A quan­tum in­ter­net may be only 10 years away, which raises an im­por­tant ques­tion: what is a quan­tum in­ter­net? MICHAEL LUCY in­ves­ti­gates.

Cosmos - - Front Page - MICHAEL LUCY is fea­tures editor of Cos­mos. IM­AGES 01 Xin­hua / Jin Li­wang / MCG 02 VCG / Getty Im­ages 03 Stu­art Hay / ANU

FOR A FEW MIN­UTES each night in cer­tain parts of China, the bright­est light in the sky is the lurid green glow of the Mi­cius satel­lite, shoot­ing a green laser down to Earth as it swings through space 500 kilo­me­tres above. When con­di­tions are right, you might also see a red beam lanc­ing back through the dark­ness from one of the ground sta­tions that send sig­nals in re­ply.

MI­CIUS IS NOT YOUR av­er­age telecom­mu­ni­ca­tions satel­lite. On 29 Septem­ber 2017, it made his­tory by ac­com­plish­ing an as­ton­ish­ing feat, har­ness­ing the mys­te­ri­ous qual­i­ties of quan­tum en­tan­gle­ment – what Ein­stein called ‘spooky ac­tion at a dis­tance’ – to ‘tele­port’ in­for­ma­tion into space and back again. In do­ing so, it en­abled the first in­ter­con­ti­nen­tal phone call – a video call, in fact, be­tween Bei­jing and Vi­enna – that was com­pletely un­hack­able.

The weird science of quan­tum physics that pow­ers Mi­cius is at the heart of a tech­nol­ogy arms race. On one side are quan­tum com­put­ers, still in their in­fancy but with enor­mous po­ten­tial once they grow in power. Among their most prized, and feared, ap­pli­ca­tions is the ca­pac­ity to cut through the com­plex math­e­mat­i­cal locks that now se­cure com­puter en­cryp­tion sys­tems – the ones that mean you can con­fi­dently con­duct fi­nan­cial trans­ac­tions over the in­ter­net. On the other side is the only sure de­fence – en­cryp­tion tech­niques that also rely on the laws of quan­tum physics.

Un­til re­cently sci­en­tists had man­aged to make quan­tum en­cryp­tion work only across dis­tances of a hun­dred kilo­me­tres or so. The Chi­nese sci­en­tists be­hind Mi­cius have now reached around the world. It brings the ul­ti­mate prize tan­ta­lis­ingly closer. “I en­vi­sion a space-ground in­te­grated quan­tum in­ter­net,” says Pan Jian­wei, whose team be­came fron­trun­ners in the quan­tum com­mu­ni­ca­tions race af­ter Mi­cius switched on.

That quan­tum in­ter­net will be both un­ques­tion­ably se­cure and dis­con­cert­ingly strange, open­ing new win­dows for science and com­put­ing.

PAN JIAN­WEI IS used to think­ing small. The Chi­nese physi­cist made his name with ground­break­ing ex­plo­rations of quan­tum en­tan­gle­ment, that cu­ri­ous kind of telepa­thy be­tween sub­atomic par­ti­cles that Ein­stein fa­mously de­rided.

At the same time Pan thinks very big. He has led China’s mas­sive quan­tum tech­nol­ogy pro­gram for more than a decade.

Af­ter Mi­cius launched from Ji­uquan space­port on the re­mote plains of In­ner Mon­go­lia in Au­gust 2016, it be­gan to per­form a se­ries of ex­per­i­ments that steadily es­ca­lated in com­plex­ity. At their core was a crys­tal­based gad­get that pro­duces pairs of en­tan­gled pho­tons and sends them via tightly fo­cused laser beams to re­ceiv­ing sta­tions on the ground.

Pan’s team first es­tab­lished long-range en­tan­gled con­nec­tions be­tween ground sta­tions in­side China. Then they suc­ceeded in trans­mit­ting the quan­tum state of a par­ti­cle – so-called quan­tum tele­por­ta­tion, which will be a vi­tal tech­nique for quan­tum com­put­ers to com­mu­ni­cate. An ex­tra­or­di­nary year was capped with the un­hack­able in­ter­na­tional video­con­fer­ence, in which dig­ni­taries from the Chi­nese and Aus­trian acad­e­mies of science ex­changed con­grat­u­la­tions.

