Quantum teleportation over 143 kilometres using active feed-forward

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1 doi:.38/nture47 Quntum teleporttion over 43 kilometres using tive feed-forwrd Xio-Song M, {, Thoms erst,, Thoms Sheidl, Dqing Wng, Sestin Kroptshek, Willim Nylor, Bernhrd Wittmnn,, Alexndr Meh,, Johnnes Kofler,3, Elen Anisimov 4, dim Mkrov 4, Thoms Jennewein,4, Rupert Ursin & Anton Zeilinger, The quntum internet is predited to e the next-genertion informtion proessing pltform, promising seure ommunition,3 nd n exponentil speed-up in distriuted omputtion,4. The distriution of single quits over lrge distnes vi quntum teleporttion 5 is key ingredient for relizing suh glol pltform. By using quntum teleporttion, unknown quntum sttes n e trnsferred over ritrry distnes to prty whose lotion is unknown. Sine the first experimentl demonstrtions of quntum teleporttion of independent externl quits 6, n internl quit 7 nd squeezed sttes 8, reserhers hve progressively extended the ommunition distne. Usully this ours without tive feed-forwrd of the lssil Bell-stte mesurement result, whih is n essentil ingredient in future pplitions suh s ommunition etween quntum omputers. The enhmrk for glol quntum internet is quntum teleporttion of independent quits over free-spe link whose ttenution orresponds to the pth etween stellite nd ground sttion. ere we report suh n experiment, using tive feed-forwrd in rel time. The experiment uses two free-spe optil links, quntum nd lssil, over 43 kilometres etween the two Cnry Islnds of L Plm nd Tenerife. To hieve this, we omine dvned tehniques involving frequeny-unorrelted polriztion-entngled photon pir soure, ultr-low-noise single-photon detetors nd entnglementssisted lok synhroniztion. The verge teleported stte fidelity is well eyond the lssil limit 9 of two-thirds. Furthermore, we onfirm the qulity of the quntum teleporttion proedure without feed-forwrd y omplete quntum proess tomogrphy. Our experiment verifies the mturity nd ppliility of suh tehnologies in rel-world senrios, in prtiulr for future stellite-sed quntum teleporttion. Signifint progress hs een mde reently in the field of quntum ommunition sed on optil free-spe links 9, whih potentilly llow muh lrger propgtion distnes ompred to the existing fire networks euse of the lower photon loss per kilometre. To enle quntum ommunition on glol sle nd mong prties not hving ess to ny fire network, it is foreseele tht experiments will involve ground-to-stellite,,5 nd inter-stellite links. Previous experiments foused on the distriution of quntum sttes of single photons or entngled photon pirs over optil free-spe links. owever, the reliztion of more sophistited multiphoton quntum informtion protools, suh s quntum teleporttion, remined n experimentl hllenge under rel-world onditions. Quntum teleporttion is sed on the simultneous retion of t lest three photons, whih for rndom photon pir soures redues the ount rte y severl orders of mgnitude ompred to experiments using only two photons 6,9. This dereses the signl-to-noise rtio nd requires long integrtion time, suh tht high system stility is neessry. Moreover, the omplexity nd environmentl requirements of quntum teleporttion set-up re inresed signifintly ompred to previous two-photon experiments, whih provides signifint experimentl nd tehnologil hllenges. The work presented in ref. 8 ws signifint hievement in long-distne quntum ommunition, hrnessing the entnglement etween different degrees of freedom of single photon. owever, euse the teleported quit ws not provided independently from the outside, the ppliility of tht sheme is limited. Most erlier experiments on teleporttion of quits nd squeezed sttes 6 8 were in-lortory demonstrtions, nd hene the ommunition distnes were rther short. Although fire-sed teleporttion hs een demonstrted experimentlly,, the mximum trnsmission distne is limited y the intrinsi photon loss in optil fire, unless quntum repeters re involved. In omprison to these previous studies, the experiment presented