The Nobel Prize in Physics this year will be shared between three scientists from the USA, Germany, and Sweden, in recognition of their contributions to the study of electronic processes lasting a billionth of a billionth of a second.
A question that has intrigued humanity since ancient times is:"Does a galloping horse ever lift all four of its legs off the ground at any given moment?"
It was not until the 19th century that the answer to this was provided by the pioneer of photography, Eadweard Muybridge, who managed to photograph a galloping horse using a sufficiently high frame rate. He discovered that, indeed, the answer is "yes."
This year's Nobel Prize in Physics commemorates a fundamental realization: the indispensable need for swift capturing tools to capture fleeting phenomena.The award will be given to three researchers who contributed to the development of methods to produce ultra-short short light flashes - specifically flashes lasting, a billionth of a billionth of a second. The award is to be shared equally between Pierre Agostini from Ohio University, USA, Ferenc Krausz from the Max Planck Institute for Physics in Munich, Germany, and Anne L’Huillier from Lund University, Sweden. Notably Krausz and L’Huillier were recipients of the Wolf Prize last year for their contributions to this field.
These ultra-short light flashes allow for the measurement and study of processes transpiring on the order of attoseconds, that is, 10-18 seconds. Such rapidly occurring processes include, for instance, the movement of electrons within molecules, changes in the electrical charge of materials, quantum phenomena, and more, and they have a profound influence across all facets of life.
Contributed to the development of methods for producing ultra-fast light flashes, used in the study of rapid processes. From right to left: L'Huillier, Kraus, and Augustini | Nobelprize.org
High-Order Harmonics: The Pathway to Attosecond Pulses
For many years, the creation of attosecond laser pulses was considered an insurmountable challenge. The solution was discovered thanks to a phenomenon called high-order harmonics: when a ponent laser is directed at certain materials, they emit radiation at a frequency that is a multiple (harmony) of the original frequency.
One of the pioneering researchers to produce such high-order harmonics consistently was Anne L'Huillier (L’Huillier) from France, who then worked at the French Institute for High Energy and Nuclear Energy. L'Huillier later moved to Sweden and continued to develop methods for creating even shorter laser pulses in the attosecond range. Prior to L'Huillier's experiments, high-order harmonics were primarily described using perturbative nonlinear optics. This theory suggested that when high-frequency photons are produced, their relative intensity would be too weak compared to the laser field applied to the system. In reality, L'Huillier was discovered to find that the relative intensity of high-order harmonics was much stronger than that predicted by perturbative optics, necessitating a more precise theoretical explanation of the phenomenon.
In 1993 and 1994, three seminal papers were published attempting to explain this phenomenon. The first paper, by Ken Kulander (Kulander) and colleagues, proposed the existence of a "cutoff frequency" for each high-order harmonics spectrum. This frequency depended on the specific atom or molecule under investigation and the intensity of the electric field (laser) applied. Kulander’s team argued that high-order harmonics could be explained using classical physics in a two-step process.
Kulander retired from science shortly after due to personal reasons, and the more memorable paper was by Canadian plasma physicist Paul Corkum, whose contribution to the field remains indisputable, and he shared the Wolf Prize with Ferencz and L'Huillier. Corkum proposed the most widely accepted model today, the three-step model: In the first step, the laser induces the release of part of the electron charge through a mechanism termed quantum tunneling. After ionization, this electron packet is accelerated by the electric field, allowing their motion to be entirely described using classical mechanics according to Newton's second law. In the third step, the same field that released part of the electron also returns the electron to its place. The collision of the electron with itself (a phenomenon possible in quantum mechanics) is accompanied by the release of kinetic energy in the form of a high-frequency photon. Thus, this process transformed a low-frequency infrared photon into a collection of photons with frequencies reaching into the deep X-ray range.
The third paper, by the way, was co-authored by L'Huillier, Corkum, and others. The model they proposed, now known as the "Lewenstein Model", offers an elegant mathematical framework as an alternative to the three-step model. Their model is based on a well-established physical approximation known as the "stationary phase."
How small is an attosecond? To the same extent that a second is small compared to the age of the universe. © Johan Jarnestad/The Royal Swedish Academy of Sciences
From Theory to Practice
The first to actually produce attosecond laser flashes were the Austro-Hungarian physicist Ferenc Krausz and French physicist Pierre Agostini, in their independent research studies in 2001. Both Krausz and Agostini demonstrated that the high harmonics indeed produce ultra-short laser pulses (Attosecond Pulse Train) and even went on to record phenomena such as atom ionization, i.e., the release or capture of an electron that changes the electric charge of the atom.
The works of L'Huillier and Krausz over the last thirty years have effectively laid the foundation for attosecond physics, enabling a profound understanding of the most basic processes occurring in the atom. In the future, their research promises to enable control over these processes and advance a range of developments in various fields, from cutting-edge electronics to innovative methods for drug production.
With the help of the ultra-short flashes, one can measure phenomena such as the ionization of atoms. Illustration of a light beam capturing atoms during ionization.© Johan Jarnestad/The Royal Swedish Academy of Sciences
Quanta, Complex Systems, and Black Holes
Last year, the Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger for their research demonstrating the existence of entangled quantum states. In 2021, the prize was awarded to three scientists whose research focused on topics related to climate change: Half of it was shared between Syukuro Manabe, who demonstrated how rising carbon dioxide concentrations in the atmosphere cause global warming, and Klaus Hasselmann, who showed how climate changes can affect the weather. The other half was awarded to Giorgio Parisi for developing theoretical models allowing the study of complex systems, including climate systems. The 2020 prize dealt with black holes. Half of it was awarded to Roger Penrose for research showing that black holes are the natural end result of the lives of large stars, and the other half to Reinhard Genzel and Andrea Ghez for astronomical observations of the black hole at the center of our galaxy, which has since been captured in photos.