one foot in a nanosecond.
And, um, that, that means that if you wanna know
where you are on the earth to within, say, a meter,
you need the timing to within
a little more than three nanoseconds.
Um, and for that you need atomic clocks.
And the better the atomic clocks, the better the timing,
which means the better you can do
with things like the global position. So
- How was all this discovered?
You know, what, what were the beginnings of this as a
- Science?
The story of laser cooling really starts in the,
the 1960s when people first, you know, invented lasers and,
and started playing around with them.
And, and, uh, they noticed that, that you would see sort of,
uh, specks of dust sort of popping in and out of the beam,
and it looked like they were kind of getting pushed
around by the light.
Uh, so a guy named Art Ashkin, uh, did some back
of the envelope calculations and,
and worked out that, you know, you could actually use light
to exert, uh,
fairly substantial forces on very small objects.
Um, and this had actually been measured
before in the, in the thirties.
Uh, there's, uh, uh, a fabulous set of,
of very early experiments, uh, done that, that demonstrated
that you could use light to exert forces on atoms,
that you could transfer this momentum from photons to,
to atoms and deflect them by a tiny amount.
But nobody could really do anything with it until you,
you had a laser, which allows you to throw, you know,
an essentially infinite number of photons at,
uh, at something.
So, uh, art Ashkin at Bell Labs started playing around
with this and demonstrated that they could, uh, use this,
these forces from light to, to manipulate small, uh,
beads basically, uh, and,
and push them around, uh, with these, these light forces.
Um, which, you know, then it's a new way to,
to manipulate microscopic objects.
And, and they started doing this in the, the early
to mid 1970s.
Um, this gets, uh, connected up to, to Adams, uh, thanks to,
to two, uh, people who are now at, at NIST in, in the us.
Um, uh, one of them is, is Dave Weinland, uh, who's one of,
uh, four people who, who, um, were part
of the original proposals of, of doing, uh,
laser cooling of atoms.
Uh, it's, it's Dave Weinland
and his, his PhD advisor, Hans Day Melt, wrote a paper on,
um, using these light forces
to manipulate specifically Adams.
Uh, and the other is, uh, art Sallow
and, uh, Ted Hench, uh, Theodore Hench won a Nobel Prize
for, uh, work on, on, um, high frequency lasers.
Uh, they also had a, a proposal both of these in a,
around 1975, looking at the idea of using these forces
specifically to cool atoms and,
and selectively slow the motion of atoms.
Um, so in the, in the mid 1970s, uh, weinland and day melt,
and, uh, Shiloh
and hench, uh, come up with this, this idea
of using these forces to slow down atoms.
Uh, and then, uh, wineland went to the,
then the National Bureau of Standards, now NIST in Boulder,
to, uh, start doing these, these experiments, uh,
which was a, a side project to
what he was actually hired to do.
But they, they brought him in with the promise
that he could do some of his own thing.
Uh, and, uh, around the same time, a few years later,
bill Phillips, um, ended up going to, uh, the,
the other National Bureau
of Standards Lab in Gaithersburg, Maryland.
And he also was hired with the promise that he could, uh,
do some side projects of, of his own choosing,
and decided to pursue laser cooling.
Wineland did, uh, laser cooling of, of ions,
because that's what he had been working on, uh,
for his thesis is, is trapping these, uh, charged particles.
And so he looked at, at ways to use, uh, lasers to,
to slow the motion of those.
Uh, Phillips, uh, looked at at doing neutral atoms.
He had been studying, uh, properties of atoms as part of,
uh, of his thesis.
And so he said, you know, we could do the same thing with,
with, uh, neutral atoms.
And there are pros and cons to, to both of those.
But, um, that's really,
or where the, the, the study of this, uh, applying this
to Adams really, really gets going. You
- Mentioned the Nobel Prize, and it's an area of physics
that has actually won quite a few of them.
There are a few stories
that you share in your three features.
Do you have a particular favorite,
- Like my, my favorite story relating
to Nobel Prizes in this is a, a story that, that,
that Bob Drollinger told.
He was a colleague of, of Dave Weinland's at, uh,
at NIST in, in Boulder.
And, uh, they were working together on this.
They had, uh, Wineland was doing the vacuum system,
and, uh, the ion trap to, to, you know, initially hold the,
the sample of atoms and dinger was the laser guy.
He was, he put together a laser system that could,
could do the extremely inconvenient frequencies
that you needed to, to do to, to cool these,
these trap ions.
