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Landry Bretheau
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Quantum computers: how do they work?

Landry Bretheau, Lecturer in quantum physics at École Polytechnique in the Laboratory of Condensed Matter for Physics (PMC*)

Uni­ver­si­ties and lab­o­ra­to­ries around the world, as well as the big tech com­pa­nies, Google, IBM, Intel and Microsoft, are study­ing and devel­op­ing quan­tum com­put­ers at break­neck speed. The stakes are high. But what exact­ly is this technology?

A quan­tum com­put­er is not real­ly a ‘com­put­er’ as such, but rather a super-cal­cu­la­tor, capa­ble of run­ning spe­cif­ic pow­er­ful quan­tum algo­rithms much faster than an ordi­nary proces­sor. It does this using the prin­ci­ples of quan­tum mechan­ics, which are respon­si­ble for the behav­iour of ele­men­tary par­ti­cles such as pho­tons, elec­trons and atoms, but also of larg­er sys­tems such as super­con­duct­ing circuits.

Such sys­tems allow for the imple­men­ta­tion of quan­tum bits (‘qubits’): two-state quan­tum sys­tems that rep­re­sent the fun­da­men­tal com­pu­ta­tion­al bricks of quan­tum information.

Unlike con­ven­tion­al com­put­ers, which encode infor­ma­tion in bina­ry form, qubits are not lim­it­ed to ‘0’ and ‘1’ but can be in any com­bi­na­tion (or ‘super­po­si­tion’) of the two. This abil­i­ty, com­bined with the fact that N qubits can be com­bined or ‘entan­gled’ to rep­re­sent 2N states simul­ta­ne­ous­ly, allows cal­cu­la­tions to be per­formed in par­al­lel on a mas­sive scale. A quan­tum com­put­er could there­fore, in prin­ci­ple, out­per­form a clas­si­cal com­put­er for some impor­tant tasks, such as sort­ing large unsort­ed lists or for ‘prime fac­tori­sa­tion’. The lat­ter forms the basis of most encryp­tion algo­rithms in use today – in par­tic­u­lar for bank­ing operations.

Quan­tum com­put­ers don’t look any­thing like their clas­si­cal coun­ter­parts either. Cur­rent mod­els look more like large cans that are sus­pend­ed from the ceil­ing, cooled to near absolute zero (-273.14 °C), with hun­dreds of cables hang­ing from them.

The bane of quantum computers: decoherence

Any­one wish­ing to build a quan­tum com­put­er today must first over­come a big prob­lem: the fact that qubits are extreme­ly frag­ile. Almost any inter­ac­tion with exter­nal ‘noise’ in their envi­ron­ment can cause them to col­lapse like a souf­flé and lose their quan­tum nature in a destruc­tive process known as deco­her­ence. If this hap­pens before an algo­rithm has fin­ished run­ning, the result is a com­plete mess (and not the result of a cal­cu­la­tion) because any infor­ma­tion stored in the qubit is lost (imag­ine a com­put­er that has to restart every sec­ond). And, the more qubits a quan­tum com­put­er has, the more dif­fi­cult it is to keep them coher­ent.  So much so that even the most advanced quan­tum proces­sors today strug­gle to sur­pass 60 phys­i­cal qubits. A real device would require sev­er­al thou­sands, however.

Because of this prob­lem, a quan­tum com­put­er must be well iso­lat­ed from its envi­ron­ment. This requires very strin­gent con­di­tions: sim­ple and very cold sys­tems, pro­tect­ed from the out­side world. Such extreme con­fine­ment cre­ates a para­dox­i­cal sit­u­a­tion, how­ev­er, because the more iso­lat­ed the com­put­er is, the more dif­fi­cult it is for us to actu­al­ly com­mu­ni­cate with it (to access the results of its cal­cu­la­tions) and con­trol what it does.

Tackling decoherence

In recent years, qubits have been made from a num­ber of iso­lat­ed sys­tems that remain coher­ent while algo­rithms are being run. These include trapped ions (atoms with an elec­tron removed or added), ultra-cold ‘Ryd­berg’ atoms, and pho­tons (light particles).

A trapped-ion based com­put­er stores infor­ma­tion in the ener­gy lev­els of indi­vid­ual ions. This is how qubits are formed in this sys­tem. Infor­ma­tion is shared between these qubits, and laser puls­es are then used to manip­u­late their states and cre­ate entan­gle­ment between them. This tech­nol­o­gy is quite advanced and researchers have recent­ly suc­ceed­ed in cre­at­ing a ful­ly entan­gled ‘24-qubit-GHZ’ (GHZ for ‘Green­berg­er-Horne-Zeilinger’) state using cal­ci­um ions1.  

