[MUSIC PLAYING] KAI-MEI FU: Hello. My name is Kai-Mei Fu, and I’m an experimental

physicist in the University of Washington physics department here, and I’ve been given

the honor of introducing tonight’s guest, David Wineland. I’d first like to thank everyone for joining

us for our fourth Frontiers in Physics public lecture. As you may know, this series was started to

provide our community inspiring, free lectures on the latest advances in physics given by

the people who directly contributed to these advances. The broad range of topics that we have heard

over the past two years mirror the broad impact that physics has on how we view the world,

and also how we impact and affect our world. In our last lecture we learned from Dr. John

Preskill how the spooky and nonintuitive nature of quantum mechanics may lead to unprecedented

computational power. And in the spring, stay tuned. We will hear from leading particle theorist

Dr. Lisa Randall who’s well-known for her work showing that extra dimensions can help

solve some of the fundamental problems in physics. Today we are excited to hear from Dr. David

Wineland, our third Nobel laureate in this series, on his contributions to the measurement

of time. Before introducing David, I would like to

first take a moment to thank Dr. Patrick O’Hara and his wife Dr. Catarina Randolph, who unfortunately

can’t be with us here tonight. It was their vision to start this series. They approached our department thinking that

what our community needed was a public lecture series, bringing the public to learn about

what’s happening in physics today. And because of their generosity it has become

a reality. I would also like to think Phil Ekstrom, who

wrote the program that you all now have on a short history of time keeping. Phil was a PhD student of Hans Dehmelt and

overlapped with David Wineland during his time as a post-doc at the University of Washington. On that note, I would actually also like to

add that if you’re interested in getting involved in the physics department, we would love to

hear from you. There is contact information on the back of

your program. So getting involved with physics can be attending

a lecture. It can be getting added to an email list so

you can hear about all the colloquium that we have and other public and outreach events

that we have going on. And also it can be helping to support our

students’ individual research projects and outreach events that we have for community. And so now onto the introduction. David Wineland– he grew up in California

and graduated from Berkeley with a bachelor’s degree in physics. He then completed his PhD with Norman Ramsey

at Harvard before joining us at the University of Washington for his post-doctoral work with

our own Nobel laureate, Hans Nobel– or, Hans Dehmelt. This was before– [LAUGHS] This before Norman

Ramsey and Hans Dehmelt would later be awarded the 1989 Nobel Prize in physics. And so if you could say one thing, you could

say that David really knew how to choose an adviser, since both of his advisors were chosen. After leaving University of Washington, Dr.

Wineland joined the now National Institutes of Standards and Technologies, and for the

next decade pioneered precision techniques to measure and control individual atoms. The main motivation for this work was precision

time-keeping. But in 1995 there was a theoretical proposal

by Cirac and Zoller on a new way to process information, quantum information processing

of ions. And because of this very fundamental work

on clocks that he had been performing, within months of that proposal he was able to demonstrate

a quantum gate with single– or two ions. This seminal work, this high-precision control

which led to his work in clocks and quantum information processing was honored in 2012

with the Nobel Prize. As some of you know, David Wineland spent

his post-doc here at University of Washington with Hans Dehmelt. And Hans Dehmelt passed away this past year. It is fitting that we’re able to hear today

about new scientific results that, in part, were influenced by Dehmelt’s work. Just as one day in the future we’ll hear of

discoveries that are enabled by David Wineland’s work, which he’ll discuss today. Thanks. [APPLAUSE] Well, thanks for the introduction, Kai-Mei. And first of all, thanks for all of you coming. I mean, one of the nice things about giving

this kind of lecture is to see the large-scale interest from the public. And so I can’t teach you everything about

what we do, but I hope to give you an impression of what we do and some of the simple ideas

that will hopefully come across. So you can see my outline there, what I’ll

talk about. And so I can start by saying, well, what’s

the use of clocks? And throughout history it’s been primarily

for navigation. And that application is still true today. So just going back on a little bit of time–

I’m not a sailor, but any sailor will know what I’m talking about here. And the basic ideas for navigation is you

want to determine your latitude and longitude, and therefore your position on the Earth. And the easy part is the latitude, where the

basic idea is if there’s some distant star– in this case, the North Star– if you measure

the angle of the direction of the star relative to the level, the tangent of the earth, you

can easily determine the latitude that you’re at. The harder part is longitude. And there you need time. And the simple reason is because the Earth

is rotating. So the principle is the same. You’re going to be looking at some distant

star that’s relatively-speaking fixed in your view, and you determine where you are by measuring

the angle between the star and the tangent on Earth where you are. The problem is, because the Earth is rotating,

you need to know time to know exactly what the angle– how the angle corresponds to your

position and longitude. And I’m not going to– I won’t go through

this simple math. The scientists out there will understand this. But the idea is, what are the errors due to

the error in time. And just a simple example I show here. And the idea is that– so a relative distance,

say, the error in the angle can be related to the error in time. And so this section of distance in latitude

is given by this simple formula here. And so the idea, of course, this angle is

changing in time and there’s an imprecision given by the imprecision in time. So in this simple expression, where we’re

rotating at once per day, the radius of the earth is about 4,000 miles. And so for an error of one second in time,

that gives a relative error in latitude position of about a half a kilometer. And so just a little history. Of course, sailors going back many centuries

relied on time to tell, to be able to navigate. And there was several incidents in the early

1700s where the Brits lost ships at sea. And so the British parliament decided to sponsor

a prize, the so-called “Longitude Act,” and the prize was given to someone who could demonstrate

a clock which would allow navigation to about 30 nautical miles, which translated to an

error in time of about two minutes. And there’s a famous story that– many of

you may have read the book. There’s a couple of good books about this–

John Harrison, who came up with the clock. It was actually like a large pocket watch. It was a mechanical clock that he came up

with. And he was able to demonstrate that it was

within the errors that were required by this to win this prize. And part of the story is that the parliament–

you know, they wouldn’t give him his money. And I think it was a couple decades before

he finally– the King stepped in and got him his money, and then I think he died a year

later or something like that. But anyway, it’s a good story. There’s a couple of good books. It’s a good story to read. But of course these days the way we navigate

is via GPS. We kind of take it for granted these days. And the positions can be much, much higher. And the basic idea there is that, to give

you the idea, you want to determine your distance from some satellite. And again, I won’t go through the math, but