Pan has no short­age of re­sources at his dis­posal. Quan­tum tech­nol­ogy is a key re­search pri­or­ity for the Chi­nese gov­ern­ment, as for many oth­ers.

The best es­ti­mate of the scale of global ef­forts comes from con­sult­ing firm Mck­in­sey & Com­pany. It re­ported in 2015 that about 7,000 re­searchers world­wide were work­ing in the field, with about US$1.5 bil­lion a year be­ing spent. Those num­bers are un­doubt­edly big­ger now, and will only grow as gov­ern­ments and cor­po­ra­tions chase the ad­van­tages of quan­tum tech­nol­ogy.

High on their list of mo­ti­va­tions: pro­tect­ing se­crets. “Se­cu­rity is the big sell­ing point,” says Jacq Romero, a pho­ton­ics ex­pert at the Univer­sity of Queens­land (see p.130). A quan­tum net­work could also be used to re­alise more ex­otic pro­pos­als, such as su­per-tele­scopes that com­bine light from mul­ti­ple tele­scopes to mas­sively en­hance astro­nom­i­cal ob­ser­va­tions.

THE WORK PAN and other sci­en­tists are do­ing now is part of what some call “the sec­ond quan­tum rev­o­lu­tion”. The first quan­tum rev­o­lu­tion be­gan in the early decades of the 20th cen­tury with the dis­cov­ery of the bizarre laws of the sub­atomic realm – in which an ob­ject can be both a wave and a par­ti­cle – by pi­o­neer­ing sci­en­tists like Heisen­berg, Schrödinger and Ein­stein. Ap­plied to tech­nol­ogy, these ideas ush­ered in the era of mod­ern elec­tron­ics with de­vices such as the tran­sis­tor, the laser and the so­lar cell.

In the sec­ond quan­tum rev­o­lu­tion, sci­en­tists are ap­ply­ing the quan­tum rules to the ba­sic ideas of in­for­ma­tion tech­nol­ogy.

Clas­si­cal com­put­ing re­lies on bi­nary in­for­ma­tion, rep­re­sented by bits that are ei­ther 1s or 0s. Quan­tum in­for­ma­tion uses quan­tum bits, or qubits, which can be in both the 1 and 0 states at the same time. This can be done us­ing the mag­netic spin of elec­trons, for ex­am­ple, which can be ‘up’ , ‘down’ or some com­bi­na­tion of up and down.

This com­bi­na­tion quan­tum state, known as a ‘su­per­po­si­tion’, is the first of sev­eral con­cepts that form the foun­da­tion of the sec­ond quan­tum rev­o­lu­tion. A qubit only ‘chooses’ one state or the other – at ran­dom, though the prob­a­bil­ity de­pends on how much up and down are in the su­per­po­si­tion – when it is mea­sured. Un­til then qubits in­side a quan­tum com­puter can ef­fec­tively per­form mul­ti­ple cal­cu­la­tions si­mul­ta­ne­ously.

The sec­ond im­por­tant con­cept is en­tan­gle­ment, where the be­hav­iour of dis­tant par­ti­cles can be in­ex­tri­ca­bly con­nected – or ‘en­tan­gled’. When one en­tan­gled par­ti­cle is mea­sured – and hence ‘chooses’ a state – its part­ner is im­me­di­ately bound by that choice, no mat­ter how far away it is. En­tan­gle­ment is the key to quan­tum com­mu­ni­ca­tion.

The third con­cept is the ‘no-cloning the­o­rem’, which says the in­for­ma­tion in a quan­tum par­ti­cle can never be fully copied with­out chang­ing the state of the par­ti­cle. A hacker can make a copy of your email now with­out you ever know­ing; a hack of a quan­tum sys­tem, how­ever, is bound by the laws of physics to leave traces.

