Wednesday, July 18, 2012

Watching Ytterbium

Absorption image of Yb ion.
Image source. Via.
If you know anything about atoms you know they're small. And if you know a little more you know that the typical size of an atom sounds Swedish - it's a few Ångström, or 10-10 meters.

First actual images of atoms went around the world two decades or so ago, taken with scanning tunnel microscopes. These microscope images require careful preparation of the sample, and also take time. It is highly desirable to find a method that works faster and is more flexible for small samples, ideally without a lot of preparation and without damaging the sample.

Taking an image with a scanning tunnel microscope doesn't have a lot in common with watching something the way that we are used to. For the average person "watching" means detecting photons that have been scattered off objects. Quantum mechanics sets a limit to how well you can "watch" an atom absorbing and releasing photons of some energy. That's because the absorption of a photon will excite an electron and temporarily put it into a level with higher energy. Alas, these excited levels have some lifetime and don't decay instantaneously. As long as the electron is in the excited state it can't absorb another photon.

So you might conclude it's hopeless trying to watch a single atom. But a group of experimentalists from Australia have found a nifty way to do exactly that. Their paper was published in Nature two weeks ago
So how do you do it? First, get some Ytterbium. Strip off an electron, so you have a positively charged ion, and put it into an ion trap in ultra high vacuum. Then laser cool your ion to a few mK (that's really, really cold).

Ytterbium has a resonance at 370 nm (in the near ultraviolet). At that frequency you can excite an Yb electron from the S ground-state to the P excited state. Alas, if it decays, the electron has a probability of 1/200 to not go back into the ground state, but end up in a metastable D state of intermediate energy. The lifetime of the excited P state is some nanoseconds, but that of the metastable state is much much longer, about 50 microseconds. So if you just keep exciting your atom at 370 nm, after some nanoseconds you'll have kicked it into the metastable state where it stays and you can't watch anything anymore at that frequency. So what's the experimentalist to do? They stimulate the emission with the right wavelength, in this case at 935.2 nm (in the near infrared), to get the electron back from the metastable state into the ground state.

Actually, to excite the atom you don't need incident light of exactly the right frequency, and in fact that's not what they use. The absorption probability has a finite width and is not exactly peaked. That means there's a small probability the atom will absorb light of slightly smaller frequency and then emit it at the resonance frequency. The actual light the experimentalists used is thus not at 370 nm, but at 369.5 nm. That has the merit that you can in principle tell (with a certain probability) which light was absorbed and reemitted and which one was never absorbed to begin with. The detuning also gives you a handle on how strongly you can afford to disturb your atom, for every time a photon scatters off it, it gets a recoil and moves. You don't want it too move too much, otherwise you'll get a blurry image.

So here's then how you take your image. Shine the slightly detuned light on the ion while driving the transition back from the metastable state to the ground state, and measure the photons at the resonance frequency. Do the same thing without driving the transition back from the metastable state. This has the effect that the probability that the ion can absorb anything is really small and you get essentially a background image. Then subtract both images, and voila. While you do that, you better try not to have too much fluctuations in the intensity of the light.

The merit of this method is its flexibility and it's also reasonably fast with illumination times between 0.05 and 1 second. The authors write that with more improvement this method might be useful to study the dynamics of nucleic acids.

19 comments:

Phil Warnell said...

Hi Bee,

A most clever technique rendering an image reminiscent of a fingerprint. The scanning tunneling method was more like someone feeling the shape of an object where this is more like looking at one in stopped action. The image reveals what looks like standing waves emanating like something you get with a slow motion photography of a water drop impacting on a pool of water. It will be interesting what other images will be produced using this method over the coming years.

Best,

Phil

Ulla said...

http://zone-reflex.blogspot.fi/2012/02/about-protons-and-atoms.html

stefan said...

Ytterbium also sounds Swedish ;-)

Plato Hagel said...

Like some Swedish family name:)

Plato Hagel said...

Of course we are talking about refractive indexing right?

Best,

DocG said...

Are we looking at concentric circles or some sort of spiral configuration?

Christine said...

Diffraction pattern.

Bee said...

Hi DocG,

Christine beat me to it. I find it somewhat misleading to speak of "absorbtion". When I think of absorbtion I think of a material absorbing photons in, say, the visible spectrum and converting it into heat, emitting in the infrared or so. Best,

B.

Plato Hagel said...

The Element Ytterbium

Plato Hagel said...

The breakthrough achievement was accomplished through combining two different techniques: the first involves trapping, or holding, an atom in “free space” in a nano-scale chamber (and applying an electrical field to control it). The second technique involves bombarding the atomic ion with a highly specific frequency of light. This light causes the atom to cast a shadow onto a detector that is dark enough to digitally photograph (as a general rule in microscopy, the darker the image, the easier it is to see). See: Scientific Breakthrough: First Ever Photo of the ‘Shadow of a Single Atom’ Taken

Georg said...

""First actual images of atoms went around the world two decades or so ago, taken with scanning tunnel microscopes. ""

Hello Bee,
What do You think of this apparatus
or its close relative, the field ion microscope?

http://de.wikipedia.org/wiki/Feldelektronenmikroskop

Bee said...

Right... now that you say it, I recall that from my undergrad lectures. I guess that might count as image of an atom, or atomic structures at least, though it takes some imagination, or interpretation respectively. That having been said, technically speaking one could probably also count Rutherford scattering as images of atoms. Best,

B.

Georg said...

Hello Bee,
this leads to the question, what we accept as a picture, or what we have "seen". Would Ernst Mach accept this new picture of a Ytterbium atom or Müllers "Ameisenhaufen"?
Regards
Georg

Phil Warnell said...

Hi Bee,

As it always is with the respect to the quantum world one ends up finding what one is looking for. That is if it's lumps your seek it's lumps you get and if it is waves you find waves.

Best.

Phil

Plato Hagel said...

Hi Phil,

I think the thing to remember though is that there is a finer disposition of information that exists for the material, and that it is not just a material world?:)

I wonder if that would "pop" a song into your head? haha

Best,

Arun said...

Off topic - re: Tegmark - what makes some mathematical object "quantum"?

Logically:
1. "Quantum" is not meaningful OR
2. "Quantum" applies to all mathematical objects, OR
3. "Quantum" applies to some, not all mathematical objects.

Bee said...

Logically, I think it must be 3. There are different types of logic, some quantum, some not. They all "exist" in the same way, so if you believe that all mathematics "exists" in the same way, you must have universes obeying all types of logic "somewhere."

Eric said...

While politically me and Lubos Motl do not agree on probably anything i am very conservative about physics. He is also conservative in that way. I disagree with him that string theory as a doctrine is conservative so i think he is way off there. But I think he is correct in his previous statements that quantum logic is the ground on which all other logic rests.

We do not see quantum logic too much in the everyday world because we live in a macroscopic world. But at the heart of the world are the individual quantum processes. They combine in stochastic processes to create the logic we see on an everyday level. I assume you were kidding about multiple universes.

Bee said...

Eric,

That's the sensible pov if you look at the universe we inhabit. But Arun was asking about Tegmark's mathematical universe. If you believe that everything that exists in mathematics exists in reality, you are forced to believe that classical logic exists as well as quantum logic. Best,

B.