Atomic Ion Clocks
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ElementarteilchenphysikWocheUhrVorlesung/Konferenz
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Transkript: Englisch(automatisch erzeugt)
00:16
Thank you Rainer and also thanks to all the organizers
00:21
This has been a really nice experience for me and and my wife who's here with me So it's been nice to meet some of the students so far and I hope to meet more through the week so what I'm going to talk about here is some some ideas about how to make clocks out of atoms and Just to summarize. I'll first talk about why we need precise clocks
00:44
I'll give some basic ideas of how we make the clocks and In this limited time. I'll only use examples From the kind of things we do in our lab, but there's it is on atomic ions But neutral atoms are very good, too
01:02
And I'll try to give some idea the state of the art and where we might go in the future so I think it's certainly one of the Applications for precise clocks over many centuries has been in navigation and that's still true today. I think certainly one
01:22
one system we take take for advantage is the rather take for granted is the is the is the GPS and the These days GPS can be very precise as you know, and but the end the idea is very simple and that is that The protocols are more complicated than what I say here, but the basic idea assume you have a you agree that
01:47
You on satellite and on the ground you have to synchronize clocks And let's say you agree that the satellite is going to send a pulse every second then by measuring the time delay just through the speed of light then the
02:03
Then that gives the distance from this this expression here and Of course there can be errors in the in the clock. So for example, if if we If the clocks are synchronized to about 10
02:20
nanosecond 10 the minus 9 seconds then that gives an uncertainty of about 30 centimeters and to give what to say what that means in terms of frequency precision is That one a nanosecond over Wayne one day is about a part in 10 to the 14 and that Says also that our clock the frequency must to maintain the synchronization that the frequency
02:45
Must be maintained to that same level And of course, it's a bit more complicated than that that the one satellite gives us the distance from that satellite but as you know with a network of satellites the the system becomes
03:00
Overdetermined and in fact the the clocks can synchronize their position and as well time and so we get three dimensional navigation on the surface of the earth So we we all have simple notions about the how clocks Work and that is that basically we we rely on on having some steady
03:26
periodic event generator and then we use a counter to to to measure elapsed time and of course that can then give us the time we're used to and the historically the the two traditional
03:42
periodic event generators for example has been the rotation of the earth and the And then later pendulum clocks which were shown to be more Accurate enough to do fairly precise navigation but we're gonna we're gonna use the
04:01
Periodic events of oscillations and atoms and so a simple case may might be a Electric dipole Interaction where over on the right side you see the the electron density versus time and the of course we one thing to think about is that we set up an oscillation a
04:23
superposition of two states say the ground and first excited state in an atom and the wave function is described by the By the simple expression I've shown there whether there's a phase evolution between the two states that goes proportional to the oscillation frequency and of course this this this frequency is given by the energy difference
04:45
divided by Planck's constant So one mode of operation for a clock then is to surround these radiating atoms with with an electromagnetic cavity and
05:01
Have some probe Say for the microwave region that that that samples of radiation or in the optical Case we just let the radiation escape through out of two mirrors in the case of a laser, and then we have some counters that that measures the oscillation of the
05:24
of the radiating atoms Just a sort of a personal little bit of personal History is actually I was a student of Norman Ramsey who you who you see here?
