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A highly directional room-temperature single photon device Nitzan Livneh, Moshe G. Harats, Daniel Istrati, Hagai S Eisenberg, and Ronen Rapaport Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00082 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 11, 2016

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A highly directional room-temperature single photon device Nitzan Livneh,†,¶ Moshe G. Harats,‡,¶ Daniel Istrati,‡ Hagai S. Eisenberg,‡ and Ronen Rapaport∗,†,‡ Applied Physics Department, The Hebrew University of Jerusalem, Jerusalem 9190401, and Racah Institute for Physics, The Hebrew University of Jerusalem, Jerusalem 9190401 E-mail: [email protected]

Abstract One of the most important challenges in modern quantum optical applications is the demonstration of efficient, scalable, on-chip single photon sources, which can operate at room temperature. In this paper we demonstrate a room-temperature single photon source based on a single colloidal nanocrystal quantum dot positioned inside a circular bulls-eye shaped hybrid metal-dielectric nanoantenna. Experimental results show that 20% of the photons are emitted into a very low numerical aperture (N A < 0.25), a 20-fold improvement over a free standing quantum dot, and with a probability of more than 70% for a single photon emission. With an N A = 0.65 more than 35% of the single photon emission is collected. The single photon purity is limited only by emission from the metal, an obstacle that can be bypassed with careful design and fabrication. The concept presented here can be extended to many other types of quantum emitters. ∗

To whom correspondence should be addressed Applied Physics Department, The Hebrew University of Jerusalem, Jerusalem 9190401 ‡ Racah Institute for Physics, The Hebrew University of Jerusalem, Jerusalem 9190401 ¶ These authors contributed equally to the paper †

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Such a device paves a promising route for a high purity, high efficiency, on-chip single photon source operating at room temperature.

Keywords Single Photon Source, Nanoantenna, Colloidal Nanocrystals, Quantum Dots, nano-optical devices, plasmonics Over the last decade, significant theoretical and technological progress set the ground for the emerging field of quantum technologies. Harnessing quantum properties opens new possibilities in many fields, such as in computation, encryption, simulations, communications, sensing and metrology. The basic building block for these technologies is the quantum bit (qubit). In many applications, efficient and reliable high-rate transmission of quantum information encoded in qubits is required over long distances. The best known ”flying” qubits suitable for such long distance operations are single photons, and thus it is of a particular importance to devise good sources for single photons. A good single photon source 1,2 (SPS) should be as deterministic as possible, it should produce on-demand, high rate single photons with small uncertainty of their emission time, and with a photon collection efficiency as close as possible to unity. Furthermore, for practical realizations, the SPS must be scalable so that it could be integrated densely into a complex, preferably on-chip device, and very importantly, it should support room temperature operation. These requirements are not at all trivial to achieve, especially in parallel. Single photons are emitted from a variety of nano-emitters such as single neutral atoms or ions, 3 defect centers in pure crystals, 4 self assembled semiconductor quantum dots 5 and colloidal nanocrystal quantum dots (NQDs). 6 NQDs are promising as single photon emitters if integrated into a single photon source device. They are substrate free and can be incorporated into a variety of designs and platforms, with a tunable photon emission wavelength ranging from the visible spectrum throughout the entire telecommunications wave-

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lengths range. Very importantly, they are high quality single photon emitters even at room temperature. 7 One problem with using NQDs for reliable SPS’s is that they usually suffer from blinking and photo-bleaching problems. 8 Recent efforts however have been successful in strongly reducing these problems 9,10 and these efforts show the promise for producing stable, non-blinking NQDs in the near future. An intrinsic problem common to all nano-emitters, including NQDs, is their isotropic angular emission pattern. This emission pattern strongly reduces the ability to collect the emitted photons efficiently, especially using standard, low numerical aperture (NA) optics which is detrimental for practical SPS applications. Several different approaches for collecting single photon emission have been demonstrated, however most of them are limited for the specific type of the source used 11–13 and are complex and hard to scale up. One promising, general approach for directing light is the coupling of nano-emitters to highly directional nano-antennas. In this approach the emitted light is coupled into the antenna mode and then re-emitted by the antenna to the far field in the desired direction, allowing efficient collection as schematically drawn in Fig. 1a. The main challenges for realizing such coupled device are the design and fabrication of a highly directional nanoantenna which can have an efficient coupling to the emitter, and then the precise positioning of a single emitter in the desired location. Recently, directing light emission from emitters using metallic nano-antennas was demonstrated, 14–18 in which the light is coupled to a surface-plasmon polariton. However, surface plasmons are inherently lossy, and also they require the emitter to be in close proximity to the metallic surface for efficient coupling, which leads to quenching of the emission 19 and enhances the probability for the emission of two photons from biexcitons. 20,21 All these problems intrinsically inhibit the efficiency and purity of plasmon-based directional SPS. Another strategy to direct single photons is to use purely dielectric cavity antennas or photonic crystals. 22–28 While these antennas are indeed low loss and can be highly directional, they are usually much more suitable for emitters that are intrinsically grown inside the host dielectric material of the antenna

