Quantum physics explained by a Nobel Prize winner

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

Read it here

Quan­tum physics is the set of phys­i­cal laws that gov­ern the behav­iour of the world at the lev­el of elec­trons, atoms, mol­e­cules and crys­tals. The laws of New­ton­ian mechan­ics that we are famil­iar with on our own scale are no longer valid at the nanome­tre scale (one bil­lionth of a metre), which cor­re­sponds rough­ly to the size of an atom. Quan­tum physics began to be devel­oped in the first quar­ter of the 20^th Cen­tu­ry with Planck and Ein­stein, and from 1925 the great physi­cists Heisen­berg, Schrödinger and Dirac devel­oped a math­e­mat­i­cal for­mal­ism that has been in use ever since.

Quan­tum physics is essen­tial, for exam­ple, to explain why mat­ter is sta­ble. Since the end of the 19^th Cen­tu­ry, we have known that mat­ter is made up of pos­i­tive and neg­a­tive charges, and that these pos­i­tive and neg­a­tive charges attract each oth­er. Mat­ter should there­fore col­lapse in on itself. This is not the case thanks to the quan­tum behav­iour of the elec­tron, which is not just a par­ti­cle, but also a wave. When you try to con­fine an elec­tron, you are forced to con­sid­er an ever-small­er wave­length and there­fore ever-greater ener­gy. As this ener­gy is unavail­able, the elec­tron can­not be con­fined to a dimen­sion small­er than the size of the atom. Quan­tum physics can also be used to under­stand the chem­i­cal bonds between atoms.

Its for­mal­ism makes it pos­si­ble to describe the elec­tric cur­rent in mate­ri­als at the micro­scop­ic lev­el too, some­thing that has enabled physi­cists to invent and man­u­fac­ture tran­sis­tors and inte­grat­ed cir­cuits, which are the basis of com­put­ers. It also allows us to under­stand how pho­tons (par­ti­cles of light) are absorbed or emit­ted by mat­ter, which was essen­tial for invent­ing the laser.

The con­cept of the quan­tum com­put­er emerged over the last two decades or so and was trig­gered by sev­er­al exper­i­men­tal break­throughs made from the 1970s onwards: the first was that we learned to observe and con­trol indi­vid­ual micro­scop­ic objects. Pre­vi­ous­ly, we could only manip­u­late large ensem­bles of par­ti­cles. Today, we can trap an elec­tron or an atom and observe and con­trol it. We can also emit a sin­gle pho­ton and make use of it.

The sec­ond series of advances is linked to quan­tum entan­gle­ment, described for the first time in the 1935 arti­cle by Ein­stein, Podol­sky and Rosen, and which is only con­ceiv­able with­in the frame­work of quan­tum physics.

Entan­gle­ment occurs when two par­ti­cles, hav­ing inter­act­ed in the past and then sep­a­rat­ed in space, form an insep­a­ra­ble quan­tum whole that con­tains more infor­ma­tion than that con­tained in the sum of the infor­ma­tion of each par­ti­cle. It is this prop­er­ty that opens the door to quan­tum com­put­ing: if instead of hav­ing just two entan­gled quan­tum bits in which to encode quan­tum infor­ma­tion, you have three, four, five, 10 or 100, the addi­tion­al quan­ti­ty of infor­ma­tion, com­pared with a clas­si­cal mem­o­ry, con­tained in these par­ti­cles is gigan­tic because it grows exponentially.

Today, how­ev­er, we are still a long way from a per­fect quan­tum com­put­er because the quan­tum bits (qubits) we have are not sta­ble and under­go what is known as “deco­her­ence” when they inter­act with their envi­ron­ment. This means that after a cer­tain time they behave like clas­si­cal objects and lose the quan­tum infor­ma­tion they con­tain. Deco­her­ence is an obsta­cle to the cre­ation of a quan­tum com­put­er and will require a major tech­no­log­i­cal effort to solve. But there is noth­ing to pre­vent us from over­com­ing the dif­fi­cul­ty faster than expect­ed. For exam­ple, we could find a sub­space of quan­tum states pro­tect­ed from deco­her­ence. If that were the case, we could see a quan­tum com­put­er in my lifetime.