the basic idea is that because clocks are so precise, that basically you can think of

a simple protocol. It’s a little bit more complicated than this. But if you agree that this satellite is–

that first of all your clocks have to be synchronized. And if you establish a protocol, say, where

the satellite emits a pulse of radiation every second tick, then there will be a delay because

of the speed of light before it reaches you. And so to give you an idea of the precisions

that are involved, for an error in time of about a nanosecond, 10 to the minus 9 seconds,

that gives an error in distance here of about 30 centimeters. And where clocks come in is, basically we’d

like the clocks to be autonomous, at least over extended periods. And so if we have an error of a nanosecond

over one day, that corresponds to a relative frequency uncertainty of the clocks of about

one part in 10 to the 14. And these are the kind of numbers we’re able

to reach today. Of course, it’s a little bit more complicated

to get three dimensional navigation. There’s a network of satellites. And, in fact, there’s enough redundancy in

the system that the satellites can not only know their position, but the clocks can be

synchronized together so we can get three dimensional position at this level of precision. So anyway, a bit about clocks. I think most physicists’ notion about time

is very similar to the non-scientists. It’s just sort of a measure of the progression

of events. And the basic idea of making a clock then

is, we have some periodic event generator and a counter. And with that then we can generate time. And traditional event generators are, of course,

the rotation of the Earth. And also later the pendulum clocks were invented,

and the precision was quite a bit better than given by the rotation of the Earth. But that’s basically it. So these days though we think of using oscillations

and atoms, and a simple picture you can think about is, say, an electron orbiting around

the nucleus. That’s a pretty good picture. Quantum mechanically it’s a little bit more

complicated. What we think about is, say, an electron orbiting

around the nucleus. The quantum mechanics tells us that the electron

can’t be– its position can’t be precisely defined. But nevertheless, there’s a very precise timing

of the orbit of the electron around the nucleus. And this picture over here is meant to show

a dipole where the electron is actually oscillating through the nucleus. But the basic idea then is that we establish

this oscillation in the atom. And the key part is that, as the early quantum

mechanics told us, is that atoms don’t exist in any arbitrary kinetic energy state. There is this certain discrete energy levels

associated with a electron orbiting around the nucleus. And what the basic idea is, that the frequency

of the oscillation is then given by the energy difference between these two quantum states

divided by Planck’s constant. So one simple mode of using these oscillations

is called a maser or a laser, and we’re more familiar with the laser, actually. The first devices that operated on this principle

were called masers. M stands for “microwave” versus “light.” And the simple picture is, we have atoms that

we stick them inside of a cavity which contains the radiation. We sample a little bit of that, and then we

have a counter to generate time. So a little bit of personal history. I was a graduate student at Harvard with Norman

Ramsey, who’s right here. And Norman and his colleague, Daniel Klepner

had recently invented and demonstrated the first hydrogen masers. So the basic idea of what I showed on the

previous slide. So this was the group in 1967. I started in 1965. That’s me getting to the boss there. You can see us there. [LAUGHTER] Anyway. So Norman wanted to have precise measurements

of all the hydrogen isotopes, the maser oscillation frequency of all the isotopes. So my project turned out to be to make a maser

based on the deuterium isotope. And the experiment wasn’t anything very astounding,

but it taught me a lot of nice techniques in atomic physics. And so this was the result of my thesis here. I’m probably the only one that has this number

memorized. [LAUGHTER] But anyway. But certainly, what came into play all through

my career and the things I’ll talk about that we did later, was basically that the name

of the game here is there’s– although the atoms are inside this cavity, there’s various

perturbations that they can undergo. And so one of the requirements is to precisely

control the environment they’re in. The other thing is that, as they were radiating–

the atoms live in so-called superposition states where they’re radiating. And these superposition states are actually

rather long on the grand scale of superpositions. They lasted about a second. So I’ll give you– there’s another mode of

operation. I showed you the laser or maser mode of operation

of a clock. And the other is where we actually, say, would–

again, here’s the energy structure. And what we’d do in this case is we would

apply radiations to atoms, say, in a cell here. And basically we just look for the condition

where the atoms absorb the radiation maximally. Then we know that the frequency of this oscillator

is equal to the frequency given by the energy level separation. So a recipe for an atomic clock is very simple. The basic idea is, we have some oscillator

here which supplies radiation. And in the previous example we had looked

for the condition of the frequency of this oscillator where the absorption is maximum. And we can think of– I won’t describe the

electronics here, but we can think of a simple servo system that feeds back that forces the

oscillator frequency to be equal to the frequency given by the energy difference. And when we reach that condition we just count

cycles of the oscillator. Now, in fact, a little subtlety here is that

we get a curve that registers this sharp absorption. It’s not infinitely narrow. There’s– one of the reasons it’s not is that

the atoms in the excited state will decay, and that limits the resolution we have with

this absorption feature, we say. And so the only other ingredient to actually

the way we work it is, if we sit right on the top we’re not very sensitive to small

changes. The top of this curve is flat, basically. So the only difference that we do is, we actually

do it in a stepwise section where we’ll first probe on the left side of this absorption

feature, and then we’ll step over to the other side and probe on that side. And when we get an equal response then we

know that the mean of those two frequencies is equal to the resonance frequency. So that’s the only wrinkle beyond what I said

before. But it’s actually no more complicated than

that. So again, a bit more common mode of operation. That was sort of for looking for continuous

absorption. The way most clocks work is just slightly

different. We basically start the atom in the lowest

energy level, and then we apply radiation for a short time. And it’s basically the same idea, is that

when the radiation frequency of the oscillator is equal to the resonance frequency of the

atom given by this expression here, then we know we’re on resonance. And of course we step from side to side of

the absorption feature. And basically then to measure this absorption

we just– ideally, we could look at the maximum absorption probability. But of course we step from side to side of

the line. So it’s just a slight difference from what

I said before. So why atomic clocks? And there’s a couple of strong reasons why

atoms are nice. And in this view graph I’m going to compare

to the pendulum clock, which actually is still a very– good pendulums are extremely good,

almost as good as quartz crystals. But you can think of quartz crystals like–

they have the same conditions– or, some of the same conditions we have to worry about. But let’s take the pendulum clock. So the frequency of a pendulum clock, as students

in physics learn early in their careers, is given by this simple formula where this is