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rather than substrate-free emitters. They also have high quality factors and are therefore generally limited for narrow-band sources, mostly those working at cryogenic environments, and they also require very demanding design and fabrication to overcome the problem of substantial emission into the host substrate rather than to free space. Therefore, it was recently suggested to utilize the advantages of both metallic and dielectric nano-antennas while minimizing their disadvantages by using hybrid metal-dielectric nano-antennas. 15,29–32 These nano-antennas exhibit high directionality of emission from substrate-free emitters over a broad spectral range with small-size antennas, thanks to the efficient diffraction induced by the metallic nanostructure, but also avoid metal related non-radiative losses. 29 The other major challenge common to all the emitter-antenna solutions discussed above is how to position a single emitter with nanometric, subwavelength accuracy to allow for an effective, well-controlled coupling to the nano-antenna. Indeed, different complex techniques are currently being developed to allow such accurate positioning, 33–36 but the challenge of a simple and accurate incorporation of a single nano-emitter into a highly-directional hybrid metal-dielectric antenna for an efficient SPS is still an outstanding one. In this work we present a hybrid metal-dielectric antenna with a single NQD embedded at the center of the device. We developed an accurate top-down positioning technique, which allowed us to position a single NQD at an exact location inside a dielectric thin layer on top of a bulls-eye shaped metal-dielectric nano-antenna. The hybrid antenna was designed to achieve efficient coupling of single photons emitted from the NQD to the nano-antennas optical modes, which results in a highly directional emission of single photons into a narrow angular cone. The directional emission allows an efficient collection of single photons into a numerical aperture as low as that of a simple optical fiber, demonstrating the ability to collect single photons from such a source without the need of any additional optics. The emitter-nano-antenna concept used here is based on our high collection efficiency hybrid metal dielectric nano-antenna design, demonstrated for multiple NQDs emission in Ref. 29 Here we make a crucial step and present a method which enabled us to deposit a

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Figure 1: (a) Left side - without a nano-antenna the angular emission of the NQD is isotropic, leading to a low collection efficiency. Right side - the nano-antenna directs the emission for increased collection efficiency. (b) A schematic diagram of a hybrid metal-dielectric antenna, with a single NQD in the middle of the hybrid antenna. (c) A cross section of the structure, defining the geometrical parameters: period Λ = 600nm, the slit width a = 200nm, the thickness of the dielectric layer h = 410nm and the height of the NQD above the surface d ≈ 300nm.