Um, and so they were, uh, they were working on this,
and they, they set up their first experiment
and, uh, they're, they're in the lab late at night
because these first experiments always happen late at night.
And, uh, they, they turn on the laser
and they saw exactly the signal.
They expected they had this sample of trapped ions.
They could measure the temperature by
how much electrical noise these, these ions were making.
And they turned on the laser, the noise level went down,
which told you the, the ions were getting cold
and worked exactly as they expected, exactly
where they expected it to be.
Um, and so Dinger says that they're in the lab,
it's late at night, they've just done this.
And he said, you know, like,
there's this wonderful feeling of excitement.
He said, but I didn't know what we were gonna do next.
And so, you know, I said to Dave, what do we, you know,
where do we go from here?
And he said, he looked across the, the laser table at,
at wineland is lit only by the, the, the glow of the lasers.
And he said, this, this smile comes over his face,
and he says, Stockholm, oh,
- .
- So, 'cause they knew right away that they had something
that was, that was just absolutely fantastic.
- But it took a while, didn't it, for them
to actually win the prize.
- It took a while to, to get the, the Nobel Prizes, uh, the,
the first of the Nobel Prizes actually went to, to Phillips
and Steve Chu
and Claude Cohen to Nugi for laser cooling of, of neutrals.
Um, Wineland, uh, got the, got the prize, uh,
several years later, uh, for, for his experiments with ions.
Uh, and the, the ion experiments are really amazing
'cause they can get down to, to, you know,
incredibly low temperatures
and trapping these single ions that are now a platform for,
for quantum computing.
And really completely revolutionized that, that whole field
of, of, of things.
- Okay. So we have quantum computers such as they are
at the moment, and such as they will be because of this
- In part.
Yeah. One of the big things that, that, uh,
kicks off the field of, of quantum computing is it's, it's
around 1994, I think.
There's, uh, a paper by, uh, RAC
and Solar, uh, that it's a theoretical study,
but they looked at the, the trapped ion system that Wineland
and his, his team had built in Boulder,
and they said, Hey, you know, if you had several
of these ions in a trap, uh, there's this, uh,
collective motion of the, the several ions back
and forth that you can use to connect the states together,
and you can actually do the operations you need to do
to make a quantum computer with this system.
And, uh, the, the ability to laser cool these to the lowest
possible energy state gives you just this
unprecedented fidelity for, for this.
And they can do all of these manipulations.
The, um, RAC
and solar paper on this really gets
people thinking very seriously about, uh,
about quantum computing specifically in ions.
And, and they start weinland
and his group really start pursuing that, that field
and that that's become one of the, the, the best areas of,
of, uh, quantum computing in terms of, you know,
your ability to manipulate these, these cubits and,
and read out the, the signal and all that.
- It is tempting to think of, you know, Nobel Prize winners
of as getting all the science right, you know,
from the get go.
But there are a couple of fun stories in your features
about, you know, the,
the experiments not necessarily going wrong,
but could you just share a couple with me?
- Oh, yeah. There, there, there's,
there's two different versions of that there.
There's, uh, the, uh, so there's, there's one with, uh,
Wineland and, and Dinger were doing the experiment.
And they were, they were using, uh,
magnesium ions is what they were trying to trap.
And, you know, the first time they, they set it up,
they've got this trapped ion signal, they can see
that the ions are kind of hot from the amount
of electrical noise that they're picking up.
They turn on the laser, the ions get cold, it's great.
Um, they do it the, the next day.
And, uh, they turn on the ions
and the, the lasers get colder,
but not as cold as they did the first time around.
And they're like, oh, that's, that's weird.
And then they try it again and it doesn't work at all.
And, um, so they end up and, and nothing worked.
And they, uh, they end up tearing the whole system apart,
rebuilding it from the ground up.
Uh, dinger says he, he, he is convinced
that Weinland thought he had just completely
screwed up the laser system.
'cause Dave, uh, you know, had built the ion traps
and knew what he was doing with the ion traps,
and knew that that was working, uh, perfectly.
Um, and so, but they rebuilt both systems completely.
And, uh, when they turned it back on, they had a,
a much better laser system,
and they knew the laser was in exactly the right
place, and it still didn't work.
And it turned out that they had used,
they had used up all of the magnesium.
They, they were, they had,
and were, in fact, they had heated up the, the oven
that produced the ions so much that they were boiling
sodium out of the glass,
and they could trap sodium ions,
which are about the same mass as magnesium.