Oth­er sys­tems are based on sol­id-state mate­ri­als that might be inte­grat­ed into tra­di­tion­al elec­tron­ic devices. These micron-sized struc­tures are small on every­day scales, but large com­pared with atoms and can behave like quan­tum par­ti­cles, such as elec­trons or atoms. Such ‘arti­fi­cial quan­tum objects’ include ‘quan­tum dots’ (tiny pieces of semi­con­duct­ing mate­r­i­al), super­con­duct­ing cir­cuits, and dia­monds con­tain­ing spe­cial type of defects called ‘nitro­gen vacan­cies’. A quan­tum com­put­er based on super­con­duct­ing qubits, for exam­ple, is cooled down to mil­likelvin tem­per­a­tures (which is cold­er than inter­stel­lar space) and is con­trolled using microwaves2.

Researchers are also try­ing to work out which of these sys­tems would make the best qubits. An impor­tant para­me­ter is, of course, a qubit’s resis­tance to deco­her­ence, which can be assessed in terms of the ‘fideli­ty’ of a quan­tum oper­a­tion. While the fideli­ty does not have to be a per­fect 100%, any­thing low­er will even­tu­al­ly lead to errors after mul­ti­ple oper­a­tions; most of today’s quan­tum com­put­ers are very sus­cep­ti­ble to errors.

Although ‘quan­tum error cor­rec­tion’ pro­to­cols can mit­i­gate deco­her­ence, these are cost­ly from a hard­ware point of view and any prac­ti­cal sys­tem must have a suf­fi­cient­ly high fideli­ty to begin with. Researchers are mak­ing progress in this area, how­ev­er, and recent work has shown that a two-qubit gate can be made from two sil­i­con quan­tum dots3. This gate can achieve 98% fideli­ty for the CROT oper­a­tion, which an essen­tial com­po­nent of a quan­tum computer.

What can quantum computers realistically achieve today and what lies ahead?

The idea of a quan­tum com­put­er was first put for­ward by the late physi­cist and Nobel lau­re­ate Richard Feyn­man in the 1980s to sim­u­late the com­pli­cat­ed equa­tions of quan­tum mechan­ics, which take too long to solve on a clas­si­cal com­put­er. Today, appli­ca­tion areas are much more var­ied: cryp­tog­ra­phy; sim­u­lat­ing the prop­er­ties of mate­ri­als (with a view to improv­ing them); solv­ing dif­fer­en­tial equa­tions extreme­ly quick­ly; and opti­miz­ing machine learn­ing. Progress has been impres­sive and researchers have gone from being able to entan­gle just three qubits to more than 50 qubits in recent years, with error rates of 1 in 10004.

Although it is dif­fi­cult to pre­dict what the future holds, a ful­ly func­tion­al, com­mer­cial­ly viable quan­tum com­put­er is unlike­ly to see the day of light any­time soon. The same goes for any kind of ‘per­son­al quan­tum com­put­er’. A quan­tum device will more like­ly be used for fun­da­men­tal research, R&D or gov­ern­ment and mil­i­tary pur­pos­es to begin with.

Today, researchers in this field are not only advanc­ing hard­ware (i.e. the machines them­selves), but are also devel­op­ing inno­v­a­tive soft­ware with new types of algo­rithms espe­cial­ly adapt­ed to quan­tum computing.

While quan­tum com­put­ing relies on the prin­ci­ples of fun­da­men­tal physics, it is a tremen­dous oppor­tu­ni­ty for sci­en­tists from many fields – includ­ing com­put­er sci­ence, math­e­mat­ics, mate­ri­als sci­ence and engi­neer­ing – to work togeth­er. The road to a real-world quan­tum com­put­er will cer­tain­ly be long, but there will be many excit­ing dis­cov­er­ies along the way. 

Interview by Isabelle Dumé

  

1http://​dx​.doi​.org/​1​0​.​1​1​0​3​/​P​R​X​Q​u​a​n​t​u​m​.​2​.​0​20343
2https://www.nature.com/articles/s41586-021–03268‑x
3https://www.nature.com/articles/s41586-019‑1197‑0
4https://www.nature.com/articles/s41586-019‑1666‑5

Contributors

Landry Bretheau
Landry Bretheau
Lecturer in quantum physics at École Polytechnique in the Laboratory of Condensed Matter for Physics (PMC*)

Landry Bretheau graduated from Ecole polytechnique in 2005 and then completed his Ph.D. at CEA Saclay. Next, he conducted two successive post-docs at ENS (France) and MIT (USA). Since 2017, he has been building-up a new laboratory – QCMX Lab – together with his colleague Jean-Damien Pillet, which explores the physics of Hybrid Quantum Circuits. To develop this new activity, Landry Bretheau was awarded a Young Team Fellowship from l’X, a Young Researcher Grant from the French National Research Agency and an ERC Starting Grant from the European Research Council. His work has led to major contributions in the fields of Mesoscopic Superconductivity and Quantum Circuits and was awarded the X Thesis Award and the 2020 Nicholas Kurti Science Prize. *PMC: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris