Asolid-state optical cavity in a semiconductor photonic crystal

Transcription

1 PUBLISED ONLINE: 7 MARC 3 DOI:.38/NPOTON.3. Quntum control of spin quit coupled to photonic crystl cvity Smuel G. Crter, Timothy M. Sweeney, Mijin Kim 3, Chul Soo Kim,DmitrySolenov, Sophi E. Economou, Thoms L. Reinecke, Lily Yng, Alln S. Brcker nd Dniel Gmmon * A key ingredient for quntum network is n interfce etween sttionry quntum its nd photons, which ct s flying quits for interctions nd communiction. Photonic crystl rchitectures re promising pltforms for enhncing the coupling of light to solid-stte quits. Quntum dots cn e integrted into photonic crystl, with opticl trnsitions coupling to photons nd spin sttes forming long-lived quntum memory. Mny reserchers hve now succeeded in coupling these emitters to photonic crystl cvities, ut there hve een no demonstrtions of functionl spin quit nd quntum gtes in this environment. ere, we hve developed coupled cvity quntum dot system in which the dot is controllly chrged with single electron. We perform the initiliztion, rottion nd mesurement of single electron spin quit using lser pulses, nd find tht the cvity cn significntly improve these processes. Asolid-stte opticl cvity in semiconductor photonic crystl memrne hs mny dvntges for nnophotonics nd cvity quntum electrodynmics (CQED),. It cn hve smll volume nd high qulity fctor Q (ref. 3), it cn e comined with wveguides into extended nd complex photonic rchitectures 3 5, nd it cn e integrted with semiconductor electronic devices 6 8. In the lst decde there hs een rpid progress oth in the development of photonic crystls themselves nd in the study of solid-stte emitters coupled to photonic crystl cvities 3. Both quntum dots nd nitrogen-vcncy centres cn e incorported into photonic crystl cvities nd cn hve long-lived spin sttes,5. This work hs led to the vision of quntum network 6, similr to tht eing developed for tomic systems 7, ut in sclle solid-stte pltform 8,9. Proposls for quntum networks lmost lwys involve quntum memories with three energy levels in L configurtion, tht is, with two ground sttes nd n opticlly excited stte 6,8,9. The two ground-stte spin levels ct s long-lived quntum memory, nd the opticlly excited stte serves to connect the ground-stte spin coherence to opticl coherence. Yet, lmost ll experimentl studies to dte involving quntum dots in photonic crystl cvities nd wveguides hve used unchrged quntum dots, which essentilly ct s short-lived two-level systems. One wy to otin the L-type three-level system envisioned for lrge-scle quntum network is to chrge quntum dot with single electron. Recently, diodes hve een incorported into photonic crystl memrnes 6 8 to serve this purpose, nd one group hs demonstrted controlled chrging of quntum dot in cvity. Although opticl initiliztion, redout nd quntum gtes hve lredy een demonstrted on spin quits, doing so in the presence of photonic crystl cvity presents oth dvntges nd dditionl chllenges. In prticulr, the photon density of sttes in photonic crystl cvity is strongly modified from free spce nd is polriztion-dependent. ere, we demonstrte complete quntum control of quntum dot spin quit coupled to photonic crystl cvity. We show () controlled chrging of quntum dot in cvity with single electron using diode incorported into the photonic crystl; () the necessry L-type three-level system resulting from mgnetic field pplied trnsverse to the smple; (3) spin mesurement nd initiliztion using resonnt lser spectroscopy; nd () single quit gtes with picosecond opticl pulses. In fct, the polriztion-dependent coupling of the cvity to the quntum dot hs significnt dvntges for spin mesurement nd redout, nd single quit gtes re demonstrted y detuning the pulses from the cvity resonnce, thus voiding polriztion-dependent coupling. Chrged quntum dot coupled to cvity A photonic crystl memrne with defect opticl cvity (L3) ws ptterned