On-Demand Single Photons with High Extraction Efficiency and Near-Unity Indistinguishability from a Resonantly Driven Quantum Dot in a Micropillar

1.3 MB·

by  Xing Ding

On-DemandSinglePhotonswithHighExtractionEfficiencyandNear-UnityIndistinguishability
fromaResonantlyDrivenQuantumDotinaMicropillar
XingDing,1,2,3 YuHe,1,2,3 Z.-C.Duan,1,2,3 NielsGregersen4,M.-C.Chen1,2,3,S.Unsleber5,S.Maier5,
Christian Schneider5, Martin Kamp5, Sven Ho¨fling1,5,6, Chao-Yang Lu1,2,3 and Jian-Wei Pan1,2,3
1 ShanghaiBranch,NationalLaboratoryforPhysicalSciencesatMicroscaleandDepartmentofModernPhysics,
University of Science and Technology of China, Shanghai, 201315, China
2 CASCenterforExcellenceandSynergeticInnovationCenterinQuantumInformationandQuantumPhysics,
University of Science and Technology of China, Hefei, Anhui 230026, China
3 CAS-Alibaba Quantum Computing Laboratory, Shanghai, 201315, China
4 DTU Fotonik, Department of Photonics Engineering,
Technical University of Denmark, Building 343, DK-2800 Kongens Lyngby, Denmark
6 5 TechnischePhysik,PhysikalischesInstita¨tandWilhelmConradRo¨ntgen-CenterforComplexMaterialSystems,
1 Universitat Wu¨rzburg, Am Hubland, D-97074 Wu¨zburg, Germany and
0 6 SUPA,SchoolofPhysicsandAstronomy,UniversityofSt. Andrews,St. AndrewsKY169SS,UnitedKingdom
2 (Dated:submittedtoPRLon29/09/2015,acceptedon01/12/2015,publishedon14/01/2016.)

y Scalablephotonicquantumtechnologiesrequireon-demandsingle-photonsourceswithsimultaneouslyhigh
a levelsofpurity,indistinguishability,andefficiency. Thesekeyfeatures,however,haveonlybeendemonstrated
M
separatelyinpreviousexperiments. Here,bys-shellpulsedresonantexcitationofaPurcell-enhancedquantum
dot-micropillarsystem,wedeterministicallygenerateresonancefluorescencesinglephotonswhich,atπpulse

3 excitation,haveanextractionefficiencyof66%,single-photonpurityof99.1%,andphotonindistinguishability
1 of98.5%.Suchasingle-photonsourceforthefirsttimecombinesthefeaturesofhighefficiencyandnear-perfect
levelsofpurityandindistinguishabilty,andthusopenthewaytomulti-photonexperimentswithsemiconductor
]
quantumdots.
h
p
-
t Single-photon devices [1] that deterministically emit one broadening of the excited state [15] and uncontrolled emis-
n
a and only one photon at a time are central resources for scal- sion time jitter from the non-radiative high-level to s-shell
u able photonic quantum technologies [2]. In particular, they relaxation [16]. The detrimental effect of the time jitter can
q are of considerable interest in Boson sampling [3], an inter- be more severe when using a QD-microcavity system where
[
mediatequantumcomputationwhereitisestimatedthatwith the Purcell-reduced radiative lifetime (T1) of the QD, which
2 20-30singlephotonsonecandemonstratecomplextasksthat boundsthesinglephoton’scoherencetime(T )byT (cid:54)2T ,
2 2 1
v canbedifficultforclassicalcomputers. Tobeusefulforthese iscomparabletoorsmallerthanthetimejitter[16].
4
applications,itiscrucialthatthesingle-photonsourcesimul- To overcome these shortcomings, increasing efforts have
8
2 taneously possesses high efficiency, near perfect photon an- been devoted to resonant excitation of QDs [12, 13, 17–19].
0 tibunchingandindistinguishability—alongsought-aftergoal By s-shell resonant laser excitation on a single QD with pi-
0 inquantumphotonics. cosecond laser pulses, near-background-free resonance fluo-
.
