Whenever we open a new window on the universe we discover something new.
Whether it's figuring out how to see to greater distances like with telescopes,
or down to smaller size-scales like with microscopes, or perhaps expanding our vision
to new wavelengths of light or via exotic means such as in neutrinos or gravitational waves. Well,
the 2023 Nobel prize in physics has been awarded to three physicists for opening just such a new
window—but it's not a window to a new size scale or a new mode of seeing—-it’s for a new window in
time. It’s for attosecond physics—the billionth of a billionth of a second that represents the
timescale of the insides of atoms. This year’s Nobel in physics is for a microscope in time
Every 230 million years the solar system completes one orbit around the Milky Way.
Every 243 years Venus passes between the Sun and the Earth.
Every year monarch butterflies migrate between the United States and Mexico. Every 3 seconds
a Kinesin protein travels down one of your cytoskeletal filaments.
As we look to smaller and smaller scales, we find faster and faster processes. Makes
intuitive sense—big things surely have a hard time moving faster than their constituents. The
trend continues to the smallest scales—the motion of individual atoms during chemical reactions,
or of individual electrons moving in atoms. The timescale of their motion is not measured
in microseconds nor nanoseconds, they're measured in attoseconds. And this is a very
small amount of time indeed. There are as many attoseconds in a second as there are
seconds in the entire history of the universe. Probably there are some things to be discovered
buried in that universe of phenomena that play out in every single second.
And this is why the Swedish Academy of Sciences made a very reasonable choice
in awarding Anne L'Hullier, Pierre Agostini and Ferenc Krausz the 2023
Nobel Prize in Physics for bringing into existence the field of attosecond physics.
Before we get to how our newly minted laureates managed this, let’s talk about why it was so
difficult in the first place. Think about what it takes to observe something. Observing means watching things bounce
off other things, whether it’s light bouncing off an apple into our eyes or electrons bouncing off
a tardigrade into our electron microscope. The smaller the object you want to observe,
the smaller the particle you need to fling at it. For example, you could get a rough
idea of my shape by hurling beach balls at me and seeing how they scattered. But
you’d get a better picture pelting me with ping pong balls. Or even better, photons.
Resolving small distances requires scattering many particles with small spatial separation,
into a well-defined spatial location. Resolving very small times requires scattering
many particles with small separations in time and well-defined temporal locations.
Imagine you want to take a video of a fast process—say, a hovering hummingbird. If
your camera aperture stays open too long you’ll only capture a blur over the span of the wings’
motion. The aperture opening needs to have a well-defined temporal location—in other words,
it needs to be quick. But quick exposures aren’t going to give you a very good movie of the
humminbird if they aren’t separated by less than a wing beat. Then you’ll only get a random seeming
collection of stills. You need short temporal separation to get a meaningful sequence. So how
do you get tight temporal location and short temporal separation down to the attometer?
Let’s start by thinking about the temporal location of a photon. That’s limited by its
period—you can’t clock a photon to any time smaller than a single up-down cycle of its
electromagnetic wave Light with a period of attoseconds is in the X-ray part of
the electromagnetic spectrum. We routinely use X-rays in imaging, so we should be fine there,
right? Not quite. A typical laser fires one photon every few femtoseconds - that’s 1000
times longer than an attosecond. So even if you make a movie of attosecond motion with
a conventional laser, the abysmal frame rate would turn the motion into a hopeless blur.
It is possible to build extremely fast lasers—for example, Free-Electron lasers… But they require
an electron accelerator to work, and they are honestly too powerful
and way too dangerous to give to the average physicist to play around with,
and besides, blasting your sample with an X-ray laser of the power required would be
like taking an x-ray using a nuclear explosion, it's just not practical.
Enter our first Nobel laureate. Back in the 1980s Anne L’Huillier and her colleagues
were playing around with irradiating Argon gas with infrared lasers. They observed something
pretty weird. Normally, when you irradiate a gas you’ll get different types of light
out. That includes some of the light you put in, some light corresponding to the electron
transitions of the atom, as well as some random thermal light from the jiggling of
the atoms. But L’Huillier’s team noticed that the outgoing beam consisted of the original
in-going frequency plus some higher frequencies that didn’t correspond to any known process.
So where did these other frequencies come from? It’s pretty cool actually. As a laser pulse passes
by a gas atom, it can nudge the electromagnetic field holding the electrons in place in such a way
that electron escapes by quantum tunneling.
The electron can then be pulled straight back to its atom and will release the energy it
gained in a single photon, think of it like a stretching a spring and letting it go. That one
photon will have a higher energy than each of the many photons that made up the laser pulse.