nd etched into n n-i-p (n-type, intrinsic, p-type) GAs diode, into which InAs quntum dots hd een grown in the intrinsic region, closer to the n-type lyer (shown schemticlly in Fig. ). Chrging of the quntum dots with controlled numer of electrons ws performed y pplying forwrd is cross the diode. The chrge stte of quntum dot ws determined from its photoluminescence s function of is. A photoluminescence is mp of quntum dot in cvity (lelled QD-C) (Fig. c) shows the neutrl exciton (X ), negtively chrged exciton (X ) nd cvity lines. The X line, indicting single electron in the quntum dot, is stle for is from.7 to.95 nd is detuned from the cvity y..3 me over this rnge. The cvity linewidth of.3 me corresponds to qulity fctor Q of,. Resonnt lser excittion of the quntum dot nd the cvity mode ws used to opticlly ddress nd mesure the spin sttes of the electron. The differentil reflectivity, (DR) of lser scnned cross the X, cvity nd X resonnces is presented in Fig. d,e; this ws tken with two lser polriztions, prllel () nd perpendiculr () to the cvity polriztion. The dominnt feture t,3.93 me for polriztion corresponds to the X line, which hs dispersive lineshpe due to the proximity to the cvity mode nd interference effects in the memrne. The cvity mode ppers t,3. me s rod (.3 me) feture, Nvl Reserch Lortory, Wshington, District of Columi 375, USA, NRC Postdoctorl Associte Residing t the Nvl Reserch Lortory, Wshington, District of Columi 375, USA, 3 Soter Defense Solutions, Inc., Annpolis Junction, Mrylnd 7, USA; These uthors contriuted eqully to this work. *e-mil: dn.gmmon@nrl.nvy.mil NATURE POTONICS ADANCE ONLINE PUBLICATION 3 Mcmilln Pulishers Limited. All rights reserved.

2 Report Documenttion Pge Form Approved OMB No Pulic reporting urden for the collection of informtion is estimted to verge hour per response, including the time for reviewing instructions, serching existing dt sources, gthering nd mintining the dt needed, nd completing nd reviewing the collection of informtion. Send comments regrding this urden estimte or ny other spect of this collection of informtion, including suggestions for reducing this urden, to Wshington edqurters Services, Directorte for Informtion Opertions nd Reports, 5 Jefferson Dvis ighwy, Suite, Arlington A -3. Respondents should e wre tht notwithstnding ny other provision of lw, no person shll e suject to penlty for filing to comply with collection of informtion if it does not disply currently vlid OMB control numer.. REPORT DATE 7 MAR 3. REPORT TYPE 3. DATES COERED --3 to --3. TITLE AND SUBTITLE Quntum control of spin quit coupled to photonic crystl cvity 5. CONTRACT NUMBER 5. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Nvl Reserch Lortory,Wshington,DC, PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES). SPONSOR/MONITOR S ACRONYM(S). DISTRIBUTION/AAILABILITY STATEMENT Approved for pulic relese; distriution unlimited 3. SUPPLEMENTARY NOTES. ABSTRACT. SPONSOR/MONITOR S REPORT NUMBER(S) 5. SUBJECT TERMS 6. SECURITY CLASSIFICATION OF: 7. LIMITATION OF ABSTRACT. REPORT unclssified. ABSTRACT unclssified c. TIS PAGE unclssified Sme s Report (SAR) 8. NUMBER OF PAGES 6 9. NAME OF RESPONSIBLE PERSON Stndrd Form 98 (Rev. 8-98) Prescried y ANSI Std Z39-8

3 NATURE POTONICS DOI:.38/NPOTON.3. PL 6 3 Cvity.78,3,3,3,36,38,3 Energy (me) kc.p.s. c 6 Bis ().8.6 X 3 Cvity.,3,3,3,36,38,3 Energy (me) d 6 e Fit X p-gas i-gas i-gas n-gas InAs QDs Bre cvity Figure Chrged quntum dots in cvity., Top: scnning electron microgrph of n L3 photonic crystl cvity with n illustrtion of n electron spin in the centre (ctul quntum dot loction is unknown). Scle r, nm. Bottom: illustrtion of the lyers tht form the n-i-p diode., Photoluminescence of the cvity quntum dot system t is of.78 nd excittion t,39 me. c, Photoluminescence is mp of the cvity quntum dot system, mesured in, counts per second (kc.p.s.). orizontl lines correspond