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