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single core/shell CdTe/ZnS NQD 37 inside a dielectric layer over the center of a circular bulls-eye shaped Ag circular grating with both vertical and axial accuracy. Schematics of the device and its cross section are shown in Fig. 1b, c, respectively. The dielectric layer, sandwiched between the metal circular bulls-eye grating and air, is designed as a single-mode slab-waveguide layer for the NQD emission wavelength range. The photons emitted from the NQD are thus preferably emitted into the dielectric layer and then propagate in a radial direction. The circular grating is designed such that the photons will be efficiently diffracted by it, with a very low divergence angle, to a direction perpendicular to the surface of the device. Both the efficient coupling and the good directionality of the photons are very sensitive to the positioning of the NQD, and the ability to have an SPS depends also on the ability to have only a single NQD inside the device. The fabrication process described here, which enabled us to achieve the above requirements with standard industrial semiconductor processes, is based on a spin-coating of a very thin layer of PMMA with embedded NQD density of ≈ 2 NQD /µm2 , on top of a metallic bulls-eye lens covered with a layer of SiO2 . The SiO2 layer, which is index-matched to the PMMA, has the correct thickness to allow an accurate vertical positioning of the PMMA/NQDs layer, which is optimal to the final antenna design. Next, by e-beam re-alignment lithography the PMMA layer is removed, except for an area of ≈ 0.5µm2 over the center of the grating. This leaves on average a single NQD above the center of the lens. Finally, another spin coating process is performed, to bury the NQD and to achieve the desired dielectric layer thickness required for optimal performance. This yields a final device where a single NQD is optimally positioned both vertically and axially inside a dielectric layer at the center of the circular grating. (A detailed description of the principle of operation, the design, fabrication, and the characterization of this hybrid metal-dielectric nano-antenna can be found in the Supporting Information and in Ref. 29 ). As a reference for the performance of our SPS, we first measured the emission properties of a single NQD deposited in a thin layer of PMMA on top of a SiO2 covered flat Ag

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Figure 2: Measurements of a reference single NQD, without a nano-antenna: (a) The PL spectrum of a single NQD (blue) and of an ensemble of NQDs (red). (b) Telegraphic blinking of the emission intensity of the single NQD. (c) A single NQD lifetime measurement, showing a nearly perfect single exponential lifetime. (d) A 2nd order correlation measurement (g (2) (τ )), showing a clear anti-bunching behavior with g (2) (0) = 0.12 ± 0.03. substrate, i.e. in exactly the same dielectric environment as the NQD on our nano-antenna device, but without the metallic circular bulls-eye nanostructure. An example for a single NQD spectrum compared to the spectrum of an ensemble of NQDs is plotted in Fig. 2a. The spectral FWHM of a single NQD at room temperature is measured to be ≈ 30nm, while the NQDs ensemble is spectrally broadened (∼ 70nm FWHM) by the size distribution of the NQDs. Next the blinking time trace of the single NQD was recorded over a 10s period (see Fig. 2b). The PL intensity of these NQDs is telegraphic, with nearly 0 emission in ”OFF” states and a constant intensity during ”ON” states. This type of blinking is a signature of a single NQD. 39 Figure 2c plots the emission decay measurement, showing an almost perfect single exponential decay with a decay time ≈ 170ns, typical for a single exciton 7


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Figure 3: (a) A SEM image of the nanoantenna, taken before the deposition of the dielectric layers and the positioning of the NQD. (b) A confocal scan of the emission from the hybrid antenna, whose edge is marked by the green dashed line, showing PL only from the center of the nano-antenna, where the NQD is located. The scan was taken with a bandpass filter (750-850nm). The bright area on the right to the nanoantenna is PL of many NQDs attached to a marker of the nanofabrication process, with collected intensity similar to that of the single NQD at the center. (c) The angles θ and ϕ of the PL angular intensity distribution are defined as the polar and azimuthal angle respectively, measured in the far field. 18,38 (d) A schematic representation of the angular PL intensity distribution I(θ, ϕ) measurements as a polar PL intensity plot where the radial direction is the polar angle θ and the azimuthal angle is ϕ, defined in (c). The maximal measured polar angle, θN A ≃ 40o is given by the NA of the objective lens used in the experiment and is marked by the dash-dotted red circle. (e) Angular PL intensity distribution I(θ, ϕ) of the SPS device showing a slightly asymmetric directional cone. (f) The measured angular PL intensity distribution I(θ, ϕ) of a reference single NQD without the nanoantenna showing isotropic (non-directional) emission. radiative recombination with only a small probability of a faster process, usually attributed to recombination of a biexciton. 40 Finally, a second order correlation measurement (g (2) (τ )) of the PL of a single NQD shown in Fig. 2d, was performed with an optical excitation of a non-resonant (λ = 405nm) pulsed laser at a repetition rate of 1MHz. The NQDs shows a g (2) (0) = 0.12 indicating that it is a nearly perfect single photon emitter at room temperature. 8


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Some of the residual peak at τ = 0 is due to a small probability for a simultaneous emission of a photon from the NQD and a background photon from the Ag substrate (This is in contrast to radiative biexciton recombination which is less significant in this case, as the residual peak does not appear for NQDs on a dielectric substrate [not shown]).

