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Jens-Christian Drawer, Victor Nikolaevich Mitryakhin, Hangyong Shan, Sven Stephan, Moritz Gittinger, Lukas Lackner, Bo Han, Gilbert Leibeling, Falk Eilenberger, Rounak Banerjee, Sefaattin Tongay, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Christoph Lienau, Martin Silies, Carlos Anton-Solanas, Martin Esmann, Christian Schneider

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[Monolayer-Based Single-Photon Source in a Liquid-Helium-Free Open Cavity Featuring 65% Brightness and Quantum Coherence](https://mdr.nims.go.jp/datasets/b53a4425-f89a-4f7f-ae33-77e0e887d57a)

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Monolayer-Based Single-Photon Source in a Liquid-Helium-Free Open Cavity Featuring 65% Brightness and Quantum CoherenceMonolayer-Based Single-Photon Source in a Liquid-Helium-FreeOpen Cavity Featuring 65% Brightness and Quantum CoherenceJens-Christian Drawer, Victor Nikolaevich Mitryakhin, Hangyong Shan, Sven Stephan, Moritz Gittinger,Lukas Lackner, Bo Han, Gilbert Leibeling, Falk Eilenberger, Rounak Banerjee, Sefaattin Tongay,Kenji Watanabe, Takashi Taniguchi, Christoph Lienau, Martin Silies, Carlos Anton-Solanas,Martin Esmann, and Christian Schneider*Cite This: Nano Lett. 2023, 23, 8683−8689 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Solid-state single-photon sources are central building blocks in quantum informationprocessing. Atomically thin crystals have emerged as sources of nonclassical light; however, they performbelow the state-of-the-art devices based on volume crystals. Here, we implement a bright single-photonsource based on an atomically thin sheet of WSe2 coupled to a tunable optical cavity in a liquid-helium-freecryostat without the further need for active stabilization. Its performance is characterized by high single-photon purity (g(2)(0) = 4.7 ± 0.7%) and record-high, first-lens brightness of linearly polarized photons of65 ± 4%, representing a decisive step toward real-world quantum applications. The high performance of ourdevices allows us to observe two-photon interference in a Hong−Ou−Mandel experiment with 2% visibilitylimited by the emitter coherence time and setup resolution. Our results thus demonstrate that thecombination of the unique properties of two-dimensional materials and versatile open cavities emerges as aninspiring avenue for novel quantum optoelectronic devices.KEYWORDS: two-dimensional materials, quantum dots, single-photon source, open microcavitySolid-state single-photon sources are devices of centralimportance to enable scalable quantum optical applica-tions. They play a pivotal role in quantum communication,metrology, and quantum computing.1−5 As such, it is crucial toengineer and characterize these devices according to theirrequirements in these real-life applications. For the vastmajority of such applications, three performance parametersof a single-photon source are of exceptional importance: thesingle-photon purity, which is characterized via the second-order autocorrelation function g(2)(0), the first lens brightness,which reflects the probability of a single photon emission fromthe device following an excitation, and finally the capability ofthe emitted single photons to display quantum interference.3In addition, the scalable and cost-effective implementation ofsuch devices with top performance is highly desirable but thusfar elusive.In recent years, a large palette of solid-state single-photonemitters has emerged, featuring different degrees of material-processing versatility and a wide range of emission wave-lengths, operation temperatures, and polarization properties.6Single InAs quantum dots, which are coupled to photoniccavities, have advanced as the most mature solid-stateplatform; however, their cost-effective implementation isstrongly hindered by expensive growth and nanofabricationroutines, and their recent implementation in deterministictunable cavities still relies on nonsustainable liquid-heliumcryostats.Atomically thin semiconductors, on the other hand, arepromising candidates for optoelectronic and quantumapplications:7,8 They combine a low-cost synthesis withmaximum compatibility in terms of integration into hetero-structures. An example of this class of ultimately thin materialsis the inorganic transition-metal dichalcogenide (TMDC)WSe2, which features single-photon emission from tightlylocalized excitons in monolayers at cryogenic temper-atures.9−13 TMDC single-photon emitters provide highquantum efficiency,14−16 charge tunability,11 and polarizationcontrol,17,18 and most notably, they can be seeded at preciselocations by engineering local mechanical strain in themonolayer.19,20The success of solid-state single-photon emitters in generalrelies on photonic cavities to shape the optical density of statesaround the emitter, i.e. increasing