05:41
right Norman was a very famous atomic physics physicist and at When I started graduate school in his group, I see now 50 years ago. That's kind of scary, but Anyway the that he and his colleague Dan Kleppner had invented and demonstrated the
06:03
first hydrogen mazers so mazers like a laser just M stands for microwave and Norman wanted to have a precise measurement of all three of the hydrogen isotopes so my project was to
06:21
was to Measure that make a maser out of deuterium and measure its frequency you can see that that's me They're trying to get close to the boss. This is Norman here Anyway, so the the result of this project was that to measure this frequency and
06:40
you can see that even the the the Features are the pardon me the precision is fairly high But still not as high as we'd like to say for have on board a satellite But this nevertheless this is one idea for for making an atomic clock, and I should say this You know for the students out there certainly this was you know a time of personal uncertainty
07:05
For me and where I would fit in into the physics world, but I but I knew By that time that I really like this kind of stuff these precision measurements I think one of the thing that still remains that the Interesting for me is kind of the detective work you go through to figure out all the things that can cause errors and
07:28
And be able to improve on that So let me come back to this as there's coming back to the basic idea of the clock There's a second mode of operation which tends to mean the more common way We do the measurements of the atomic frequencies one of the problems in the maser or the lasers
07:46
that the radiating atoms are coupled to this cavity which also has a resonance frequency and the and the the Those two those two objects the cavity and the atom can pull the frequency of one or another so it tends to
08:02
this problem tends to Shift the the observed frequency in such a way that we can't always Control it well enough to Be just the atomic frequency so another way to To to make a clock is to say have an atoms the atoms in a container
08:22
I'll come back to this in a minute, but And then what we do is we think about starting the atom in the ground state as you see on the lower left And then we apply radiation it'll be near the atomic resonance frequency for a short time and then basically all we do is we measure the
08:42
The probability of the atom being excited and when the when the problem when the maximum transition probability is maximum then we know that the frequency of the Of the radiation we're applying is equal to the resonance frequency of the atom so the the the basic recipe is
09:01
For making our one basic recipe for making an atomic clock is is then to have a have a an Atoms contained in some sort of container and in fact some some of the early clocks based on microwave radiation were Exactly this it was a glass cell with some typically the interior say a rubidium
09:24
clock would be The interior of the cell would be coated with some low polarizability material like like paraffin the atoms would bounce around in there and We'd apply microwave radiation here
09:40
and if as the as the frequency that radiation was tuned near the resonance of the atoms and the the transmitted radiation would would be decreased and so if you in this picture up here the The the idea is that there'd be some absorption feature. It's of course. It's not infinitely narrow may be limited by the
10:02
typically by the lifetime of the atoms and the excited state But nevertheless you can we can make a simple servo to basically make the frequency of the radiation Be that of the of the atomic resonators in fact It's a little slightly more complicated that if we sit right on the top of this
10:24
Absorption feature of course there's the the slope the discriminator it has less sensitivity so typically all the only Slight complication then as we typically measure on one side of the line and then on the other side and basically We just make this the signals out be exactly the same
10:44
and then we know that the mean frequency of those of those two frequencies is equal to the resonance frequency and That it's literally no more complicated than that the way the way we typically make clocks Based on on the second mode of operation where we look for absorption
11:02
So okay, so what's what's good about atomic clocks and? But I'm here comparing to a pendulum clock which Things like quartz crystals have the same same issues I'll describe here, and so the first the pendulum clock is
11:20
Given the frequency is given by this expression there We can think of it what first of all one thing we have to worry about various environmental Effect and we for example look at temperature It even if we have a fairly low expansion material say that has a
11:41
temperature coefficient of about 10 to minus 8 per degree C And that's too that's typically about more than a hundred times better than most metals for example even with that fairly low expansion material the the the Frequency Shift due to temperature changes is given by the expression on the on the lower left there about part intended a little less than a
12:07
part in 10 to the 8th per degree C The we also have to worry about temperature of our atoms In our container, and this is certainly one of the most more interesting Kind of effects we we know we learned from Einstein about
12:23
Relativistic time deletion if the atoms are are moving time Relative to us in the lab the time moves slower for the atoms and that gives the so-called second-order Doppler time Delicious shift that that that Einstein told us about and for example for cesium
12:43
Which is the current definition of a second based on a microwave transition and cesium that then gives rise to a Temperature coefficient of about a part in 10 to the 15 per degree C, so substantially better than we can do with a pendulum clock
13:01
Okay, the last point is maybe Is reproducible Reproducibility of the clocks and for a pendulum clock for example depends on the the length depends on the manufacturing tolerances and certainly the local value of the Acceleration of gravity and and also on where if the if the if the bearing for that holds the
13:26
Pendulum, where's of the length changes a little bit, so there's some changes Due to that over time the nice thing about atoms Is that as far as we know any any gap any atom of a particular?
