# 2023 Nobel Prize in Physics: Attosecond flashes: a camera fast enough to freeze an electron > The 2023 Physics prize honours the camera shutter for the electron. Electrons move so fast that ordinary flashes of light only blur them, so the laureates built bursts of light measured in attoseconds, billionths of a billionth of a second. Those flashes are finally brief enough to freeze and follow electrons inside atoms and molecules. - Laureates: Pierre Agostini (share 1/3, The Ohio State University, Columbus, OH, USA); Ferenc Krausz (share 1/3, Max Planck Institute of Quantum Optics, Garching, Germany); Anne L’Huillier (share 1/3, Lund University, Lund, Sweden) - Official citation: “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter” - Course page: https://nobelexplained.com/physics/2023-attosecond-flashes - Last updated: 2026-06-21 ## Explained simply (ELI5) To photograph a hummingbird's wings without a blur, you need a camera with a very fast shutter. The faster the thing you want to freeze, the shorter the flash of light has to be. Electrons are the tiny particles that whizz around inside every atom, and they move almost unimaginably fast. To take a sharp picture of one, an ordinary camera flash is far too slow. You need a flash that lasts only an **attosecond**, a billionth of a billionth of a second. > **A shutter fast enough for an electron** This year's laureates learned how to make flashes of light so short they can freeze an electron in mid-motion. For the first time we can watch what electrons actually do, instead of only guessing from a blur. An attosecond is so short it is hard to picture. There are as many attoseconds in one second as there have been seconds since the universe began, about 13.8 billion years ago. That is the tiny stopwatch you need to keep up with an electron. ## Explained for undergraduates Different things in nature move at different speeds, and each needs its own shutter speed. Atoms in a vibrating molecule shift over **femtoseconds** (10⁻¹⁵ s), and filming that already won a Nobel Prize. But the electrons themselves are roughly a thousand times faster, moving on the **attosecond** scale (10⁻¹⁸ s). For decades the femtosecond was treated as the shortest flash light could make, so electrons stayed a blur. The breakthrough came from a process called **high-harmonic generation**. In 1987 Anne L’Huillier sent an intense infrared laser into a noble gas and found that the gas emitted not just the original colour but a long ladder of higher overtones, much the way a plucked guitar string sounds its fundamental note plus a series of higher tones. > **Many waves add up to one spike** Each overtone is a pure wave of a different frequency. When many of them overlap in step, they cancel almost everywhere and reinforce only in one brief instant, producing a sharp spike of light. The more overtones you combine, the shorter the spike, until it lasts only attoseconds. It is the same idea that lets a wide band of frequencies build a sharp pulse. - **1987**: L’Huillier shows that an infrared laser passing through a noble gas produces a broad ladder of high harmonics, or overtones. - **1990s**: L’Huillier and colleagues work out why the harmonics appear, turning the effect into something you can control. - **2001**: Pierre Agostini generates and measures a train of pulses, each about 250 attoseconds long. - **2001**: Ferenc Krausz isolates a single 650-attosecond pulse and uses it to watch electrons leave an atom. With these flashes in hand, physicists can do something genuinely new. They can measure **how long** it takes an electron to be pulled off an atom, and watch the cloud of electrons in a molecule slosh from place to place. Before, those positions could only be recorded as a smeared average. ## Explained for experts Electron motion sits at the attosecond scale because that is the natural clock of bound charge. Where atomic vibrations and chemical bonds evolve over femtoseconds (10⁻¹⁵ s), the electrons themselves rearrange roughly a thousand times faster, over tens to hundreds of attoseconds (1 as = 10⁻¹⁸ s). Resolving that motion in time demands a probe pulse of comparable length, well below the femtosecond floor that conventional mode-locked lasers could reach. Improving existing lasers was not enough; an entirely different physical process was needed. > **Rip, accelerate, recombine** An intense infrared field bends an atom's potential enough to tunnel-ionise one electron. The freed electron is accelerated away by the oscillating field, then driven back when the field reverses, and on recombining with its parent ion it releases its gained kinetic energy plus the ionisation energy as a single high-energy photon. Repeated every half-cycle of the laser, this emits high harmonics of the drive frequency that extend in a broad plateau into the extreme ultraviolet. This semiclassical recollision picture is the engine behind every attosecond source. The harmonics matter because they are phase-locked. A coherent comb of equally spaced frequencies is, by Fourier synthesis, a train of pulses in time, and the wider the plateau, the shorter each burst. Left alone the emission is chirped, because for a given harmonic energy two electron trajectories, a 'short' path and a 'long' path, recombine at slightly different times. Compensating that phase is part of squeezing a pulse down to its limit. > **A pulse train versus a single shot** Pierre Agostini characterised a train of consecutive bursts of about 250 attoseconds each, reconstructing them by letting the pulse train ionise atoms alongside the original infrared field and reading the resulting electron sidebands. Ferenc Krausz instead gated the process down to one isolated pulse of about 650 attoseconds, then used it as a pump-probe shutter to clock electrons being emitted from atoms. A train gives periodic strobe-like access; a single pulse gives one clean start-the-clock event. **What the attosecond shutter opens up** - Photoemission timing: measuring the delay, often tens of attoseconds, between light striking an atom and an electron actually leaving it, and how that delay depends on how tightly the electron was bound. - Charge migration: following how the electron cloud in a molecule redistributes immediately after ionisation, before the much heavier nuclei have had time to move. - Materials and electronics: watching, and eventually steering, how electrons respond inside solids, which matters for faster signal processing. - Diagnostics: using the characteristic attosecond response of matter to identify molecules, with proposed applications in medical sensing. > “We can now open the door to the world of electrons. Attosecond physics gives us the opportunity to understand mechanisms that are governed by electrons.” > (Eva Olsson, Chair of the Nobel Committee for Physics, 2023) ## Think first **Q:** To freeze a hummingbird's beating wings in a photo you change one camera setting. Which one, and why does it matter even more for an electron? **A:** **The shutter speed, which is really the length of the flash.