the spontaneous emissionrate via the Purcell effect into a specific photonic mode,Received: July 11, 2023Revised: September 1, 2023Published: September 9, 2023Letterpubs.acs.org/NanoLett© 2023 The Authors. Published byAmerican Chemical Society8683https://doi.org/10.1021/acs.nanolett.3c02584Nano Lett. 2023, 23, 8683−8689This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on October 22, 2023 at 03:02:29 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jens-Christian+Drawer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Victor+Nikolaevich+Mitryakhin"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hangyong+Shan"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Sven+Stephan"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Moritz+Gittinger"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Lukas+Lackner"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Lukas+Lackner"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Bo+Han"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Gilbert+Leibeling"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Falk+Eilenberger"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Rounak+Banerjee"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Sefaattin+Tongay"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Christoph+Lienau"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Martin+Silies"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carlos+Anton-Solanas"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Martin+Esmann"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Martin+Esmann"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Christian+Schneider"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.3c02584&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/23/18?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/18?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/18?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/18?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c02584?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/ensuring optimal light collection.21 The scalable anddeterministic integration of solid-state quantum emitters intophotonic microcavities in general and TMDC QDs inparticular is still one of the most delicate tasks in quantumengineering. While techniques based on combining nano-lithography, nanoimaging, and emitter site control have beenwidely explored to integrate III/V QDs22−24 and TMDC QDsinto optical resonators, more powerful and versatile approacheswere recently developed.Among those, the concept of open photonic cavitiesrepresents an ad hoc, fully deterministic approach forinterfacing a microcavity with single-photon emitters in two-dimensional materials. In these reconfigurable Fabry−Perotresonators, the two opposing mirrors allow relative displace-ments in three dimensions of space, facilitating precise controlof quantum light emission.25−29 Open-cavity-based solid-statesingle photon sources have so far been implemented in liquid-helium bath cryostats or in highly engineered cavity platformsrequiring active phase-locking of the cavity length and fiber-based mirrors.30−36In this work, we demonstrate a high-performance single-photon source based on a WSe2 monolayer QD that isdeterministically coupled to the optical resonance of an opencavity. The reconfigurable open cavity is implemented in a low-vibration, helium-free cryostat without any active stabilization.Additionally, the cavity modes are inscribed in planar mirrorsvia focused ion beam (FIB) milling, providing superior controlon the photonic mode engineering, as opposed to fiber-basedcavities. The photonic resonator tunability allows us todeterministically position the single emitter of a wrinkledmonolayer at the cavity center and tune the cavity resonance tothe corresponding photon emission wavelength. Our single-photon source displays a high single-photon purity with ag(2)(0) value as low as 4.7 ± 0.7% and a first-lens brightness ashigh as 65%, which translates to a single-photon emission rateof 49.8 MHz utilizing a pump laser of 76.2 MHz. Itfurthermore displays statistically significant signs of quantumcoherence in Hong−Ou−Mandel experiments with 2%interference visibility limited by the emitter dephasing timeof 23.1 ps and the temporal resolution of our measurementapparatus.The design of the open cavity sample is graphically sketchedin Figure 1a. It is based on an asymmetrical mirror design toenhance the single-photon collection in the same direction asthe excitation: the bottom part of the cavity consists of adistributed Bragg reflector (DBR) with a high reflectivityhosting the monolayer flake. The monolayer is capped by athin layer of hexagonal boron nitride, guaranteeing spectralstability. The top part of the cavity is built from a glass mesacontaining concave hemispherical indentations of