I’m con­vinced that soon­er or lat­er an ide­al quan­tum com­put­er that works per­fect­ly will exist, because in my expe­ri­ence, when some­thing that seems fea­si­ble is not for­bid­den by the fun­da­men­tal laws of physics, engi­neers even­tu­al­ly man­age to find a way of mak­ing it hap­pen. Real­is­ti­cal­ly, though, I’d be sur­prised if this hap­pened in the near future.

A quan­tum inter­net, or if we want to be more pre­cise, a quan­tum net­work, would make use of two or more quan­tum com­put­ers com­mu­ni­cat­ing with each oth­er by send­ing quan­tum infor­ma­tion direct­ly from the quan­tum state, with­out hav­ing to pass through an inter­me­di­ate clas­si­cal state. This would enable a very large amount of infor­ma­tion to be trans­mit­ted. This can be achieved by a process known as quan­tum tele­por­ta­tion, which has already been demon­strat­ed for indi­vid­ual par­ti­cles and small ensem­bles of par­ti­cles, but over dis­tances of no more than a few tens of kilometres.

If the deco­her­ence prob­lem is not solved in the near term, a class of “degrad­ed” quan­tum com­put­ers will prob­a­bly appear first. These inter­me­di­ate-scale machines will be much more effi­cient than a con­ven­tion­al com­put­er for cer­tain tasks, such as opti­mi­sa­tion prob­lems (the famous trav­el­ling sales­man prob­lem, for exam­ple, or the opti­mi­sa­tion of elec­tric­i­ty networks).

Today we often hear talk about “quan­tum 2.0”, but I pre­fer to call it “the sec­ond quan­tum rev­o­lu­tion”, because it is a rad­i­cal rev­o­lu­tion. The first quan­tum rev­o­lu­tion was pri­mar­i­ly con­cep­tu­al and sci­en­tif­ic, the imple­men­ta­tion of a new math­e­mat­i­cal for­mal­ism to describe wave-par­ti­cle dual­i­ty. It led to a much bet­ter under­stand­ing of the phys­i­cal world, and to appli­ca­tions that have rev­o­lu­tionised soci­ety. The sec­ond rev­o­lu­tion is based on two new con­cepts: our abil­i­ty to iso­late and con­trol indi­vid­ual quan­tum objects; and the pos­si­bil­i­ty of entan­gling these objects and exploit­ing this entan­gle­ment in real applications.

Will these new quan­tum tech­nolo­gies rev­o­lu­tionise our soci­ety in the same way as the tran­sis­tor and the laser? It’s too ear­ly to say, but I think it’s impor­tant for com­pa­nies to invest in these tech­nolo­gies, because if they real­ly do bring the rev­o­lu­tion­ary advances we’re expect­ing, those who don’t invest will be out of the game. It will be impor­tant to have in-house experts in quan­tum physics capa­ble of rapid­ly exploit­ing these advances. Today, there is a short­age of peo­ple with skills in quan­tum physics, and I think that entre­pre­neurs need to join forces with uni­ver­si­ties to deal with this prob­lem more effectively.

At Paris-Saclay, for exam­ple, we have a pro­gramme called ARTeQ, with which Ecole Poly­tech­nique (IP Paris) is asso­ci­at­ed. Sev­er­al indus­tri­al­ists pro­vide finan­cial sup­port. ARTeQ enables stu­dents in the sci­ences, but not nec­es­sar­i­ly in the quan­tum sci­ences, to acquire a good ground­ing in quan­tum cul­ture so that they can apply the knowl­edge they acquire in their future careers.

We need quan­tum researchers, engi­neers and tech­ni­cians. So, my mes­sage is: invest in research, tech­nol­o­gy and training.

Interview by Isabelle Dumé