To­gether, these phe­nom­ena pave the way for quan­tum com­put­ers able to crunch through big data prob­lems that in­volve find­ing op­ti­mum so­lu­tions from vast num­bers of op­tions. That in­cludes ef­fi­ciently re­verse-en­gi­neer­ing the en­cryp­tion keys that pro­tect your in­ter­net bank­ing ses­sions. At the same time, they make pos­si­ble hack-proof quan­tum com­mu­ni­ca­tion, in which eaves­drop­ping can al­ways be de­tected.

THE SEEDS OF A quan­tum in­ter­net were first sown in the 1970s by a physi­cist named Stephen Wies­ner. As a grad­u­ate stu­dent at Columbia Univer­sity in New York, Weis­ner re­alised how the strange laws of quan­tum me­chan­ics could be used for new kinds of com­mu­ni­ca­tion.

Wies­ner’s ideas were de­vel­oped into a de­tailed pro­to­col for se­cure com­mu­ni­ca­tion in 1984 by Charles Ben­nett and Gilles Bras­sard. Many cryp­to­graphic schemes in­volve a piece of in­for­ma­tion – known as a key – that is shared by the sender and the re­cip­i­ent of a mes­sage, but by no one else. The Ben­nett and Bras­sard scheme sought to solve the prob­lem of shar­ing the key it­self in a se­cure way.

Their idea in­volved a sender (con­ven­tion­ally known as Alice in cryp­tog­ra­phy) send­ing a long string of 1s and 0s to a re­cip­i­ent (call him Bob) that is en­coded in pho­tons in such a way that if an eaves­drop­per (Eve, nat­u­rally) con­ducted any mea­sure­ments on it, Alice and Bob would know (be­cause mea­sur­ing a quan­tum par­ti­cle changes its prop­er­ties). They would then throw out any af­fected 1s and 0s, and be left with an ideal cryp­to­graphic key – a long ran­dom num­ber they both know but no one else does.

Quan­tum cryp­tog­ra­phy sud­denly be­came more rel­e­vant in 1994, when math­e­ma­ti­cian Peter Shor showed that quan­tum com­put­ers might one day be able to use quan­tum in­de­ter­mi­nacy to break through ex­ist­ing cryp­to­graphic schemes with alarm­ing ease. Crack­ing such schemes – like the ones that keep your in­ter­net bank­ing ses­sions safe from pry­ing eyes – in­volves find­ing the fac­tors of ex­tremely large num­bers. Shor showed that a quan­tum com­puter would be able to do it much more quickly than a clas­si­cal one.

Mean­while, fur­ther de­vel­op­ments in the the­ory of quan­tum com­mu­ni­ca­tion – the prac­tice was still some years off – made use of the even stranger phe­nom­e­non of en­tan­gle­ment, which can bind to­gether the fates of ob­jects sep­a­rated by any dis­tance.

This quan­tum con­nec­tion turns out to be very handy for Alice and Bob in their quest to have a quiet chat with­out Eve in­ter­rupt­ing. A pair of en­tan­gled par­ti­cles is in a sense a sin­gle en­tity, no mat­ter how far apart they are. This in­sight was ex­tended to its log­i­cal yet ab­surd con­clu­sion by the­o­rist David Bohm, who noted that, as a con­se­quence of quan­tum me­chan­ics, “the en­tire uni­verse must, on a very ac­cu­rate level, be re­garded as a sin­gle in­di­vis­i­ble unit”.

In 1991, Ox­ford physi­cist Ar­tur Ekert fig­ured out ex­actly how en­tan­gle­ment could im­prove on the Ben­nett-bras­sard scheme. Sup­pose Alice gen­er­ates a stream of en­tan­gled pho­tons and keeps one of each pair for her­self, send­ing the other to Bob. She mea­sures the po­lar­i­sa­tion of her own pho­tons, and writes down a 1 ev­ery time it is hor­i­zon­tal and a 0 ev­ery time it is ver­ti­cal. Even­tu­ally she will have a string of num­bers. Thanks to en­tan­gle­ment, if Bob has done the same mea­sure­ments he will have the iden­ti­cal string. If Eve has in­ter­cepted any pho­tons, if will make de­tectable changes to the cor­re­la­tions be­tween Alice and Bob’s mea­sure­ments.