differentdiameters; a 33 nm thick layer of gold is evaporated onto thisstructure to finalize the top mirror (for further details onsample preparation see Section S1 in the SupportingInformation). A scanning electron microscope (SEM) imageof the cavity top mirror (before the gold coating) is shown inFigure 1b (top). The hexagonal boron nitride capped WSe2monolayer flake on the DBR is shown in Figure 1b (bottom).We placed the open-cavity device inside a low-vibration closed-cycle exchange-gas cryostat and kept it at 3.2 K. With therecent dramatic increase in helium prices, sustainable solutionsnot relying on liquid helium bath cryostats have become anurgent need for the quantum photonics community. However,especially the performance of spectrally tunable open cavitiesthus far has relied on their implementation in a vibration-freebath cryostat. Our implementation geometry of the cavity,which is sufficiently robust to not be impeded in itsperformance by the pulse-tube cooler of our exchange-gascryostat, can be found in Figure S1 of the SupportingInformation.To assess the possible performance of our cavity device, weperformed finite-difference time-domain (FDTD) simulationsof the experimental resonator configuration. Figure 1cFigure 1. WSe2 monolayer in an open cavity. (a) Graphical representation of single-photon emission from a monolayer source in a plano-convexopen cavity under optical excitation. The relative position of the top and bottom mirrors is adjustable by nanopositioners. (b) (top) Scanningelectron microscope image of the mesa-type cavity top mirror with hemispherical indentations of different diameters etched by focused ion beamlithography (before gold layer deposition) and (bottom) optical microscope image of the WSe2 monolayer placed on a SiO2/TiO2 DBR. Thesingle-photon source is located at the edge of the flake near a wrinkle. (c) (top) Transmission and Purcell factor (red and blue lines) of the open-cavity system used in the experiments derived using FDTD simulation of the electric field of a dipole located at the monolayer position and(bottom) real space intensity distribution inside the cavity. The surface of the top and bottom mirror is indicated by dashed white lines.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c02584Nano Lett. 2023, 23, 8683−86898684https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c02584?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as(bottom) shows the resonant real space intensity distributioninside the open cavity at a wavelength of 786 nm. For a lensdiameter of 5 μm and a depth of 300 nm (corresponding to aradius of curvature of 6.8 μm), the field is laterally confined toa diameter of ∼2 μm at the emitter position. Figure 1c (top)shows the calculated transmission through the top mirror for apoint dipole source and the corresponding Purcell factor as afunction of wavelength.Interestingly, the simulation predicts an on-resonant Purcellenhancement of up to 1.5 (blue line in Figure 1c), inconjunction with an off-resonant suppression of spontaneousemission up to a factor of 3.8. The latter is a clear indicator ofthe strong suppression of so-called leaky modes in our cavityimplementation. From our simulation, we can directlyanticipate photon extraction efficiencies (also referred to as“first-lens brightness”) beyond 65%, under the preconditionthat the internal quantum efficiency of the emitter approachesunity.The experimentally studied quantum dot (QD) like emitter,which evolves in our WSe2 monolayer, emerges at an emissionenergy of 1.5707 eV (789.3 nm). It is interesting to note thatthis wavelength, which is widely tunable via piezo strain,37 isvery close to the technologically relevant Rb-87-D2 line, withthe potential for a quantum memory in future repeaternetworks.38 Moreover, the emission wavelength is alsocompatible with free-space quantum communication applica-tions.39 The spectral line width of the QD is limited by theresolution of our detection system of 200 μeV (see Figure S5of the Supporting Information for a high-resolution spectrum).As a first important parameter of our source, a polarization-resolved measurement, carried out without the top mirror ofthe cavity (Figure 2a), reveals that our QD emitter displaysclose to perfect linear polarization up to a degree of 96.8 ±2.5%. We attribute this remarkable feature to the emergence ofthe studied QD from a monolayer wrinkle,17 creating a localand quasi-one-dimensional strain potential40,41 which results ina strongly aligned dipole. As a next step, we added the topmirror and studied the performance of the coupled cavity−emitter system. We first use nonresonant continuous wavelaser excitation (532 nm) and record the sample PL for acontinuously varying cavity length. The resulting color heatmap is plotted in Figure 2b. In our experiment, we observelongitudinal mode families, each