to the reflectivity scns elow. d, Differentil reflectivity ner X t is of.785 (solid lines), t which the quntum dot is chrged with single electron, nd t.7 (dshed red line), t which the quntum dot is unchrged. nd correspond to the lser polriztions. e, Differentil reflectivity of X t.7. The mgnetic field is zero, nd the temperture is 7 8 K. The energy scle in d nd e is reltive to X t,3.93 me. 5 3 K.75 3 K c 8 K.6 8 K K.5 3 K 5 erticl B = 8 K 3 K 7 K.3.5 oriz. B = 8 K 3 K 7 K d 3 No pulse B x = T With pulse Figure Resonnt lser spectroscopy.,, DR for nd polriztions, respectively, for series of tempertures, vrying the detuning of X from the cvity. The dshed lue lines in re fits to the reflectivity. The spectr re offset verticlly t ech temperture for clrity. c, Level digrm showing the electron spin sttes nd X spin sttes in oigt mgnetic field. Single (doule) rrows represent electron (hole) spins. d, DR t B x ¼ T for circulr polriztion with (upper curve) nd without (lower curve) short, resonnt pulse tht defets opticl pumping. The energy scles re reltive to X t 7 K. lso with highly dispersive lineshpe, due primrily to the is modultion in the differentil technique. The coupling of X to the cvity is mde evident y the order of mgnitude increse in signl compred to X nd polriztion nisotropy for the X signl. The opticl response for polriztion is 7 times greter thn for. The DR for X shows nisotropic exchnge splitting 3, ut the polriztion nisotropy in the signl strength is essentilly gone, ecuse X is detuned y 6 me from the cvity resonnce. NATURE POTONICS ADANCE ONLINE PUBLICATION 3 Mcmilln Pulishers Limited. All rights reserved.

4 NATURE POTONICS DOI:.38/NPOTON.3. The linewidth of X is 3 me, much greter thn the 8 me linewidth of X nd 3 times greter thn the expected rditive limit for quntum dot outside the cvity. We ttriute this linewidth increse to the cvity quntum dot interction. Figure, presents plots of DR for nd polriztions for series of tempertures, showing the ehviour s function of cvity detuning. The X feture for polriztion decreses y fctor of t 3 K ( me detuning), nd the X line for polriztion decreses y fctor of only, reducing the polriztion nisotropy. The X linewidth lso decreses y s much s 3% with incresing temperture, consistent with decrese in coupling to the cvity. The decrese in linewidth continues until 8 K, when the linewidth egins to increse, presumly due to phonon rodening. The DR for polriztion ws fitted to model tht descries scttering of light from the coupled cvity quntum dot system, including interference with ckground reflections (Supplementry Section SII.A). From these fits we otined cvity dot coupling g C of 5 me. This reltively smll coupling strength is proly due to the modest sptil overlp etween the quntum dot nd cvity mode. Spin initiliztion nd mesurement To chieve spin mesurement nd initiliztion, trnsverse mgnetic field ws pplied to split the electron nd X energy levels. This resulted in the four trnsitions shown in Fig. c, which cn e considered two L-type three-level systems (the two systems differing in the X spin stte). In Fig. d, the lower DR signl shows the cvity mode with only some smll ripples ner the expected X trnsitions. This result is n indiction of opticl pumping 6. When lser is resonnt with one of the trnsitions, smll chnge in reflectivity occurs if the system is in the spin stte eing opticlly driven, thus mesuring the spin stte. owever, recomintion to the other spin stte cn occur, so the quntum dot system is quickly pumped out of the spin stte eing driven nd into the other, eliminting sorption from the quntum dot. This effect is used to initilize the spin quit. The four opticl trnsitions re experimentlly oserved in Fig. d only when n opticl pulse resonnt with ll four trnsitions (upper curve) is present to counter the effects of opticl pumping. The presence of some residul signl from the quntum dot without