in this Letter hieves long-distne free-spe teleporttion of n independent quntum stte, thus pving the wy for stellitesed glol quntum ommunition. Quntum teleporttion relies on using oth quntum hnnel nd lssil hnnel etween two prties, usully lled Alie nd Bo 5 (here loted in L Plm nd Tenerife respetively; Fig. ). The quntum hnnel is used y Alie nd Bo to shre the entngled uxiliry stte 3 Y { j i 3 ~ p ffiffi ji j j i 3 i 3 { ji whih is one of the four mximlly entngled Bell sttes ( Y + ~ pffiffi ðjiji+ jijiþ nd W + ~ pffiffi ðjiji+ jijiþ). jæ nd jæ denote the horizontl nd vertil polriztion sttes. Alie nd Bo shre this entngled stte, where photon is with Alie nd photon 3 is with Bo. Chrlie provides the input photon to e teleported to Alie in generl polriztion stte jwi ~ji zji ðþ where nd re omplex numers (jj jj 5 ), unknown to oth Alie nd Bo. Alie then performs Bell-stte mesurement (BSM), projeting photons nd rndomly onto one of the four Bell sttes eh with the sme proility of 5%. As onsequene, photon 3 is projeted onto the input stte jwæ, up to unitry trnsformtion (U), whih depends on the outome of the BSM. When Alie feeds the outome of the BSM forwrd to Bo vi the lssil hnnel, he n implement the orresponding unitry opertion in rel time nd thus otin photon 3 in the initil stte (eqution ()) of photon. If jy æ is deteted, then U orresponds to the identity opertion, whih mens tht Bo needs to do nothing. If, on the other hnd, jy æ is deteted, Bo hs to pply p phse shift etween the horizontl nd the vertil omponent of his photon 3. ðþ Institute for Quntum Optis nd Quntum Informtion (IQOQI), Austrin Ademy of Sienes, Boltzmnngsse 3, A-9 ienn, Austri. ienn Center for Quntum Siene nd Tehnology, Fulty of Physis, University of ienn, Boltzmnngsse 5, A-9 ienn, Austri. 3 Mx Plnk Institute of Quntum Optis, ns-kopfermnn-strße, Grhing/Munih, Germny. 4 Institute for Quntum Computing nd Deprtment of Physis nd Astronomy, University of Wterloo, University Avenue West, Wterloo, Ontrio NL 3G, Cnd. {Present ddress: Deprtment of Eletril Engineering, Yle University, New ven, Connetiut 65, USA. 3 SEPTEMBER OL 489 NATURE 69 Mmilln Pulishers Limited. All rights reserved

2 RESEARC LETTER L Plm Tenerife φ 3 φ BSM Ψ / Ψ + 3 Clssil feed-forwrd hnnel Quntum hnnel 43 km I/π N SP EPR Ψ 3 EPR/Alie 44 nm fs pulsed 8 Mz BSM/Alie BBO 3 BBO t SP/Chrlie Tx 88 nm m CCD 53 nm Clssil feedforwrd hnnel 43 km Trking Quntum hnnel 53 nm CCD Ψ + Rx f GPS e Bo TTU/Logi GPS d Ψ + TTU/Logi Ψ,64 nm Pump Bem pths Mirror WP QWP IF 8 nm IF 3 nm Bem dump BBO PBS APD EOM Fire Coil Coupler FPC FBS Lser PD Cle DM 7 m dim. lens 4 m dim. lens Figure Quntum teleporttion etween the Cnry Islnds L Plm nd Tenerife over oth quntum nd lssil 43-km free-spe hnnels., Experimentl sheme. Alie nd Chrlie re situted in L Plm, nd Bo in Tenerife. Chrlie prepres the teleporttion input photon in wæ, using herlded single-photon (SP) soure with trigger photon (photons re indited y lk numerls on red irles). An Einstein Podolsky Rosen (EPR) 3 soure genertes n entngled pir of photons nd 3 in the stte Y æ 3. Alie then performs Bell-stte mesurement (BSM) on photons nd, nd projets them onto two of the four Bell sttes ( Y æ / Y æ ), nd sends the result vi the lssil feed-forwrd hnnel to Bo. Photon 3 is sent vi the free-spe quntum hnnel to Bo, who pplies unitry trnsformtion (identity opertion or p phse shift) on photon 3 depending on the BSM result nd thus turns its stte wæ 3 into repli of the initil quntum stte wæ., Set-up. In L Plm, frequeny-unorrelted polriztion-entngled Our experiment ws onduted etween Alie s trnsmitter sttion (t the Jous Kpteyn Telesope (JKT) of the Is Newton Group on L Plm) nd Bo s reeiver sttion (t the Optil Ground Sttion (OGS) of the Europen Spe Ageny on Tenerife), seprted y 43 km, oth t ltitudes of out,4 m. Our experimentl set-up is shown in Fig.. At L Plm, ner-infrred femtoseond pulses (with entrl wvelength of 88 nm) emitted from mode-loked Ti:spphire lser, with repetition rte of 8 Mz, were up-onverted to lue pulses (with entrl wvelength