How to prepare for the acceleration of quantum technologies

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

Read it here

Laurent Sanc­hez-Palen­cia and his team are inter­ested in under­stand­ing the organ­isa­tion of mat­ter at the quantum scale, where both wave and particle effects, such as inter­fer­ence and entan­gle­ment, are inter­woven, as well as strong inter­ac­tions between particles. Laurent Sanc­hez-Palen­cia is also involved in devel­op­ing new train­ing pro­grammes in quantum tech­no­lo­gies at the École Poly­tech­nique and the Insti­tut Poly­tech­nique de Paris.

His team is cur­rently work­ing on quantum sim­u­la­tion to mod­el and under­stand the beha­viour of ‘quasi-peri­od­ic’ mater­i­als at low tem­per­at­ures. These mater­i­als are too com­plex to be fully described at the atom­ic scale and could be bet­ter under­stood using quantum sim­u­la­tion. Oth­er examples include high-tem­per­at­ure super­con­duct­ors and quantum magnetism.

The the­or­et­ic­al res­ults obtained for min­im­al mod­els can be tested in real exper­i­ments in con­trol­lable sys­tems known as ‘optic­al lat­tices’. These are made up of atoms at extremely low tem­per­at­ures – on the order of a few tens of bil­lionths of a Kelvin – and held togeth­er by laser beams. When the right num­ber of laser beams point in the same dir­ec­tion on a plane, the res­ult is an exot­ic sys­tem, halfway between order and dis­order (ordered over a long-range but non-peri­od­ic­ally ordered), some­thing that is known as quasi-periodic.

Research­ers are study­ing what hap­pens when inter­ac­tions between atoms lead to the appear­ance of new quantum phases called Bose glasses. These glasses are a spe­cial type of insu­lat­or that, in the­ory, should only appear in struc­tures that are either dis­ordered or quasi-peri­od­ic. A stand­ard insu­lat­or has an energy gap between its fun­da­ment­al state and its first excited states. This means that only a power­ful elec­tric field can excite the charges and set them in motion. In a Bose glass, on the oth­er hand, there is no such energy gap, but the charges are con­fined to very loc­al­ised regions that do not allow for a cur­rent of particles to flow.

Bose glasses were first pre­dicted at the end of the 1980s, but have nev­er been observed unam­bigu­ously in an exper­i­ment, even in sys­tems of cold atoms. Indeed, cold atoms are nev­er per­fectly cold and, even at tem­per­at­ures as low as a few bil­lionths of a Kelvin, thermal fluc­tu­ations can des­troy quantum phases. How­ever, Laurent Sanc­hez-Palen­cia and col­leagues recently pre­dicted a situ­ation in which, des­pite these thermal fluc­tu­ations, a Bose glass could be sta­bil­ised and observed. They are cur­rently dis­cuss­ing with their exper­i­ment­al col­leagues how to design an exper­i­ment in which they could actu­ally observe these exot­ic glasses.

These sys­tems are fas­cin­at­ing in many ways. For example, they are often non-ergod­ic, as opposed to con­ven­tion­al ergod­ic. Ergod­ic sys­tems explore all the space at their dis­pos­al and can thus reach ther­mo­dy­nam­ic equi­lib­ri­um, a situ­ation well described by Boltzman­n’s the­ory, developed at the end of the 19^thCen­tury. Indeed, the beha­viour of ergod­ic sys­tems is com­pat­ible with most of the obser­va­tions made to date on objects ran­ging from the size of a micron to that of stars and galax­ies. This the­ory is based on the idea that the sys­tem fluc­tu­ates between all pos­sible states along paths that allow it to move very quickly from one state to anoth­er. In non-ergod­ic sys­tems, on the oth­er hand, this ergodi­city is pre­ven­ted by inhomo­gen­eit­ies in the sys­tem. The sys­tem remains there­fore trapped in a sub­set of its pos­sible con­fig­ur­a­tions, far from equilibrium.

Entan­gle­ment is anoth­er purely quantum phe­nomen­on of interest to the team. Here, two or more particles can have much stronger cor­rel­a­tions than clas­sic­al phys­ics allows. For example, the observ­able prop­er­ties of a quantum particle are gen­er­ally inde­term­in­ate, so that meas­ure­ment res­ults are ran­dom. How­ever, when particles are entangled, determ­in­ing the state of one particle instant­an­eously fixes the state of the other(s), regard­less of the dis­tance between them. This power­ful “spooky action at a dis­tance”, as Ein­stein called it, seems to tran­scend space and time, so that we can determ­ine the state of one particle simply by meas­ur­ing that of its entangled part­ner. For example, if you meas­ure the spin of one particle, say an elec­tron, you can determ­ine the spin of the oth­er without ever observing it.

We are only just begin­ning to exploit the applic­a­tions of this spec­tac­u­lar cor­rel­a­tion-at-a-dis­tance effect, even though it is already used in the cryp­to­graphy of cer­tain tele­com­mu­nic­a­tions. In simple terms, sup­pose that the sender and receiv­er share an entangled pair, so that the res­ults of their meas­ure­ments are ran­dom but identic­al. To inter­cept the com­mu­nic­a­tion, an eaves­drop­per has to per­form a meas­ure­ment, the res­ult of which is ran­dom but, more import­antly, changes the state of the pair, which is no longer entangled. The meas­ure­ments of the sender and receiv­er are no longer cor­rel­ated, and they can veri­fy this by com­par­ing the res­ults of their meas­ure­ments. The strength of such an encryp­tion meth­od is that it is not based on the dif­fi­culty of spy­ing without being detec­ted, but on an impossib­il­ity based on the fun­da­ment­al laws of the quantum world.

Anoth­er applic­a­tion of the ran­dom nature of the meas­ure­ment is the pos­sib­il­ity of mak­ing per­fect ran­dom num­ber gen­er­at­ors and totally ran­dom cryp­to­graph­ic keys.

These ran­dom num­ber gen­er­at­ors are already on the mar­ket and there is even at least one example of a mobile phone that uses quantum tech­no­logy of this type. Quantum entan­gle­ment can also be exploited in a pro­cess known as dense cod­ing, which is linked to the fact that an entangled state con­tains a phe­nom­en­al amount of inform­a­tion com­pared with the inform­a­tion car­ried by each indi­vidu­al particle.

Each particle con­tains inform­a­tion about its own state, but the inform­a­tion on cor­rel­a­tions is dis­trib­uted among all the infin­itely lar­ger sub­sets of particles. This effect makes it pos­sible to encode an immense quant­ity of inform­a­tion in struc­tures called quantum bits, or qubits. These dif­fer from stand­ard com­puter bits, which can take the value either 0 or 1. Qubits, how­ever, can take both val­ues at once, or any com­bin­a­tion of 0 and 1.

Qubits are the basic build­ing blocks of future quantum com­puters. Exploit­ing their quantum prop­er­ties, in par­tic­u­lar entan­gle­ment, can be applied to solve com­plex com­pu­ta­tion­al prob­lems, mak­ing it pos­sible to per­form cer­tain com­pu­ta­tion­al oper­a­tions much faster than the most power­ful com­puters avail­able today. This would res­ult in an expo­nen­tial increase in com­put­ing power. Qubits can be fab­ric­ated from a vari­ety of cur­rently avail­able mater­i­al plat­forms, such as super­con­duct­ing qubits or trapped atoms and ions. Oth­er future meth­ods include photon­ic quantum pro­cessors that use light.

Spec­tac­u­lar pro­gress has been made in recent years. How­ever, a real ‘quantum advant­age’ is not expec­ted until quantum com­puters are oper­at­ing with – depend­ing on the estim­ate – between a few hun­dred thou­sand and a few mil­lion qubits. We only know how to build machines with around a hun­dred qubits today, so there is still a long way to go.

We only know how to make machines with a hun­dred or so qubits today.