So it looked like they had ions in the trap, uh,
but they were the completely the wrong element.
And so the lasers didn't affect them at all.
Um, and so it wasn't D Jinger's fault with the laser system.
It was, uh, it was, they'd run outta magnesium,
and so they got more magnesium, they put,
they reloaded the system,
and then it worked great from there, there on.
Um, the other version of that is, is, uh, Phillips, uh,
bill Phillips and Hal Metcalf were doing, uh,
magnetic trapping of sodium atoms.
So they, they had a laser, they're slowing down a beam
of sodium atoms, and then they would turn on this collection
of magnetic fields that would trap the, the atoms.
Uh, and, and then they, you know, they'd hold them there
for a while, and then, then they, you know,
flash on some light, and they would see
that these atoms are sticking around for a very long time.
And they were, uh, they, they were working on this,
and, you know, they, they got a signal.
They saw these, these, these atoms trapped.
And then, uh, and then just nothing worked for a while.
And, and it, uh, it, it wasn't working.
And they said, you know, okay, this is, you know,
uh, something's wrong here.
We gotta go think about this. So they went
and they got, they got fast food.
They turned everything off in the lab.
They went out, they got a,
a very late dinner at like McDonald's
or someplace like that.
Uh, and they came back, uh, you know, an hour or so later,
and they turned everything on
and they get this fabulous signal of trapped atoms.
Uh, and they're like, you know, hooray.
And then the signal slowly got worse over a couple hours.
And what it turns out is, uh, they were,
they were heating the system up.
Uh, so they were laser cooling the atoms successfully.
And they were, they were getting 'em to the area,
but the, the, to make the magnetic trap, they had to run
so much current through the, the coils that they were,
they were heating the whole system.
And then the vacuum, uh, system would degrade, uh,
to the point where they, they, uh,
had all this stray background gas in there
that would knock the, the sodium atoms out of the trap.
And they couldn't trap anything.
'cause the vacuum was terrible.
Um, and so they, they figured that out,
and they got it, got it fixed up,
and they, they ended up, um, they ended up working
through the night getting this, this data
and showing that they were, they were magnetically trapping
these, these sodium atoms.
And then, uh, the, the, the story they tell is that, uh,
you know, they, they worked all night
and they're like, at six in the morning, they go
to Bill's house and they root
around in the refrigerator looking for something to eat,
and they just end up eating ice cream.
And so it's in the morning, uh,
bill Phillips' wife comes down and, and her husband
and Hal Metcalf are sitting in the, in the kitchen,
uh, eating ice cream.
And she's like, what, what are you doing?
It's six in the morning and that ice cream is for the kids.
And they're like, no, no, no, no, we had a good night
- .
- So, you know, so they're celebrating that, you know, what,
what turned into a Nobel Prize Later on.
- I hope you're enjoying this conversation
with Chad Azel so far.
And just a reminder that he has written a three part feature
series for Physics World, all about the history
of laser cooling.
In the first part of this conversation,
Chad mentioned Stephen Chu, who shared the 1997 Nobel Prize
with Bill Phillips and Claude Coi, not content
with reaching this pinnacle of science.
Chu also went on to serve as the US Secretary
of Energy from 2009 to 2013
during Barack Obama's first presidential team.
Perhaps part of Chu's suitability to politics is
that he built a reputation as the guy
who gets difficult experiments. Done.
- Yeah. So, uh, so Steve Chu comes in, he was, uh,
a colleague of Art Kin's at, at Bell Labs.
And, and Ashkin had this, uh, idea of,
of doing laser cooling
and using, um, you know, did the first demonstrations
of using light to push around microscopic objects.
But he is using these like little glass beads and,
and things like that, uh, to, to push stuff around.
Then they had the idea to do some of this with Adams,
but he particularly wanted to do, um, trapping, uh,
using only light, right?
And so using, using just the forces from laser beams to trap
Adams, um, and, and cool them.
And, uh, that turns out to be a,
a really tricky thing to do.
Um, Ashkin and, and his colleague John Olm worked on this,
and they did some, some preliminary experiments,
but they didn't really have the, the setup to do it.
And, and in fact, their bosses at Bell Labs started
to tell them, you know, okay, this has been fun,
but do do something else, right?
Uh, stop, stop working on this so much.