Figure 4: (a) The measured photon collection efficiency as a function of the NA of the collecting lens of the SPS device (blue), and of the reference NQD (red), extracted from Fig 3c and d respectively. The dashed black (green) lines are corresponding theoretical calculations taken from Ref. 29 (b) Blinking time trace of the total PL intensity from the single NQD embedded in the SPS device. (c) g (2) (τ ) of the SPS device measured during ON periods only. The red dashed line marks (g (2) = 0.5). The solid red line shows the calculated g (2) (τ ) using a model that includes the emission from the metal, as is detailed in the Supporting Information. (d) g (2) (τ ) of the SPS device measured during the OFF periods only. Next we characterize a fabricated NQD-antenna device where a single NQD is positioned close to the center of the nanoantenna. A SEM image, presented in Fig. 3a shows the metallic bulls-eye grating. A confocal spatial scan of the emission from the device, using a focused pulsed laser excitation (at λ = 405) is presented in Fig. 3b. The measurement shows light emission only when the center of the antenna is excited, indicating the central location 9


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of the NQD. The blinking time trace of the PL intensity, plotted in Fig. 4b again reveals a telegraphic blinking typical for a single NQD emission. While there is only one NQD in the device, We note here that there is still a constant PL signal during the ”OFF” times, which is about 1/4 of the signal during the ”ON” times. This background PL is similar to the weaker background seen for the NQD without the nanoantenna presented earlier, and is again a result of a residual broadband emission of the corrugated metallic part of the nanoantenna, as will be discussed later. 41 Next, we characterized the directionality of the photons emitted from the device. This was done in a Fourier-plane imaging setup, 18,38 resulting in a 2D angular PL intensity distribution map, I(θ, ϕ), defined in Fig. 3c,d, where θ and ϕ are the polar and azimuthal angles, respectively. The measured range of θ is limited by the N A = 0.65 of the objective lens used for light collection out of the sample. Fig. 3e plots such angular intensity distribution image taken during the ”ON” times of the NQD in the device. A movie showing blinking during the Fourier plane imaging measurement can be viewed in the Supporting Information. The resulting angular emission pattern from the device is shaped as a directional hollow cone at θ ∼ 8◦ , with an slightly asymmetric shape in ϕ and, importantly, with most of the emission directed within θ ≈ 15o , corresponding to a N A < 0.25. This is in clear contrast to a similar measurement done for the reference single NQD sample, plotted in Fig. 3f, where the emission pattern has no preferred direction and is essentially isotropic. The angular emission pattern of our device is slightly different than the Lorentzianshaped narrow angular emission pattern (less than 3.5◦ FWHM) expected in the ideal case at perfect resonance. 29 The reason for the observed angular cone rather then the expected Lorentzian is a mismatch between the designed wavelength of the nanoantenna and the actual emission wavelength λQD of this specific NQD (arising from the random size distribution of the NQDs). Such a hollow cone was also observed for a device with many NQDs out of perfect resonance. 29 The asymmetry of the beam in ϕ is a result of imperfect axial centering of the