In fact it’ll have a frequency that’s some integer multiple of the laser photons, and
a cool detail is that it'll be an odd multiple, for complex reasons regarding symmetry
This process is called high harmonic generation, and it basically adds overtones to the laser beam.
Play a note on any musical instrument and you’ll get the fundamental frequency plus
higher frequency overtones. The fundamental corresponds to the longest wavelength standing
wave that can fit along the resonating string or air column, while the overtones correspond
to all shorter wavelength standing waves that also fit. The balance of the strength of the
overtones determines the precise character of the sound of an instrument—its timbre.
So, the timbre of the laser was altered by the argon cloud. That’s a cute effect,
but we’re not quite at Nobel-level science just yet. To get to attosecond physics, we need one more
analogy with acoustics. Listen to this. It’s a very low G note. And this is a very low A.
Individually there’s nothing unusual, but if we play them together we hear a sort of "wawawa".
This is known as a "beat" in acoustics, and it’s due to the fact that the sine waves of the G and
the A line up perfectly at certain points, making them louder with constructive interference, while
they’re perfectly out of alignment elsewhere, and so cancel out with destructive interference.
For a pair of waves the beats are quite spread out,
with a lower frequency than both of its parents. But if you add more and more waves,
it’s possible to narrow the width of the beats so you end up with sharp pulses and very little
in between. For that you need a large number of frequencies of similar intensity—for example,
the rich spectrum of overtones produced in Anne L’Huillier’s experiment. In fact,
it proved possible to get pulses that were mere hundreds of attoseconds in their temporal width.
Our new attosecond-resolved pulses weren’t ready for application just yet. The potential
value is that they would allow us to measure attosecond-scale events. But if the pulses
themselves are attosecond-scale events, how can we calibrate them to start with? It's
like trying to measure your height with a series of rules whose individual length you don’t know.
And here we get to our second Nobel Laureate, Pierre Agostini. Agostini
was able to calibrate the pulse train by again using constructive and destructive
interference—this time in reference to the ingoing laser beam. He did this by
deflecting part of the beam and adding a delay to it before recombining it with the
now-frequency-multiplied beam. In this way he could measure the width of the pulses—clocking
them at 250 attoseconds. He also found that the pulses were what we call phase locked,
which means the beats were nice and consistent and just what we needed for attosecond measurements.
These pulse trains provided pulses with attosecond temporal locality and separation,
just as we required. But for some applications it would be preferable to have single,
isolated attosecond pulses. And that effort is thanks to our third Laureate,
Ferenc Krausz. I won’t go into the gory details, but for your viewing pleasure here’s their
Rube-Goldberg experimental setup. With intricate phase and amplitude manipulation, they were able
to create isolated pulses of 650 attoseconds, whose width was known to 150 attosecond precision.
OK, attosecond resolution achieved. So what can we do with it? As Krausz himself states
in an interview, they invented this technology because looking at Nature
in a new way is wonderful, literally, it fills you with wonder. But we did just
give ourselves a new superpower, so it’d be a shame not to use it.
The first application of attosecond pulses was to look at electron motion in atoms and
molecules. Electrons travel the breadth of their orbitals in a handful of attoseconds,
or rather the fuzzy quantum clouds that define the electron in an atom change on that timescale. By
hitting these clouds with attosecond pulses we can study the shapes and dynamics of these electrons.
Attosecond pulses can also be used to manipulate electrons on tiny timescales,
which has a number of powerful applications. One effort that Ferenc Krausz’s team is working on
at the Max Planck institute for Quantum Optics is in molecular fingerprinting,
in which attosecond pulses are frequency-tuned to cause vibrations in specific molecules. In
this way the detailed molecular composition of a sample can potentially be cataloged. The
Krausz team are using this to develop molecular fingerprinting devices for medical diagnosis.
Another very exciting possibility is the creation of ultrafast electronics. If we have two metal
plates with opposite electric charge and we shoot an isolated attosecond pulse at one of
the plates it can be absorbed by an electron, which hops to the second plate. This is the
photoelectric effect, and we’ve known about it for 120 years. But in the configuration I described
it’s also a transistor, a key component in most of our technology. Regular transistors control
the flow of electricity between two charged plates by changing the charge in a third plate,
but this new type of transistor controls the flow using light itself, which can in
principle be much, much faster—especially if the pulse width is measured in attoseconds.
Krausz claims that it may be possible to increase the power of computers by a
factor of 100,000 this way. That sounds … optimistic. But even if we get a fraction
of that it’s an incredible achievement, and may save Moore's law for a few more decades.
Those are just a couple of applications in medicine and electronics, but as we look at
the universe with this new tool we are bound to find new applications and to make new discoveries.
After all, every time we open a new window to the universe new mysteries are revealed—whether
that window is to the largest, the smallest, and now the fastest phenomena in Space Time