** A blurry photo means the subject moved while the shutter was open. Electrons move far faster than any wing, so freezing one needs a flash lasting only an attosecond, a billionth of a billionth of a second. That very tool is what the 2023 laureates learned to build. **Q:** A single pure colour of light is one smooth wave. How could adding many different colours together ever give you a flash shorter than any one of them? **A:** **By interference.** Line up many waves of different frequencies so that they all peak at the same instant. Everywhere else their crests and troughs cancel, but at that one instant they add up to a sharp spike. The more colours, or overtones, you stack, the narrower the spike. That is how high-harmonic generation turns a long laser pulse into attosecond flashes. ## A second holds as many attoseconds as the universe holds seconds An attosecond is 10⁻¹⁸ of a second. There are as many attoseconds in a single second as there have been seconds since the universe began, roughly 13.8 billion years ago. A flash of light crossing an ordinary room already lasts about ten billion attoseconds, so an attosecond pulse is short even by the standard of light itself. ## Key terms - **Attosecond**: One quintillionth of a second, 10⁻¹⁸ s. It is the natural timescale on which electrons move inside atoms and molecules. - **High-harmonic generation**: A process in which an intense laser driven into a gas makes the gas re-emit a ladder of much higher frequencies, or overtones, whose combination can form attosecond pulses. - **Overtone (harmonic)**: A wave whose frequency is a whole-number multiple of a fundamental frequency. In light, stacking many overtones in phase produces an ultrashort pulse. - **Femtosecond**: 10⁻¹⁵ s, a thousand attoseconds. The timescale of atomic vibration and chemical bonds, and long thought to be the shortest flash of light that could be made. - **Electron dynamics**: The fast rearrangement and motion of electrons within atoms, molecules and materials, the very processes attosecond pulses are built to observe. - **Pulse train**: A regular series of repeated light flashes. Agostini measured a train of attosecond pulses; an isolated single pulse, as Krausz made, acts like one camera shutter. ## Check yourself 1. Why do you need attosecond flashes to study electrons, when femtosecond flashes were enough for chemistry? - Electrons are larger than whole atoms - **Electrons move about a thousand times faster than atoms do** (correct) - Femtosecond lasers are too expensive - Electrons only move in the dark - Why: Atoms shift over femtoseconds, but the electrons themselves move on the attosecond scale, roughly a thousand times faster. A blur-free snapshot needs a flash as brief as the motion you are trying to freeze. 2. What does high-harmonic generation do when an intense laser passes through a noble gas? - It cools the gas to absolute zero - It turns the gas into a mirror - **It makes the gas emit a ladder of higher-frequency overtones of the laser light** (correct) - It slows the laser light down - Why: The gas re-emits the laser light as a broad comb of high harmonics, or overtones, at multiples of the original frequency. Combined in phase, these overtones build the attosecond pulse. 3. In 2001 Agostini and Krausz both reached the attosecond scale, but in different ways. What was the difference? - **Agostini measured a train of pulses while Krausz isolated a single pulse** (correct) - Agostini used sound and Krausz used light - One worked with electrons and the other with protons - There was no real difference - Why: Agostini characterised a repeating train of pulses about 250 attoseconds each, while Krausz isolated one single pulse of about 650 attoseconds. Both proved the attosecond world could be produced and measured. ## The laureates ### Pierre Agostini In 2001 Pierre Agostini generated and measured a train of consecutive light pulses, each lasting only about 250 attoseconds. His method proved that the ultrashort flashes promised by high-harmonic generation were real and could be timed. Born in Tunis in 1941, he is a professor at The Ohio State University. ### Ferenc Krausz Also in 2001, Ferenc Krausz and his group isolated a single light pulse lasting about 650 attoseconds, like uncoupling one carriage from a moving train. He used it to watch electrons being torn from their atoms in real time. Born in Mor, Hungary, in 1962, he directs the Max Planck Institute of Quantum Optics in Garching. ### Anne L’Huillier In 1987 Anne L’Huillier showed that sending an infrared laser through a noble gas produces a ladder of light overtones, the raw material from which attosecond pulses are built. Over the following years she did much of the work to explain why the effect happens, which is what made the field possible. Born in Paris in 1958, she is a professor at Lund University in Sweden. ## Sources - [The Nobel Prize in Physics 2023, popular science background (NobelPrize.org)](https://www.nobelprize.org/prizes/physics/2023/popular-information/) - [The Nobel Prize in Physics 2023, press release (NobelPrize.org)](https://www.nobelprize.org/prizes/physics/2023/press-release/) - [High harmonic generation (Wikipedia)](https://en.wikipedia.org/wiki/High_harmonic_generation) - [Attosecond (Wikipedia)](https://en.wikipedia.org/wiki/Attosecond) --- From Nobel Explained (https://nobelexplained.com), a free, independent course on every Nobel Prize since 2020. Not affiliated with the Nobel Foundation. Facts are pinned from the official Nobel Prize API.