Attosecond spectroscopy wins 2023’s Nobel Prize in Physics

By starting with a femtosecond laser and compressing the pulses, and then passing those pulses through a noble gas, extreme ultraviolet (or XUV) pulses can be generated with attosecond-level speeds. This enables scientists to probe the behavior of individual electrons within atoms and molecules, using the technique of attosecond spectroscopy. The 2023 Nobel Prize in Physics was awarded for advances made in precisely this field. (Credit: Attosecond Research Center, Milan Polytechnic)

If you want to understand and measure the world around you, one of the most important tools at your disposal is the ability to image — or take a snapshot — of precisely what’s occurring. In the 19th century, photography meant holding your subject perfectly still while you accumulated large amounts of light: several seconds worth of it. In more modern times, we can perform high-speed photography, using a shorter “pulse” of light to image an individual, brief moment in the life of objects as they naturally occur, including objects in motion. We can do this with visible light for macroscopic objects, but we can do with in a variety of wavelengths on microscopic scales with a special type of technology: high-speed laser pulses.

Because individual atoms and molecules are so small, as little as an Angstrom (or ~10^-10 meters) across, that means that changes/transitions occurring within them — driven by electrons — can occur in as little as that distance, 10^-10 meters, divided by the speed of light, or 3 × 10⁸ m/s. That works out to a few attoseconds, where a single attosecond is just 10^-18 seconds. Can we perform imaging that quickly? We can if we can generate short-enough laser pulses, and that’s exactly what the 2023 Nobel Prize in Physics is for, awarded to Pierre Agostini, Ferenc Krausz, and Anne L’Huillier. Here’s the science behind this incredible advance.

By firing a pulse of light at a semi-transparent/semi-reflective thin medium, researchers can measure the time it must take for these photons to tunnel through the barrier to the other side. Although the step of tunneling itself may be instantaneous, the traveling particles are still limited by the speed of light. By taking high-speed images of this light pulse, we can construct a movie that appears continuous. (Credit: J. Liang, L. Zhu & L.V. Wang, 2018, Light: Science & Applications)

When you watch the world unfold with your own eyes, your brain interprets the images you see as though reality were being written in a continuous, unbroken stream: like everything smoothly moves from one moment to the next. Down at the quantum level, however, we know this isn’t true. What’s actually happening, inside your anatomy, is that:

• individual quanta of light are arriving at the rods and cones in your retina,
• stimulating the photoreceptor molecules that live inside,