13:42
Isotope they're they're exactly identical Of course atoms don't wear out we can continue to use to the same atoms and they they won't change Well, I actually atomic clocks are not a new idea this is this is One I'm not sure how far this goes back, but it goes back to at least this
14:05
This this this work by Lord Kelvin and his colleague Tate and They attribute this idea to Maxwell But basically at that time they were they were they were realizing the the prop starting to realize the properties of atoms having vibrations and so in this case they were
14:24
This they were thinking about sodium And and that Actually meant the optical oscillations that they were thinking of here when the idea was that that They realized that that that sodium atoms would
14:40
Be exactly reproducible and They can be excused a little bit on this lap last part about being independent of the position They didn't know about relativity yet But they certainly had the basic idea for atomic clocks and or so we're playing off of these early ideas So this was actually when I after
15:01
After graduate school and then a postdoc I went to the to what's what was then the National Bureau standards now called NIST the National Institute of Standards and Technology This was our group at that time. We were starting to do experiments on atomic ions with the idea of
15:21
of making clocks and Unfortunately, I see again this is a this is after quite a while and my my my two colleagues Wayne Atano and Jim Burke was on the left there. They look pretty much the same but those other guys didn't Didn't fare so well over time
15:41
So anyway, there was one thing I would say there too. I want it was nice Although that was the that was the size of our group when we started at that time one nice thing for me and I think I think for all of us we we basically spent have spent our whole careers together and And I'll say a little bit about where we are now, but anyway and around 1981 we
16:06
We were thinking Mercury we were using playing with mercury ions and mercury was interesting because the it it has a Fairly high hyperfine frequency which we could make a microwave clock around 40 gigahertz, but it also had this optical transition
16:25
which would which would be interesting and the basic eye the basic idea here or at least one of the Features of using optical transitions that is that the oscillation rate the tick rate of the clock is much faster So you can define any unit of time into much finer
16:43
Into much finer increments if you use higher frequencies. So anyway, we We were interested in this transition this electric Quadrupole transition the upper state has a lifetime of about a tenth of a second So the the line width is around a Hertz at about 10 to the 15 Hertz and the basic idea
17:04
We've been playing with single Ions and this I won't describe this trap structure but we make a simple electrode structure that uses oscillating and static electric fields to hold the atoms and then we can irradiate the the atoms with
17:22
In this case ultraviolet light and try to excite this this transition here and We you might ask well why just one ion You're obviously we want more atoms to have a higher signal rate and
17:43
I'll comment a little later This is one of the advantages of the the current neutral atom clocks as they typically deal with More atoms than we do but in any case the reason we stuck with one ion is that for is just because of the Systematic shifts and in this case with our with our mercury ions if we have two ions in the in the trap
18:05
the the the upper state in the in this transition of mercury is a has a quadrupole shape like a like a soccer or pardon me like an American football and the the the In the electric field gradients from one ion acting on this quadrupole causes a shift
18:25
It's about a kilohertz for typical conditions where the answer in these traps for the two ions we'd be separated by a few microns and we'd like to get down to about a millihertz and so Precision and so that's why we've stuck with one ion so far
18:42
Another feature of this is that that with the way and to lead into how we detect transitions is that we also have a transition in mercury where the where they The lifetime of the upper state is very short so we can scatter a lot of photons on that transition It does two things versus allows us to cool the ions laser cool the ions to about a milli kelvin. We can also
19:07
See our ions here this since a fluorescent the ultraviolet we can't see with our eyes, but we can use a simple video Ultraviolet video camera to see the ions And so the basic idea is that we can use this other transition did to detect
19:25
transitions on our on our favorite clock transition and the basic idea is that is that if we if we start the atom in the in the ground state the s-state and then we apply radio the the radiation on this clock transition near that transition frequency
19:42
If the atom had if the radiation was mistuned and the atom remains in the ground state Then we turn on this this Cooling and detection light and and if it remains in the ground state Then we see fluorescence on the other hand if it's been promoted to the excited state Then when we turn on this this cooling detection laser
20:04
We don't see fluorescence. And in fact we can we can get fairly good discrimination You can see in the lower signal there that that when the atom fluoresces It's quite a bit different than we see a little bit bit of background light But we can essentially tell with a hundred percent efficiency whether the atom has made the transition that doesn't mean the signal to noise is perfect because we're always left with the
20:27
Quantum fluctuations we make a superposition state after we apply the radiation But then when we project or measure the ion, there's always the quantum fluctuations Of which state it's in even though we detect the state with a hundred percent efficiency
20:42
Well, anyway as I mentioned that the one reason for going after optical transitions is that the tick rate is is much higher Here's the this on the upper part of the figure there that we show we measure the the temperature the the frequency of the ion to precision of about a part in 10 to the 15 and actually this was
21:03
We were kind of proud of this was is about around 2005 and we were proud of this because it was the first time Over many decades that Another type of a comet clock would actually have higher higher Accuracy than that of the the cesium clock the other thing which I'm leaving out of this
21:23
to count these very high frequencies both Ted hench and Jan Hall and their colleagues really Made this wonderful Frequency comb and it was a it was an astounding Development because the the way to count frequencies
