Table Of Content

Electrically driven and electrically tunable quantum light sources J. P. Lee,1,2 E. Murray,1,3 A. J. Bennett,1,∗ D. J. P. Ellis,1 C. Dangel,1,3,4 I. Farrer,3,† P. Spencer,3 D. A. Ritchie,3 and A. J. Shields1 1Toshiba Research Europe Limited, Cambridge Research Laboratory, 208 Science Park, Milton Road, Cambridge, CB4 0GZ, U.K. 2Engineering Department, Cambridge University, 9 J. J. Thomson Avenue, Cambridge, CB3 0FA, U.K. 3Cavendish Laboratory, Cambridge University, J. J. Thomson Avenue, Cambridge, CB3 0HE, U.K. 4Physik Department, Technische Universität Mu¨nchen, 7 85748 Garching, Germany 1 (Dated: January 17, 2017) 0 2 Compact and electrically controllable on-chip sources of indistinguishable photons are desirable for the development of integrated quantum technologies. We demonstrate that two quantum dot n a lightemittingdiodes(LEDs)incloseproximityonasinglechipcanfunctionasatunable,all-electric J quantumlightsource. LightemittedbyanelectricallyexciteddrivingLEDisusedtoexcitequantum dots the neighbouring diode. The wavelength of the quantum dot emission from the neighbouring 6 driven diode is tuned via the quantum confined Stark effect. We also show that we can electrically 1 tune the fine structure splitting. ] l l a Sources of indistinguishable single photons or entan- ing as it allows for individual tuning of many closely h gled photon pairs have applications in quantum key dis- spaceddevicessuitableforproducingcompactintegrated - s tribution [1, 2], quantum teleportation [3], imaging [4] quantumtechnologies. Wenotethatalthoughstraintun- e and linear optical quantum computing [5]. ingisusuallyappliedtoawholedevice,recentlyadevice m Self assembled quantum dots (QDs) can be used in wasdevelopedthatwouldinprinciplebesuitableforthe . solid-state quantum light sources. In an InAs QD the production of multiple strain tuned diodes [13]. Strain t a decay of a bright neutral or singly charged exciton re- tuningcanalsobeusedintandemwithelectrictuningin m sultsintheemissionofasinglephoton[6]. Furthermore, order to allow independent control over the exciton and - the biexciton cascade of a QD has been used to gener- biexciton energies [14]. d ate on-demand entangled photon pairs [7]. These QDs n are produced using well-developed semiconductor fabri- o cation techniques, allowing them to be integrated into c [ semiconductor devices and photonic cavities. An ongoing challenge of working with self assembled 1 quantum dots is their random distribution in the size, v shape and chemical composition, which leads to a ran- 5 5 dom distribution in the wavelength of the emitted pho- 2 tons. The distribution in transition energies is over 104 4 times the width of each individual line, consequently the 0 probability of two randomly chosen quantum dots hav- . 1 ing emission lines at the same wavelength is less than 0 10−4 [8]. Any difference in wavelength between photons 7 makes them distinguishable. Clearly this obstacle must 1 be overcome in order to realise the many applications of : v quantumlightsourcesthatmakeuseofHong-Ou-Mandel FIG. 1. An illustration of the devices used. The devices i (HOM)interference,whichrequiresthephotonstobein- X have a p-doped region (red), an intrinsic region (clear) and distinguishable. an n-doped region (blue). A LED driven strongly in forward r a Severalmethodsoftuningtheemissionwavelengthsof bias (left) emits light (shown as a blue beam), which excites quantumdotsin-situhavebeenexplored,includingvary- quantumdotsinaneighbouringdevice(right). Thequantum ing the temperature [9], magnetic field [10], strain [11] dots emit anti-bunched light (green). andelectricfield[12]. Thelatterapproachisverypromis- Applying a vertical electric field to a QD causes the transitions to undergo a Stark shift, which tunes the wavelength of the emitted photons. In most devices, ap- ∗ [email protected] † Current affiliation: Department of Electronic & Electrical En- plyinganelectricfieldofover−60kVcm−1 canresultin gineering, University of Sheffield, Mappin Street, Sheffield, S1 the quenching of emission [15]. This can be increased by 3JD,U.K. anorderofmagnitudebyembeddingthequantumdotsin 2 Diode 1 Diode 2 Normalised Intensity 850 Wemeitstisniogn layer Wabestotirnpgti olanyer 935 11 5000 4000 Wavelength (nm)900 Ndeocna-ryadiative Wavelength (nm)930 X X - 0 0123.000000000000 + X 950 Quantum dot emission 925 -350 -300 -250 -200 -150 -100 -50 Emission (a.u.) Emission / Absorption (a.u.) -1 Electric Field (kV cm ) FIG.2. Adiagramshowingtheprincipleofoperation-Light FIG. 4. The emission spectrum of Diode 2 (driven by Diode emitted from the wetting layer of Diode 1 is absorbed by the 1)asafunctionofappliedelectricfield. Tuningofover5nm wetting layer of Diode 2, generating charge carriers in Diode is visible. 2. The charge carriers can be captured by quantum dots in Diode 2 resulting in quantum light emission. The wetting layeremission(left)andthequantumdotemission(right)are realdata,thewettinglayerabsorptionshownisaduplicateof theemissiondataandisincludedsolelytoshowtheprinciple of operation of the device. nt through Diode 1 (mA)1122....0505 Quantum light emission from Diode 2 (a.u.)2.0 Voltag2e.5 across Diod3.0e 1 (V) 3.5 FetimInGgis.soi5of.nalinnTeeuhfterroasmleceoDxncidoit-dooenrd2ea.rsIncaosrefruten:l)acttTiioohnnefofiufnnetchtsietorneulceotcfutrraiecssifipnelgilltde- e urr across Diode 2. C0.5 0.0 generationofphotonsisdesirableandhasleadtothede- 0 1 2 Voltage across Diode 1 (V) velopment of a single photon [22] and entangled photon [23] light emitting diodes. Combining the advantages of FIG. 3. A graph showing the current through Diode 1 as a wavelength tuning and electrical excitation of quantum function of voltage. The device shows diode like behaviour. dots would allow the electrically triggered generation of Inset: Quantum light emission from Diode 2 as a function indistinguishable photons from different sources. of the voltage across Diode 1. Note that the emission and In this paper we demonstrate a method of producing current both ‘turn on’ at ∼2.25 V. electrically triggered anti-bunched light from an electri- callytunablesource. Thedeviceisfabricatedfromasec- a GaAs/AlGaAs quantum well, resulting in an increased tionofasinglewaferandcontains16individuallytunable tuning range [16]. These structures have been used to diode structures on a single chip. demonstrate electric control of a spin [17], tuning of the Thedevicesusedinthisstudyare180µm×210µmpla- g-factoroftrappedspins[18]andtuningofthefinestruc- nar microcavity LEDs containing a layer of InAs quan- ture splitting (FSS) [19, 20]. A small FSS is required to tumdotsembeddedina10nmGaAsquantumwellwith produce entangled photons from the bi-exciton cascade. Al Ga As barriers. Distributed Bragg Reflectors 0.75 0.25 The key advantages of electrically tunable devices are (DBRs) grown above (4 repeats) and below the InAs that many individually tunable devices can be produced (15 repeats) quantum dot layer and quantum well are on a single chip using standard photo-lithographic semi- used to form a half-wavelength cavity. This unbalanced conductor processing techniques and, unlike piezo-tuned cavity both increases the portion of QD light emitted systems, they do not suffer from long term drift [21]. vertically and acts as a horizontal waveguide for optical For the development of compact and scalable quan- emission from the InAs wetting layer. The top DBR is tumtechnologies,thecapabilitytoelectricallytriggerthe doped p-type and the bottom DBR is doped n-type to 3 form a diode structure suitable for electrical excitation. the output. The key idea is to use light produced by one LED to Finally, we demonstrate that we can tune the FSS as excite the QDs in the neighbouring diode. We run one a function of electric field. The inset in figure 5 shows LED (Diode 1) in forward bias, resulting in broadband finestructuresplittingofanexcitoninDiode2asafunc- emission from the InAs wetting layer. The emitted light tion of the electric field across Diode 2, indicating that is guided horizontally due to the Bragg reflectors above these devices could be used as sources of entangled pho- and below the wetting layer. A portion of the emitted ton pairs. light is incident on the neighbouring LED (Diode 2) and Inthefuturewecanenvisageanumberofchangesthat will be absorbed by the wetting layer, generating exci- would increase the efficiency of the device. For example, tons. These will be subsequently captured by quantum Diode1couldbestructuredtoincreasethedirectionality dots in Diode 2 resulting in quantum light emission (see ofemissiontowardsDiode2bymeansofaunidirectional figures1and2). Thison-chipinplaneexcitationscheme antenna. Similarly,awaveguidebetweentheLEDscould is similar to the scheme presented in [24], which makes increase the cross-coupling efficiency, with in principle useofawhispering-gallery-modeQDlasertoexciteaQD onedrivingLEDexcitingmanytunableLEDs. Theuseof embedded in a nearby micropillar. fast electronics and low RC constant devices could allow Thecavitymodeoftheplanarmicrocavitymatchesthe ‘on-demand’emissionbyeithervaryingthebiastodiode emission wavelength of the quantum dots of interest, in- 1tomodulatethe‘pump’orbyvaryingthebiastoDiode creasingtheproportionofQDemissiondirectedupwards 2 to modulate the wavelength. into the collection optics. Varying the bias across Diode Wehavedemonstratedanall-electrictunablequantum 2 allows wavelength tuning by Stark shifting the tran- source fabricated using a simple photo-lithographic pro- sitions. The quantum dots are embedded in a quantum cess. While this is a proof-of-principle device intended well(asin[16])inordertoincreasethemaximumvoltage to demonstrate the possibility of an electrically tunable that can be applied without quenching the emission. and electrically driven quantum light source, we antici- The intensity of light emission from Diode 2 can be pate that future work will improve the coupling between controlled by varying the voltage across Diode 1 and the driven and driving LEDs and lead to the production of devices show good diode-like behaviour (figure 3). We devices suitable for compact multiplexed on-chip quan- apply a voltage of 2.5V (corresponding to a current den- tum optics technologies. sity of ∼ 23 A cm−2) to Diode 1, resulting in a broad emission spectrum with a peak at around 870 nm corresponding to the InAs wetting layer (figure 2). Figure 4 Acknowledgments shows the observed emission spectrum of a QD in Diode 2 as a function of the applied electric field - a Stark shift The authors acknowledge funding from the EPSRC of over 5 nm is visible. for MBE system used for the growth of the QD-LED. A diffraction grating was used to spectrally filter the E.M. and C.D. acknowledge support by the Marie Curie outputofDiode2topickoutlightfromasingleemission Actions within the Seventh Framework Programme for line. The filtered light was then directed into a Hanbury Research of the European Commission, under the Ini- Brown and Twiss setup in order to perform a second- tial Training Network PICQUE (Grant No. 608062) J. order correlation function (g(2)(τ)) measurement. The L. gratefully acknowledges financial support from the result can be seen in figure 5). We observe a dip in EPSRC CDT in Photonic Systems Development and g(2)(τ) as τ approaches 0, giving g(2)(0) = 0.06, below Toshiba Research Europe Ltd. the g(2)(0) < 0.5 threshold, which shows the output is dominated by light from a single quantum emitter. This is an order of magnitude improvement on the values of Data Access g(2)(0) measured in prior demonstrations of on-chip in- plane excitation [25]. We note that g(2)(0) (cid:54)= 0, even The experimental data used to produce the figures in though we have accounted for the detector response. We thispaperispubliclyavailableathttps://doi.org/10. attribute this to the leakage of light from Diode 1 into 17863/CAM.6837. [1] C.H.Bennett,inInternationalConferenceonComputer [4] L.A.Lugiato,A.Gatti, andE.Brambilla,JournalofOp- System and Signal Processing, IEEE, 1984 (1984) pp. ticsB:QuantumandSemiclassicalOptics4,S176(2002). 