Nobel Prize in Physics 2023: Attosecond Light Pulses and Electron Motion

This note covers the Nobel Prize in Physics 2023: who won it, how attosecond light pulses are generated and used to watch electrons move inside atoms and molecules, how the discovery unfolded over four decades, why it matters for science and technology, and quick facts for exams.

What was the Nobel Prize in Physics 2023 awarded for?

The Royal Swedish Academy of Sciences awarded the prize "for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter". This is the official citation, quoted exactly as given by the Nobel committee.

In plain words, the three laureates found ways to make extremely short flashes of light, so short that they are measured in attoseconds, and used these flashes to take snapshot-like measurements of electrons as they move inside atoms and molecules.

An attosecond is one billionth of a billionth of a second. Before this work, no light pulse was short enough to freeze such rapid motion, so electron movement inside matter could not be directly observed in real time.

The official name of the award is the Nobel Prize in Physics, and the 2023 prize was shared equally among the three laureates, each receiving one third of the prize money and recognition.

Who are the laureates?

Pierre Agostini

Born on 23 July 1941 in Tunis, in what was then the French protectorate of Tunisia.

At the time of the award he was affiliated with The Ohio State University, Columbus, Ohio, USA, and held a one-third share of the prize.

In 2001, Agostini's team in France created and timed a repeating series of light flashes, a "pulse train", in which each pulse lasted just 250 attoseconds.

He used a technique later refined into a method called RABBIT to measure how these pulses were timed.

Ferenc Krausz

Born on 17 May 1962 in Mór, Hungary. At the time of the award he held affiliations with the Max Planck Institute of Quantum Optics in Garching, Germany, and Ludwig-Maximilians-Universität München, both in Germany, and he too held a one-third share.

Around the same time as Agostini's work, Krausz's group in Austria developed a technique to isolate a single light pulse lasting 650 attoseconds, which they used to track electrons being pulled away from atoms.

Anne L'Huillier

Born on 16 August 1958 in Paris, France. At the time of the award she was affiliated with Lund University, Sweden, holding the remaining one-third share.

In 1987, L'Huillier discovered that passing infrared laser light through a noble gas produced many different "overtones" of light. She continued studying this effect through the 1990s, building the theoretical foundation that later made attosecond pulses possible.

What problem were the laureates trying to solve?

Human senses cannot perceive very fast events; rapid motion simply blurs together, much like a hummingbird's wingbeats appear as a blur rather than distinct movements. To capture something, the "camera shutter" must be faster than the thing being photographed.

Atoms themselves move relatively slowly, on the scale of femtoseconds (millionths of a billionth of a second), because their nuclei are heavy. But electrons inside atoms and molecules move far faster, changing position and energy on timescales of attoseconds.

For decades, a single cycle of laser light, around one femtosecond, was considered a natural and unbreakable limit for how short a light pulse could be.

This meant physicists could study how atoms rearrange in chemical reactions, but the faster inner world of electrons, the entities responsible for chemical bonding and energy exchange between light and matter, remained invisible in real time.

As the Nobel committee's scientific background explains, the German physicist Werner Heisenberg, when formulating quantum mechanics in 1925, argued that quantities like an electron's exact position were, in principle, unobservable.

The challenge the 2023 laureates eventually met was to make something once considered unobservable into something that could be measured directly.

Simple comparisons made the difficulty obvious: the natural atomic unit of time is only about 24 attoseconds, while a single-cycle optical pulse lasts about one femtosecond, a far longer interval.

For many years the shortest pulse that any laser laboratory could produce was around 6 femtoseconds, nowhere near fast enough to catch an electron in the act.

Earlier laser technology had already allowed scientists to watch how whole atoms move and rearrange during chemical reactions, work recognised by the 1999 Nobel Prize in Chemistry awarded to Ahmed Zewail.

Reaching electrons, though, needed more than refining existing lasers; it required a genuinely new physical mechanism, since all earlier progress in measuring brief time intervals since around the year 1600 had simply improved existing technology rather than breaking a fundamental barrier.

The roots of the eventual breakthrough go back to the late 1970s and early 1980s, when several groups studied highly charged atomic ions produced when powerful lasers stripped electrons from atoms.

In 1979, Pierre Agostini and co-workers discovered a related effect called above-threshold ionisation, in which a strong laser field drives an escaping electron through extra energy steps matching the laser wavelength, a clue that would later feed into the rescattering theory.

How are attosecond pulses of light actually generated?