the acceleration of gravity and this is the length of the pendulum bob. And so what are the sensitivities? What can cause the frequency of the oscillation

to change? And one is, say, temperature. The temperature is not precisely controlled

and can be fluctuating. So what happens in this example of the pendulum

is that most suspension rods are a metal usually. And, of course, with temperature changes the

metals usually expand and contract as the temperature cools. And again, I won’t go through the details

of this simple derivation, but the basic idea is that even with materials of very low expansion

coefficients– so this represents the fractional change in the length of the pendulum versus

temperature. We get to about– anyway, we work through

this simple expression here and we get to sensitivity that’s expressed fractionally

of about a part in 10 to the 8th per degree C frequency change of the pendulum clock. Now to atoms. What it’s nice is that– we still have to

worry about temperature effects. And actually, one of the more interesting

one is due to Einstein. And that is the fact that, typically in this

container that we’re holding, the atoms aren’t at rest. They’re moving around. And what Einstein told us was that with two

frames of reference that move relative to each other, time runs at a different rate. It’s not just as simple as saying the clock

here based on the atoms runs at a different rate, but actually time runs at a different

rate. So this was, I mean, an amazing revelation

that changed our notion about nature. And it was due to Einstein. So anyway– but to give you an idea of the

size of the effect, we do have to worry about this in our high-precision clocks. And again, I won’t go through the simple math

here. But for a cesium atom which has a mass of

133 mass units, the frequency of the oscillation will change due to this relativistic time

dilation. And the fractional change per degree C is

given by– is about a part in 10 to the 15th. So many orders of magnitude smaller sensitivity

to temperature changes then this pendulum. And there’s a simple– of course we’re more

familiar these days with quartz crystals in our watches. But also in that case the temperature sensitivity

of those is quite a bit larger than what we can get with atoms. OK. So the other thing we have to think about

is if we make different– if we realize different versions of the clock, how reproducible are

they? And of course with a pendulum clock we have

manufacturing tolerances to worry about. The length of this pendulum for different

realizations can be different, which gives a different oscillation frequency. Of course there can be wear of the pendulum,

the bearing on the pendulum may wear a little bit which will effectively lengthen the distance

of the length of the pendulum bob. And, of course, it depends on the local value

of gravity, which can change around the world and also fluctuates with Earth tides and things

like that. So the nice thing about atoms is that all

atoms of a particular kind– as far as we know so far– they’re absolutely identical. So if we can agree on an atom whose frequency

we measure then we, in principle, can get exactly the same frequency within these environmental

perturbations that we have to worry about. And the other thing is, atoms don’t wear out. I mean, we can take the same atom and repeat

this absorption process, in principle, an infinite amount of times and then they preserve

their properties throughout. So these days, actually starting– the first

cesium clocks– which measured the so-called hyperfine transition at a frequency of about

9 billion cycles per second– it was developed in the early ’50s. And by the mid-’50s, by international agreement,

we used that to the oscillations of the cesium hyperfine transition to define the second. And this definition still holds true today. What I’m going to show you is at least the

performance of the clocks we can make now are better than we can realize with the cesium–

measuring the cesium transition. Of course, cesium was always getting better

through the decades, so it was like a moving target. But we finally were able to overtake their

performance about 10 years ago. So why atomic optical clocks? And one of the simple reasons is that the

oscillation frequencies of electrons around the nucleus are much higher than this so-called

hyperfine transition. In this case, compared to cesium the oscillation

of a typical optical oscillation are about 100,000 times faster than the oscillations

of the cesium clock. And so what that means very simply is, we

get more ticks in any given interval of time, say, the second. So we can define that interval– divide that

interval of time into finer and finer increments. So that’s the simple reason we want to think

about high-frequency. The other thing is that, for certain transitions–

and I’ll say a little bit more about that in a minute– these transitions– the frequency

over which they absorb can be extremely narrow compared to the actual frequencies. So we get a very high relative precision. For most of the cases I’ll talk about, this

width then– which is not infinitely narrow– is given by the lifetime of the upper state

and the transition. So is it a new idea? And the answer is no. And, in fact, actually a colleague at NIST

in Boulder– he did some research, and he published this paper in the early ’70s. And he dug up a text from Thompson Lord Kelvin

and his colleague Peter Tate. And in their paper they acknowledge this idea

was due to Maxwell, the kind of the inventor of the formalism of electromagnetism a long

time ago. Anyway, they wrote this couple of sentences

here. And their idea was basically what we’re talking

about with these atomic clocks. And they basically said, well, there’s recent

discoveries that are seeing the different wavelengths of emission of different atoms. You know, so they had the idea of, well, you

could really– atoms such as hydrogen or sodium, which are relatively available and in nearly

infinite numbers, they’re alike in every physical property. So this was what I was saying. They’re absolutely identical. So they had that idea. And so they were actually thinking about the–

when they say vibration of sodium particles– this was actually optical oscillations that

we’re thinking about. So the basic idea here has been around for

an extremely long time. And, of course, it’s the technology that had

to catch up to be able to realize this idea. Actually, one interesting thing in this same

paragraph– they say, this oscillation of these modes is known to be absolute independent

of its position in the universe. And they can be excused because this was before

Einstein came along and said no, that the rate of time is not absolute. But anyway, they certainly had the basic idea

many, many years ago. So anyway, after my graduate career I came

to the University of Washington and I worked with Hans Dehmelt. And his main interests– I was mostly attracted

by– I’ll say this in a minute. But anyway, he and his colleagues had done

spectroscopy on helium ions, and I was interested by that, and potentially the application in

atomic clocks. But anyway, Hans wanted to focus on an experiment

to measure the electron magnetic moment. And the basic idea there is an electron, in

addition to its charge, has the property that it behaves also like a little magnet. And the reason this experiment was so important

was that the theory of quantum electrodynamics can predict the value of this magnetic moment

to a precision of about– uncertainty of about a part in 10 to the 12th. And so this experiment was very important

because eventually it was able to measure this precision to about that level. And that’s a whole separate story. But anyway, two things to say here is that–

actually, Bob Van Dyke, who’s in the front row down here was the person that actually

led the experiment starting at the time I was here. And I actually didn’t stay around for the

actual important measurements. So these first very precise measurements were

done by Bob Van Dyke and other colleagues here at University of Washington. Another thing to say is that, because Hans

was developing these techniques to confine, in this case, electrons– and previously the

ions– he shared the Nobel Prize with Wolfgang Powell, who invented a slightly different

kind of ion trap, we say, that we actually used for our clock experiments. And I won’t say anything very much about actually

how the trap works. You can see this electrode structure that

I’ve drawn in cartoon form that looks pretty much like this, for the non-technical people

in the audience. A very simple analogy is it’s like this–

when we apply electric fields to this electrode structure we create– for a single ion in

the trap, it’s like a marble in a bowl. We create a so-called harmonic potential. But this analogy to a marble in a ball that

rolls back and forth is actually very good. So the other thing was that I mentioned that

I was attracted– earlier on, actually, Hans and his colleagues– one being Norvell Fortson

who was a post-doc before my time. He’s actually a Ramsay student before my time,

too, but he came to work for Hans. And they were measuring the same kind of so-called

hyperfine structure in helium. And the resolution was very high, so this–

in some sense people gave the ideas and I was certainly interested in how these ideas

might be applied for atomic clocks. So what I’m going to show here is one exper–

focus on one experiment we’ve done in our lab. And I have to say, and I’ll repeat it later,

that this kind of work goes on in many labs throughout the world, and I’m going to focus

on just this example of how we used a mercury ion to realize an atomic clock. And I should also say, we did many different

kinds of experiments with these ions. But this project here, making a frequency

standard out of the mercury ion, was led by Jim Burgquist in our group. So I’ve been a colleague with Jim for the

last 42 years, and so we’ve done a lot of things together. Anyway. So the basic idea here is, again, coming back

to this simple picture. I mentioned before that basically we start

the atom in the ground state, and then we apply radiation for a short amount of time. And then the idea is, we measure whether the

ion has made the transition up to the excited state. And we look for the condition where it does

with the frequency of the laser where it makes that transition with highest probability. This transition is actually in the ultraviolet,

which is a bit of a technical problem. But we can make radiation at that frequency. Anyway, the idea– one of the issues is, how

do we do we detect when this atom has made the transition? Well, let me first say, these experiments

we’ve done for quite a while have been just with a single atomic ion in the trap. So why do we focus on one ion? And there’s one reason– with mercury it turns

out the upper level in the mercury ion– it kind of has– the shape of the electron cloud

around it is kind of like a football. And it turns out that the the inhomogeneous

electric fields on one ion as seen by the other ion interact with this football-shaped

charge distribution and can give a frequency shift. And it’s actually fairly large compared to

the resolution we’re trying to achieve. So that’s one of the reasons, at least our

experiments mostly up until now, have used just one single ion. In any case, the way we detect is to look

at another transition. And this one I should have mentioned here

the lifetime of the upper state in this mercury ion for this clock transition is about a tenth

of a second, which gives this very high resolution we can achieve. So the way we detect our ions– or, at least,

first of all to even identify that they’re in the trap– is we would use another transition

in mercury. And this one can scatter photons at a very,

very high rate, on the order almost– several hundred megahertz per scatter rate. And the nice thing about that is we can not

only identify atoms– and, in fact, in these experiments we can, with a camera that works

in the ultraviolet, we can actually make pictures of our single ions. And actually Dehmelt and his colleagues–

this was after I left, but– they also were working on single-ion experiments. And most of the ions, because there’s one

electron removed to be able to excite the next electron, the transitions are out of

the visible spectrum in the ultraviolet. But there’s a couple ions, and one of those

barium. And so Dehmelt and as colleagues– they actually

made fluorescence of a– or could see it make an experiment like this. And the light that was scattered was in the

blue part of the spectrum. But in these experiments you can actually

see with your eye a single atom, which is pretty amazing when you think about it. It kind of looks like a faint star, but you

can actually see it with your eye. So one of the issues we had to think about

is that– in fact, in the early experiments on helium that I described that had been done

by Norval Fortson and Dehmelt and their colleagues, was that the atoms were actually moving fairly

quickly and bouncing around fairly quickly inside this trap. And so one of the limitations was this time

dilation shift that Einstein told us about. So one of the messages from that was, it would

be nice to have a way to slow down the motion of the ions then to cool them, basically. And this was, again, to suppress this time

dilation effect. So the way this simple form of laser cooling works is the following. And actually when I was here as a post-doc

with [INAUDIBLE], we published a little paper on how this would work. And at the same time, independently, two colleagues,

Ted Haensch and Art Schawlow, who later also won Nobel prizes. Anyway, we had basically the same idea. And the idea, if you’ll bear with me a little

bit– it’s not too complicated. And the basic idea is that I’ve already told

you that atoms, rather than the energy of, say, the electrons rather than existing in

a continuous spectrum of energy states, the energy is confined to discrete energies. So that’s a key part of this. And then, as I said, just for a clock, I haven’t

said much about the motion, but if the atoms are absorbed then when the lasers tune to

this so-called resonance frequency given by the energy difference, then they absorb maximally

at that condition. Now of course what we have to think about

is, in general even though the atoms are confined in this trap, we say they’re moving around

with some amount of kinetic energy. And the idea is that, if the atom is moving

against the laser beam, one thing we have to worry about is this so-called first-order

Doppler shift. And the common example that we all experience

is if a, say– in my day the example was a train that would go by. And the train whistle, as it approached you

would be higher pitch than it was receding from you. And, of course, the same effect with a car

going by or a motorcycle going by, that you hear this change in the pitch of the sound. And the same idea applies to light. So the idea here is that when the atom is

moving against the photons from this laser beam, they actually absorb. But not the frequency they absorb at with

the rest, but the frequency they absorb at is shifted by the velocity divided by the

speed of light. And we can take advantage of that because

the idea is, then what we do is we– a laser comes in from this side and we tune the frequency

lower than the frequency it would absorb at at rest. And the idea is, then when the atoms move

into this resonance condition here the atoms will absorb the light. And when they absorb the photon they get a

momentum kick which, in this case, is against their motion. And then when they re-radiate, generally do

it in all directions. So on average every time they absorb and re-emit

a photon, the momentum is reduced by the momentum of the photon. And so we can repeat this process, do many

scattering events, which gives the slowing process, which allows us to cool the atoms

down. And so– OK. That’s what I just said. And so this has become a standard technique

now in all atomic clocks, because the precisions are high enough that if the atoms are at room

temperature the shift is just too big from this time dilation effect. So we have to invoke this cooling idea. So anyway, a little bit of personal history