1 Self-assembledquantumdots(QDs)[5]havebeenshownto rescence(RF)hasbeenobtainedandallowedtheobservation
0 possesthehighestquantumefficiencyinallsolid-statesingle- of Rabi oscillations [12]. Under π pulse excitation, single
6
photondevicessofar,andthusarepromisingasdeterministic photons have been deterministically generated with a single-
1
single-photonemitters. Tremendousprogresshasbeenmade photonpurityof98.8(2)%andindistinguishabilityof97(2)%.
:
v inthepastdecadesindemonstrationsofvarioussingle-photon However,theplanarcavitystructureusedinRef.[12]hasal-
i
X sources [6–13], however, none has allowed a scalable exten- lowedonly∼6%ofthegeneratedsinglephotonscollectedby
siontomulti-photonexperiments.Thereasonisthatmostpre- thefirstlens.
r
a viousexperimentseitherreliedonnon-resonantexcitationof In this Letter, we report the first single-photon source that
aQD-microcavitythatdegradedthephotonpurityandindis- combines near perfect single-photon purity, indistinguisha-
tinguishability,orusedresonantexcitationofaQDinaplanar bility and high extraction efficiency. By pulsed strict res-
cavitythatlimitedtheextractionefficiency. onant excitation of a Purcell-enhanced QD-micropillar sys-
In the non-resonant excitation experiments, single-photon tem, at π pulse we obtain pulsed RF single photons with
generation efficiencies typically increased asymptotically a count rate of 3.7 million per second on a silicon single-
with pump power, and a trade-off between efficiency and photondetector,whichcorrespondstoanextractionefficiency
single-photon purity and/or indistinguishability were ob- of ∼66% from GaAs. The RF photons show an antibuching
served [6–11]. It has been shown that the recapture of carri- ofg2(0) = 0.009(2)andaraw(corrected)two-photoninter-
ersintotheQD-micropillarduetonon-resonantpumpingcan ference visibility of 96.4(3)% (98.5(4)%). Our work opens
lead to a significant degradation of the single-photon purity thewaytoscalablemulti-photonquantuminformationexper-
[14].Furthermore,thephoton’sindistinguishabilitycanbere- imentswithsolid-statedevices.
duced by non-resonant excitation that induces homogeneous Our experiments were performed on an InAs/GaAs self-
2
a b c
2.5μm
700
24
} 600
15 pairs s)
20 p
K) e ( 500
}25.5 pairs Temperature ( 1126 ulsed RF lifetim 234000000 RF counts (a.u.)0.11
P
8
100
0.0 0.5 1.0 1.5 2.0
Time delay (ns)
0
896 897 898 899 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Wavelength (nm) QD-cavity detuning (meV)
FIG.1:Purcell-enhancedQD-micropillarsystem.(a)AnillustrationofasingleQDembeddedinamicropillar.TheQDisgrownviamolecular
beamepitaxy,embeddedinaλ-thickGaAscavityandsandwichedbetween25.5lowerand15upperDBRstacks. Micropillarswith2.5µm
diameterweredefinedviaelectronbeamlithography. (b)2Dintensity(inlogscale)plotoftemperature-dependentmicro-photoluminescence
spectra.Theexcitationcwlaserisat780nmwavelengthandthepoweris∼3nW.(c)PulsedRFlifetimeasafunctionofQD-cavitydetuning
byvaryingthetemperature. Thetime-resolveddataismeasuredusingasuperconductingnanowiresingle-photondetectorswithafasttime
resolution of ∼63 ps. The orange curve is a fit using the standard theoretical formula from ref.[20]. The inset shows two examples of
time-resolvedRFcountsatQD-cavityresonanceandatfardetuning.
assembledQDembeddedinsidea2.5µmdiametermicropil- mogeneouslinewidthisveryclosetothelifetime-limitedline
lar cavity which has 25.5 (15) λ/4-thick AlAs/GaAs mirror width of 1.89 GHz. The inhomogeneous (Gaussian) compo-
pairs forming the lower (upper) distributed Bragg reflectors nent in the spectrum can becaused by spectral diffusion due
[19](seeFig.1a). Thedeviceiscooledinsideacryogen-free tochargefluctuationsinthevicinityoftheQD[21].
bathcryostatwithatemperaturethatcanbefinelytunedfrom Figure2b shows the detected pulsed RF photon counts on
4.2 K to 30K. We first characterize the QD through micro- asiliconsingle-photondetectorasafunctionofincidentfield
photoluminescence measurements with 780 nm laser excita- amplitude. WeobserveaRabioscillationwhichisduetoco-
tion. Figure1b shows photoluminescence spectra of a single herent control of the QD two-level system [22]. The single-
QD as a function of temperature tuning. The photolumines- photon counts reaches the maximum for a π pulse with an
cence intensity reaches a plateau at a temperature range of pumping laser power of 24 nW. We note that this is in stark
4.5-10K.Thequalityfactorofthemicropillarcavityismea- contrast with previously experiments with non-resonant ex-
suredtobe6124. citation where the generated single-photon counts typically
For pulsed resonant excitation, a Ti:Sapphire laser is used growasymptoticallywithexcitationlaserpower,thusanear-
togeneratelaserpulsesatacentralwavelengthof897.44nm unityefficiencywouldneedveryhighpowerpumping.
and a pulse width of ∼3 ps. The excitation laser is further At the π pulse, we observe a count rate of 3,700,000 per
filtered with an etalon with a linewidth of 45 GHz to match secondonasiliconsingle-photondetector,under81MHzrep-
the micropillar cavity. A confocal microscope is operated in etitionratelaserexcitation,whichgivesanoverallsystemef-
across-polarizationconfiguration[12],wherebyapolarizeris ficiencyof4.6%.Thesignal(RF)tobackground(mainlylaser
placed in the collection arm with its polarization perpendic- leakage)ratiois40:1. Aftercorrectingforindependentlycal-
ular to the excitation laser suppressing the laser background ibrated photon detection efficiency (∼33%), polarization ex-
byafactorexceeding107. TodeterminethePurcellfactorof tinction(∼50%),transmissionrateintheopticalpath(∼60%,
themicropillarcavity,weperformtime-resolvedRFmeasure- includingopticalwindow, polarizer, andtwobeamsplitters),
mentsfordifferentcavity-QDdetunings,asplottedinFig.1c. and single-mode fiber coupling efficiency (∼72%), excited-
TheshortestRFlifetimeis83.9psat7.8K(bluecurveinthe statepreparationefficiency(∼96%)atπpulse,andassuming
insetofFig1c). At28Kwithalarge(>1meV)detuning,the perfect internal quantum efficiency of the QD, we estimate
decaytimeconstantincreasesto587.8ps(redcurveinthein- that 66% of the generated single photons are collected into
setofFig1c).Themeasuredlifetimeasafunctionofdetuning thefirstobjectivelens(NA=0.68).
iswellfittedbythestandardweak-couplingtheoreticalmodel
Tofurtherimprovethesignaltobackgroundratio,wepass
(orangecurve)[20]whichallowsustodeduceaPurcellfactor
the RF through a 3-GHz etalon and then characterize its pu-
of6.3(4).
rityandindistinguishability. Thesingle-photonnatureofthe
Figure2a shows a high-resolution spectrum of the pulsed collected RF at the π pulse is evident from second-order
RF, which can be best fitted using a Voigt profile with a ho- coherence measurement [23] shown in Fig.2c where nearly
mogeneous(Lorentzian)linewidthof1.91(7)GHzandinho- vanishing multi-photon probability is observed at zero delay
mogeneous (Gaussian) line width of 1.14(9) GHz. The ho- (g2(0) = 0.009(1)). Figure2dsummarizethecombinedper-
3
a b
8000 4,000,000
6000 3,000,000
s)
nts (/0.05 4000 ounts (/s) 2,000,000
ou C
C
2000 1,000,000
0 0
-12 -8 -4 0 4 8 12 0 3 6 9 12
0.5
Frequency (GHz) Excitation field amplitude (nW )
c d
1.0 0.7 efficiency 0.020
e 2
s (a.u.) 0.8 er puls 00..56 g(0) Secon
nt p 0.015 d
u s -o
o 0.6 n 0.4 r
c o d
malized 0.4 ed phot 00..23 0.010 er corre
or ct la
N 0.2 olle 0.1 tion
C
0.005
0.0 0.0
-37.5 -25.0 -12.5 0.0 12.5 25.0 37.5 0.0 0.5 1.0
Delay (ns) Pulse area ( )
FIG.2:CharacterizationofthepulsedRFsingle-photonsource.(a)Ahigh-resolutionRFspectrumwhenexcitedbyaπpulse,obtainedusing
ahome-builtFabry-Pe´rotscanningcavityhasafinesseof170,alinewidthof220MHz(fullwidthathalfmaximum),afreespectralrangeof
37.4GHz,andatotaltransmissionrateof61%. TheorangelinewasfittedusingaVoigtprofile. (b)DetectedpulsedRFcountsonasilicon
single-photondetectorasafunctionofthesquarerootofexcitationlaserpower. Thebluecurveisaguidetoeyes. (c)Intensity-correlation
histogramofthepulsedRFphotonsunderπpulseexcitationobtainedusingaHanburyBrownandTwiss-typeset-up[23]. Thesecond-order
correlationg2(0) = 0.009(1)iscalculatedfromtheintegratedphotoncountsinthezerodelaypeakdividedbyitsadjacentpeak. (d)The
photonscollectedintothefirstlensperpulse(generation+extractionefficiency)versusthemeasuredsingle-photonpurity(seetext)versus
pumppower.
formanceoftheefficiencyandsingle-photonpurityasafunc- experiments. We first adopt a similar free-space set-up as
tion of pump power. We emphasize that the high generation shown in Ref.[12] with a time delay of 2.1 ns between two
andextractionefficiencyareobtainedwithlittlecompromise pulses. Figure3a and 3b show time-delayed histograms of
ofthesingle-photonpurity,whichisimportantforrealappli- normalizedtwo-photoncountsfororthogonalandparallelpo-
cationsinphotonicquantuminformationprocessing. larization,respectively. Analmostvanishingzero-delaypeak
Anothercrucialdemandisthatthephotonsshouldpossess is observed for two photons with identical polarization. In
a high degree of indistinguishability, which is at the heart of contrast, for two photons with cross polarization, the zero-
opticalquantumcomputing[2],Bosonsampling[4]andsolid- delay peak has the same intensity as its adjacent peaks. We
state quantum networks [25]. Pulsed s-shell resonant exci- obtain a raw two-photon quantum interference visibility of
tation has been demonstrated to yield a near-unity indistin- 0.964(3).Aftercorrectingwiththeresidualmultiphotonprob-
guishability by eliminating emission time jitter and dephas- ability(g2(0)=0.009(1)),weobtainedthecorrecteddegrees
ings, however, only for QDs in planar cavities. We note that ofindistinguishabilitytobe0.985(4).
the pulsed RF technique is more critically needed for QDs WefurthertesttheHong-Ou-Mandelinterferencebetween
withlargePurcellfactorswherethereducedradiativelifetime twoconsecutivelyemittedsinglephotonsatanincreasedtime
approachesthetimejitter[12,16]. delayof12.4ns—thelaserpulseseparation(seeRef.[10]for
Thesinglephotons’indistinguishabilityistestedusingnon- asimilarsetup). Theresultinghistogramsfororthogonaland
postselective two-photon Hong-Ou-Mandel interference [24] parallel polarized two photons are shown in Fig.3c-d, from
4

a c
1.0 1.0
0.8 0.8
)
a.u.) 0.6 (a.u. 0.6
unts ( 00..24 ounts 00..24
o c
alized c 10..00 b malized 01..00 d
rm 0.8 or 0.8
o N
N
0.6 0.6

0.4 0.4
0.2 0.2
0.0 0.0
-6 -4 -2 0 2 4 6 -40 -30 -20 -10 0 10 20 30 40
Time delay (ns) Time delay (ns)
FIG.3: QuantuminterferencebetweentwopulsedRFsinglephotonsfromaPurcell-enhancedQD-micropillarsystem. Thetimeseparation
betweenthetwosinglephotonsemittedfromthesingleQDaresetat2.1nsina-band12.4nsinc-d,respectively.Theinputtwophotonsare
π-pulseexcitedandpreparedincross(a,c)andparallel(b,d)polarizations,respectively.Thefittingfunctionistheconvolutionofexponential
decay(emitterdecayresponse)withGaussian(photondetectiontimeresponse). Theareaofthefittedcentralpeaksareextractedandusedto
calculatetherawvisibility,whichis0.964(3)and0.959(3)forthetimedelayof2.1nsand12.4ns,respectively.Allthedatapointspresented
arerawdatawithoutbackgroundsubtraction.