Light is a wave, and the length of its shortest possible pulse is tied to its wavelength: the pulse cannot easily be shorter than one cycle of the wave.

Ordinary laser light has wavelengths that limit pulses to around a femtosecond. The breakthrough insight was that combining many different, shorter wavelengths of light together can build a much shorter pulse, similar to the way different musical overtones combine to give an instrument its distinctive sound.

To create these extra wavelengths, physicists send intense infrared laser light through a noble gas, such as argon or neon. This process is called high-harmonic generation (HHG). The mechanism, confirmed theoretically by L'Huillier and colleagues and later explained through a "three-step" or rescattering model, works as follows:

1. The strong laser field distorts the electric field holding an electron to its atomic nucleus, allowing the electron to tunnel free (tunnelling ionisation).
2. The laser's oscillating electric field then accelerates this free electron away from the atom, giving it extra kinetic energy.
3. As the laser field reverses direction in the next half-cycle, the electron is driven back towards the ion it left behind.
4. If the electron recombines with the ion, its accumulated kinetic energy is released as a single burst of high-energy ultraviolet light, which is the source of the attosecond pulse.

Draw an atom with an electron escaping through a tilted energy barrier (tunnelling ionisation), then a curved arrow showing the electron being pushed away and pulled back by the oscillating laser field, and finally a small burst of light labelled "extreme ultraviolet photon" released when the electron recombines with the ion.

Once many of these bursts combine from many atoms, their light waves interfere: peaks reinforcing peaks make the light stronger, while a peak meeting a trough cancels it out. Under the right conditions, this interference produces a train of extremely brief pulses, each only a few hundred attoseconds long.

The discovery that set this whole field moving began in 1987, when Anne L'Huillier and colleagues at a research centre in Paris-Saclay sent intense infrared light (1064 nanometres) through rare gases and recorded many high-order overtones at once, work published in 1988.

The surprise was that, instead of fading away smoothly, the emission intensity dropped sharply for the lowest overtones, then stayed roughly constant across a wide range from about the fifth up to the thirty-third harmonic for argon, forming a flat plateau before finally falling off, an unexpectedly broad spread of usable wavelengths.

This plateau mattered because a wide, flat band of overtones is exactly what is needed to build a very short pulse by combination.

Several groups suggested that the plateau's bandwidth should, in principle, allow attosecond pulses, but nobody yet understood the physical mechanism producing it.

That explanation came from L'Huillier, Kenneth Schafer and Kenneth Kulander, who in 1991 solved the governing quantum equation numerically and correctly predicted the plateau's shape, while Kulander's group derived a simple formula for the highest photon energy reachable, Ec = Ip + 3Up, where Ip is the atom's ionisation energy and Up is the extra energy the laser field gives an oscillating electron.

In 1993, Kulander's team presented the full rescattering picture, and a similar "three-step model" was proposed independently around the same time by the physicist Paul Corkum.

How are these attosecond pulses measured and used?

Producing a pulse that short is only half the challenge; physicists also needed a way to measure a timescale even their fastest instruments could not directly resolve.

Agostini's team developed a metrology approach, later refined into a technique called RABBIT (reconstruction of attosecond beating by interference of two-photon transitions), which focuses the ultraviolet pulse and light from the original "drive" laser onto a target gas and analyses the resulting photoelectrons to work out pulse duration.

Krausz's group instead developed methods to isolate a single attosecond pulse rather than a train, comparable to uncoupling one carriage from a moving train.

They used spectral filtering with a multilayer mirror to select the right range of harmonics, then measured the kinetic energy of electrons ejected from krypton atoms exposed simultaneously to the isolated pulse and the original laser light, a technique called streaking.

Krausz's route to an isolated pulse depended on earlier technical groundwork carried out with Mauro Nisoli's group in Milan, whose collaboration compressed laser pulses using a gas-filled hollow fibre and, by around 1997, produced what were then the shortest light pulses ever recorded: about 4.5 femtoseconds using krypton and 5 femtoseconds using argon in the fibre.

Building on this, the Vienna team generated a broadened high-harmonic spectrum reaching a cutoff near 300 electron-volts, giving them enough bandwidth to isolate a single burst rather than a whole train.

With these tools, researchers could later measure things once thought immeasurable, such as a small time delay of about 21 attoseconds between electrons leaving different energy levels of a neon atom when it is hit by light, an effect linked to the century-old photoelectric effect first explained by Albert Einstein, for which he won the 1921 Nobel Prize in Physics.