is that after my post-doc position in Dehmelt’s lab I got a position at what was then called

the National Bureau of Standards, now called this NIST, the National Institute of Standards

and Technology. And my first job there when I went there was–

there was a cesium beam clock. That’s what this is here. Basically cesium atoms are made to go travel

down this tube, which has a vacuum, and make a stream of cesium atoms. And when they go down this tube we measure

the radiation of this so-called hyperfine transition. But anyway, my group leader at that time,

the person that hired me, had a vision for NIST that we should be doing more research. So luckily he got us some money to try this

idea of laser cooling. And sort of an interesting personal aside

was the fact that– so after Dehmelt and I had this idea– and Dehmelt had taken a sabbatical. It was after I left. And he went to work in Peter Toschek’s lab. He was then in Heidelberg. And I knew they were going to try to demonstrate

this cooling. But we got some money at NIST to try this

experiment. And so they didn’t know about our experiment,

but I knew they were trying to do this. So we were racing, at least. And so what’s interesting is basically without

any– I knew they were working on it, but I had no idea where they were in terms of

their progress. But, interesting– the papers on these first

demonstrations were published about the same time. You can see that ours was published a little

bit earlier. But to be fair, if you look at the dates these

were received at the journal, they beat us by one day. So anyway, as most of you know, I think–

I mean, these experiments are relatively long-term. And if you do it within a few months of each

other it’s certainly a tie. So anyway, both groups got credit for doing

this at the same time. But this was an amazing near coincidence here. So OK. So I’ll say a little bit more about this. So I have already alluded to this. For this mercury ion optical clock we’re going

to use this transition here in the ultraviolet at about 282 nanometers. And how do we measure the transition? So one way we describe this process– again,

we start the atom in the ground state. We applied this radiation for a little while. And then we make what’s called a superposition

state. So the atom is– this is kind of a standard

notation for wave functions, but the wave function in this case after we apply this

radiation, is that the atom is in, we say, a superposition of a lower state and the first

excited state. And the one nice thing– we have a very good

way of measuring when the atom makes the transition. And it’s the idea of the following. When we turn on this other laser then we tend

to– this superposition state that we made– it can, we say, “collapse” into either the

ground state or the excited state. And it does it with a probability that’s related

to these coefficients in front of these components of the wave function. But the key idea is that, when the atom–

suppose we try to drive this transition, and suppose that the frequency of the laser is

a bit off, then the atom remains in the ground state. And we can tell when that happens because

then when we turn on this laser here we’re going to see scattering like in the picture

I showed. And we can pick a bit of that up in, say,

a photomultiplier, some detector. On the other hand, if we’re close to resonance

then there’s a high probability that when we turn on this– [COUGHS] pardon me– this

laser here, that the atom will actually collapse into the upper state. And when that happens we don’t see any scattered

light when we turn on this laser here. So the nice thing is that we can easily discriminate

which state the atom has been detected in. So, for example, this data here is– we were

causing transitions, but the data that we took we would average the fluorescence for

about a millisecond. That you can easily see that if we put a discriminator

here we can tell with essentially 100% efficiency what level the atom is measured to be in. So there’s a lot of details in this. But anyway, some of the interesting ones in

this mercury ion experiment was, basically the electrode structure I showed in cartoon

form– it’s a little hard to see here, but that’s this same structure that I showed in

the cartoon. In this case one of the unfortunate things

is that with mercury the way we would create the ions is, we’d just leak in a little bit

of mercury vapor, and then in those days we’d– very crude experiment– we would just make

a crude electron beam with a homemade filament, and stream electrons through that ring electrode

in the trap. And then when the neutral mercury atoms were

inside, occasionally one would be ionized by this electron beam. And when that would happen the ion would be

trapped. But one of the problems we had was that, it

turns out mercury– if you leak it into a metal vacuum system, which you can see that

this is– the container without the lid– is that the mercury, it turns out, when you

leak it into a vacuum system like that what it does is it amalgamates with copper. But on the other– so what it has, it basically

diffuses into the copper. But the problem is there’s always– even if

we try to pump all the mercury away, the mercury is effusing out of the vacuum. And the problem that made was it turns out

that when the mercury ions were in the excited state, if they collided with a neutral mercury

atom in the background, they would [INAUDIBLE] associate, we’d say, and it would make a mercury

molecule, a dimer, a two-atom molecule. And basically that was the end of our atomic

ion cubits– or ion for the clock. And so it was just a horribly-annoying problem,

because we’d get all the lasers tuned up, and then after 10 [INAUDIBLE] this collision

process would happen and we’d have to reload the ion and tune everything up again. So basically we just hit it with a sledgehammer. And that is, we would we put our ion trap

in its enclosure. We attached it to a liquid helium reservoir,

and basically the liquid helium temperature under normal conditions is at about 4 Kelvin

absolute. So basically everything freezes out, except

for maybe some helium. But helium is not a problem. So literally we went from storage times–

we could keep the iron earlier for only about 10 minutes. And we went for– we could literally increase

the lifetime to about six months. And that was just due to that we’d do something

stupid, which would kick the ion out. But there’s other advantages to going to low

temperature. One is that– I’ve already alluded to. We’d basically freeze out most of the background

gas so the collisions are almost reduced to nothing. Another subtle thing is that it turns out

the clocks are perturbed by thermal radiation, so-called black body radiation. And I was always– when we were first thinking

about this, it turns out the thermal radiation in the room, the electric fields due to that

radiation, are not insignificant. They’re about 10 volts per centimeter. So they’re actually fairly strong fields. And we’re all bathed in this radiation. Anyway, this radiation could shift the frequency

of the clock, and so it would be nice to reduce that. And one way to do it is to put an atom at

a very cold temperature. OK. So there’s other issues. Now, the way I mentioned that the absorption

range of this mercury ion– it would be about 1 Hertz, one cycle per second wide out of

about a million billion cycles per second. So it gave very high resolution. But one of the problems we had going into

this– everybody had– was the laser– although we knew how this should work, the lasers were

not stable enough. In other words, the wavelength– the frequency

and wavelength of the radiation would fluctuate around. So the standard technique that people used

for many decades– and it’s still the way people do these experiments– is basically

we form a resident cavity with two mirrors. And what we do is we shine our laser radiation