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NQD, resulting in axial symmetry breaking and a preferential angular emission direction. Even with this slight deviation from the perfect design, the device still showed a high collection efficiency into low numerical apertures, and much better than a bare NQD. This is summarized in Fig. 4a, where the photon collection efficiency as a function of the collection numerical aperture is presented for both the device (blue) and the reference bare NQD sample (red). This was done by measuring the photon count rate for all emission angles up to a given NA, and then extracting the collection efficiency of photons into this NA as described in detail in the Supporting Information. The experimentally extracted collection efficiencies are in good agreement with the calculated collection efficiencies (black and green dotted lines, correspondingly), based on Comsol simulations of point dipole emitters with and without the nanoantenna. 29 It can be seen that with a moderate NA=0.65, the collection efficiency of the device exceeds 35%, three times better then the reference sample. The difference is even more significant at lower NA’s. For example, 20% of the photons are emitted into an N A < 0.25, corresponding to the NA of a Multi-Mode fiber without any additional optics. This is a 21-fold improvement over our reference sample and a 12-fold improvement over the calculated best achievable collection efficiency of a bare point source. It proves that using such a hybrid device is a promising route for efficient directional SPS’s requiring only very simple, cheap, and compact low-NA optics. To check the quality of the single photon emission, we have performed a g (2) (τ ) measurement on the directional emission of photons from the device. This is shown by the blue curve in Fig. 4c, where a non-resonant pulsed excitation of the NQD was used at a pulse repetition rate of 1MHz, and the data shown is collected during the ON times of the NQD. The measurement reveals the typical anti-bunching signal of an SPS, with g (2) (0) ≃ 0.37 < 0.5. This is translated to single photon emission events with a probability larger than 70% (see supporting information and Ref. 42 ). From the above measurements, the effective brightness of the SPS device, which is the rate of single photons reaching the collection lens, is given by: (excitation rate)×(collection

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efficiency)×(single photon probability)= 106 × 0.35 × 0.7 = 0.245 × 106 photons/s. This high brightness is limited only by the relative long lifetime of this specific type of NQD. One option for increasing the photon emission rate is by placing the NQD in proximity to the metallic nanostructure. This could enhance the total photon flux from the NQD but also result in an increased non-radiative loss through surface plasmons 18,19,43 which will reduce the photon emission regularity, thus increasing the probabilistic nature of the emission. Moreover, the probability of emission into the waveguide mode will be reduced, leading to a reduced collection efficiency. 29 A more complex design having a plasmonic nanoantenna combined within the current device might yield a significant lifetime shortening without significant plasmonic losses and collection efficiency, thus maintaing or improving the deterministic photon emission. This is a subject for future work. Another possible strategy is replacing this NQD with a different type of NQDs having a shorter lifetime. For lifetimes in the few ns range, the excitation rate will increase to ∼100MHz and resulting in a two-orders-of-magnitude increase in brightness, bringing it close to a 108 photons/s. We note that the measured value of g (2) (0) ≃ 0.37 is larger than that of a single NQD placed inside a similar dielectric layer over a flat Ag film. The residual correlation counts around τ = 0 for NQDs placed over the metallic film arises from the residual weak excitation of the rough Ag surface by the pulsed laser excitation, which then results in a weak broadband metal emission that has a very short lifetime compared to the NQD PL lifetime. This weak emission gives rise to the background emission seen in OFF times of the NQD, and to a non-vanishing probability of two photon coincidence count, either by detecting two or more simultaneous photons coming from the metal or simultaneous emission of one photon from the NQD and one from the metal (see supp. information for more details on the different terms in the g (2) (τ ) measurement). The metal excitation becomes more efficient as the metal corrugation and roughness increases. Therefore the metal excitation and thus the background photon emission is even larger in the corrugated surface of the antenna, resulting in an even larger residual peak in g (2) (0).

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To make this point even more clear, in Fig. 4d we show the g (2) (τ ) measurement results during the ”OFF” times of the NQD, where only background emission is measured. Indeed, no anti-bunching is seen, and the emission decay time is limited by the temporal resolution of the detectors, indicating uncorrelated emission from the metal with lifetime shorter than 250ps. To confirm that this residual emission is indeed coming from the metal, we have repeated such a measurement on a similar nanoantenna without NQDs, and found a residual emission very similar to the one plotted in Fig.4d. Finally, the red curve in Fig. 4c shows a calculation of the expected g (2) (τ ) measurement of the device assuming a single photon emitting NQD and an uncorrelated photon emission from the metal (see Supporting Information for the details of the model). The calculation fits the experimental results very well when we assume that the flux of the uncorrelated photons from the metal is ∼ 1/4 of the NQD single photon flux, exactly matching the observed flux ratio measured in Fig. 2a. This confirms our interpretation that the residual emission from the metal corrugations attributes to the observed peak at g (2) (0) and limits the single photon quality of the current device to 70%. This rather general limitation of using metallic nanoantennas for SPS devices was not measured nor discussed previously to the best of our knowledge. The broadband PL of metals is known for many years, 44 and more recent studies have shown that this PL is strongly related to excitation and emission from surface plasmons on rough surfaces. 45–48 While this seems rather detrimental on first site, a few simple solutions can overcome this limitation. Optimizing metallic composition and improving metal deposition, hence making surfaces with very high quality, as was recently demonstrated, 49 will reduce the metal PL and result in a very high quality nanoantenna. Another approach is to design the nano-antenna with the NQD positioned above a flat metal area of a size comprable to the excitation spot, thus avoiding the excitation of corrugated metal areas. Even for non-ideal antennas, since the metal PL is both broadband and with a very short lifetime, tight spectral filtering around the NQD emission line, and temporal filtering of the emission in a short time window after the excitation (either in post processing or with real-time electronics 50,51 ) should reject almost