That first measurement, made by the Krausz group using 100-electron-volt photons, compared electrons leaving the neon atom's inner "2s" level with those leaving its "2p" level and found the 2p electrons arrived 21 attoseconds later.

Several theoretical groups later calculated a delay roughly half this size, and L'Huillier's own team in Lund eventually traced the mismatch to an overlooked side effect called shake-up, in which one departing electron briefly knocks a neighbouring electron into a higher orbit, slightly distorting the original Garching measurement.

How did the discovery unfold?

Why does this discovery matter?

Attosecond pulses open a window onto a world that was, for most of the twentieth century, considered beyond direct observation. They let scientists measure how long it takes an electron to be pulled away from an atom, and how that time depends on how tightly the electron is bound.

The committee pointed to practical possibilities too. In electronics, understanding and controlling electron behaviour inside materials is central to designing better devices.

In medicine, attosecond pulses can be used to probe molecules, producing a signal with a structure that acts like a fingerprint identifying the molecule, with potential use in medical diagnostics.

The scientific background also notes early steps towards analysing biofluids for disease-related molecular changes, and experiments extending the technique to liquids and solids, such as measuring a 50 to 70 attosecond delay between photoelectrons from liquid water compared with water vapour.

Researchers linked this delay mainly to solvation, the way water molecules interact with their neighbours, which slows the electrons' path to the surface compared with isolated gas-phase molecules.

A separate experiment on solid tungsten found a delay of about 100 attoseconds between photoelectrons coming from tightly bound, atom-like inner states and those coming from the metal's freely moving conduction-band states.

This showed that attosecond methods can track electron behaviour inside solids as well as inside single atoms and molecules, which matters for materials used in solar cells, batteries, catalysts and electronic devices.

As Eva Olsson, Chair of the Nobel Committee for Physics, put it, "We can now open the door to the world of electrons", adding that the next step would be learning to put that access to practical use.

The scientific background underlines that attosecond science, once a narrow field of multiphoton atomic physics, has since grown to touch molecular physics, physical chemistry, the physics of condensed matter, and technologies for generating light, with many questions about electron behaviour in complex materials still open.

Krausz's group has also begun combining broadband optics and precision laser timing to build an electric-field "molecular fingerprinting" method that can detect subtle changes in the molecular make-up of biofluids such as blood, an early step towards a non-ionising diagnostic tool for spotting disease-related molecular traces.

The committee's background report stresses that many molecules can be monitored at once this way, with the radiation involved causing no ionising harm to tissue.

At the award ceremony, Eva Olsson noted that attosecond chemistry allows specific chemical bonds to be selectively broken or formed, and that attosecond physics can track charge-transfer processes important for how solar cells, batteries, catalysts and electronic devices work and can be improved.

She described the origin of the prize as reaching back to work on atoms in strong laser fields in the late 1970s and early 1980s, from which the laureates eventually learned to generate and measure both pulse trains and isolated attosecond pulses.

How does this connect to what you study?

This prize links directly to topics in school physics such as waves, light and the photoelectric effect, as well as to basic ideas about atomic structure taught in chemistry.

The idea that light consists of waves with a given wavelength, and that producing a very short pulse needs a mix of many different wavelengths added together, builds directly on basic wave concepts such as wavelength, frequency and the superposition of waves, where crests reinforcing crests make a stronger wave and a crest meeting a trough cancels part of it out.

The attosecond pulses in this prize are created exactly this way, by combining several high-harmonic wavelengths of light.

The photoelectric effect, explained by Albert Einstein in 1905, for which he received the 1921 Nobel Prize in Physics, is a standard topic in modern physics.

This prize shows how a century-old idea, once assumed to happen instantly, could finally be tested on its true, extremely fast timescale, revealing a small but real delay of around 21 attoseconds in one measured case.

The quantum idea that electrons occupy definite energy levels within an atom, and that moving between levels or escaping the atom involves absorbing or emitting energy as light, also underlies how the overtones and attosecond pulses in this work are produced, measured and interpreted in experiments described in the scientific background.

Finally, the discovery illustrates how theoretical physics, such as Werner Heisenberg's 1925 argument about what can and cannot be observed, can later be overtaken by new experimental technology, a useful example when studying how scientific understanding develops over time.

Quick facts for exams

The Nobel Prize in Physics 2023 was awarded jointly to Pierre Agostini, Ferenc Krausz and Anne L'Huillier for developing experimental methods that generate attosecond pulses of light, used to study how electrons move inside atoms and molecules.

