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.