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all metal emission but with a negligible loss of single photons from the NQD. For NQDs with a high nonlinear response, excitation by two photon absorption can also result in separation of the NQD short-wavelength spectrum from the long-wavelength background. To summarize, we demonstrate a promising prototype for an efficient and bright roomtemperature single photon source based on a single NQD positioned inside an on-chip hybrid metal-dielectric nanoantenna. Our measurements have shown that more than 20% of the photons are emitted into a small numerical aperture of N A = 0.25, and more than 35% of the emission is collected with a larger N A = 0.65, allowing large collection efficiency with very simple collection optics, or even directly into a fiber. Our device has a rather high single photon probability of more than 70%, limited by residual metal emission. Several improvements can be done to strongly reduce the residual metal emission, which should yield a much higher single photon probability approaching 100%. Initial calculations performed recently on improved designs predict that the collection efficiency can be significantly increased to over 70% into an NA as small as 0.12, which will allow an efficient direct coupling to a Single Mode fiber. This type of nanoantenna design is also very flexible: it can be easily tuned to any chosen resonant wavelength in the visible and IR spectral range, and it can easily incorporate other high quality room temperature single photon nano-emitters (such as other types of non-blinking NQDs, defect states in nano-diamonds, 4 or defects in the new 2D materials 52–57 ). With such improvements and the use of highly stable suitable emitters (NQDs or others), we believe that this type of device is promising and practical for real applications benefiting from simple integrated room-temperature single photon sources, such as for quantum key distribution as an example.

Acknowledgement This work was supported in parts by the Einstein foundation Berlin, by the U.S. Department of Energy: Office of Basic Energy Sciences - Division of Materials Sciences and Engineering, the European Cooperation in Science and Technology through COST Action MP1302 Nano14


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spectroscopy and by the Ministry of Science and Technology, Israel.

Supporting Information Available Details of the fabrication procedure, description of the experimental setup, explanation of the different terms composing the g (2) (τ ) function, calculation of the efficiencies of the optical setup and device, explanation on the simulation of the g (2) (τ ) measurement, a movie showing real-time blinking during the Fourier plane measurement. This material is available free of charge via the Internet at http://pubs.acs.org/.

Notes and References (1) Buckley, S.; Rivoire, K.; Vuˇcković, J. Rep. Prog. Phys. 2012, 75, 126503. (2) Eisaman, M. D.; Fan, J.; Migdall, A.; Polyakov, S. V. Rev. Sci. Instrum. 2011, 82, 071101. (3) Keller, M.; Lange, B.; Hayasaka, K.; Lange, W.; Walther, H. Nature 2004, 431, 1075– 1078. (4) Kurtsiefer, C.; Mayer, S.; Zarda, P.; Weinfurter, H. Phys. Rev. Lett. 2000, 85, 290–293. (5) Santori, C.; Pelton, M.; Solomon, G.; Dale, Y.; Yamamoto, Y. Phys. Rev. Lett. 2001, 86, 1502–1505. (6) Michler, P.; Imamo˘glu, A.; Mason, M. D.; Carson, P. J.; Strouse, G. F.; Buratto, S. K. Nature 2000, 406, 968–970. (7) Brokmann, X.; Messin, G.; Desbiolles, P.; Giacobino, E.; Dahan, M.; Hermier, J. P. New J. Phys. 2004, 6, 99. (8) van Sark, W. G. J. H. M.; Frederix, P. L. T. M.; Van den Heuvel, D. J.; Gerritsen, H. C.;

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