The award was announced on 3 October 2023 by the Royal Swedish Academy of Sciences. Each laureate received an equal one-third share of the 11 million Swedish kronor prize.

Agostini was born in Tunisia, Krausz in Hungary and L'Huillier in France; at the time of the award they were based at institutions in the USA, Germany and Sweden respectively.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Attosecond — one billionth of a billionth of a second, the timescale on which electrons move inside atoms and molecules.
• Femtosecond — one millionth of a billionth of a second, the timescale on which whole atoms move in a molecule.
• High-harmonic generation (HHG) — a process where laser light passing through a gas produces light at multiples of the original laser frequency.
• Overtone — a wave that completes several full cycles for every one cycle of the original wave, giving extra wavelengths of light.
• Tunnelling ionisation — the escape of an electron from an atom when a strong laser field distorts the electric field holding it in place.
• Rescattering model — the three-step theory describing how an electron is freed, accelerated, and then recombines with its ion to emit an attosecond photon.
• RABBIT — reconstruction of attosecond beating by interference of two-photon transitions, a method to measure attosecond pulse duration.
• Streaking — a measurement technique using the kinetic energy of photoelectrons exposed to both an attosecond pulse and the drive laser.
• Photoelectric effect — the emission of electrons from matter after it absorbs light, explained by Einstein and central to this prize's background.
• Pulse train — a series of consecutive, repeated short light pulses, as produced by Agostini's method.
• Isolated pulse — a single attosecond light pulse separated out from a train, as achieved by Krausz's method.
• Electron dynamics — the motion and energy changes of electrons inside atoms, molecules and solids over time.

Common errors and misconceptions

• Misconception: The three laureates worked as one single team. Correct: They worked in separate research groups, L'Huillier's foundational discovery came first in 1987, and Agostini's and Krausz's pulse-generation breakthroughs both occurred independently in 2001.
• Misconception: An attosecond pulse is a single photon. Correct: It is an extremely brief burst of light formed by many overlapping wavelengths interfering constructively, not a single particle of light.
• Misconception: Attosecond pulses let us watch atoms moving. Correct: Atoms move on the slower femtosecond scale; attosecond pulses are needed specifically to observe the faster motion of electrons.
• Misconception: Agostini produced a single isolated pulse. Correct: Agostini's group produced a train of consecutive 250-attosecond pulses; Krausz's group isolated a single 650-attosecond pulse.
• Misconception: This work has no practical use beyond pure research. Correct: The Nobel committee pointed to potential applications in electronics and in medical diagnostics through molecular identification.
• Misconception: Heisenberg's 1925 argument was proven permanently correct. Correct: The scientific background notes that what Heisenberg thought could never, even in principle, be measured is now being measured directly in laboratory experiments using attosecond science.

Exam-style questions with model answers

Q1. In which year was the Nobel Prize in Physics 2023 announced? [1 mark]

1. The prize was announced on 3 October 2023 by the Royal Swedish Academy of Sciences.

Q2. State the official citation for the Nobel Prize in Physics 2023. [2 marks]

1. The citation reads "for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter", awarded jointly to Pierre Agostini, Ferenc Krausz and Anne L'Huillier.

Q3. What is an attosecond, and why was a new timescale needed for this research? [3 marks]

1. An attosecond is one billionth of a billionth of a second.
2. Electrons inside atoms and molecules move and change energy on timescales of hundreds of attoseconds, far faster than whole atoms, which move on the femtosecond scale.
3. Before this work, even the best laser laboratories could only produce pulses of about 6 femtoseconds, too slow to capture electron motion, so a new, much shorter pulse was needed to observe it directly.

Q4. Describe the three-step rescattering process used to generate attosecond pulses of light. [4 marks]

1. A strong laser field distorts the electric field binding an electron to its nucleus, allowing the electron to tunnel free.
2. The oscillating laser field then accelerates the free electron away from the atom.
3. When the field reverses direction, the electron is driven back towards the ion.
4. On recombining with the ion, the electron releases its extra kinetic energy as a burst of high-energy ultraviolet light, forming the attosecond pulse.