in there. And, for the physics students here– they

know this problem– is, basically the cavity will only transmit– will build up radiation

and only transmit radiation through when there is an integer number of half-wavelengths of

this radiation given by the distance between– that fit inside the distance spanned by these

two mirrors. And so basically we can– although there’s

many frequencies that can happen, we basically stabilize our lasers to one of these transmission

conditions, and then this stability is governed just by how well we can control the distance

between the mirrors. And actually, this has a lot of similarities–

you know, this last year with the big splash in the last year or two was to be able to

detect gravity waves by looking at the oscillations of these mirrors. And we’re not nearly as sensitive as those

instruments, but the idea is very much the same. We can stabilize our lasers as long as the

spacing between these mirrors is very precise. So anyway, Jim Bergquist and I had the idea

very simple idea to get rid of mechanical vibrations, as he basically supported. What this cartoon is meant to show here is

one mirror. Another mirror is hidden, but there’s a class

spacer here that’s very rigid and provides a rigid distance between the ends of these

mirrors. One thing– again, it’s no comparison to the

sensitivity of the gravity wave detectors, but what I always found interesting was that

when we’d set this down on the floor to tune up the optics on the small optical table,

is we’d always see some noise very close to the center frequency where the laser would

be locked to this resonance given by the spacing. And we always see some noise on the side at

much less than a Hertz, about a seventh of a Hertz. And that turns out to be the waves crashing

on the beach in California that gives this broad sort of noise spectrum at one seventh

of a Hertz. Anyway, the state of the art these days–

we’re still bothered by vibrations. And this is– we’re now affected by the same

things that, in part, limit the detection sensitivity of the gravity wave detectors. We still have to worry about mechanical vibrations. But what it comes down to now is, we do have

to worry about the spacing of the mirrors, but the main thing we’re left with is actually

the coatings on the mirrors that give the reflectivity. They’re also mechanical systems. They vibrate and they give some noise, which

limits how well we can stabilize, in our case, the lasers to this cavity. OK. We’re coming back– I’m just going to summarize

the many years of work. And first of all, the fact that we’re trapping

the ions– their average velocity is 0, so we get rid of what’s called this first-order

Doppler effect. And what we call the second-order Doppler

shifter time dilation is suppressed highly by this idea of laser cooling. And I’ve already mentioned we suppress several

other effects by going to low temperature in this apparatus. So the one thing that we felt proud about–

and it’s mainly due to Jim Bergquist– was that we had been chasing the performance of

cesium for decades. And so in about 2006 then this was the first

clock that could demonstrate that an optical clock was actually better in terms of performance

than the cesium clocks. And so nowadays all the standards labs for

sure– we’ve all gone to optical clocks. Some of the other ions that people are using–

we’re actually using aluminum these days. It’s a bit better performance than mercury. But there’s many choices. And of course there’s many neutral atom choices. One of the interesting possibilities that

people are looking towards is the thorium nucleus is– has a nuclear transition which,

by coincidence, [? their ?] energy levels, which are separated by close to an optical

transition, as time evolves it appears it’s pretty far in the ultraviolet. But nevertheless this is an interesting idea. Rather than using atomic transitions, to be

able to use nuclear transitions. So let me just come back to these so-called

systematic effects, the environmental effects that cause frequency issues. I’ve told you about the first-order Doppler

shift where we– because the atom is trapped, its velocity goes to zero. And then there’s this so-called time dilation,

or second-order Doppler shift. Now, I lied to you a little bit. Although the atoms is trapped, and it’s true

that its average velocity goes to 0, one thing we have to worry about is that the laser will–

we have an optical table that the laser sits on, and then our trap is, say, at the other

end of the optical table. And what we have to worry about is that the

temperature in the lab changes, the table is shrinking and contracting a little bit. And with the velocities we’re sensitive to

at the level of precision we’re at now are about 3/10 of a nanometer per second. So we have to stabilize the distance between

the laser and the the clock atom in this trap. And what we do there is we actually borrowed

a technique from satellite ranging where basically the satellite ranging is done by sending a

signal out reflecting it off– effectively reflecting it off the satellite. And the signal that comes back is shifted

by the Doppler shift, by basically twice the first-order Doppler shift. And from that we can measure the velocity,

in that case, of the spacecraft. We do the same thing here, only the velocities

are quite a bit smaller than the velocity of a satellite. Or rather, in this case, a deep-space probe

that’s– But anyway, we use exactly this same technique to get to subtract out this Doppler

shift. And I’ve also talked about the time dilation

shift, this famous so-called twin paradox by Einstein. And we suppress that with laser cooling. There’s another effect which is also– well,

anyway. So the net result of our experiment so far

is we’re down to a real fractional precision, which is how we characterize all the clocks. Because doing it that way it’s only– the

real measure of the performance is the relative precision. So we’re down to a level of about 8 parts

in 10 to the 18 with these experiments. So one of the effects we have to worry about

is this so-called– again, due to Einstein’s so-called gravitational potential redshift. And in addition to the time dilation shift

due to movement [? at the ends, ?] Einstein also showed us that clocks run at different

rates in different gravitational potentials. And so when– one example I can give you is

that, suppose you had a twin brother or sister and you were separated at birth and suppose

your twin lived in Boulder, Colorado where we are, about a mile above sea level, and

you lived at sea level. It turns out that there is an effect, but

it’s nothing to really worry about too much. And in fact after 80 years your twin will

be about a millisecond older than you are due to this potential. So it’s a very small effect. Nevertheless, we can see this effect. So kind of as more of a stunt, but kind of

a fun thing to do is to demonstrate this effect on kind of a more everyday scale. So just to kind of show this effect, this

is our optical climate clock. This one is based on aluminum, but the principles

are very much the same on what I described for mercury. And so obviously this is not the one you wear

on your wrist yet. But nevertheless we made a clock like this

in one lab, and in an adjacent lab we had another clock based on aluminum which was,

as far as we could, tell was identical. Anyway, we measured the ratio of the frequencies

of those two clocks, and it was about– I forget how many– on the order of 17 digits,

anyway, that they were running at the same frequency. And then it just says that, as a demonstration,

we– James Chou, who is running this experiment, is basically– you can see he’s put some jacks

under the table here, and he raises the table up by about a foot, 33 centimeters, and we

could actually see that we could resolve this so-called gravitational potential redshift. So the precisions now are such that we do