of 44 nm). They were used to generte two photon pirs vi type-ii spontneous prmetri down onversion (SPDC) in two nonliner -rium orte (BBO) rystls pled in sequene. The first SPDC soure ws ligned to emit the entngled uxiliry photon pirs (photons nd 3) in the jy æ 3 stte 4, eqution (), while the seond rystl ws herlded photon pir soure generted photons nd 3 in BBO (EPR/Alie) nd olliner photon pir soure generted photons nd in BBO (SP/Chrlie). All single photons were oupled into single-mode fires. For implementing the BSM, photons nd interfered in fire em splitter (FBS) followed y polriztion-resolving single-photon detetion (BSM/Alie). Photon 3 ws guided to the trnsmitter telesope vi -m single-mode fire nd sent to Bo in Tenerife, where the unitry trnsformtion ws implemented using n eletro-optil modultor (EOM) nd photon 3 s polriztion ws mesured. A rel-time feed-forwrd opertion ws implemented y enoding the Y æ BSM result in,64-nm lser pulses, whih were then sent to Bo vi the feedforwrd hnnel. On Bo s side, they were seprted y dihroi mirror (DM), deteted with photodetetor (PD) nd used to trigger the EOM to perform the required p phse shift opertion. See min text for detils. single-photon (SP) soure providing Chrlie s photon to e teleported. Tht soure delivered pirs of horizontlly (photon ) nd vertilly (photon ) polrized photons in produt stte. The detetion of photon y the vlnhe photodiode (APD) t served s trigger to herld the presene of photon. All photons were spetrlly filtered using interferene filters (IF) nd oupled into single-mode fires for spetrl nd sptil mode seletion. To relize the BSM, Alie s photon ws overlpped on fire em splitter (FBS) with the teleporttion input photon, whose polriztion ws ritrrily prepred y Chrlie using hlf- nd qurter-wve pltes (WP nd QWP). In eh output port of the FBS, polrizing em splitter (PBS) ws used to projet these photons on either horizontl or vertil polriztion. Fire polriztion ontrollers (FPCs) were used to ompenste for unwnted polriztion rottion indued 7 NATURE OL SEPTEMBER Mmilln Pulishers Limited. All rights reserved

3 RESEARC y the fires. Our BSM n identify two out of the four Bell sttes, whih is the optimum hievle with liner optis only 5. For more detils on our BSM, see Supplementry Informtion. While the BSM ws eing performed t Alie, photon 3 ws guided to 7-m-perture trnsmitter telesope through -m-long single-mode fire nd then sent vi 43-km free-spe quntum hnnel over to Bo in Tenerife. There, it ws olleted y the -m-perture OGS telesope, nd guided through its Coudé pth to Bo. In the first stge of our experiment, we only onsidered the ses where Alie deteted jy æ in the BSM, whih results in photon 3 eing lredy in the stte of the input photon, jwæ, nd hene Bo ws required to perform n identity opertion, tht is, to do nothing t ll. We verified the suess of the teleporttion proess y nlysing the polriztion stte of photon 3, whih ws omplished y polriztion nlyser, onsisting of qurter- nd hlf-wve plte nd two free-spe oupled Si-APDs (silion vlnhe photodiodes) pled in eh output mode of polrizing em splitter. In the seond stge of our experiment, we implemented rel-time feed-forwrd opertion. When Alie otined jy æ, she sent this lssil informtion to Bo. On reeiving this informtion, Bo hd to pply p phse shift etween the jæ nd jæ omponents of photon 3 to otin the repli of the input stte jwæ. For further detils on the feed-forwrd implementtion, see Supplementry Informtion. The relevnt events on Alie s nd Bo s sides were reorded with seprte time-tgging units eh disiplined to the glol positioning system (GPS) 4. First, Alie in L Plm identified the threefold oinidene events orresponding to the jy 6 æ outomes of the BSM. This ws done using oinidene logi iruit feturing two seprte output signls relized with trnsistor-trnsistor logi (TTL) pulses. These TTL pulses were fed into time-tgging unit whih reorded the ext time nd the BSM result into inry file. Similrly, Bo fed either the signls of oth detetors (stge, without feed-forwrd) or the oinidene