which we extract raw and corrected visibilities of 0.959(3) multaneouslyachievesagenerationefficiencyof96%,extrac-
and0.978(4),respectively. InstrongcontrasttoRef.[10]that tionefficiencyof66%,single-photonpurityof99.1%andin-
usednon-resonantexcitation,increasingthetimedelayby10 distinguishabilityof98.6%. Suchasingle-photonsourcecan
ns has negligible effect on the photons’ indistinguishability, be readily used to perform multi-photon interferometric ex-
indicatingthattheinhomogeneousbroadening(seeFig.2a)of perimentswithasolid-stateplatform. Immediateapplications
thepulsedRFphotonsaremainlyfromspectraldiffusionsat includeimplementationofBosonsampling[4]withtime-bin
timescalesmuchslowerthan10ns.Thisdemonstratesthepo- encoding using a loop-based architecture [26]. In addition
tentialofthisQD-micropillardeviceasanefficientsourceofa tothephotonicapplications,thehigh-efficiencyextractionof
stringofindistinguishablesinglephotons, whichareparticu- transition-selectiveRFwouldalsoallowafast(nanoseconds),
larlysuitableforlinearopticalquantumcomputingandBoson high-fidelitysingle-shotreadoutofsingleelectronspins[29].
sampling experiments with time-bin encoding [26]. Alterna- The time-jitter free, pulsed RF method is compatible with
tively, onecanalsodemultiplexthesingle-photonstringinto higher Purcell factor, which can allow photon extraction to
multiplephotonsatseparatespatialmodes. be further improved in the future by optimized microcavity
We compare the performance of the single-photon source fabrications,withoutcomprisingthesingle-photonpurityand
created in this work with heralded single photons produced indistinguishability. The current overall system efficiency—
by parametric down conversion [27] which have served as 4.6%,thehighestreportedinQDs—canalsobeimprovedus-
the workhorse for multi-photon interferometric experiments ingtechniquessuchasorthogonalexcitationanddetectionof
inthepastdecades[2]. Inthepreviousdemonstrationofthe RF [17, 18], near-unity-efficiency superconducting nanowire
largest, eight-photon entanglement [28], sources of triggered single-photondetection[30],andantireflectioncoatingsofthe
singlephotonsweregeneratedwithacountrateof310,000/s, opticalelements.
multi-photon emission probability of 2.9% and raw indistin- note: Afterthispaperwassubmitted,webecameawareof
guishability of 76%, under a laser pump power of 880 mW. arelatedworkonarXiv[31].
For applications such as Boson sampling [4] that needs sin- Acknowledgement: ThisworkwassupportedbytheNational
glephotonsasinput,thesingle-photonsourcerealizedhereis Natural Science Foundation of China, the Chinese Academy
superior,asitistentimebrighter,nearperfectlyindistinguish- of Sciences, and the National Fundamental Research Pro-
able,andrequiresapumppowerthatis7ordersofmagnitude gram. We acknowledge financial support by the State of
lower. Bavaria and the German Ministry of Education and Re-
In summary, by pulsed s-shell pumping a QD-micropillar search(BMBF)withintheprojectsQ.com-HandtheChist-era
system with a Purcell factor of 6.3, we have realized a high- project SSQN. NG acknowledges support from Danish Re-
performancesingle-photonsourcethatatπpulseexcitationsi- searchCouncilforTechnologyandProduction.
5
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