consisting of three transversemodes, separated by a spacing of 26.3 meV. The modes arevisualized by the guide to the eye in Figure 2b and emphasizedin a logarithmic representation in Section S3 of the SupportingInformation.The quality factor of these cavity resonances is around 600for the chosen mirror separations of ∼5.5 μm (the separation isdirectly extracted from the longitudinal mode spacing).Importantly, and as reflected in the simulation in Figure 1c,the mode of lowest transverse order is a Gaussian mode, whichis optimally suited for coupling to a commercial single-modefiber.The open nature of the cavity allows us, in a straightforwardmanner, to study the coupled cavity−emitter system undervarious detunings by changing the resonance condition via thecavity resonator length. As reflected in Figure 2b, onresonance, the photoluminescence intensity of the emitter isenhanced by more than a factor of 10, which clearly reflects thestrong impact of the resonator structure on the performance ofthe coupled emitter−cavity system. To further improve theperformance of our source, we optimize the photon injectionefficiency into our WSe2 QD by choosing a double-resonancecondition (see Figure S4 in the Supporting Information).While we maintain the spectral resonance condition betweenthe emitter zero-phonon line and the cavity mode, we tune ourpulsed excitation laser (2 ps pulse length, 76.2 MHz repetitionrate) on resonance to the next higher order longitudinal cavitymode spectrally located at 740 nm with identical (lowest)transverse order. This condition allows us to inject lightefficiently into the cavity to pump the emitter quasi-resonantlyinto a higher resonance shell.13 It is also important to note thatquasi-resonant pumping of WSe2 emitters below the freeexciton resonance is very important to guarantee a highquantum efficiency, which can otherwise be impeded vianonradiative losses into the free exciton bath.To quantify the enhancement of spontaneous emission inour device more rigorously, we performed time-resolvedphotoluminescence measurements under varying emitter−cavity detunings. For these experiments, the QD emissionline has been optically filtered via a coarse bandpass (∼2.5meV bandwidth) and was directly detected by an avalanchephotodiode connected to a time correlator. The correspondingdecay dynamics for the off- and on-resonant cases are shown inFigure 2c (left). The characteristic decay times have been fittedwith a single-exponential decay function and unambiguouslyreflect the speeding up of the spontaneous emission rate in theFigure 2. Cavity mode detuning dependent PL and lifetime. (a) Polar plot of polarization-resolved PL intensity of the emission under 532 nmcontinuous wave excitation. The sinusoidal fit reveals a degree of linear polarization of 96.8 ± 2.5%. (b) Color map of PL spectra when tuning thecavity optical length while the sample is strongly excited above the band gap and outside of the stopband of the microcavity by a 532 nmcontinuous wave laser. Cavity modes are highlighted by dashed lines. (c) (left) Lifetime in on- and off-resonant cases (blue, on resonance; red,−4.11 meV detuning; purple, +3.81 meV detuning) and (right) cavity mode detuning dependent radiative lifetime. Error bars in the right panelrepresent the standard error resulting from fitting the time-resolved photoluminescence data (as shown in the panel on the left) with an exponentialdecay function. The fitting method utilizes a damped least-squares algorithm.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c02584Nano Lett. 2023, 23, 8683−86898685https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c02584?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asresonant case. The overall detuning dependence of thespontaneous emission decay is plotted in Figure 2c (right),reflecting the interplay of the spontaneous emission rate withthe optical resonance bandwidth. In addition, we have alsoanalyzed the lifetime of our emitter without the top mirror,yielding a decay time of 2.3 ns. The values in Figure 2c,therefore, indicate a cavity-induced reduction in lifetime of25% on resonance, whereas off resonance the emitterexperiences a more than 2-fold inhibition of spontaneousemission due to the presence of the open cavity. Thisobservation is in excellent agreement with the theoreticallypredicted changes in the lifetime shown in Figure 1c. Thepredicted ratio of enhanced to inhibited emission is 3, whereasin the experiment we find 2.71 ± 0.08.To assess the purity of the single-photon pulses emitted byour device, we measured the second-order correlation functionvia a standard Hanbury−Brown−Twiss (HBT) setting. Fromthe correlation histogram in Figure 3a, we can extract a photonantibunching of g(2) = 4.7 ± 0.7% (details on the analysis canbe found in the Supporting Information). It is worth notingthat in these