An­other use for en­tan­gle­ment was dis­cov­ered in 1993, when Ben­nett and Bras­sard, along with oth­ers, fig­ured out that it could be used to trans­port the quan­tum state of a par­ti­cle – a qubit, es­sen­tially – from one place to an­other. If Alice has a pho­ton in some un­known su­per­po­si­tion – the par­tic­u­lar com­bi­na­tion of 1 and 0 states – this ‘quan­tum tele­por­ta­tion’ tech­nique lets her send in­for­ma­tion to Bob so he can cre­ate an iden­ti­cal pho­ton. To col­lect this in­for­ma­tion, Alice must de­stroy the quan­tum state of her pho­ton. Bob then uses that in­for­ma­tion to cre­ate a pho­ton with the same at­tributes as Alice’s, in­clud­ing any en­tan­gle­ments.

Physi­cists call this tele­por­ta­tion be­cause the prop­er­ties of a sub­atomic par­ti­cle, such as its po­si­tion, mo­men­tum, po­lar­i­sa­tion and spin, are all there is to know about it. If a par­ti­cle with a par­tic­u­lar set of prop­er­ties dis­ap­pears at one lo­ca­tion and one with ex­actly the same prop­er­ties ap­pears else­where, how can any­one say they are not the same par­ti­cle?

This kind of weird­ness high­lights the deep con­nec­tions be­tween cryp­tog­ra­phy, in­for­ma­tion the­ory and fun­da­men­tal physics that the quan­tum in­ter­net will ex­ploit. An­ton Zeilinger, an Aus­trian physi­cist who was Pan Jian­wei’s men­tor and is now his col­lab­o­ra­tor, put it bluntly in a 2005 es­say in Na­ture: “the dis­tinc­tion be­tween re­al­ity and our knowl­edge of re­al­ity, be­tween re­al­ity and in­for­ma­tion, can­not be made.” WHILE THE THE­ORY be­hind the quan­tum in­ter­net is mind-bend­ing, building it is largely an en­gi­neer­ing ex­er­cise. Even John Ste­wart Bell, the Belfast­born physi­cist who dreamed up the en­tan­gle­ment ex­per­i­ments that killed the idea of any kind of common-sense re­al­ity be­neath quan­tum me­chan­ics, de­scribed him­self as a “quan­tum engi­neer”, and said he only had time to con­tem­plate prin­ci­ples on Sun­days.

So it is for to­day’s prac­ti­cal quan­tum sci­en­tists. De­vices must be cal­i­brated, ex­per­i­ments must be re­fined, noise must be re­duced. Ques­tions of why give way to fig­ur­ing out how.

It is the abil­ity to solve those dis­crete en­gi­neer­ing prob­lems that im­presses Vikram Sharma, head of Quintessence Labs, a com­pany based in Can­berra, Aus­tralia, that builds quan­tum se­cu­rity sys­tems.

Quintessence Labs is putting quan­tum tech­nol­ogy to use in a net­work se­cu­rity sys­tem built around a de­vice that uses quan­tum un­pre­dictabil­ity to spit out a bil­lion ran­dom num­bers a sec­ond.

One of the com­pany’s key achieve­ments is to shrink the de­vice. “We used to do it on an op­tics ta­ble with lasers and elec­tron­ics and all kinds of equip­ment,” Sharma ex­plains. “It was prob­a­bly a me­tre by a cou­ple of me­tres. Now we have re­duced it to about the size of a cell phone.” He says it with an engi­neer’s pride. “It just slots in to a stan­dard server.”

If a par­ti­cle with a par­tic­u­lar set of prop­er­ties dis­ap­pears at one lo­ca­tion and one with ex­actly the same prop­er­ties ap­pears else­where, how can any­one say they are not the same par­ti­cle?

Next on the agenda is to “fully ma­ture” a se­cure sys­tem that uses the prop­er­ties of a whole laser beam to trans­port en­cryp­tion keys, rather than sin­gle pho­tons, mak­ing it a lit­tle less frag­ile. Sharma says he hopes to have a ver­sion on the mar­ket in early 2019.