175–179. [5] E. Knill, R. Laflamme, and G. J. Milburn, Nature 409, [2] A. K. Ekert, Phys. Rev. Lett. 67, 661 (1991). 46 (2001). [3] C. H. Bennett, G. Brassard, C. Crépeau, R. Jozsa, [6] S. Buckley, K. Rivoire, and J. Vuckovic, Reports on A. Peres, and W. K. Wootters, Phys. Rev. Lett. 70, Progress in Physics 75, 126503 (2012). 1895 (1993). [7] R. M. Stevenson, R. J. Young, P. Atkinson, K. Cooper, D.A.Ritchie, andA.J.Shields,Nature439,179(2006). 4 [8] R. Trotta, J. Mart´ın-Sańchez, I. Daruka, C. Ortix, and (2010). A. Rastelli, Phys. Rev. Lett. 114, 150502 (2015). [17] A.B.DeLaGiroday,A.Bennett,M.Pooley,R.Steven- [9] D. G. Gevaux, A. J. Bennett, R. M. Stevenson, A. J. son, N. Sko¨ld, R. Patel, I. Farrer, D. Ritchie, and Shields, P. Atkinson, J. Griffiths, D. Anderson, G. A. C. A. Shields, Physical Review B 82, 241301 (2010). Jones, and D. A. Ritchie, Applied Physics Letters 88, [18] A.J.Bennett,M.A.Pooley,Y.Cao,N.Sko¨ld,I.Farrer, 131101 (2006), http://dx.doi.org/10.1063/1.2189747. D.A.Ritchie, andA.J.Shields,Naturecommunications [10] S. Reitzenstein, S. Mu¨nch, P. Franeck, A. Rahimi-Iman, 4, 1522 (2013). A. Lo¨ffler, S. Höfling, L. Worschech, and A. Forchel, [19] A. Bennett, M. Pooley, R. Stevenson, M. Ward, R. Pa- Phys. Rev. Lett. 103, 127401 (2009). tel, A. B. de La Giroday, N. Sko¨ld, I. Farrer, C. Nicoll, [11] S. Seidl, A. Ho¨gele, M. Kroner, K. Karrai, A. Badolato, D. Ritchie, et al., Nature Physics 6, 947 (2010). P. Petroff, and R. Warburton, Physica E: Low- [20] M. Ward, M. Dean, R. Stevenson, A. Bennett, D. El- dimensionalSystemsandNanostructures32,14 (2006), lis, K. Cooper, I. Farrer, C. Nicoll, D. Ritchie, and proceedings of the 12th International Conference on A. Shields, Nature communications 5 (2014). Modulated Semiconductor StructuresProceedings of the [21] J. Zhang, J. S. Wildmann, F. Ding, R. Trotta, Y. Huo, 12th International Conference on Modulated Semicon- E. Zallo, D. Huber, A. Rastelli, and O. G. Schmidt, ductor Structures. Nature communications 6 (2015). [12] F.Findeis,M.Baier,E.Beham,A.Zrenner, andG.Ab- [22] Z. Yuan, B. E. Kardynal, R. M. Stevenson, A. J. streiter, Applied Physics Letters 78, 2958 (2001). Shields, C. J. Lobo, K. Cooper, N. S. Beattie, D. A. [13] R. Trotta, P. Atkinson, J. Plumhof, E. Zallo, R. O. Ritchie, and M. Pepper, Science 295, 102 (2002), Rezaev, S. Kumar, S. Baunack, J. Schro¨ter, A. Rastelli, http://science.sciencemag.org/content/295/5552/102.full.pdf. and O. Schmidt, Advanced materials 24, 2668 (2012). [23] C. Salter, R. Stevenson, I. Farrer, C. Nicoll, D. Ritchie, [14] R. Trotta, E. Zallo, E. Magerl, O. G. Schmidt, and and A. Shields, Nature 465, 594 (2010). A. Rastelli, Physical Review B 88, 155312 (2013). [24] E.Stock,F.Albert,C.Hopfmann,M.Lermer,C.Schnei- [15] R. Oulton, J. J. Finley, A. D. Ashmore, I. S. Gregory, der, S. Ho¨fling, A. Forchel, M. Kamp, and S. Reitzen- D. J. Mowbray, M. S. Skolnick, M. J. Steer, S.-L. Liew, stein, Advanced Materials 25, 707 (2013). M. A. Migliorato, and A. J. Cullis, Phys. Rev. B 66, [25] P. Munnelly, T. Heindel, M. M. Karow, S. Ho¨fling, 045313 (2002). M.Kamp,C.Schneider, andS.Reitzenstein,IEEEJour- [16] R. B. Patel, A. J. Bennett, I. Farrer, C. A. Nicoll, D. A. nal of Selected Topics in Quantum Electronics 21, 681 Ritchie, and A. J. Shields, Nature photonics 4, 632 (2015).