experiments the emission was only filtered by acoarse bandpass (∼2.5 meV bandwidth); this furtheremphasizes the emission purity of the open-cavity device.A critical parameter in the performance of single-photonsources is the probability of delivering a single-photon state perexcitation pulse, which is usually benchmarked by thebrightness at the first collection lens. To quantify this criticalperformance indicator, which is of central importance forquantum communication implementations, we studied theemission flux of the single-photon sources as a function of thepulsed pump power (76.2 MHz repetition rate). As shown inFigure 3b, we detect more than 1 MHz of single-photon countsin our single-photon detectors. After carefully assessing thetransmission and detection efficiencies of our collection setup(see Table S1 of the Supporting Information), this valuedirectly translates into an emission frequency of 49.8 MHzsingle photons that are emitted from our source and into arecord first-lens brightness of 65 ± 4% in a linearly polarizedmode. It is worth noting that this value approaches the currentstate of the art in solid-state single-photon sources based onIII/V QDs42−44 and widely outperforms monolayer-basedtriggered single-photon sources reported in any implementa-tion.15,39,45,46 It furthermore is interesting to note that thecongruence of directly measured first-lens brightness andtheoretically calculated source extraction efficiency suggeststhat the internal quantum efficiency of our emitters approachesunity. We believe that this encouraging result was facilitated bythe combination of utilizing high-quality TMDC materials andcapping via hexagonal boron nitride, the resonant coupling tothe photonic cavity, and finally the applied quasi-resonantpumping scheme, which does not allow for losses via high-momentum free-exciton states or relaxation into long-liveddark exciton states. A full set of measurements for a secondWSe2 QD including cavity tuning, degree of linear polarization,first-lens brightness, and single photon purity can be found inSection S4 of the Supporting Information.The final benchmark of the quantum-optical properties ofour TMDC single-photon source is the temporal coherence ofthe emitted single-photon wavepackets, reflected in theircapability to display genuine quantum interference. Thisproperty is of capital importance for quantum applications,since it guarantees the capability of photons to interfere and,thus, propagate entanglement along quantum nodes. It isfurthermore of profound fundamental interest since suchquantum interference from atomically thin emitters has notbeen observed thus far.We implement the two-photon Hong−Ou−Mandel (HOM)interference in a path-unbalanced Mach−Zehnder interfer-ometer (see the scheme of the setup in Figure S2 of theSupporting Information), interfering two photon wavepacketssuccessively emitted by the source (with an initial temporaldelay of 13 ns and eventually corrected by the delay in theFigure 3. Single-photon source characterization. (a) Second-order autocorrelation function of single photons measured in an HBT experiment with76.2 MHz pulsed excitation in the saturation regime. The data are fitted by a double-exponential decay convoluted with the system responsefunction (details in the Supporting Information). (b) Brightness of the source as a function of optical pump power measured before focusing on thesample. Errors are the standard errors as the result of averaging over 10 samples for each point. (c) Second-order correlation function of singlephotons in an HOM setup discretized by the pulsed excitation for three different temporal postselection window sizes in the case of parallel (HH)and perpendicular (HV) polarization. Error bars show standard deviations of the assumed underlying Poissonian distributions of counts in eachintegrated window. (d) HOM interference visibility for varying temporal postselection window size. The shaded area shows the error bounds.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c02584Nano Lett. 2023, 23, 8683−86898686https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c02584?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asinterferometer). The quantum interference is extracted via themeasurement of the second-order correlation function betweenthe two detectors at the output of the interferometer (gHOM(2) ).The perfect bosonic quantum interference features completeantibunching (gHOM(2) = 0).To quantify the quantum interference from the photonsemitted by our source, we measure the HOM correlationbetween photons with parallel/orthogonal polarizations(gHOM,HH(2) /gHOM,HV(2) ). Figure 3c shows the correspondingnormalized correlation histograms, measured using the sameexcitation conditions as in the HBT measurement.We compare the critical cases of photons of orthogonalpolarization (HV) in the two