Even when care­fully en­gi­neered to ma­tu­rity, how­ever, Quintessence Labs’ sys­tem will be limited by an ob­sta­cle that is very dif­fi­cult to work around, one that hin­ders all the com­peti­tors in the race to take quan­tum com­mu­ni­ca­tions to the world.

THE MA­JOR OB­STA­CLE that must be over­come to cre­ate a global quan­tum net­work is in the ‘global’ part: long dis­tances are a real prob­lem.

As en­tan­gled pho­tons are beamed through air or an op­tic fi­bre, they are slowly picked off by en­coun­ters with other par­ti­cles. Af­ter at most a cou­ple of hun­dred kilo­me­tres, 99.99% will be gone and the sig­nal will be too weak to use for com­mu­ni­ca­tion.

One way around this is Pan Jian­wei’s scheme: make con­nec­tions via a satel­lite that or­bits the world and fires pho­tons down from space via laser beam.

An­other ap­proach is to use re­peaters to re­trans­mit faded sig­nals. A ‘half-quan­tum’ sys­tem es­tab­lishes quan­tum con­nec­tions along a chain of ‘trusted nodes’ that de­code and re-en­code the sig­nal. The long­est such link in op­er­a­tion is a 2,000 km long pipe­line run­ning from Bei­jing to Shanghai via Ji­nan and He­fei, also built by Pan’s team. These trusted nodes are use­ful for key dis­tri­bu­tion – a po­ten­tial hacker could only read the key by ac­cess­ing a node it­self. How­ever the nodes do not ex­tend the reach of en­tan­gle­ment.

That will re­quire the cre­ation of a so-called ‘quan­tum re­peater’: a de­vice that can re­ceive a quan­tum sig­nal and trans­mit it again with­out de­stroy­ing the quan­tum state, like a re­lay sta­tion that passes a pack­age from a tired courier to a fresh one with­out open­ing it.

Some of the most promis­ing re­search is be­ing done at the Aus­tralian Na­tional Univer­sity, where Matthew Sel­lars and Rose Ah­le­feldt have found a way to use crys­tals doped with er­bium atoms to store and re­lease pho­tons with a wave­length (about 1550 nanome­tres) that works neatly with ex­ist­ing fi­bre-op­tic ca­bles.

When a pho­ton is ab­sorbed, its quan­tum state is mapped on to changes in the spin of the nu­cleus of the er­bium atom. “If you put the in­for­ma­tion on a nu­clear spin, it can hold for much longer,” Ah­le­feldt says. This is be­cause the nu­cleus of the atom is in­su­lated from the out­side world.

The atom can then be stim­u­lated to re­lease a new pho­ton iden­ti­cal to the one that went in. “You can store the po­lar­i­sa­tion, the ar­rival time, the pulse shape, the di­rec­tion,” Sel­lars says. “The pho­ton that went in is the pho­ton that comes out.”

Cru­cially, this in­cludes any en­tan­gle­ment of the orig­i­nal pho­ton. A chain of re­peaters con­nected with op­tic fi­bre could ex­tend en­tan­gle­ment in­def­i­nitely.

Sel­lars and Ah­le­feldt are hop­ing to demon­strate the ba­sic func­tions of a re­peater in the next year or two. Af­ter that, says Sel­lars, “It be­comes a case of en­gi­neer­ing and how much money you throw at it.” One un­cer­tainty is how much de­mand there will be: “No one’s had a quan­tum in­ter­net be­fore.”

Sim­i­lar tech­nol­ogy will be needed to plug quan­tum com­put­ers in to the quan­tum in­ter­net. “If we set up this global-scale quan­tum net­work, we want to be able to con­nect things to it,” Ah­le­feldt says.

Get­ting qubits out of a quan­tum com­puter – where they might be stored as elec­tron spins or the mag­netic flux of a su­per­con­duct­ing loop – is a feat in it­self.

