Représentation abstraite d’ordinateur quantique. Futuriste, composants quantique.
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How quantum technology is changing the world

Quantum computing: 15 minutes to understand everything

Loïc Henriet, CTO at Pasqal and Landry Bretheau, Professor at Ecole Polytechnique, Quantum Physicist and Researcher in the Laboratory of Condensed Matter Physics (PMC*)
On September 13th, 2023 |
5 min reading time
Loic Henriet
Loïc Henriet
CTO at Pasqal
Landry Bretheau
Landry Bretheau
Professor at Ecole Polytechnique, Quantum Physicist and Researcher in the Laboratory of Condensed Matter Physics (PMC*)
Key takeaways
  • Unlike conventional computers, qubits can represent both 0 and 1. They perform several calculations at the same time thanks to their superimposed state, speeding up the resolution of complex problems.
  • At present, a quantum processor is still at the exploratory stage: it takes up a lot of space and the sophisticated optics needed to control the qubits consist of lasers, lenses and mirrors.
  • For a quantum computer to work, it must be able to correct the errors caused by the imperfect nature of current hardware, which prevent the final result of the calculation from being achieved.
  • The quantum computer will not replace the personal computer or the smartphone, and its first customers will certainly be governments and large companies rather than the general public.

Quan­tum com­put­ers work with quan­tum bits, or qubits, which, unlike stan­dard com­put­er bits that can have a val­ue of either 0 or 1, can be in a super­po­si­tion of both 0 and 1. This char­ac­ter­is­tic means that quan­tum com­put­ers could be much faster than con­ven­tion­al com­put­ers for cer­tain tasks. They could also be used to solve cer­tain prob­lems that a con­ven­tion­al com­put­er cannot.

“A quan­tum com­put­er will manip­u­late many qubits in a mas­sive­ly super­posed state: 0000 plus 1111, for exam­ple, » explains Landry Bretheau. « In this ‘entan­gled’ state, sev­er­al cal­cu­la­tions can be car­ried out in par­al­lel. A con­crete exam­ple: imag­ine that the cal­cu­la­tion, the prob­lem, is to get out of a maze. How would you go about that? A human being or a com­put­er pro­gramme will try out dif­fer­ent pos­si­ble tra­jec­to­ries one by one. Each time, a dead end is reached, they will then retrace their steps. In this way, they will test all the paths until they emerge from the maze. Since a quan­tum sys­tem can be in a super­po­si­tion of states, that is, it can be in sev­er­al places at the same time, it can there­fore try to explore all the dif­fer­ent paths in par­al­lel and escape from the maze more quickly. »

Qubits can be built from dif­fer­ent plat­forms or mate­r­i­al build­ing blocks, such as super­con­duct­ing qubits, ele­men­tary par­ti­cles or trapped ions. Oth­er meth­ods in the pipeline are pho­ton­ic quan­tum proces­sors that use light. « We use the term quan­tum com­put­er, but it would be more accu­rate to talk about quan­tum proces­sor, because the entire cal­cu­la­tion can­not be imple­ment­ed on a quan­tum com­put­er, only a frac­tion of it, » explains Loïc Hen­ri­et. « We will always need a clas­si­cal proces­sor to coor­di­nate all the steps of the calculation. »

At present, a quan­tum proces­sor is still in the exper­i­men­tal stage: for one, it takes up a lot of space – for exam­ple, the one Löic Hen­ri­et’s team is work­ing on sits in a large box mea­sur­ing 3 metres by 2 metres by 2 metres. A very high vac­u­um, of 10-11 mbar, is also required to place the qubits in well-defined posi­tions. This cor­re­sponds to around the same pres­sure as can be found on the sur­face of the Moon.

The sophis­ti­cat­ed optics need­ed to con­trol the qubits con­sist of lasers, lens­es and mir­rors. To coor­di­nate the oper­a­tion of each of these dif­fer­ent pieces of equip­ment (which make up the hard­ware) and to syn­chro­nise them, on-board soft­ware is need­ed. This soft­ware is the quan­tum proces­sor’s oper­at­ing system.

Potential applications

There are many areas in which a quan­tum com­put­er could prove more use­ful than a con­ven­tion­al com­put­er, either in terms of com­put­ing time or the qual­i­ty of the results obtained. The best-known exam­ple is Shor’s algo­rithm, which enables large num­bers to be effi­cient­ly fac­torised into prime fac­tors, for appli­ca­tions in cryp­tog­ra­phy and com­put­er secu­ri­ty, for exam­ple. Quan­tum com­put­ers will also be very good at mak­ing use of spe­cial algo­rithms to solve com­plex opti­mi­sa­tion prob­lems, such as those linked to sched­ul­ing, rout­ing and logis­tics. These prob­lems involve find­ing the opti­mum solu­tion from a large num­ber of pos­si­bil­i­ties – the most famous being the « trav­el­ling sales­man » prob­lem, which involves find­ing the short­est pos­si­ble route between sev­er­al cities. Deliv­ery and logis­tics com­pa­nies will cer­tain­ly want to adopt quan­tum tech­nol­o­gy for this type of application.