But, um, Chu uh, got transferred into, uh,
a different Bell Labs research facility,
ended up in an office next to Ashkin and started talking
and then said, no, you know, I think we can make this work.
And so he was instrumental in, in really, uh,
taking the ideas that that Ashkin
and Bjork home had had put together
and, you know, building up a, a much fancier vacuum system
to, to really contain these.
And coming up with, uh, the idea of, of what they dubbed,
um, optical molasses.
This, uh, idea of overlapping laser beams going in,
in opposite directions that will slow down atoms no matter
what direction they're moving in.
And, uh, that gives you the ability to, to,
to really cool the atoms.
The atoms feel like they're in a viscous fluid,
thus optical molasses a very colorful term.
And, um, and,
and it was true that that really got that,
that whole thing working experimentally.
Um, so he was, uh, he had been doing some experiments on,
uh, positrons
and positron in, uh, in other systems,
something very different, uh,
but got interested in this, in this idea.
And one of the, the neat things, you know, bell Labs, uh,
had this, this very open culture at the time of, uh,
I think it's Bjork home, who, who put it that, you know,
you could work on anything you wanted as long
as it was world class, right?
So whatever you wanted to study, you were,
you were pretty much free to study anything
that seemed interesting, as long
as you were gonna be one of the best in the world at it.
And so they were, they were able to, to take on this,
you know, this idea of laser cooling doesn't really seem
like it has much to do with, you know, bell Labs
as a telecommunications company doesn't seem directly
connected to that, but at the time, they were allowed to,
to do kind of whatever they wanted.
And that freedom really helped launch things.
Uh, something very similar at the NIST too, I should say.
The, the National Bureau of Standards.
You know, they hired Dave Weinland in Boulder.
They hired Bill Phillips in, uh, in, in Gaithersburg.
Uh, both of them were hired to work on other things.
Uh, Weinland's job was to, to help evaluate
and improve an existing cesium atomic clock.
Uh, but he was told he could spend some
of his time on doing these experiments with trapped ions.
Um, and Phillips was hired to, to do some, uh, uh,
electrical measurements that eventually, uh, became the,
the new standard for the, the volt,
and then the, the kilogram on.
He was hired to work on that,
but again, was told he could have some, some time to do, uh,
you know, his own thing when,
and do these laser cooling of, of neutral atoms.
Uh, and they were pretty much, uh, left alone to, to do,
they, they both cite their, you know, their initial bosses
as being extremely supportive of, of exploring, you know,
whatever they were, they were most interested in.
Uh, and later on, they both, uh, particularly praised, um,
Katherine Gebe, who was eventually the director
of the physics laboratory at, at nist, and,
and had a really good, uh, line about, um,
that really captures sort of the ethos of, of that,
which was she felt that her job was to, to hire good people
and stay out of their way so that, uh, you know, really, uh,
supported, uh, Wineland and Phillips and, and Jan Hall
and, uh, some other people to, to really explore
very broadly into exciting, uh, areas of research.
And that's really paid off.
- That's an interesting thing, isn't it?
Because that freedom giving great researchers freedom
to do their work led to, as you say, you know,
Nobel Prize winning physics.
Is that freedom something
that we're seeing today in physics still?
- It's certainly, um, you know, that
that still exists in a lot of places.
There's still, the, the NISTs are still very much run
that way, that they have, uh, you know, people in the,
in the physics labs that are exploring, uh,
really exotic things in, in, in a lot of ways.
There, there's a bit less of that now.
Uh, the, certainly the, you know, industrial
labs like Bell Labs are, are, you know,
there's still a Bell Labs around,
but it's, it's a shell of
what it was in the glory days when they were really, uh,
you know, operating in kind of the infinite money limit
where they could, could go off
and explore literally anything that they wanted to.
Um, but there's, there's still a lot of that, that really,
the pursuit of basic fundamental science that's foundational
to so many other things, uh,
but it does, it, it, it gets a lot of times, uh, sort
of disparaged, people will talk about, well, you know,
why are you, why are you studying that?
That's so arcane and,
and weird that nobody's,
that's never gonna be useful for anything.
Uh, but in fact, a lot of the times, that's the stuff
that turns out to be foundational to
the next leap forward, right?
Where, you know, somebody goes out
and figures out a way to make atoms cold,
and then suddenly we've got, you know, atomic clocks
that are factors of a hundred or a thousand better.
And then we've got these, you know, this,
these ultra cold b Einstein condensates firmi gasses
that you can study these really weird exotic, uh,
condensed matter phenomenon.
- So, Einstein plays a role in all this
and features in the third installment of your features,
which is called, well, in brief, coldest.
Can you tell us a bit about that?
- The idea of, of, uh, Bose Einstein condensate is
that there's this, this very strange phenomenon
that happens, uh, as you approach absolute zero.
Uh, so, you know, one way to think about temperature is to,
to think of, of temperature as, you know, the average speed
of an atom and a gas,
and they're all moving in random
directions and, and that sort of thing.
But, um, as you get colder and colder and colder, right?
Quantum mechanics kicks in,
and these things behave like waves.
And the proper way to think about, um, a collection
of things that behave like waves is to think
of them in terms of, of discrete allowed states.
And this is what happens in an atom, right?
You have an atom, you have an electron that's going
around the, the nucleus of the atom.
It can only go around at cer in certain orbits, right?
Um, very, very loosely.
You can think about it as, you know, if it's going
around in an orbit, the wave associated
with the electron has to come back to where it started, uh,
when it, when it completes an orbit.
And that gets you the right i basic idea.
Uh, same thing happens if you have a bunch of atoms
that you're just holding in a, in a trap, right?
You can think of that as a, as a whole bunch of,
of discreet states that have, uh, very particular energies.
So there's only a limited number
of possible energies the atoms in the trap can have.
So one way to think about the temperature is you can think
about the temperature as the,
the speeds at which the atoms are moving.
Another way you can think about it is it's a distribution
of atoms over all of these possible energy states in a trap.
Um, and, you know, as you lower the temperature,
either you're lowering the average speed,
or you're, you're decreasing the number of states
that these, these atoms can occupy.
Um, what happens is, as you get really, really cold,
then there's another property of the atoms
that comes into play, which is this thing called spin, uh,
which is an intrinsic, uh, ag angular momentum
that's associated with these atoms.
Purely quantum mechanical thing, not predicted
by any classical theory,
and I am legally required to note
that they're not literally spinning.
Um, but, uh, the, uh, atoms have this property called spin.
And if the spin is, uh, an integer value,
an integer multiple of plunks constant, then these atoms,
uh, are called boons.
They have this property that, that allows them
to be in the same energy state.
Uh, and then what happens is you get colder
and colder, the number
of states they can possibly occupy decreases.
And then at some point, it, it reaches a, a situation
where the atoms are so close to each other
that they're aware of the presence of other atoms,
and they all will condense into a single energy state,
generally the lowest state that's available to them.
Uh, and this is just because, you know, they,
they realize collectively that, wait a minute,
like if we were all in the lowest energy state,
that would be a big drop in the, the energy of, of the gas.
And we're always looking to, to, you know,
go in the lowest possible state.
So, uh, this is a phenomena called Bo Einstein condensation.
It's predicted by the, um, Indian, uh,
mathematical physicist.
Uh, RA Na Bose, uh, came up with this, uh,
in thinking about a way to, to understand the spectrum
of light emitted by a hot object.
Um, he couldn't get his paper published, uh,
because he was, you know, nobody in, in India.
Uh, and so he sent, uh, a copy of the paper to Einstein,
who he had met once before, uh, Einstein read it
and said, this is amazing, and also realized
that this condensation thing would happen.
Uh, and so he wrote it a paper of his own
and sent the both of them to Zeit for physics
and said, publish these.
And so this, this, um, got published,
and this is why these, uh, atoms
with integer spin are called boons.
It's an honor of, of Bose.
And this phenomenon is Bose Einstein condensation,
because Einstein took Bose's idea and,
and pointed out that, Hey, you can do this thing where all
of the atoms will collapse into a single state.
And that's pretty, pretty neat.
And it, it's a purely quantum thing.
It has nothing to do with interactions
between the, the atoms at all.
Uh, just the, the fact that they're there and their waves
and those waves sort of overlap with each other, um,
triggers this, this process
because of this quantum statistical character of this spin
that they have, uh, will put them into this,
this low energy state.
- You say that they have awareness of each other,
but they, they're not aware of each other, are they?
Because atoms don't have awareness? Yeah,
- It's, uh, you know, it, it, we, we have a tendency
to anthropomorphize, uh, you know, microscopic
inanimate objects, uh,
and say that, you know, they want
to be in the lowest energy state and,
and whatnot, um, that kind of thing.
But yeah, they, they, you know, that's, that's
how we tend to talk about it.
The, the, um, the presence
of these different things becomes, uh, significant.
- Well, as we've said, it takes a little time
for the Nobel Prize to come to people
for the work that they've done.
Um, are there discoveries that have been made that will win
future or may win future Nobel Prize physics for this area?
Is, are those discoveries already happened,
- Uh, in this field?
Yeah, I think there's probably a, a, a few future ones.
The, the Nobel Prizes in laser cooling
and, uh, related fields are actually, uh, on the scale
of Nobel Prizes pretty quick, right?
The, uh, the stuff that, uh, Phillips did is,
uh, happens in the, the early 1980s, uh,
around 1983, is the magnetic trapping and,
and Zaman slowing, uh,
and he gets a share of the Nobel in 1997.
Uh, Weinland's Nobel is in,
I'm forgetting the, the year now.
Uh, let me look this up.
Uh, Weinland's Nobel is a, is a little later.
It's in, uh, 2012.
Um, and the work that, that he is being rewarded
for starts in, in 1978.
Um, so that's, you know,
that's relatively quick on the scale of Nobel Prizes.
Um, the, the Bo Einstein common State Nobel is in 2001.
Um, and that's for, for stuff that was done only,
only like six years earlier in, uh, in 1995.
So that, that happened really fast
on the scale of these things.
- Why was that one so fast? Well,
- It's fast on the experimental side, right?
The experiments that that did it, uh, were in 1995,
that's Cornell and Wyman and, and Ketley, uh,
and they get the Nobel in 2001.
You know, the prediction of B Einstein condensation
that this ought to be possible is made in, in 1924.
So, uh, so it's, you know, the, it's a realization
of a theory that had been around for a very long time,
and that makes it a little,
you know, come a little bit faster.
- So, what's the physics that's going on today that might be
worthy of a Nobel Prize in the future?
- Uh, there are these experiments with optical latts
and optical lattice clocks where they, they, uh, take atoms
and they trap them in, um, in light, you know, uh,
arrangements of light beams
that are lasers going in opposite directions.
They interfere with each other to make a, a periodic array
of bright and dark spots.
Uh, and if you arrange this very cleverly, you can, uh,
trap atoms in the bright spots
or trap atoms in the dark spots in this.
Um, and, uh, this turns out to be a really good way to,
to make, uh, atomic clocks with neutral atoms.
You can hold them very, very tightly in these.
So they're not moving at all, really.
Uh, they stick around for a very long time,
so you can interrogate them, um, over, over long periods
of time and make incredibly precise measurements.
These are the, these are the clocks that are good to,
you know, one second in more than the age of the universe.
And, uh, that's a, uh,
that's been some really spectacular work in,
in those fields.
That is probably the kind of thing that,
that down the road would be, uh, would be Nobel worthy.
Um, there's also, uh, a lot of, of,
of work in, uh, these, uh,
degenerate fermi gasses is another area that is, um,
so the other category of things,
you have b Einstein condensation happens if you have atoms
whose spin is an integer multiple of plan constant.
Um, you also have, uh, this pheno this,
the other possibility is you can have atoms whose spin is,
uh, a half integer multiple.
So one AVEs, one and a half, you know, two
and a half times plunks constant.
And those particles are called phons,
and those are absolutely forbidden from being
in the same energy state.
And so they can be, um, they can also be cooled down,
and as you cool them, you, you limit the number
of states they can, can be in.
But unlike the boons, they fill up the states, right?
Once there's one phon in, uh, in a allowed state,
no other phons can occupy that.
So at some point, as the temperature gets cold enough,
you reach a point where all
of the available states are already occupied,
and, uh, no more atoms can go in there.
Uh, and at that point, the, the system sort
of in some sense stops cooling, right?
It can't get any more compressed.
It can't get any, get any smaller, it can't get any colder
because all of the available states are, are occupied.
Um, and this is, uh, phenomenon.
It's, it's closely related to what happens in metals, in,
in solids that determines whether something's, uh,
a conductor or an insulator, uh, determines, is determined
by this same physics, this filling up of energy states.
But you can demonstrate, it doesn't depend on interactions
between the particles at all.
You can demonstrate it with, with ultra cold atoms.
Uh, and this is done in 1999 by, uh, at, at NIST in Boulder
by, uh, Debbie Gin, uh, who was, uh, a new, uh,
staff scientist at, at nist.
And, uh, her grad student, Brian DeMarco, um,
got this lab up and running to do, uh,
degenerate Fermi gasses.