“There are three prob­lems to solve,” ac­cord­ing to Sven Rogge, who works on quan­tum com­put­ers at the Univer­sity of New South Wales. “First you have to be able to con­trol one qubit and read it out. Then you have to cou­ple two of them that are close to­gether, for two-qubit op­er­a­tions in­side the com­puter. Then you have to do that two-qubit op­er­a­tion over a much larger dis­tance. That’s the holy grail, the re­ally hard part.”

HOW LONG BE­FORE a ma­ture global quan­tum net­work is pos­si­ble? Though many of the un­der­ly­ing tech­nolo­gies are still in pro­to­type form, Pan be­lieves that progress will be rapid. “Maybe it will take 10 years,” he guesses.

A team based at the Delft Univer­sity of Tech­nol­ogy in the Nether­lands, how­ever, hopes to have a small net­work con­nect­ing four Dutch cities – over dis­tances in the tens of kilo­me­tres that will not re­quire quan­tum re­peaters – op­er­at­ing by 2020. Af­ter that? Even vi­sion­ar­ies like Pan can only spec­u­late about the the even­tual uses of the quan­tum in­ter­net.

How long be­fore a ma­ture global quan­tum net­work is pos­si­ble? Pan be­lieves that progress will be rapid. “Maybe it will take 10 years,” he guesses.

Right now se­cure com­mu­ni­ca­tion is the killer app – the thing that makes gov­ern­ments and banks pour cash into re­search. An­other likely use is con­nect­ing to quan­tum com­put­ers, which are ex­pected to be ex­pen­sive and cum­ber­some ma­chines for some time to come. Much as peo­ple once di­alled in to mas­sive main­frames to get their com­put­ing done, a quan­tum link would al­low re­mote ac­cess to quan­tum com­put­ers with the added twist of ‘blind com­put­ing’, in which the quan­tum com­puter can never know what cal­cu­la­tions it has per­formed or what sen­si­tive data it has han­dled.

Quan­tum com­mu­ni­ca­tion will also al­low dis­tant clocks to be syn­chro­nised within 10– 20 sec­onds, about a thou­sand times as pre­cise as the best cur­rent atomic clocks. This pre­ci­sion will al­low or­bit­ing satel­lites to im­prove GPS sys­tems, map Earth’s grav­i­ta­tional field in un­prece­dented de­tail and even catch the tiny rip­ples of pass­ing grav­i­ta­tional waves.

Bet­ter op­ti­cal tele­scopes are an­other po­ten­tial fringe ben­e­fit. Ra­dio tele­scopes such as the nascent Square Kilo­me­tre Ar­ray com­bine sig­nals from dis­tant dishes to ef­fec­tively form a sin­gle, huge tele­scope. A quan­tum in­ter­net could make this pos­si­ble for vis­i­ble­light tele­scopes, too, by tele­port­ing pho­tons from dis­tant tele­scopes.

Pan sees his work as part of a con­tin­uum in the hu­man im­per­a­tive to com­mu­ni­cate and ex­change in­for­ma­tion. It was, he says, the defin­ing char­ac­ter of early Homo sapi­ens. “They cre­ated ba­sic sym­bols and lan­guages so that they could in­ter­act ef­fec­tively and form a co-op­er­a­tive group. In­for­ma­tion ex­change is a key fac­tor in hu­man evo­lu­tion”.

The next stage in that evo­lu­tion will oc­cur through the pa­tient labour of small, in­cre­men­tal steps: im­prov­ing the Mi­cius tech­nol­ogy to make the satel­lite work in day­light, repli­cat­ing it in other satel­lites, learn­ing how to make mul­ti­ple satel­lites func­tion to­gether. He may have opened the door to a global quan­tum in­ter­net, but Pan still thinks in the mea­sured terms of an engi­neer. “We will study how to build a more ef­fi­cient net­work.”

The Tian­gong-2 space lab­o­ra­tory, launched af­ter the ground­break­ing Mi­cius satel­lite, will ex­tend China’s quan­tum com­mu­ni­ca­tions pro­gram, bring­ing a global quan­tum net­work closer to re­al­ity.

Rose Ah­le­feldt and Matthew Sel­lars at work on a ‘quan­tum re­peater’ to ex­tend the range of quan­tum com­mu­ni­ca­tion.

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