A Quantum Photonic Interface for Tin-Vacancy Centers in Diamond
Alison E. Rugar, Shahriar Aghaeimeibodi, Daniel Riedel, Constantin Dory, Haiyu Lu, Patrick J. McQuade, Zhi-Xun Shen, Nicholas A. Melosh, Jelena Vu?kovi?
AA Quantum Photonic Interface for Tin-Vacancy Centers in Diamond
Alison E. Rugar, ∗ Shahriar Aghaeimeibodi, ∗ Daniel Riedel, ∗ Constantin Dory, Haiyu Lu,
2, 3, 4
Patrick J. McQuade,
4, 5
Zhi-Xun Shen,
2, 3, 4, 6
Nicholas A. Melosh,
4, 5 and Jelena Vuