Quantum physics has already changed the world

This arti­cle is part of our spe­cial issue « Quan­tum: the sec­ond rev­o­lu­tion unfolds ».

Read it here

Quan­tum physics explains the behav­iour and inter­ac­tions between par­ti­cles, as well as the force fields that dri­ve them. Born over a cen­tu­ry ago, it is prob­a­bly also the least intu­itive of all the the­o­ries avail­able to sci­en­tists to describe and under­stand the world.

In the uni­verse of the infin­i­tes­i­mal, the most obvi­ous con­cepts of our every­day expe­ri­ence are shat­tered. A par­ti­cle, for exam­ple, has both par­ti­cle and wave prop­er­ties. Its loca­tion is not deter­mined by a pre­cise posi­tion but by a “cloud of prob­a­bil­i­ties” that makes it exist almost every­where at once, with greater or less­er chances of find­ing it if we final­ly try to observe it.

What’s more, the very con­cept of mea­sure­ment takes on a com­plete­ly dif­fer­ent mean­ing. In the quan­tum world, we can­not mea­sure a prop­er­ty of a par­ti­cle with infi­nite pre­ci­sion. Worse still, accord­ing to the prin­ci­ple of com­mu­ni­cat­ing ves­sels, the more pre­cise we are about cer­tain prop­er­ties (its posi­tion, for exam­ple), the less pre­cise we will be about oth­ers (its ener­gy, for exam­ple). And these lim­i­ta­tions do not come from our mea­sur­ing instru­ments: on the con­trary, they are fun­da­men­tal­ly inscribed in the rules that gov­ern the world of the infinitesimal.

Quan­tum mechan­ics also describes the exchange of ener­gy between particles.

Final­ly, and this is what gives it its name, quan­tum mechan­ics also describes the exchange of ener­gy between par­ti­cles. And unlike our clas­si­cal macro­scop­ic world, where the ener­gy of a ten­nis ball or a car can take on any val­ue, an elec­tron in an atom can only emit or absorb pre­cise­ly deter­mined quan­ti­ties of ener­gy. Each “pack­et” of ener­gy that the elec­tron absorbs or emits is called a “quan­tum” of ener­gy (hence the name “quan­tum” physics). These exchanges take place in suc­ces­sive bursts, rather than con­tin­u­ous­ly as we are used to on our own scale.

All these strange rules lead to sit­u­a­tions that may seem para­dox­i­cal, such as the fact that a quan­tum object can exist in sev­er­al states simul­ta­ne­ous­ly, or that two so-called “entan­gled” par­ti­cles are so fun­da­men­tal­ly linked that if you make a change to one, the oth­er will instant­ly suf­fer the con­se­quences, regard­less of the dis­tance sep­a­rat­ing them.

These bizarre sit­u­a­tions are observed every day in research lab­o­ra­to­ries around the world. And, far beyond the doors of research insti­tutes, these phe­nom­e­na are used to oper­ate the many devices we use every day.

One of the most aston­ish­ing dis­cov­er­ies of quan­tum physics is the famous “wave-par­ti­cle dual­i­ty”. In the 19th cen­tu­ry, numer­ous exper­i­ments had shown the wave-like nature of light, but it was not until 1905 that Albert Ein­stein demon­strat­ed a so-called “pho­to­elec­tric” effect, which proved that light could strike elec­trons and eject them like boc­ce balls. It was not until 20 years lat­er that the French physi­cist Louis de Broglie realised that, far from being a prob­lem, light (and any mate­r­i­al par­ti­cle) behaves like both a wave and a par­ti­cle. This dis­cov­ery led to a num­ber of every­day appli­ca­tions, such as pho­to­volta­ic pan­els and the CCD sen­sors in our cameras.

Sim­i­lar­ly, the quan­tifi­ca­tion of ener­gy exchanges between elec­trons in mat­ter has led to sev­er­al fun­da­men­tal inno­va­tions with­out which mod­ern tech­nol­o­gy would not exist.

Let’s start with the laser, which is used in CD play­ers, in indus­try to cut mate­ri­als, in astron­o­my to mea­sure the dis­tance between the Earth and the Moon, in med­i­cine to cut or cau­terise tis­sue, in super­mar­kets to read bar­codes, in laser print­ers and in opti­cal fibres to com­mu­ni­cate from one con­ti­nent to another.

This very spe­cial light, made up of iden­ti­cal pho­tons (the name giv­en to the par­ti­cles of light), is pro­duced by forc­ing the atoms to all emit the same quan­ta of ener­gy. The result is a spe­cial kind of light that we would be hard pressed to do with­out today.

Anoth­er appli­ca­tion of quan­tum the­o­ry is noth­ing less than… all mod­ern elec­tron­ics! This tech­nol­o­gy, found in our mobile phones, watch­es, vehi­cles, com­put­ers, med­ical devices (pace­mak­ers, bath­room scales, blood pres­sure mon­i­tors, car­diac defib­ril­la­tors) and an infi­nite num­ber of oth­er every­day appli­ca­tions, works thanks to an under­stand­ing of the behav­iour of elec­trons in a cat­e­go­ry of mate­ri­als called “semi­con­duc­tors” – i.e. mate­ri­als that are nat­u­ral­ly insu­lat­ing but can eas­i­ly become con­duc­tive if a small elec­tri­cal volt­age is applied to them. This prop­er­ty, which allows the pas­sage (or not) of an elec­tric cur­rent to be con­trolled at will, is used to build the diodes and tran­sis­tors that are the basic build­ing blocks of all electronics.

And when you mix con­trolled light emis­sion and semi­con­duc­tors, you build LEDs (light-emit­ting diodes), which are cur­rent­ly replac­ing a large pro­por­tion of the old, much more ener­gy-con­sum­ing light bulbs.

Quan­tum mechan­ics is all around us: in the tech­no­log­i­cal appli­ca­tions we have devel­oped, but also in all the nat­ur­al phe­nom­e­na that sur­round us and that we can­not under­stand with­out using quan­tum theory.

If the Sun shines, it’s because of the nuclear fusion that takes place in its core, which in turn is made pos­si­ble by anoth­er quan­tum quirk: the tun­nel effect, which allows par­ti­cles to ‘jump’ poten­tial bar­ri­ers that would oth­er­wise be impass­able in the clas­si­cal world. As for the blue of the sky, this is due to the way in which sun­light inter­acts with the mol­e­cules in the Earth­’s atmosphere.

Even pho­to­syn­the­sis (the process by which plants trans­form the ener­gy they receive from the Sun into organ­ic mat­ter, which is then absorbed by her­bi­vores which are in turn con­sumed by car­ni­vores) is sus­pect­ed, in the most recent research, of owing its exis­tence to quan­tum phe­nom­e­na, the mys­tery of which biol­o­gy has yet to unravel.

Quan­tum physics has rev­o­lu­tionised the way humans under­stand and shape the world. But since the end of the twen­ti­eth cen­tu­ry, a “sec­ond quan­tum rev­o­lu­tion” has been under­way, in which the most fun­da­men­tal process­es of quan­tum mechan­ics are being exploit­ed to take our tech­nolo­gies to a new level.