etween these signls nd the BSM results sent vi,64-nm lser pulses (stge, with feed-forwrd) into his time-tgging unit. After mesurement run ws ompleted, oth time-tgged dt files were ompred y ross-orreltion, nd the detetion events ssoited with simultneous detetion of four photons originting from the sme pump pulse were identified. The rel-life long-distne environment provided numer of hllenges for the present teleporttion experiment. These hllenges resulted most signifintly in the need to ope with n extremely low signl-to-noise rtio when using stndrd tehniques indeed, too low to perform suessful experiment. To enhne the signl-to-noise rtio to level tht mde the experiment possile, we used the following dvned tehniques: frequeny-unorrelted polriztionentngled photon pir soure 6 8, ultr-low-noise lrge-tive-re single-photon detetors t Bo 9, nd entnglement-ssisted lok synhroniztion 3,4,7. Wheres ll these tehniques hve een implemented individully, our work is (to our knowledge) the first tht omines ll of them simultneously nd moreover in n outdoor environment. See Supplementry Informtion for detils. First we present our results without feed-forwrd, where we only onsidered the BSM outome jy æ. The input stte jwæ ws lwys pproximtely one of the four idel input sttes jw idel i[ ji, ji, jpi~ ðjizjiþ p ffiffi pffiffi, jli~ ðji{ijiþ.we performed tomogrphi mesurements on three onseutive nights, therey umulting dt over 6.5 h. Figure shows the stte tomogrphy results of quntum teleporttion. The mesured density mtrix r for eh of these teleported sttes ws reonstruted from the experimentlly otined dt using the mximum-likelihood tehnique 3.The fidelity of the teleported stte is defined s the overlp of the idel teleported stte jw idel æ with the mesured density mtrix: f 5 Æw idel jrjw idel æ. For this set of sttes, the teleported stte fidelities re mesured to e f 5.89(4),.865(46),.845(7) nd.85(37), yielding n verge f ~:863ð38Þ, where digits in prentheses represent s unertinties, for exmple, f ~:863+:38. During these mesurements the link ttenution vried from 8. db to 39. db, whih ws minly used y rpid temperture hnge nd strong wind. Despite suh high loss in the quntum free-spe hnnel, the lssil verge fidelity limit 9 of /3 ws lerly surpssed y our oserved fidelities, s shown in Fig. 3. (Note tht our rndom smpling of the input sttes over the mutully unised ses sttes leds to the sme lssil limit s smpling over the whole Bloh sphere. Also, it does not mtter whether the smpling is done over ll six mutully unised sis sttes or over three sttes with one stte per sis.) Therefore, we hve expliitly demonstrted quntum teleporttion over the 43-km free-spe hnnel. ρ idel = ρ idel = d ρ idel = ( ) ρ idel = ( + i i + ) Figure Stte tomogrphy results of the four quntum sttes without feed-forwrd over the 43-km free-spe hnnel with the BSM outome of Y æ. The r grphs show the reonstruted density mtries r for the four sttes teleported from Alie (L Plm) to Bo (Tenerife) over the 43-km freespe hnnel. The wire grid indites the expeted vlues for the idel ses. The dt shown omprise totl of 65 fourfold oinidene ounts in out 6.5 h. The unertinties in stte fidelities extrted from these density mtries re lulted using Monte Crlo routine ssuming Poissonin errors. 3 SEPTEMBER OL 489 NATURE 7 Mmilln Pulishers Limited. All rights reserved

4 RESEARC LETTER Stte fidelity Clssil limit: 66.7% P L P FF R FF Input sttes Figure 3 Summry of the stte fidelity results for the teleported quntum sttes with nd without feed-forwrd. Grey rs show the results otined in quntum teleporttion without feed-forwrd. Bo ws informed vi n internet onnetion when Alie s BSM outome ws Y æ, tht is, those ses where he is not required to pply ny opertion to his photon. The unertinties of these stte fidelities re the sme s presented in the min text. Red rs stnd for the results otined in quntum teleporttion with feed-forwrd (FF). Bo ws informed vi the lssil free-spe link of Alie s BSM outomes of Y æ. The unertinties of these stte fidelities re derived from Poissonin