Prob­lems linked to the reac­tiv­i­ty of mol­e­cules will also ben­e­fit. « There is a lot of research going on in this area, » explains Löic Hen­ri­et. « With a quan­tum proces­sor, we will be able to car­ry out much more effi­cient cal­cu­la­tions to deter­mine the reac­tiv­i­ty of cer­tain pro­teins, for exam­ple, which will have enor­mous appli­ca­tions for the phar­ma­ceu­ti­cal indus­try and the syn­the­sis of new drugs. We will also be able to cal­cu­late the prop­er­ties of new mate­ri­als of inter­est for many tech­no­log­i­cal fields. »

Machine learn­ing and arti­fi­cial intel­li­gence are also impor­tant appli­ca­tion areas since quan­tum com­put­ers should be able to improve machine learn­ing algo­rithms – poten­tial­ly dra­mat­i­cal­ly – by pro­vid­ing faster and more effi­cient opti­mi­sa­tion rou­tines or by explor­ing new mod­els and archi­tec­tures. This could be a mas­sive new mar­ket, but it will depend on build­ing prac­ti­cal quan­tum com­put­ers on a large scale and devel­op­ing algo­rithms and appli­ca­tions that can take advan­tage of their unique capabilities.

Towards universality

For a quan­tum com­put­er to work, it must be « uni­ver­sal », that is, it must be capa­ble of cor­rect­ing the errors caused by the imper­fect nature of cur­rent hard­ware, which pre­vents a cal­cu­la­tion from being com­put­ed com­plete­ly. The main cause of these errors is the deco­her­ence of the qubits them­selves, which destroys the quan­tum char­ac­ter of the qubits and returns them to the state of clas­si­cal bits. Deco­her­ence is caused by the inter­ac­tion of the qubits with their envi­ron­ment. The real chal­lenge is there­fore to iso­late the sys­tem effec­tive­ly. To do this, the qubits gen­er­al­ly have to oper­ate at a tem­per­a­ture close to 0 K and be shield­ed from each oth­er and the envi­ron­ment. In addi­tion to this, quan­tum error cor­rec­tion (QEC) tech­niques can be used to achieve « fault-tol­er­ant quan­tum com­put­ing ». These tech­niques involve using a large num­ber of qubits to cre­ate a « log­i­cal qubit » that is much less prone to errors. 

Accord­ing to experts, a real quan­tum « advan­tage » or « suprema­cy » will only be achieved when quan­tum com­put­ers oper­ate with a mil­lion qubits. And since the cur­rent record is still less than 100 qubits, there is still a long way to go.

Major challenges

While, in the­o­ry, there is noth­ing pre­vent­ing the cre­ation of large-scale quan­tum com­put­ers, a num­ber of major engi­neer­ing prob­lems need to be resolved first. « Com­pa­nies see quan­tum com­put­ing as a strate­gic invest­ment and don’t want to miss out, » explains Löic Hen­ri­et. « It’s no longer a ques­tion of if, but rather a ques­tion of when the quan­tum proces­sor will become an inte­gral part of IT solutions.

We are cur­rent­ly at the cusp of a tech­no­log­i­cal rev­o­lu­tion and, in France, we have the resources to be at the heart of this, both at aca­d­e­m­ic lev­el and at the lev­el of com­pa­nies and start-ups. Of course, end-cus­tomers and busi­ness­es also need to be on board.

Over the last five years or so, we’ve seen a real increase in interest

That said, a quan­tum com­put­er, uni­ver­sal or not, will not replace your per­son­al com­put­er or smart­phone any time soon, and the first cus­tomers will cer­tain­ly be gov­ern­ments and major cor­po­ra­tions rather than the gen­er­al pub­lic, adds Landry Bretheau. « Sci­en­tists them­selves will also be the first users, which is why the world of quan­tum com­put­ing is of inter­est to so many dis­ci­plines: chem­istry, mate­ri­als sci­ence, biol­o­gy and physics. Each of these dis­ci­plines will devel­op its own algo­rithms to solve very spe­cif­ic problems.

« Over the last five years or so, we’ve seen a real increase in inter­est, » he says. “Some are call­ing it a ‘quan­tum boom’, with the cre­ation of numer­ous start-ups and major fund-rais­ing campaigns.”

Psi­Quan­tum and IonQ, which have raised €600 mil­lion and €400 mil­lion respec­tive­ly, are two out­stand­ing exam­ples. « In France, we have the Quan­tum Plan, announced by Pres­i­dent Macron in ear­ly 2021, and the start-up with the most wind in its sails from a finan­cial point of view in France at the moment is Pasqal, which has just raised €100 million.

Although we prob­a­bly won’t suc­ceed in build­ing a ful­ly oper­a­tional and fault-tol­er­ant com­put­er in the next ten years, we can be sure that we will make unthought-of dis­cov­er­ies along the way that will be poten­tial­ly use­ful and per­haps even change the face of soci­ety, just as clas­si­cal com­put­ing has done over the last 50 years and, more recent­ly, the Internet.

« It’s a great time to be work­ing in this field, » says Landry Bretheau. « There’s a lot of excite­ment and the sec­tor is evolv­ing very quickly.”

Isabelle Dumé

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