the pulse is proly due to incomplete pumping nd interference effects. To etter chrcterize opticl initiliztion, it is useful to modertely detune the quntum dot from the cvity resonnce, mking the four trnsitions esier to resolve. Temperture tuning cn ccomplish this, ut opticl pumping is defeted t higher tempertures y fster spin relxtion. Insted, we exmine different quntum dot cvity system (QD-C) (Fig. 3), in which X is me ove the cvity resonnce t 7 K. In Fig. 3, DR for polriztion is three times lrger thn for, indicting the quntum dot is coupled to the cvity. At mgnetic field of T (Fig. 3, inset), the four trnsitions re oserved with strong polriztion dependence. We note tht in our quntum dots the polriztion xis is rndomly oriented, presumly due to vlence nd mixing 5,7. For this prticulr quntum dot, the xis is close to the cvity orienttion, with the outer (inner) trnsitions predominntly coupling to (). For QD-C the xis is closer to 58, so ll four trnsitions couple well to the cvity. To eliminte the dispersive lineshpes in DR tht mke it difficult to extrct initiliztion fidelities, we mesured the differentil resonnce fluorescence 8,9 (DRF, Supplementry Section SI.B) of QD-C y exciting with polriztion nd detecting, locking out the reflected lser. Resonnce fluorescence provides nerly ckground-free lterntive to DR s wy to mesure the spin stte. Insted of mesuring smll chnge in reflectivity when the system is in the spin stte eing opticlly driven, photons re only scttered nd detected when in tht spin stte. In Fig. 3, on the edges of the chrge stility region (.87 nd.97 ) where ΔRF (kc.p.s.). Cvity B x = T B x = T, / rpid co-tunnelling occurs 5, ll four trnsitions pper. Nominlly, the outer trnsitions should not pper for excittion, ut the polriztion selection rules re not perfectly ligned with /, nd the cvity enhnces emission for these trnsitions. As the is moves towrds the centre of the chrge stility region, the inner trnsitions dispper due to opticl pumping, giving n initiliztion fidelity of t lest 95%. The outer trnsitions never entirely dispper ecuse they re driven much more wekly with polriztion, nd ecuse the pumping rte is significntly slower for these trnsitions, s we will now show. An importnt feture of the cvity is tht the pumping rte is strongly modified, ut in n symmetric wy. This is illustrted in the insets to Fig. 3c. When driving n outer () trnsition (lower digrm), which is more strongly coupled to the cvity, emission is likely to return the system to its initil spin stte, thus giving slow pumping rte. When driving n inner () trnsition (upper digrm), emission is more likely to chnge the spin stte, thus giving fst pumping rte. We determine these rtes y temporlly resolving the leching of RF due to opticl pumping fter short c Pumping rte (ns ) No cvity ARTICLES Inner / Outer / 8,,6 Lser power (nw) Figure 3 Spin initiliztion nd mesurement in QD-C., DR for nd lser polriztions t zero mgnetic field nd is of.9. Inset: plot of DR of the X lines t B x ¼ T nd.87. The inset level digrm shows the electron spin sttes nd X spin sttes in oigt mgnetic field. Single (doule) rrows represent electron (hole) spins. Thicker red lines indicte cvity-enhnced trnsitions., DRF for polrized lser, polrized detection for series of ises ( m modultion) t B x ¼ T nd lser power of 5 nw. Spectr re offset verticlly ccording to is. c, Spin pumping rtes versus lser power for inner/outer trnsitions for QD-C t.93 nd quntum dot outside the cvity. The lser/detection polriztion is / (/) for inner (outer) trnsitions. Error rs represent the stndrd error of the nonliner lest-squres fit to n exponentil nd re shown when lrger thn the dt points. Inset level digrms illustrte the effect of cvity enhncement on pumping rtes. Solid (dshed) lines indicte the excittion lser (spontneous emission). NATURE POTONICS ADANCE ONLINE PUBLICATION Mcmilln Pulishers Limited. All rights reserved.

5 NATURE POTONICS DOI:.38/NPOTON σ ±.3 6 8,, Dely τ (ps) c Rmsey fringe mp. (%). d Rmsey fringe mp. (%). π/ PL. 3π/ π.5 Fidelity Purity π Pulse power (μw) Pulse detuning (me) Pulse detuning (me) Pulse e Fidelity/Purity. Figure Coherent spin rottion., Rmsey interference fringes t B x ¼ T for pulse detuning of.3 me, with the initiliztion/mesurement lser ner trnsition. Inset: Bloch sphere with rottion of the Bloch vector from the first pulse nd precession etween pulses., Level digrm showing the coherent Rmn process tht couples the two electron spin sttes through n X stte. The dshed line represents virtul stte. c, Fitted mplitude of the Rmsey interference fringes s function of pulse power, for detuning of.56 me. The red curve is model clcultion. d, MximumRmseyfringe mplitude (solid circles) s function of pulse detuning. The photoluminescence (PL) spectrum (t B ¼ ) nd smple pulse spectrum re displyed for comprison. Error rs in c nd d represent the stndrd error of the nonliner lest-squres fit to decying cosine. e, Theoreticl model of the fidelity of the spin rottion nd purity of the resulting stte s function of pulse detuning. pulse depolrizes the spin. In Fig. 3c, the opticl pumping rtes re plotted s functions of lser power for the lser tuned to the inner nd outer trnsitions, with pumping rtes for quntum dot outside the cvity plotted for comprison. The pumping rte t sturtion is n order of mgnitude higher for the inner trnsitions thn for the outer, even with moderte detuning. Thus, driving the trnsitions tht re not coupled to the cvity permits fst initiliztion, nd driving the trnsitions coupled to the cvity is etter for spin mesurement. For enhnced mesurement it is especilly dvntgeous to hve well-ligned polriztion xes nd lrge Purcell enhncement. Optimizing these would give cycling trnsition tht could permit single-shot redout. Ultrfst single quit gtes A functionl spin quit lso requires single quit gtes (tht is, spin rottions). This is ccomplished with short, circulrly polrized pulses tht couple the two spin sttes together through one of the two X spin sttes (Fig. ) 9,3 37. A significnt compliction is tht the cvity only couples to polriztion, nd yet the rottion requires circulr polriztion. We find tht spectrlly detuning the pulse from the cvity llevites this issue. The pulse then primrily intercts with the quntum dot directly insted of through the cvity mode. To demonstrte these coherent rottions, we mesured the spin popultion fter two rottion pulses ( 3 ps length, 5 me ndwidth) delyed with respect to ech other y vrile time t (ref. 3). The resulting Rmsey interference fringes for QD-C re displyed in Fig.. The first pulse rottes the Bloch vector ner the equtor, where it precesses (Fig., inset) for time t t the Lrmor precession frequency. The second pulse rottes the spin up or down depending on the phse, giving rise to oscilltions in the spin popultion. The decy time of the oscilltions ( ps) is due primrily to dephsing from the fluctuting nucler polriztion 38. It should e possile to recover the spin coherence using spin echo techniques 39 or y opticlly suppressing nucler spin fluctutions,. In Fig. c, the mplitude of the Rmsey fringes is plotted s function of rottion pulse power nd is indictive of dmped Ri oscilltions of the electron spin. The peks t 3 mw nd mw correspond to rottion pulses with res of p/ nd 3p/, respectively. The troughs t 7 mw nd 5 mw correspond to rottions of p nd p. The non-zero mplitude of the Rmsey oscilltions for two nominl p pulses is due to precession during the pulses, which limits rottion fidelity. Simultions of the Rmsey fringes (displyed in Fig. c) give similr