interferometer arms versus thoseof parallel polarization (HH). As we reduce the width of thetemporal selection window from 3 ns down to 1.1 ns�approaching the resolution limit of our detection setup�asignificant difference between the parallel/orthogonal polar-ization correlations cases of the gHOM(2) measurement arisesconsistently. Such an effect is depicted in the panel insets,where gHOM,HH(2) and gHOM,HV(2) correlations display differentvalues beyond the standard deviation of the correlation peaks.This fact is further visualized in Figure 3d, where we depict theinterference visibility V = (gHOM,HV(2) − gHOM,HH(2) )/gHOM,HV(2) as afunction of the postselected temporal window.The results on photon quantum interference manifest thepresence of a substantial dephasing channel in the TMDC QD.From the modest interference visibility, we can estimate adephasing time of 23.1 ps via a fit to the data in Figure 3d47taking into account the instrument response function, which isconsistent with previous studies of the line width of WSe2QDs.15 We notice that without correcting for the finite g(2)value in the HBT experiment, this yields a conservativeestimate of T2. The dephasing most likely has its roots in rapidsurface-induced charge noise and is only partly mitigated bythe capping of our monolayer with hexagonal boron nitride.Thus, in order to further improve the coherence time of theQD emission, we suggest to further stabilize the chargeenvironment via including graphene contacts to gate thesystem.48 We furthermore believe that it will be possible toboost the Purcell enhancement in our cavity beyond a factor of10 via minimizing the mode volume (e.g., further closing thecavity) and slightly improving the cavity quality factor (e.g., byemploying a top mirror with slightly enhanced reflectivity42 orresorting to nanoscale resonators).49Harnessing the high first-lens brightness of 65% and purityof our source, it is readily applicable in quantumcommunication schemes which do not rely on quantuminterference and entanglement, such as the BB84 protocol inurban networks.39 We furthermore believe that our imple-mentation of the open cavity in a liquid-helium-free exchange-gas cryostat will inspire the single-photon source and quantummaterial cavity QED community toward accelerating thetransition toward dry cryostats.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584.Additional details on the experimental setup and itscalibration, sample preparation, numerical FDTDsimulations, and additional data on the tunability ofthe open cavity (PDF)■ AUTHOR INFORMATIONCorresponding AuthorChristian Schneider − Institute of Physics, Carl von OssietzkyUniversity Oldenburg, 26129 Oldenburg, Germany;Email: christian.schneider@uni-oldenburg.deAuthorsJens-Christian Drawer − Institute of Physics, Carl vonOssietzky University Oldenburg, 26129 Oldenburg,Germany; orcid.org/0000-0002-4157-9675Victor Nikolaevich Mitryakhin − Institute of Physics, Carlvon Ossietzky University Oldenburg, 26129 Oldenburg,Germany; orcid.org/0000-0003-4765-8507Hangyong Shan − Institute of Physics, Carl von OssietzkyUniversity Oldenburg, 26129 Oldenburg, GermanySven Stephan − Institute of Physics, Carl von OssietzkyUniversity Oldenburg, 26129 Oldenburg, Germany;University of Applied Sciences Emden/Leer, 26723 Emden,GermanyMoritz Gittinger − Institute of Physics, Carl von OssietzkyUniversity Oldenburg, 26129 Oldenburg, GermanyLukas Lackner − Institute of Physics, Carl von OssietzkyUniversity Oldenburg, 26129 Oldenburg, Germany;orcid.org/0000-0002-7970-0450Bo Han − Institute of Physics, Carl von Ossietzky UniversityOldenburg, 26129 Oldenburg, GermanyGilbert Leibeling − Institute of Applied Physics, Abbe Centerof Photonics, Friedrich Schiller University Jena, 07743 Jena,Germany; Fraunhofer-Institute for Applied Optics andPrecision Engineering IOF, 07743 Jena, Germany; Max-Planck-School of Photonics, 07743 Jena, GermanyFalk Eilenberger − Institute of Applied Physics, Abbe Center ofPhotonics, Friedrich Schiller University Jena, 07743 Jena,Germany; Fraunhofer-Institute for Applied Optics andPrecision Engineering IOF, 07743 Jena, Germany; Max-Planck-School of Photonics, 07743 Jena, GermanyRounak Banerjee − Materials Science and Engineering, Schoolfor Engineering of Matter, Transport, and Energy, ArizonaState University, Tempe, Arizona 85287, United StatesSefaattin Tongay − Materials Science and Engineering, Schoolfor Engineering of Matter, Transport, and Energy, ArizonaState University, Tempe, Arizona 85287, United States;orcid.org/0000-0001-8294-984XKenji Watanabe − Research Center for Functional Materials,National Institute for Materials Science, Tsukuba 305-0044,Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − International Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Tsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Christoph Lienau − Institute of Physics, Carl von OssietzkyUniversity Oldenburg, 26129 Oldenburg, Germany;orcid.org/0000-0003-3854-5025Martin Silies − University of Applied Sciences Emden/Leer,26723 Emden, Germany; orcid.org/0000-0002-3704-2066Carlos Anton-Solanas − Depto. de Física de Materiales,Instituto Nicolás Cabrera, Instituto de Física de la MateriaCondensada, Universidad Autónoma de Madrid, 28049Madrid, SpainMartin Esmann − Institute of Physics, Carl von OssietzkyUniversity Oldenburg, 26129 Oldenburg, Germany;orcid.org/0000-0002-2329-9696Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c02584Nano Lett. 2023, 23, 8683−86898687https://pubs.acs.org/doi/10.1021/acs.nanolett.3c02584?