Q5. Compare the contributions of Pierre Agostini and Ferenc Krausz in 2001. [4 marks]

1. Both worked at roughly the same time but in separate groups, France for Agostini and Austria for Krausz.
2. Agostini's group produced a train of consecutive light pulses, each lasting 250 attoseconds, measured using a frequency-based metrology method.
3. Krausz's group instead developed a technique to isolate a single light pulse of 650 attoseconds, using spectral filtering and a streaking measurement with krypton atoms.
4. Together, these two approaches, pulse trains and isolated pulses, demonstrated that attosecond pulses could be reliably produced and measured for use in further experiments.

Q6. Explain Anne L'Huillier's contribution and why it was foundational to the later breakthroughs. [5 marks]

1. In 1987, L'Huillier discovered that passing infrared laser light through a noble gas produced many strong overtones of light, each completing several cycles for every cycle of the original laser wave.
2. This observation showed that intense infrared light interacting with gas atoms could generate a wide range of new wavelengths, a process now called high-harmonic generation.
3. Through the 1990s, working partly at Lund University, she continued studying this phenomenon and helped develop the theoretical understanding, including work explaining the shape of the harmonic spectrum and the physical mechanism behind it.
4. This theoretical and experimental groundwork was essential before anyone could reliably produce a controlled, measurable light pulse short enough to be called an attosecond pulse.
5. Without this foundation, the specific pulse-generation achievements of Agostini and Krausz in 2001 would not have been possible, which is why the prize credits all three equally.

Q7. Discuss why the Nobel committee considered attosecond physics significant for both fundamental science and practical applications. [6 marks]

1. Fundamentally, attosecond pulses let scientists directly observe electron motion inside atoms, molecules and solids for the first time, something that was considered impossible to observe even in principle when Heisenberg discussed quantum mechanics in 1925.
2. This allowed long-standing questions to finally be answered, such as the precise timescale of the photoelectric effect, first explained by Einstein in 1905, by measuring small delays of a few tens of attoseconds between electrons emitted from different atomic energy levels.
3. The technique also lets researchers study how electron distributions oscillate in molecules and materials, information previously only available as an average rather than something observed changing over time.
4. On the practical side, the committee noted potential applications in electronics, where controlling electron behaviour in materials is important, and in medical diagnostics, where molecular "fingerprints" detected using these pulses could help identify diseases.
5. Committee chair Eva Olsson described this as opening "the door to the world of electrons", suggesting the next phase of research will focus on turning this new access into practical uses.
6. Overall, the prize recognises both a scientific first, direct observation of electron dynamics, and a technology with wide-ranging future applications across physics, chemistry and medicine.

Key takeaways

• The Nobel Prize in Physics 2023 went jointly to Pierre Agostini, Ferenc Krausz and Anne L'Huillier.
• Their work generates attosecond pulses of light, letting scientists observe electron motion inside matter directly.
• An attosecond is one billionth of a billionth of a second, far shorter than the femtosecond scale of atomic motion.
• L'Huillier's 1987 discovery of laser-induced overtones in noble gases laid the theoretical foundation for attosecond pulses.
• In 2001, Agostini produced a train of 250-attosecond pulses, while Krausz isolated a single 650-attosecond pulse.
• The three-step rescattering model explains how tunnelling, acceleration and recombination of an electron create attosecond light bursts.
• Applications discussed by the committee include electronics and medical diagnostics using molecular fingerprinting.
• The prize connects back to Einstein's 1921 Nobel Prize for the photoelectric effect by finally measuring its timescale.

Test yourself

Who chairs the Nobel Committee for Physics quoted in the press release for this prize?

Eva Olsson, who said attosecond physics "can now open the door to the world of electrons".

What is the key difference between a pulse train and an isolated pulse?

A pulse train is a repeating series of consecutive short light pulses, as Agostini produced, while an isolated pulse is a single separated pulse, as Krausz achieved.

Which effect, explained by Einstein in 1905, could finally have its timescale measured using attosecond pulses?

The photoelectric effect, for which Einstein received the 1921 Nobel Prize in Physics.

Why can't ordinary laser light alone produce attosecond pulses?

A single cycle of ordinary laser light lasts about a femtosecond; shorter pulses need many combined wavelengths, produced by passing the laser through a gas.

What gas-based process generates the extra wavelengths needed for attosecond pulses?

High-harmonic generation, in which intense laser light passing through a noble gas produces overtones at multiples of the laser frequency.

Name one potential practical application of attosecond pulses mentioned by the Nobel committee.

Medical diagnostics, where attosecond pulses can reveal a molecular "fingerprint" signal that helps identify specific molecules.

At which institution was Anne L'Huillier based when she received the 2023 prize?

Lund University in Sweden.