ehviour due to this effect. To determine how the cvity ffects the pulse rottion, the mximum Rmsey fringe mplitude (tht is, for p/ rottion pulses) ws mesured s function of detuning, s shown in Fig. d. The mplitude is highly symmetric out the quntum dot trnsitions t,3.95 me. For negtive pulse detunings, where the pulses re fr from the cvity mode, the Rmsey fringes re strong, nd for positive detunings, where the pulses re nerly resonnt with the cvity mode, the fringes re quite wek. We compre this result to theoreticl model in which the incident lser field is modified y the cvity mode (Supplementry Section SII.C). Figure e plots the theoreticl fidelity of single pulse p/ spin rottion nd the purity of the resulting spin stte fter the pulse within this model. The clcultions show ehviour qulittively similr to the experiment. The poor fidelity ner the cvity resonnce rises from rel excittion of X through the cvity, whereupon recomintion destroys the purity of the spin stte. The increse in fidelity t detuning of.3 me is due to the cvity field driving the system up to X nd then prtilly ck down to the ground sttes, preventing recomintion. Both mesurement nd theory show tht the fidelity cn e quite high with lrge negtive detuning, such tht the pulse is spectrlly fr wy from the cvity. Discussion We hve demonstrted the first functionl spin quit coupled to n opticl cvity in which opticl initiliztion, control nd redout of n electron spin is chieved. ving long-lived solid-stte spin quit coupled to cvity opens up mny res of reserch including NATURE POTONICS ADANCE ONLINE PUBLICATION 3 Mcmilln Pulishers Limited. All rights reserved.

6 NATURE POTONICS DOI:.38/NPOTON.3. CQED with spin degree of freedom, spin-controlled photonics, nd quntum networks. This type of system my in fct e used s node in quntum network, with opticl pulses controlling the emission of photons into nery wveguides, trnsmitting quntum sttes to other nodes 9. An importnt step in this direction will e to provide etter control over the prmeters of the system, including spectrl nd sptil overlp with the cvity mode, higher Q-fctors, nd the polriztion xis of the quntum dot reltive to the cvity mode. Bsed on the pumping rtes mesured here, it seems possile with improved Q-fctors nd coupling strengths to drsticlly decrese the initiliztion time to picosecond timescles, significntly improving quit opertion speed. Also, with the quntum dot polriztion xis etter ligned to the cvity polriztion, the cvity-enhnced trnsitions should ct s very right cycling trnsitions tht could e used for single-shot redout. Previously, the only demonstrted quntum dot spin system with oth fst spin initiliztion nd cycling trnsitions ws the W-system otined in coupled quntum dots 6,9. We lso nticipte the incorportion of multiple spin quits within node. This cn e chieved using quntum dot molecules in cvity, where tunnelling interction provides two quit gtes 35,36. Even without tunnel coupling, multiple quntum dot spin quits within cvity cn e entngled using pulse sequences tht mke use of cvity-induced interction. These multiple quits cn e used for error correction nd form the sis of quntum repeter or quntum computer. Methods Smple structure. The smples were grown y moleculr em epitxy on n-type GAs sustrtes. A 5 nm scrificil lyer of n-al.7 G.3 As ws grown on the sustrte, followed y 5 nm n-gas, nm intrinsic GAs, 3 nm InAs quntum dots, 5 nm intrinsic GAs nd 3 nm p-gas. The quntum dots were distriuted rndomly in the growth plne with density of severl quntum dots per mm. A positive electron-em resist (ZEP5A, Zeon Chemicls) ws used to define two-dimensionl photonic crystls using Rith 