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[Neul Ha](https://orcid.org/0000-0002-7695-2193), [Takaaki Mano](https://orcid.org/0000-0002-6955-260X), [Takashi Kuroda](https://orcid.org/0000-0001-6445-7673), [Yoshiki Sakuma](https://orcid.org/0000-0001-6804-7217), [Kazuaki Sakoda](https://orcid.org/0000-0002-5530-3020)

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 This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Neul Ha, Takaaki Mano, Takashi Kuroda, Yoshiki Sakuma, Kazuaki Sakoda; Current-injection quantum-entangled-pair emitter using droplet epitaxial quantum dots on GaAs(111)A. Appl. Phys. Lett. 19 August 2019; 115 (8): 083106 and may be found at https://doi.org/10.1063/1.5103217.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Current-injection quantum-entangled-pair emitter using droplet epitaxial quantum dots on GaAs(111)A](https://mdr.nims.go.jp/datasets/b72a5445-a4f0-4f5c-a1be-a87afac4ed6d)

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Current-injection quantum-entangled-pair emitter using droplet epitaxial quantum dots on GaAs(111)AViewOnlineExportCitationRESEARCH ARTICLE |  AUGUST 21 2019Current-injection quantum-entangled-pair emitter usingdroplet epitaxial quantum dots on GaAs(111)A Neul Ha  ; Takaaki Mano  ; Takashi Kuroda   ; Yoshiki Sakuma  ; Kazuaki Sakoda Appl. Phys. Lett. 115, 083106 (2019)https://doi.org/10.1063/1.5103217Articles You May Be Interested InNanoholes fabricated by self-assembled gallium nanodrill on GaAs(100)Appl. Phys. Lett. (March 2007)Generation of maximally entangled states and coherent control in quantum dot microlensesAppl. Phys. Lett. (April 2018)Quantum key distribution with an entangled light emitting diodeAppl. Phys. Lett. (December 2015)  04 December 2025 22:24:58https://pubs.aip.org/aip/apl/article/115/8/083106/38582/Current-injection-quantum-entangled-pair-emitterhttps://pubs.aip.org/aip/apl/article/115/8/083106/38582/Current-injection-quantum-entangled-pair-emitter?pdfCoverIconEvent=citejavascript:;https://orcid.org/0000-0002-3253-0087javascript:;https://orcid.org/0000-0002-6955-260Xjavascript:;https://orcid.org/0000-0001-6445-7673javascript:;https://orcid.org/0000-0001-6804-7217javascript:;https://orcid.org/0000-0002-5530-3020https://crossmark.crossref.org/dialog/?doi=10.1063/1.5103217&domain=pdf&date_stamp=2019-08-21https://doi.org/10.1063/1.5103217https://pubs.aip.org/aip/apl/article/90/11/113120/912995/Nanoholes-fabricated-by-self-assembled-galliumhttps://pubs.aip.org/aip/apl/article/112/15/153107/35662/Generation-of-maximally-entangled-states-andhttps://pubs.aip.org/aip/apl/article/107/26/261101/235887/Quantum-key-distribution-with-an-entangled-lighthttps://servedbyadbutler.com/redirect.spark?MID=188841&plid=3385069&setID=1044459&channelID=0&CID=1622678&banID=524192615&PID=0&textadID=0&tc=1&rnd=8682890111&scheduleID=3549797&adSize=1640x440&data_keys=%7B%22%22%3A%22%22%7D&metadata=%5B%5D&mt=1764887098181495&spr=1&referrer=http%3A%2F%2Fpubs.aip.org%2Faip%2Fapl%2Farticle-pdf%2Fdoi%2F10.1063%2F1.5103217%2F14527687%2F083106_1_online.pdf&request_uuid=4467595b-f21f-4b40-ad80-e363a1f05c7c&hc=673001f558b3db4810da856b97a2da59a1518244&location=Current-injection quantum-entangled-pair emitterusing droplet epitaxial quantum dots on GaAs(111)ACite as: Appl. Phys. Lett. 115, 083106 (2019); doi: 10.1063/1.5103217Submitted: 25 April 2019 . Accepted: 1 August 2019 .Published Online: 21 August 2019Neul Ha, Takaaki Mano, Takashi Kuroda,a) Yoshiki Sakuma, and Kazuaki SakodaAFFILIATIONSNational Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japana)Author to whom correspondence should be addressed: kuroda.takashi@nims.go.jpABSTRACTA source of single photons and quantum entangled photon pairs is a key element in quantum information networks. Here, we demonstratethe electrically driven generation of quantum entangled pairs using a naturally symmetric GaAs quantum dot grown by droplet epitaxy.Coincidence histograms obtained at a temperature of 10 K reveal the generation of quantum entangled pairs that have a fidelity to the Bellpairs of 0.716 0.015, much beyond the classical limit. We study the temperature dependent device characteristics and estimate themaximum operation temperature to be � 65K, which is essentially