The main obstacle to pro­gress is quantum deco­her­ence. This res­ults from the inter­ac­tion of qubits with their envir­on­ment, which des­troys their entan­gle­ment. To avoid this – or at least lim­it it – it is gen­er­ally neces­sary to oper­ate qubits at a tem­per­at­ure close to 0 kelvin and pro­tect them from the envir­on­ment. From a fun­da­ment­al point of view, there is noth­ing to pre­vent the cre­ation of large-scale quantum com­puters, but there are still sci­entif­ic ques­tions to be resolved, such as wheth­er deco­her­ence can be fun­da­ment­ally countered. Import­ant engin­eer­ing prob­lems also need to be over­come. Pro­jects to solve these are being fun­ded by import­ant gov­ern­ment pro­grammes and private invest­ment around the world.

In the short­er term, hopes are being pinned on machines that are less sens­it­ive to deco­her­ence. These are quantum sim­u­lat­ors, which can be seen as ded­ic­ated quantum com­puters with an archi­tec­ture that optim­ises cer­tain spe­cif­ic tasks. Quantum sim­u­lat­ors are par­tic­u­larly well suited to the search for ‘min­ima of mul­tivari­able func­tions’. Such machines are there­fore of interest to com­pan­ies that use com­plex net­works and seek to optim­ise them. These net­works con­tain a large num­ber of vari­ables and a quantum sim­u­lat­or could optim­ise them in a way that a con­ven­tion­al com­puter cannot.

Most of the neces­sary tech­no­logy already exists, but the ques­tion remains as to which com­put­ing prob­lems quantum sim­u­la­tion could be of real eco­nom­ic interest. These tech­no­lo­gies are still very expens­ive and con­sume a lot of energy. The ques­tion is more open than we might think, but cur­rent advances are pav­ing the way for a host of rad­ic­ally new tech­no­lo­gies. The mes­sage that Laurent Sanc­hez-Palen­cia is try­ing to get across to his stu­dents is that we need to think not only about quantum com­puters, but also about all the asso­ci­ated tech­no­lo­gies. These may seem less spec­tac­u­lar at first glance, but they will very likely give rise to new technologies.

There is an urgent need for train­ing at all levels in the field of quantum mech­an­ics. For many years, phys­ics teach­ing at École poly­tech­nique has focused on this sub­ject. As a res­ult, all stu­dents, includ­ing those who do not intend to go on to study phys­ics or engin­eer­ing, are trained in this dis­cip­line dur­ing their first year of study. We are already reap­ing the rewards: many of the start-ups being developed in France today in the field of quantum mech­an­ics are run by former stu­dents of the École polytechnique.

In order to anti­cip­ate the devel­op­ment of quantum tech­no­lo­gies, the Quantum Sci­ence and Tech­no­logy course was cre­ated a few years ago: it focuses on the most mod­ern aspects of quantum phys­ics, in par­tic­u­lar entan­gle­ment and how this effect can be exploited. The course emphas­ises the link between fun­da­ment­al sci­ence and tech­no­lo­gic­al devel­op­ment, because at present, des­pite what we may read in the press, quantum phys­ics is still at the devel­op­ment stage, and there is still much to understand.

The Mas­ter­’s and PhD pro­grammes are run in close col­lab­or­a­tion with the oth­er schools on cam­pus, with­in the Insti­tut Poly­tech­nique de Par­is. They aim to recruit the best stu­dents from the best insti­tu­tions around the world. Stu­dents who join the insti­tute for a Mas­ter 1 dir­ectly join a research team at IP Par­is to rein­force links between train­ing and research. In addi­tion, IP Par­is recently star­ted offer­ing con­tinu­ing edu­ca­tion courses for pro­fes­sion­al engin­eers who have nev­er stud­ied quantum sci­ence. This train­ing is also import­ant for start-uppers who need very spe­cif­ic tech­nic­al skills to devel­op par­tic­u­lar devices and for man­agers of com­pan­ies, wheth­er they are SMEs or larger.

An import­ant aspect of the devel­op­ment of quantum research at IP Par­is is that it is being car­ried out hand in hand with the Uni­versité Par­is Saclay with­in the Insti­tut Quantum-Saclay. This allows the two insti­tu­tions to take advant­age of their com­ple­ment­ary strengths. At the nation­al level, a new con­sor­ti­um was cre­ated last year, fun­ded by the French Nation­al Research Agency.

   Isabelle Dumé