5 electron-em lithogrphy system, followed y Cl -sed inductively coupled plsm etching. A tringulr lttice of holes (rdii, 7 nm) with lttice spcing of nm were etched through the epilyer into the AlGAs, with three missing holes t the centre forming n L3 cvity (Fig. ). The AlGAs under ech photonic crystl ws etched wy, leving 8-nm-thick photonic crystl memrne. Ohmic contct ws mde to the p-type lyer on the surfce nd to the n-type sustrte. Mesurement techniques. The smple ws mounted on piezo-stges in mgnetoopticl cryostt, with the mgnetic field oriented prllel to the long dimension of the cvity. A.68 NA spheric lens focused lsers onto the smple nd collected photoluminescence nd reflected lser light. For DR mesurements, the is of the diode ws modulted t kz with squre-wve pek-to-pek mplitude of m, nd the modultion of the reflected light ws mesured with lock-in detection. Smll shifts in the cvity position with is give rise to DR signl from the cvity tht ws essentilly the derivtive with respect to is, giving rise to dispersive lineshpes. For Rmsey interference fringe mesurements, the c.w. lser ws used to initilize the system nd red out the spin stte. Pulses from the Ti:spphire lser were split into two 3 ps pulses with circulr polriztion opposite to the c.w. lser, so tht in detection the pulses were rejected using polrizer. The c.w. lser ws on etween the pulses, which my hve led to some decoherence. This effect ppered to e negligile compred to the effects from nucler spins. Received 9 July ; ccepted 3 Jnury 3; pulished online 7 Mrch 3 References. hl, K. J. Opticl microcvities. Nture, (3).. Nod, S., Fujit, M. & Asno, T. Spontneous-emission control y photonic crystls nd nnocvities. Nture Photon., 9 58 (7). 3. Notomi, M. Mnipulting light with strongly modulted photonic crystls. Rep. Prog. Phys. 73, 965 ().. Englund, D., Fron, A., Zhng, B. 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7 38. Ldd, T. D. et l. Pulsed nucler pumping nd spin diffusion in single chrged quntum dot. Phys. Rev. Lett. 5, 7 (). 39. Press, D. et l. Ultrfst opticl spin echo in single quntum dot. Nture Photon., ().. Greilich, A. et l. Nuclei-induced frequency focusing of electron spin coherence. Science 37, (7).. Sun, B. et l. Persistent nrrowing of nucler-spin fluctutions in InAs quntum dots using lser excittion. Phys. Rev. Lett. 8, 87 ().. Solenov, D., Economou, S. E. & Reinecke, T. L. Fst two-quit gtes for quntum computing in semiconductor quntum dots using photonic microcvity. Phys. Rev. B 87, 3538 (3). Acknowledgements This work ws supported y Multi-University Reserch Inititive (US Army Reserch Office; W9NF96), the NSA/LPS, nd the US Office of Nvl Reserch. The uthors thnk A. Greilich for contriutions during the preliminry stge of this reserch. NATURE POTONICS DOI:.38/NPOTON.3. Author contriutions All uthors were involved in prepring the mnuscript. S.G.C., T.M.S., A.S.B. nd D.G. conceived nd designed the experiments nd smples. A.S.B. grew the quntum dot smples. M.K., C.S.K. nd A.S.B. processed photonic crystls nd gtes in the smples. T.M.S., S.G.C. nd L.Y. opticlly chrcterized the cvities nd quntum dots. S.G.C. performed the differentil reflectivity nd lser control experiments. D.S., S.E.E., T.L.R. nd T.M.S. provided theoreticl insight nd clcultions. Additionl informtion Supplementry informtion is ville in the online version of the pper. Reprints nd permissions informtion is ville online t Correspondence nd requests for mterils should e ddressed to D.G. Competing finncil interests The uthors declre no competing finncil interests. 6 NATURE POTONICS ADANCE ONLINE PUBLICATION 3 Mcmilln Pulishers Limited. All rights reserved.