limited by the weak charge carrier confinement in the present dot system.Our study offers a guideline for the fabrication of quantum entangled pair sources suitable for practical use.Published under license by AIP Publishing. https://doi.org/10.1063/1.5103217Quantum entanglement is an essential resource for the imple-mentation of quantum information processing. In a quantum net-work, for example, various quantum devices are expected to be linkedvia a photonic stream that serves as flying qubits. Entangled photonpairs transmitted in the network create nonlocal correlations betweenquantum devices and enable complex processing that is not availablewith a classical architecture. A photon emitting device is therefore akey element of a quantum network.The use of a semiconductor quantum dot (QD) as an entangledpair source was initially proposed in 2000.1 Despite the concept beingstraightforward and analogous to that of an atomic cascade,2 experi-mental implementation has been problematic3 due to the presence ofoptical anisotropy inherent in commonly used dots.4–6 Several postpro-duction techniques have already been developed to recover the QD isot-ropy.7–17 An alternative way to achieve an isotropic dot is to use a C3vsymmetric {111} crystallographic surface as a growth substrate. The tri-angular arrangement of atoms on the {111} surface eliminates a sourceof structural and wavefunction elongation and leads to near-perfect isot-ropy with a vanishing bright exciton splitting.18,19 Although conven-tional Stranski-Krastanov QD growth is prohibited on a {111} surface,other techniques, such as pyramidal selective etching,20,21 core-shellnanowire growth,22,23 and droplet epitaxy,24–27 have been used to createisotropic dots on {111} for the generation of quantum entangled pairs.Among these techniques, droplet epitaxy is a self-assembled QD growthtechnique and is thus suitable for large-scale device integration.In this work, we demonstrate the electrically driven operation ofentangled pair emission from droplet epitaxy GaAs QDs. Previousstudies on the droplet QD source have focused on optical excitationwith the aid of an external laser. This scheme is easy to implement, butnot favorable for practical applications. This drawback can be overcomeby constructing an electrically driven source of entangled pairs.28,29Here, we develop a photon emitting device comprising a naturally sym-metric QD grown on a Ga-rich GaAs(111)A substrate, encapsulated ina p-i-n diode structure. Thanks to our careful optimization of the growthof doped barrier layers along the [111] direction, our entangled-pairemitters can work at temperatures of up to 65K and are simply limitedby the charge carrier confinement of the GaAs/AlGaAs heterosystem.The QD diode sample is grown on nþ doped GaAs(111)A using asolid source molecular beam epitaxy machine. After depositing Si-doped n-type GaAs buffer (n¼ 1� 1018 cm�3), we grow a Si-dopedAl0.25Ga0.75As layer (n¼ 5� 1017 cm�3, 200nm thick), which serves asan electron supplying layer. Here, we optimize the aluminum composi-tion of the AlGaAs barrier at 0.25, which makes n-type conduction pos-sible even at low temperatures, while a reasonably high barrier height ismaintained.30 The QD layer is separated from the n-doped (p-doped)region by a 120nm (80nm) thick intrinsic AlxGa1–xAs layer (x¼ 0.25).In addition, we insert an atomically thin AlxGa1–xAs layer (x¼ 0.35,1 nm thick) under the QD layer to avoid the occurrence of the localdroplet etching effect during the droplet formation process.31,32For the QD formation, 0.05 of a monolayer (ML) of gallium issupplied at a growth rate of 0.01 ML/s at 450 �C, yielding the forma-tion of gallium droplets on AlGaAs(111)A with a density of4� 108 cm�2. Then, an As4 flux (5� 10–6Torr) is supplied at 200 �Cfor the crystallization of GaAs dots from gallium droplets, followed byAppl. Phys. Lett. 115, 083106 (2019); doi: 10.1063/1.5103217 115, 083106-1Published under license by AIP PublishingApplied Physics Letters ARTICLE scitation.org/journal/apl 04 December 2025 22:24:58https://doi.org/10.1063/1.5103217https://doi.org/10.1063/1.5103217https://www.scitation.org/action/showCitFormats?type=show&doi=10.1063/1.5103217http://crossmark.crossref.org/dialog/?doi=10.1063/1.5103217&domain=pdf&date_stamp=2019-08-21https://orcid.org/0000-0002-3253-0087https://orcid.org/0000-0002-6955-260Xhttps://orcid.org/0000-0001-6445-7673https://orcid.org/0000-0001-6804-7217https://orcid.org/0000-0002-5530-3020mailto:kuroda.takashi@nims.go.jphttps://doi.org/10.1063/1.5103217https://scitation.org/journal/aplin vacuo annealing at 500 �C. With these conditions, GaAs QDs havea disklike shape with an average base diameter of 40 nm and a heightof 1.0 nm (see supplementary material Fig. S1 for an atomic micros-copy image of the QD surface).After capping the QDs with an undoped AlGaAs barrier,C-doped p-type Al0.25Ga0.75As (p¼ 5� 1017 cm�3, 200 nm thick),highly C-doped Al0.25Ga0.75As (p¼ 2� 1018 cm�3, 20 nm thick), andC-doped GaAs (p¼ 2� 1019 cm�3, 20 nm thick) layers are formed at520 �C. A top ohmic electrode with 10lm square open windows isfabricated with photolithography and Ti/Pt/Au deposition. Finally,chemical etching is performed to form a mesa structure in order torestrict the carrier flow region. A top view of this device, denoted sam-ple A, is shown in the inset of Fig. 1(a). We also study a similar diodestructure with a slightly different layer sequence, denoted sample B,and its growth details are shown in supplementary Fig. S2.In optical experiments, forward DC bias is applied between the topand bottom electrodes so that charge carriers are continuously injectedinto the GaAs QDs. Electroluminescence (EL) signals are collected byusing a micro-objective lens with a numerical aperture of 0.8 and thenpassed through a wave retarder and a Glan prism, which serves as apolarization analyzer. Then, the photon beam is fed into a monochro-mator equipped with a silicon avalanche photodiode. We count threephoton channels simultaneously, i.e., biexciton (XX) and exciton (X)photons with a given polarization state and X photons with an orthogo-nal polarization state. This setup enables us to suppress the effect ofdetection-yield fluctuation during the measurement. The number ofcoincidences is analyzed using a fast-response time-to-digital converter.Figure 1(a) shows the current-voltage (I-V) characteristics of ourdevice (sample A). It reveals an asymmetric response with a thresholdat a forward bias, which is a characteristic of a p-i-n diode structure.The threshold voltage is observed to be lower at 90K than at 10K dueto the thermal activation of free carriers.Figure 1(b) shows the EL spectrum of a single GaAs QD. Threeemission lines are observed and assigned as charge neutral X, positivelycharged X (Xþ), and charge neutral XX, from the higher energy side.The EL dependence on temperature is investigated, where the bias cur-rent is adjusted so that the X intensity reaches �80% of its saturationvalue. With increasing temperature, all the spectral lines move to thelower energy side along with the temperature shift in the bandgap.They are broadened simultaneously and accompanied by the acousticphonon band at temperatures as high as 70K.33 Note that significant Xand XX lines are observed even at 70K (although the intensity is �1/4of that at 10K). Stable QD emission at such high temperatures is notcommon in standard photoinjection devices using GaAs dots, implyingthat effective carrier injection is achieved in the p-i-n diode.Figure 1(c) shows the dependences of the X and XX intensities onbias current. Linear and quadratic dependences are confirmed in thelow current regions for X and XX signals, respectively, supporting theirspectral identification. We find that the bias current needed to reachthe saturation intensity is more than ten times higher at 10K than at70K. This is due to the quenching of the active carrier concentration atlow temperatures, as discussed in the supplementary material.The results of photon correlation measurements at 10K are sum-marized in Fig. 2, where L, R, H, and V are the left-hand circular,right-hand circular, linear horizontal, and vertical polarizations,respectively; D is linear diagonal with a polarization axis tilted by 45�from H, and A is antidiagonal where A? D. The top panel in Fig. 2(a)is a coincidence histogram for L-polarized XX photons and R-polarized X photons (denoted LR). The central coincidence peak hasan asymmetric shape that consists