have to worry about these very small effects, including this gravitational potential redshift. So where are we at these days? So I showed the number we achieved, this fractional

precision, that is taking account of all the environmental effects. So we actually had the lead for a few years,

but now there’s many other– there’s always been many other groups working on that. And so these results have been superseded

more recently by– first of all, we had some colleagues in basically the German version

of NIST where they’ve made a clock based on ytterbium ions, and the precision in their

clock is a little more than a factor of two smaller than in ours. The other thing I haven’t talked about but

it’s important is that– I’ve been talking about single ions. And we, for the reason I mentioned, that we

do want to scale up the number to larger numbers. And it’s simply, we get more signal with more

ions, and so we can improve the time it takes to reach a measurement [INAUDIBLE] precision. So we’re making traps that don’t look the

same, but basically can make strings of ions and therefore increase the numbers that we

can play with. And also, there have been a number of experiments

done with neutral atoms, and they don’t use the same kind of traps, but they’re able to

make traps by using laser beams. And kind of a cartoon version in two dimensions

is kind of like an egg crate that can hold individual atoms, and they can do this in

three dimensions. And so this work first started– the first

high-precision measurements were done by a group of Katori in Japan, but the leader in

more recent years has been Jun Ye and the group at JILA. And they’re down now to where the so-called

systematic precision is down about four times lower than we’re able to do. And the other thing is they can hold quite

a large number, a few thousand, atoms in their so-called egg crate structure. So they can much more quickly reach a high-precision

than we can with our single ion. So we have some catching up to do now. But anyway, so the best precisions are very

close to a part in 10 to the 18th. And just another thing to compare back to

this gravitational potential redshift. They shift due to this gravitational potential

redshift is about one part in 10 to the 18 for a centimeter rise in position. So we’re getting down to these very sensitive

values. Actually, coming back to this one problem

we have, is that if we can– because of this gravitational potential redshift, one of the

problems we have is that in order to compare two clocks, the problem is that– we can make

measurement comparisons between clocks, say, one at sea level and one in Boulder, Colorado. The problem is, we’re limited in precision

because we don’t know the height of Boulder in terms of this gravitational potential redshift

to only about 30 centimeters. So, in fact, with these higher precisions

we’re limited in precision simply because we don’t know the gravitational potential

redshift between sea level and Boulder. So the only way we can make precise comparisons

of these really accurate clocks is we have to bring them together where we know they’re

in the same gravitational potential. So this is kind of a headache. Right? But of course we tried to take the high road

and so one view is, in the future, is to take these very accurate clocks and be able to

map what’s called the geoid, the gravitational potential around the Earth. And I’ve just mentioned some of the groups. There’s many more around the world working

both on ions and neutral atoms pursuing these ideas. So what’s the future? And so navigation is still one of the main

applications. The other is in synchronization. So, for example, in network synchronization,

timing signals, we need these higher and higher precisions. But we still continue to think

about navigation. So at the centimeter scale you’d think, well,

who really cares. And one application that people talk about

is that, for example, a precursor to earthquakes is usually– an earthquake is preceded by

strain in the earth, meaning two relative locations that might be separated by kilometers–

they’re going to their relative height compared to the center of the Earth will change. And this strain– and eventually the strain

will cause the earth to let go and they’ll readjust. And so this– measuring this strain at this,

perhaps, centimeter level can be, maybe not a predictor, but it can be a signal that an

earthquake is building up, or the probability of an earthquake is increasing. And so we think about doing this. And I already mentioned this idea of mapping

the gravitational potential redshift around the world. There’s also a bunch of fun things we can

think about doing. And one of the interesting things is if we

take clocks that are made on different elements, it turns out that the frequencies that they

run at in general depend on different ratios of the fundamental forces, say, the so-called

strong nuclear forces and electromagnetic forces, which– the ratios of the contributions

of the frequencies different basic forces will vary. And so if we measure– if we measure the frequency

ratio of clocks based on two different elements over time, one of the interesting things to

think about is, are the fundamental “constants,” quotes, that govern the frequency of these

transitions, are they changing in time? So we’ve been able to put limits on that. And this continues on. There’s still a lot of motivation to do that. We’re always– as a popular game we’re always

trying to see deviations of Einstein’s theory of general relativity, and so far he’s doing

just fine. But nevertheless, we still try to probe with

higher accuracy to see whether there might be variations. So with that I’d like to thank the real people

doing the group. This is just our group working on these experiments. And there’s many groups around the world working

on these problems. I want to also acknowledge our laboratory

director, Katharine Gabbie, who was laboratory director, meaning the larger group of divisions,

like the Time and Frequency Division. And she was very supportive of some of these

basic ideas. For example, this idea of laser cooling. I mean, we just wanted to do it because it

would be kind of nifty to demonstrate this effect. But it became very important for clocks. And, in fact, it’s used in all high-accuracy

clocks. And she was very supportive of this kind of

exploratory work. Unfortunately she passed away a year or so

ago. But nevertheless, she made a great environment

for us. And I’m going to– for those who might know

a few people there, a list of people– actually, how much time– AUDIENCE: [INAUDIBLE] DAVID WINELAND: [LAUGHS] So just– I mean,

often– and maybe you’ve seen these things before, but I’m going to say a little bit

about the Nobel Prize here. So I think most of you know that– [CLINKING] Oops. The prize is– oh. I’ll find that later, I hope. Anyway, the prize is announced around October

10th or so. And, I mean, the whole thing is very surreal

because, you know, it gets so much attention. But anyway, so they announce the prize in

early October. And then the ceremony is actually coming up

within days here. I think it’s on the tenth of December– anyway,

on the date of Nobel’s death. And anyway, so we go there in early December. And this is– what’s nice– and Stockholm’s

a very charming city and, actually, even during the dead of winter here. You can see the snow on the ground. It was very cold. But they have these open-air markets, and

so– I mean, really, they do a Christmas in a very charming way, I must say. But anyway, actually one thing about this

figure– so we get to spend time wandering around a little bit. And one interesting thing about this figure–

I mean, Sweden is pretty far north, so this picture was at about 3 o’clock in the afternoon,

and you can see the sun has already set. So it’s getting pretty dark pretty early there. So anyway, everything is just way over the

top in these ceremonies. And so this was the awards ceremony, and the

laureates over several disciplines were all lined up in this first row here. And here is the royal family. And so basically the way the ceremony goes

is, each person– a little bit is said about the person, and they walk up and receive the

award from the King. And so you can see everything is very well

organized. We have a rigid uniform. And part of that uniform for us was that we

had– of course we had to wear tuxedos, but also we had to wear patent leather shoes. And these patent leather shoes were– this

was a firm carpet, but walking on this carpet was like walking on ice. And the whole time when I was going up to

receive my award I said, just don’t fall. Anyway. So I made it through OK. This is me receiving the award from the King. And anyway, after this very fancy ceremony

the royal family had a few people over for dinner. [LAUGHTER] And all of this is– I mean, it’s just– everything

is so much over the top. Actually, one thing I didn’t know before going

there is, Nobel actually favored the physicists out of the chemist and the other disciplines,

and so we were the ones that got to sit with members of the royal family at these different

events, which was somewhere here in the middle of this table. The other thing to say is that there’s about–

I forget– I think it was like 1,200 people here. And so what you learn is that, the Nobel laureates

and their guests enforced were only, you knows– at these fancy official events we could only–

each laureate could only invite 12 people, and there were, I think, eight of us– and

now I’m forgetting– eight of us that year to receive the prize. In these different disciplines. And so that’s about 100 guests. And so there was about 1,400 people at this

thing. And what you learn is that this is a big deal

for not only Swedish society, but officialdom. You know? It’s a big deal to go to this event. And it’s also nice they invite some students

from the local universities. But anyway, it’s this very fancy thing. And so the dinner there is extremely well-organized. So there was one of these guys here was, like,

an orchestra conductor. So, you know, there was– I don’t know– 100

or more servers, and they each would serve a few people, but he would wave his wand and

everything would be done in synchrony. And anyway, it was just, as I say, a very–

it was really a surreal thing. But of course very, very fun. So one of the nice things was that the person

I shared the award with was Serge Roche whose lab is in Paris. And I’ve gotten to know Serge– oh gosh, 35

years ago. I first knew him through the literature because

he had done some nice work. And then I got to know him personally about

25 or 30 years ago, and gradually our wives became friends as well. So it was a great pleasure to share with him. And I think we both feel the same way that–

and I think most laureates do– that, you know, the one thing to say is that the probability

of receiving this award, extremely small. And I think we both felt we were lucky to

have it happen. But there’s many qualified people and we more

feel like we represented our field rather than our individual accomplishment. But nevertheless, it was a great, real thrill

for us to share with my friend Serge. Anyway, with that I will stop for the final

time. And this– of course these are the people

in our lab doing the work. [APPLAUSE] KAI-MEI-FU: OK. So thank you very much for that wonderful

talk. I think we have time just for a couple of

questions from the audience. Are there any questions? Yeah. Come on up. There’s a mic right here in the aisle, in

both aisles. AUDIENCE: Hello. Is this mic on? OK. No? Yes. OK. I was curious how you keep time on your person. On your person, out and about, how do you

keep time? DAVID WINELAND: I have a watch here, and it’s

good to about, maybe two minutes. [LAUGHTER] So not very well. AUDIENCE: Thank you. DAVID WINELAND: But probably more seriously,

like all of you I have my cell phone and I rely on that these days to give me a better

time. AUDIENCE: So, another easy question. When you lifted the table up and then moved

it back down, did you get back to the same difference of zero? [LAUGHTER] DAVID WINELAND: Yeah. It was reproducible within our precision. You know? Actually, I must say that this was– you know,

it was a demonstration, but there’s been much more accurate demonstration of this gravitational

potential redshift. And I mentioned– I gave a reference when

we talked about compensating for the Doppler shift due to the expansion and contraction

of our table. The experiment I quoted there was one where

it goes back– it had a rocket which was suborbital. So this rocket went over. I forget where it was launched from but it

went up in this arc and then crashed into the sea. But during this– I don’t know. I forget– roughly an roughly an hour that

it was in this orbit or this trajectory, of course the gravitational potential changed

significantly. Anyway, they were able to measure the gravitational

potential redshift to about a part in 10 to the 6. And ours was only– this thing that I showed

you was about 10%. So just kind of give the the basic idea. And so this rocket on board had a hydrogen

maser, actually. And that was done quite a while ago, I think

in the late ’60s. And that’s still the most accurate measurement

of the gravitational potential redshift, was this early experiment that was done with the

hydrogen maser. AUDIENCE: I was wondering– is the mic on? OK. I can’t hear myself. What makes an atom a good candidate to be

used for an atomic clock? DAVID WINELAND: OK. I mentioned this briefly, but I mentioned

a lot of things. But again, the basic idea has to do with the

fact that all atoms of a given kind, as far as we know, are exactly identical. So we have things that perturb the frequency,

one being these esoteric things like the gravitational potential redshift. But if we bring– if we have two atoms that

undergo the same environmental perturbations, as far as we know, they should run at exactly

the same frequency. And so, as I was trying to make the case there,

for example for a pendulum clock we have to worry about these things like the pendulum,

the length of the pendulum can vary in production. But there isn’t that difference with atoms. As far as we know they’re exactly identical. So– AUDIENCE: I guess my question was, which elements? DAVID WINELAND: Oh. I see. Yeah. There’s no simple answer. In fact, there’s– I don’t know how many. There’s probably 25 different atoms or ions

that people consider. And they all have advantages and disadvantages. And some may be good for reducing some environmental

effect, and others are better for other reasons. So there’s no big winners. I would say nobody has come up with an atom

where this is the choice everybody should be using. One interesting sidenote about that, what’s

amazing is that the cesium clock I mentioned was first demonstrated in the ’50s. And then about the mid-’60s it was decided

it would become the standard for the length of time, the second. And what’s amazing to me is that it was the

best clock from basically in the mid-’60s to about 2006 where we did this optical clock

experiment. So it’s just remarkable that it was the best

choice for this very long length of time. Of course, you know, they were always working

to improvement it, but still it was amazingly good choice. It was the best clock for that length of time. KAI-MEI-FU: So it’s getting late, so unfortunately

it’ll have to end this evening. But I’d like to thank you all again for attending

this lecture. DAVID WINELAND: Yeah. Thank you! [APPLAUSE] [MUSIC PLAYING] [MUSIC PLAYING]

love this keep them comming

Starts [here](https://youtu.be/AHcOJLvpFYI?t=5m22s)