Nobel Prize in Physics 2023: an unprecedented image of the infinitely small

The 2023 Nobel Prize in Phys­ics was awar­ded for the devel­op­ment of exper­i­ment­al meth­ods for gen­er­at­ing atto­second laser pulses (10–18 seconds), flashes with a speed of around one bil­lionth of a bil­lionth of a second. They are use­ful for study­ing the dynam­ics of elec­trons in mat­ter. Three sci­ent­ists received awards: Pierre Agostini, Fer­enc Krausz and Anne L’Huillier.

Laser pulses are like cam­era flashes that allow us to freeze and observe the move­ment of mat­ter. The faster the flash, the faster the dynam­ics we can observe. Pre­vi­ously, we were able to gen­er­ate laser pulses in the femto­second range (10^-15 seconds). This is the speed at which atom­ic nuc­lei move dur­ing chem­ic­al reac­tions. By get­ting down to the atto­second scale, we are now able to observe the rearrange­ments of the elec­trons them­selves: this pro­cess is extremely rap­id – it was even con­sidered instant­an­eous in many the­or­et­ic­al models!

Elec­tron rearrange­ments take place dur­ing crit­ic­al stages in the trans­form­a­tion of atoms, molecules and mater­i­als. Under­stand­ing these dynam­ics is a fun­da­ment­al issue in phys­ics and chem­istry. Atto­second lasers have opened the way to unpre­ced­en­ted exper­i­ment­al obser­va­tion of nature. Many ques­tions can be addressed: how does the elec­tron cloud reor­gan­ise after a rap­id dis­turb­ance, and how long does it take? How does this influ­ence the move­ment of the nuc­le­us? Can we con­trol and steer the rearrange­ment of electrons?

Quantum mech­an­ics provides us with the equa­tions that describe the beha­viour of mat­ter, includ­ing elec­trons. But solv­ing these equa­tions accur­ately requires enorm­ous com­put­ing power. And that’s even for a very simple atom like heli­um, made up of a nuc­le­us and two elec­trons… So, we’re a long way from mov­ing on to more com­plex atoms, or even molecules!

The only solu­tion is to use approx­im­a­tions to sim­pli­fy the cal­cu­la­tions. These approx­im­a­tions also provide sim­pli­fied men­tal mod­els for think­ing about this com­plex phys­ics. It is there­fore essen­tial that they are accur­ate. This is where atto­second pulses come in: they provide extremely pre­cise exper­i­ment­al meas­ure­ments, which are invalu­able for estab­lish­ing and val­id­at­ing these approximations.

In elec­tron­ics, cur­rent is gov­erned by switches con­trolled by elec­tro­mag­net­ic fields. For example, an elec­tric field is applied to a tran­sist­or which, depend­ing on wheth­er or not the field is activ­ated, either lets the cur­rent through or blocks it. To explore the speed lim­it­a­tions of these switches, laser pulses are used instead of tran­sist­ors. Atto­second phys­ics has developed the fast­est and most pre­cise elec­tro­mag­net­ic fields in exist­ence. Sci­ent­ists at Garch­ing in Ger­many and Graz in Aus­tria have used them to test how quickly it is pos­sible to switch from one mode to anoth­er. The res­ult is that at around one peta­hertz, or one mil­lion giga­hertz, there is an upper lim­it for well-con­trolled opto­elec­tron­ic pro­cesses¹.

Oth­er advances relate to the time taken for an elec­tron to leave its atom after absorb­ing a photon. In 2010, Fer­enc Krausz’s team pub­lished exper­i­ments that showed a dif­fer­ence of 20 atto­seconds between the emis­sions from two elec­tron lay­ers of a neon atom². After sev­en years of sci­entif­ic debate, Anne L’Huillier and her team were able to cla­ri­fy the ori­gins of this delay as a cor­rel­a­tion between neon elec­trons³.

No, that’s a long way off. But the sci­entif­ic scope for their use is con­stantly expand­ing. Chem­ists have been tak­ing an interest in them since 2010. One of their aims is to optim­ise cer­tain chem­ic­al reac­tions. How­ever, elec­tron­ic dynam­ics are highly com­plex and dif­fi­cult to con­trol, so research is still at a fun­da­ment­al stage. Molecu­lar bio­lo­gists are using them to observe how and at what speed elec­tric charge migrates along large molecules after the sud­den remov­al of an elec­tron. This enables them to bet­ter under­stand the dam­age caused to DNA by cer­tain types of radi­ation. The semi­con­duct­or industry is also inter­ested in the ima­ging pos­sib­il­it­ies offered by these lasers.

The meth­od most widely used today is that dis­covered by Anne L’Huillier: a laser (in the near infrared or vis­ible range) is dir­ec­ted at gas atoms. Under the right con­di­tions, the laser’s elec­tric field pulls on the elec­trons, steer­ing them along tra­ject­or­ies around their atoms and caus­ing them to col­lide with their atoms. These syn­chron­ised col­li­sions between all the atoms gen­er­ate atto­second pulses. We now know that it is also pos­sible to use very thin solids, plasma mir­rors or even free elec­tron lasers. Tech­nic­ally, it’s not com­plic­ated to gen­er­ate trains of atto­second pulses, but you need to know how to char­ac­ter­ise them. As the atto­second sources developed today become more power­ful, they will be able to address new processes.

The sys­tems stud­ied are becom­ing more com­plex: molecules are lar­ger, solids are struc­tured on a nano­met­ric scale… The pulses will be short­er and short­er, and the zepto­second fron­ti­er – thou­sandths of an atto­second – will fall. At this scale, it will be pos­sible to make sim­il­ar obser­va­tions of pro­tons and neut­rons bound togeth­er in the nuc­lei of atoms.

Anoth­er poten­tial approach is to con­cen­trate energy over time to unpre­ced­en­ted powers. Quantum the­ory shows that with a suf­fi­ciently strong elec­tro­mag­net­ic field, it is pos­sible to sep­ar­ate matter/antimatter pairs from the quantum vacu­um. In oth­er words, light can be trans­formed into mat­ter. How­ever, there is cur­rently no instru­ment that can deliv­er the neces­sary power. Plasma mir­rors, which are of major interest to the award-win­ning team, are a prom­ising way⁴ of com­press­ing the most intense lasers cur­rently avail­able (pet­awatt) in time and space, such as APOLLON, which is man­aged by the labor­at­ory for the use of intense lasers at the Insti­tut Poly­tech­nique de Par­is and Sor­bonne Uni­ver­sity. The aim is to test fun­da­ment­al the­or­ies in extreme con­di­tions nev­er before encountered.

Anaïs Marechal