sttistis. All oserved fidelities signifintly exeed the lssil verge fidelity limit of /3 (66.7%). Error rs, 6s. The reonstruted density mtries of the teleported quntum sttes llow us to fully hrterize the teleporttion proedure y quntum proess tomogrphy. The four input sttes (r idel 5 jw idel ææw idel j 5 jæ Æj, jæ Æj, jpæ ÆPj, jlæ ÆLj) re trnsferred to the orresponding (reonstruted) output sttes r. We n ompletely desrie the effet of teleporttion on the input sttes r idel y determining the proess mtrix x, defined y r~ P 3 l,k~ x lk s l r idel s k, where the s i re the Puli mtries with s the identity opertor. The proess mtrix x n e omputed nlytilly from these four equtions for the four different input nd output sttes. The idel proess mtrix of quntum teleporttion x idel hs only one non-zero omponent, (x idel ) 5, mening the input stte is teleported without ny redution in qulity. Figure 4 nd show respetively the rel nd imginry omponents of x for quntum teleporttion sed on our experimentl results. The proess fidelity of our experiment ws f proess 5 Tr (x idel x) 5.7(4). This lerly onfirmed the quntum nture of our teleporttion experiment s it is five stndrd devitions ove the mximum proess fidelity of.5, whih is the limit one n reh with lssil strtegy where Alie nd Bo do not shre ny entnglement s resoure. In the seond stge of the experiment, we relized quntum teleporttion inluding rel-time feed-forwrd of the BSM result over the 43-km lssil hnnel. We set jpæ nd jræ sttes ( jri~ ðjizijiþ p ffiffi )s input sttes for whih the required p phse shift etween the jæ nd the jæ omponents of photon 3 resulted in 9u polriztion rottion. owever, for the jæ or jæ input stte, feed-forwrd is irrelevnt euse p phse shift would only result in non-detetle glol phse shift. Thus, the qulity of teleporttion of these sttes is lredy onfirmed y our first-stge experiment. Relizing teleporttion for the sttes jpæ nd jræ fully onfirms the generlity of the proedure, s these sttes elong to different mutully unised ses. In Tenerife, we nlysed photon 3 in the eigensis of the input stte, tht is, the jpæ/ jmæ (jræ/jlæ) sis when the input stte ws jpæ (jræ). ere jmi~ ðji{ jiþ p ffiffi. The resultnt fidelities of the teleported sttes re.76(5) nd.8(37) for jpæ nd jræ, respetively (red rs in Fig. 3). Both results re lerly ove the lssil fidelity ound. Note tht in our experiment, the effiieny of the lssil link ws mesured to e.3%, whih ws minly due to mplitude flututions used y tmospheri turulene. Using the rel-time feed-forwrd opertion, we unmiguously experimentlly demonstrted quntum teleporttion from L Plm to Tenerife over 43-km free-spe hnnel. We note tht from oneptul perspetive, rel-time feed-forwrd is prt of the originl teleporttion proposl 5. Ultimtely, the dvntge of long-distne teleporttion ompred to just sending the quntum stte itself my lie in the following future pplitions: if Alie nd Bo n stokpile their entngled sttes eforehnd (with the help of quntum memories), teleporttion is dvntgeous if the quntum hnnel is of low qulity or if Bo s lotion is unknown to Alie. This is euse Alie n rodst the lssil informtion with high qulity nd to ritrry, or even unknown, lotions 5. (Also, the quntum repeter, whih is of high importne for lrge-sle quntum networks, is sed on teleporttion in the form of entnglement swpping 3.) Speifilly, this dvntge lso shows the usefulness of quntum teleporttion even for quntum hnnels tht llow only smll trnsmission rtes. We note tht over the yers teleporttion soures hve een mrkedly improved. We elieve this development will ontinue nd urrently severl new shemes re eing pursued. Our work proves the fesiility of oth ground-sed nd stellitesed free-spe quntum teleporttion. Our quntum teleporttion set-up ws le to hieve oinidene prodution rtes nd fidelities suffiient to ope with the optil link ttenution resulting from vrious experimentl nd tehnil hllenges; suh hllenges will rise in quntum trnsmission etween ground-sed trnsmitter.8.8 Re( χ ).6.4. Im( χ ).6.4. σ σ σ σ 3 σ 3 σ σ σ σ σ σ σ 3 σ 3 σ σ σ Figure 4 Quntum proess tomogrphy of quntum teleporttion without feed-forwrd.,, The rel (Re(x lk ); ) nd imginry (Im(x lk ); ) vlues of the omponents of the reonstruted quntum proess mtrix, with l, k 5,,, nd 3. The results of the stte tomogrphy of the four teleported sttes, æ, æ, Pæ, Læ, re employed to reonstrut the proess mtrix of quntum stte teleporttion. The opertors s i re the identity (i 5 ) nd the x-, y-, nd z-puli mtries (i 5,, 3). For the idel se, the only non-zero omponent of the proess mtrix of quntum teleporttion, x idel,is(x idel ) 5, whih is indited y the wire grid. Our experiment lerly onfirmed tht x (identity opertion) is indeed the dominnt omponent nd surpsses the lssil limit. 7 NATURE OL SEPTEMBER Mmilln Pulishers Limited. All rights reserved

5 RESEARC nd low-erth-oriting stellite reeiver 3. In ft, some of the demnds of our experiment were even more hllenging thn in stellite ommunition sine there the tmospheri distnes to e overome re ertinly shorter thn the distne etween Tenerife nd L Plm. In ddition, quiring, pointing nd trking (APT) for diffrtion-limited trnsmitter nd reeiver telesopes in spe is well-estlished tehnology. Therefore, our experiment represents ruil step towrds future quntum networks in spe, whih require spe-to-ground quntum ommunition. During the finliztion of our mnusript, relted work ws reported 33 tht used 5-z APT system, while our APT system operted t few hertz, yet fst enough to fulfil the speifi requirements of our experiment. The most signifint distintion etween ref. 33 nd the experiment presented in this Letter is our implementtion of n tive feed-forwrd tehnique. The tehnology implemented in oth experiments hs ertinly rehed the required mturity for oth stellite nd long-distne ground ommunition. 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Supplementry Informtion is ville in the online version of the pper. Aknowledgements We thnk the stff of IAC: F. Snhez-Mrtinez, A. Alonso, C. Wrden, M. Serr nd J. Crlos; nd the stff of ING: M. Blells, C. Benn, J. Rey, O. duvesu, A. Chopping, D. González, S. Rodríguez, M. Areu, L. González; J. Kuusel, E. Wille nd Z. Sodnik; nd J. Perdigues of the OGS nd ESA. X.-S.M., T.J., R.U. nd A.Z. thnk S. Rmelow for disussions, P. Kolenderski for disussions on the SPDC soure with the Bell-stte synthesizer, S. Zotter for help during the erly stges of the experiment, nd R. Steinker for meteorologil dvie. J.K. ws supported y the EU projet MALICIA. E.A. nd.m. thnk C. Kurtsiefer nd Y.-S. Kim for detetor eletronis design, J. Skr for support, nd the Reserh Counil of Norwy (grnt No. 8439/ 3) ndindustry Cnd for support. This work ws mde possile y grnts from the Europen Spe Ageny (ontrt 448//NL/AF), the Austrin Siene Foundtion (FWF) under projets SFB F48 nd CoQuS, nd the FFG for the QTS projet (no. 8836) within the ASAP 7 progrm. We lso knowledge support y the Europen Commission, grnt Q-ESSENCE (no. 4895) nd the John Templeton Foundtion. Author Contriutions X.-S.M. oneived the reserh, designed nd rried out the experiment, nd nlysed dt. T.., T.S. nd D.W. rried out the experiment nd nlysed dt. S.K., W.N., B.W. nd A.M. provided experimentl ssistne during the erly stge of the experiment. J.K. provided the theoretil nlysis nd nlysed dt. E.A. nd.m. developed the ultr-low-noise detetors. T.J. provided experimentl nd oneptul ssistne, nd oneived nd developed the oinidene nlysis ode. R.U. oneived the reserh, plnned nd rried out the experiment nd nlysed dt. A.Z. defined the sientifi gols, oneived the reserh, designed the experiment nd supervised the projet. X.-S.M., T.., T.S., J.K., R.U. nd A.Z. wrote the mnusript with ssistne from ll other o-uthors. Author Informtion Reprints nd permissions informtion is ville t The uthors delre no ompeting finnil interests. Reders re welome to omment on the online version of the pper. Correspondene nd requests for mterils should e ddressed to X.-S.M. ([email protected]) or A.Z. ([email protected]). 3 SEPTEMBER OL 489 NATURE 73 Mmilln Pulishers Limited. All rights reserved