And, and looking at, at potassium atoms, they were able
to cool them down to the point
where they stopped getting any colder,
and they could show the other category
of weird quantum things
that happens at ultralow temperatures,
which is this degenerate amiga behavior where the, the, all
of the available states are full
and that, uh, that changes the properties of materials.
Um, and so there's a, there's a huge amount of work out of
that that's also, uh, really interesting.
And, um, you know, probably, uh, Nobel worthy someday.
Uh, it is very sad that, uh, Debbie Gin
who pioneered all this and,
and was, uh, really, uh,
enormously influential in the field.
Uh, unfortunately she passed away, uh,
several years ago of, of cancer.
Uh, and so she's not around to, to get the, the Nobel,
or would certainly be on the short list of people that,
that are expected to win one.
Um, but this is a really important area
because, uh, these phons, uh, the,
the other kind of part really important particle
that are phons are electrons
and electrons in a solid, right?
I said it determines this, this, uh, this filling up
of states determines the properties and materials.
Um, if you do this with, with atoms, you can make a system
that's analogous to, uh, any kind of, of, you know, metals
or superconductors or semiconductors, things like that.
And you can study the behavior
with atoms taking on the role of electrons.
And the nice thing about that is atoms are, you know,
thousands to millions of times heavier than electrons,
so they move a lot slower.
So you can watch phenomena that happen with electrons
that happen far too quickly to to be tracked directly.
You can set up an analog of that
where atoms are playing the roles of the electrons
and watch them move around and interact and,
and, um, you know, study these transport properties, uh,
at timescales where you can,
you can really follow this in real time
and see how these things, uh, shift around, uh, which is a,
a whole new regime for studying these interesting
properties of material. Yeah,
- It's fascinating, isn't it?
I mean, it, it's a funny one for me
because it's not something that I've thought about
that's coming to my life really knowingly, at least.
Yeah. Until I read your features.
And when I read that,
and, you know, when I've said to people,
I'm gonna be doing this interview about laser cooling,
their reaction is laser cooling.
That doesn't make sense.
And it's that this all, it's not even a little thing, is it
that peculiar nature of this means that they're going
to listen to this podcast, you know, they are going
to be interested in this because of that peculiar thing.
Is it always something that's interested you?
- It's, I, you know, I, I had exactly the, the reaction
that you described, right?
I, I got into this field
because in, um, kind of the winter of, of
19 91, 92, um, I heard, uh, Claude Cohen Nugi give a,
a talk about, uh, laser cooling.
He, he came to the, the small college where I was an,
an undergrad, and he gave this talk.
And, and, uh, Cohen Nugi is just,
he is a magnificent public speaker,
gives these just incredibly clear, um, you know,
coherent talks, uh, about things.
And it, and it absolutely blew my mind.
The, the idea that, you know, this,
this counterintuitive notion that I've got a gas of atoms,
I shine laser light on it,
and suddenly I can make these, these atoms move at,
you know, centimeter per second,
millimeter per second speeds.
Uh, get this down to, to millionth of a degree
above absolute zero was just, just absolutely incredible.
And then I found out that one of my professors had a lab
where he was trying to set up an experiment to, to do that.
And I was like, I'm in, like, I sign me up.
I wanna be part of this. Uh,
and so it, it was, it was really cool.
And, and it's exactly that counter intuitiveness that
that really drew me into the, into the field.
And I was lucky enough to, to go to grad school.
And I worked for Bill Phillips, uh, at, at NIST in,
in Gaithersburg, uh, doing laser cooling experiments for,
for my PhD thesis.
And so, you know, I was incredibly fortunate to be around,
uh, some of these, these really exceptional people, uh,
doing this, this exceptional science.
- And you can read much more about Chad Zell's conversations
and explorations of this topic on the physics world website
with his three features, the first cold,
how physicists learn to manipulate
and move particles with laser cooling.
The second Calder, how physicists beat the theoretical limit
for laser cooling
and laid the foundations for a quantum revolution,
and the third, at the time of recording, known only
and not yet published as coldest.
But by the time you are listening,
or certainly soon enough, you'll be able to read
that third part on physics world.com.
Before that, settle in and read part one
and part two of the Cold, colder, and Coldest trilogy.
I'd like to thank Chad Zel for joining me for this episode
of the Physics World Stories Podcast.
And of course, I'd like to thank you very much
for listening,
physics World.