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c02584/suppl_file/nl3c02584_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Christian+Schneider"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfmailto:christian.schneider@uni-oldenburg.dehttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jens-Christian+Drawer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-4157-9675https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Victor+Nikolaevich+Mitryakhin"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-4765-8507https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hangyong+Shan"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Sven+Stephan"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Moritz+Gittinger"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Lukas+Lackner"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-7970-0450https://orcid.org/0000-0002-7970-0450https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Bo+Han"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Gilbert+Leibeling"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Falk+Eilenberger"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Rounak+Banerjee"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Sefaattin+Tongay"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-8294-984Xhttps://orcid.org/0000-0001-8294-984Xhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3701-8119https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-1467-3105https://orcid.org/0000-0002-1467-3105https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Christoph+Lienau"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3854-5025https://orcid.org/0000-0003-3854-5025https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Martin+Silies"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-3704-2066https://orcid.org/0000-0002-3704-2066https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carlos+Anton-Solanas"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Martin+Esmann"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-2329-9696https://orcid.org/0000-0002-2329-9696pubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c02584?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asComplete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.3c02584Author ContributionsJ.-C.D., V.N.M., and H.S. contributed equally. All authors havegiven approval to the final version of the manuscript.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis project was funded within the QuantERA II programmethat has received funding from the European Union’s Horizon2020 research and innovation programme under GrantAgreement No. 101017733, and with funding organizationthe German ministry of education and research (BMBF)within the projects EQUAISE and TubLan Q.0. Financialsupport from the European Research Council within theproject unLimit2D (Grant number 679288) is acknowledged.Furthermore, the open cavity was developed with support ofthe projects SCHN1376 11.1 and SCHN1376 14.1, funded bythe German Research Foundation (DFG). We also thank theDFG for support within the program for major equipment(INST 184/220-1 FUGG). Financial support from theNiedersächsisches Ministerium für Wissenschaft und Kulturwithin the collaborative project DyNano and from theVolkswagen Foundation within the project SMART isgratefully acknowledged. M.E. acknowledges funding by theUniversity of Oldenburg through a Carl von Ossietzky YoungResearchers’ Fellowship. S.T. acknowledges primary supportfrom NSF DMR 2111812 for materials development, NSFGOALI 2129412 for scaling, and NSF ECCS 2111812fabrication. We acknowledge partial support from DOE-SC0020653 (materials texture development), NSF ECCS2052527 for electronic and NSF DMR 2206987 for magneticpurity tests. C.A.-S. acknowledges the support from theComunidad de Madrid fund “Atraccion de Talento, Mod. 1”,ref. 2020-T1/IND-19785, the project from the Ministerio deCiencia e Innovación PID2020-113445GB-I00, and the projectULTRA-BRIGHT from the Fundación Ramón Areces.■ REFERENCES(1) Pan, J.-W.; Chen, Z.-B.; Lu, C.-Y.; Weinfurter, H.; Zeilinger, A.;Żukowski, M. 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