of a dip (negative peak) at t� 0 anda positive peak at t� 0. Note that coincidence signals at positive timesare counted for the detection of an XX photon followed by that of anX photon. Thus, the measured asymmetric evolution in the coinci-dence traces confirms the XX-X radiative cascade. The XX and X pho-tons that have a polarization combination of LR are clearly correlated,resulting in a higher probability than that for detecting uncorrelatedphotons. The peak disappears for LL and RR, while it recovers for RL.Figures 2(b) and 2(c) show coincidence histograms for linear polar-izations. A positive correlation appears for parallel polarization (e.g.,HHand DD), while it disappears for perpendicular polarization (e.g., HVand DA). These polarization correlations, which are independent ofthe choice of projection basis, suggest that the measured two-photonwavefunction is approximated by one of the maximally entangled (Bell)states, jWþi ¼ fjHHi þ jVVig=ffiffiffi2p� fjRLi þ jLRig=ffiffiffi2p. It shouldbe noted that the coincidence dip at t� 0 does not reach zero, althoughwe expect no probability of observing an X photon followed by an XXphoton (reversed cascade). This nonideal signature is due to the finiteresolution of the coincidence setup. Our photon detector has a timing jit-ter of �400 ps close to the emission lifetime of GaAs dots (�600 ps).Hence, the sharp antibunching dip is smoothed out and observed to stayat a finite value at t¼ 0. This smoothing effect also influences the preciseevaluation of positive correlation intensities and makes it difficult todetermine the degree of quantum entanglement.To quantify the correlation degree, we measure coincidence sig-nals with a reduced current injection, where we expect to suppressthe probability of counting accidental (uncorrelated) photons.34,35FIG. 1. (a) I-V dependence measured at 10 and 90 K (sample A). The inset showsthe top view of our device. (b) EL spectra of a single GaAs QD measured at 10, 30,50, and 70 K (sample A). (c) X and XX intensities as a function of bias current at 10and 70 K (sample A). Broken lines behind the data points are linear and quadraticdependences drawn to guide the eye.Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 115, 083106 (2019); doi: 10.1063/1.5103217 115, 083106-2Published under license by AIP Publishing 04 December 2025 22:24:58https://doi.org/10.1063/1.5103217#supplhttps://doi.org/10.1063/1.5103217#supplhttps://scitation.org/journal/aplFigure 3 reveals the impact of current injection I on the coincidencetrace. When I¼ 40mA, close to a saturation current, we observe thecorrelation peaks superimposed on the high backgrounds associatedwith accidental coincidence [Fig. 3(a)]. When I¼ 15mA, the back-ground is greatly reduced, although a significant coincidence peakremains [Fig. 3(b); see also supplementary Fig. S3 for histograms withall polarization settings and supplementary Table S1 for the numbersof coincidence extracted from these histograms]. Thanks to thesenearly background-free traces, we can evaluate the polarization degreefor each projection setting, i.e., CRL¼ 0.706 0.03, and CHV   CDA¼ 0:566 0:03, where the polarization degree is defined asC ¼ jðnjj � n?Þ=ðnjj þ n?Þj, and njj ðn?Þ is the coincidence numbernormalized with the accidental two-photon flux for a copolarized(cross-polarized) setting. The number of coincidence is integrated overa time window of 0.9 ns, which is sufficiently longer than the radiativelifetime. Note that we observe a lower correlation in linear polarizationthan circular polarization. This is a typical signature of dephasinginduced by nuclear spin noise.36Using the above set of C values, we determine the fidelity to theBell state as f ¼ 0.716 0.015, where we adopt the simple expressionf ¼ ð1þ CRL þ CHV þ CDAÞ=4.37 Note that the fidelity parameter ischaracterized without any background subtraction (see the supple-mentary note for the calculation details). The measured fidelity to Bellpairs is higher than 0.5, which is the classical upper limit expectedunder local-objective theory. Thus, we confirm that our diode gener-ates quantum entangled pairs.We also study the impact of increasing temperature on deviceperformance. Figures 4(a)–4(c) show coincidence histograms for co-and cross-circular polarizations measured at different temperatures. Inthis measurement, we adjust the bias current so that the X intensity iskept at �80% of its saturation value. With increasing temperature, thecross-circular (LR) peak height decreases, but the peak signature isapparent even at 50K. A similar correlation is measured for the otherpolarization basis and shown in supplementary Fig. S4. The measurednumbers of coincidence are summarized in supplementary Table S2.Figure 4(d) shows the fidelity to the Bell state as a function of tempera-ture, where we integrate the coincidence counts over a time interval of512 ps, which is similar to the emission lifetime. The fidelity valuedecreases with temperature and crosses the classical limit of 0.5 at�65K, which serves as a rough guideline for the maximum operationtemperature of our device.FIG. 2. Coincidence histograms between XX and X photons emitted from a GaAsQD for different polarizations at 10 K (sample B). The two-photon projection settings(such as LR) are indicated by the first letter for XX photons and the second letterfor X photons. The bias current is set so that the EL intensity almost reaches its sat-uration value (I¼ 40mA). The histograms are integrated with a time bin of 128 ps.FIG. 3. Comparison of coincidence histograms at 10 K measured with bias currentsof (a) 40 mA and (b) 15mA (sample B). In each panel, we plot the sum of the coin-cidence traces for two equivalent projection settings, such as LR and RL.FIG. 4. Temperature dependence of coincidence histograms for circular polarizationcombinations measured at (a) 10 K, (b) 30 K, and (c) 50 K (sample A). (d) The fidel-ity to the Bell state of emitted photons as a function of operation temperature.Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 115, 083106 (2019); doi: 10.1063/1.5103217 115, 083106-3Published under license by AIP Publishing 04 December 2025 22:24:58https://scitation.org/journal/aplWe attribute the correlation reduction with temperature to the shal-low confinement of the charge carriers in our GaAs/AlGaAs QD system.Note that the confinement depth of charge carriers is estimated to bearound 50meV in the present dot. This weak confinement means thatcharge carriers at high temperatures easily escape from dots. As the timescale of carrier escape/injection becomes comparable to that of the XX-Xemission cascade, the probability of observing uncorrelated photonsincreases, and the correlation degree decreases. The robustness againstincreasing temperature could therefore be improved by using a hetero-structure material that has a higher quantum confinement, as recentlydemonstrated in a telecommunication-wavelength QD system.38In conclusion, we fabricated an electrically driven entangled pairsource using droplet epitaxy GaAs dots embedded in a p-i-n diode.Thanks to the high structural symmetry of dots grown on (111)A, weconfirmed a clear polarization correlation whose degree is well beyondthe classical limit even with a DC injection. The quantum nature ofthe emitted photon pairs is conserved at operation temperatures up to65K, which is essentially limited by the shallow confinement of chargecarriers in GaAs/AlGaAs heterosystems. Thus, the application of alarge offset heterosystem to the QD base material is a potential routeto achieving higher operating temperatures.See the supplementary material for an atomic microscopy imageof the QD surface (supplementary Fig. S1), the detailed layer sequenceof the studied diodes (supplementary Fig. S2), complete sets of coinci-dence histograms for all polarization combinations (supplementaryFig. S3) at different temperatures (supplementary Fig. S4), full lists ofthe measured coincidence numbers (supplementary Tables S1 and S2),and description about the formula used to calculate the fidelity.We acknowledge the support of a Grant-in-Aid from theJapan Society for the Promotion of Science.REFERENCES1O. Benson, C. Santori, M. Pelton, and Y. Yamamoto, “Regulated and entangledphotons from a single quantum dot,” Phys. Rev. Lett. 84, 2513–2516 (2000).2A. Aspect, P. Grangier, and G. Roger, “Experimental tests of realistic local the-ories via Bell’s theorem,” Phys. Rev. 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