21:40
I don't have time to go into is extremely complicated these very high optical frequencies But they the device they made these frequency combs allowed us to actually have a counter of these very high frequencies which then could be used to make our clock and it was as I say was amazing development because with
22:01
After they made their developments within a year or two all many labs could build these counters and have them in their lab Well, I'm leaving out a lot of details one of our favorite projects is to use aluminum ions and which has some advantages over the mercury ion and
22:22
We got down a few years ago to about eight parts and in ten to the eighteen uncertainty So they as I said before there's many things we have to worry about a lot of categories of electric and magnetic fields Shifts some of the more interesting ones. I've talked about the
22:42
Time dilation shift the second entry on that on the view graph there actually an interesting one we have to worry about also is the first-order Doppler shift and It what the way that manifests itself in the in the lab is that we will have lasers on one end of the table and the ion trap on the other and for example
23:03
There's a frequency a Doppler shift associated with the frequent fact that when the atoms or pardon me when the temperature in the room changes the table shrinks and and contracts and we're sensitive to about About a less than a nanometer per second and we have to worry about this
23:20
So actually what we do is to compensate for this we use a technique This was used in satellite ranging and we got the idea from this experiment here to to compensate for the stopler shift So there's another very interesting one also from Einstein and in addition to the to the time dilation shift from movement he in his theory of general relativity of course he explained that that
23:47
clocks also run in at different rates and different gravitational potentials and One way to say this is the I mean it's not a very Significant effect in our ordinary
24:02
day lives a good example is suppose you had your students out there if you had a twin sibling and You were separated at birth Then suppose your twin lived in Boulder, Colorado about a mile above sea level
24:20
In fact after 80 years your twin would only be about a Millisecond older than you so it's nothing that to get too worried about but nevertheless with our the precisions We we have in our atomic clocks. This is we have to take account of this so anyway one fun way to demonstrate that we had a
24:43
Way in our experiments on aluminum clocks. This shows a The table that holds the optics and ion trap for one of our clocks we had another clock in an adjacent room And we could measure the frequency ratio of the of the of the two
25:02
Transitions optical transitions and the aluminum clocks, and we measured the ratio you see on the bottom entry there So here's James Chow who is a postdoc at that time? he's going to he's going to raise one of these clocks with jacks and in fact we when we then compare the frequency we can see that the
25:24
Obviously, it's not great precision here, but in fact we can see the frequency shift due to this to the second-order Doppler shift so We can't for a while. We held the record at this eight parts into the 18, but mentioned very briefly that there's many
25:42
First of all there's many other groups working on ions. There's many groups working on neutral atoms the both all these experiments are very interesting in many ways and the The the way the neutral atom experiments work and the principle is actually not so much different than how we trap our charged atoms
26:03
But basically what they can apply certain laser fields that that that will trap the atoms Sort of a two-dimensional analog of the way these these optical lattices work is it looks like the atoms in it or held in kind of like an egg crate and
26:20
Anyway, they they use that to to hold their atoms, and I just to give you kind of an idea that This isn't these this doesn't represent all the many groups working on it, but just to give you an idea Professor Katori from Japan was the one to first
26:40
Be able to utilize the idea that if you choose the the lasers that use are used for this Trapping if you choose the wavelength appropriately the energy levels are shifted by these trapping fields but if you choose the wavelength appropriate Then the it turns out the ground and excited state are shifted in an equal way
27:01
so you get rid of the perturbing effects of these trapping fields and He he's been developing this idea for many years Jun Yi at Chilla down the street from us has also Been been working on the system and other groups are as well, so they've actually the world's record right now and
27:21
accuracy is held by Jun Yi's group at about two parts and 10 to the 18th there's a group our counterpart of NIST in in Germany the PTB they they also have an ion clock Which is it's pretty close to the to the same performance and this this game will never end We'll keep trying to push each other to increase the precision and in the future
27:47
I think you know you'd say well Do we really need better? navigation and and Certainly, it's good for most most of our daily needs One interesting idea is to if we can do this increase the this level of precision of navigation
28:04
We could make it measure the relative positions of two locations on the on the earth down to two centimeter or less Precision and for example this might be useful for Certainly, we know that that the strain
28:22
between various location between various locations is an is a precursor of earthquakes as if we could do it at this position might be a abuse and earthquake prediction The the second order pardon me the gravitational redshift might be used to use in geodesy. There's many interesting
28:44
the ideas to use precise clocks and in To look for for basic effects, and we're always trying to prove Einstein wrong as a sport and so far he's doing just fine, but But nevertheless we always want to check if they're that at some level there might be deviations to it
29:03
Einstein predicted and with that all conclude this our group Gradually grew over the years see there's quite a few people In addition to the original group now. I do want to say I think both Bill Phillips, and I we were in the same
29:22
laboratory of misfits called the precision measurement lab and Our our laboratory director Catherine Gabby basically one Measure of her success is I'm the fourth person to win a Nobel Prize under her direction including bill and Eric Cornell and Jan Hall, so
29:42
With that I'll conclude. Thank you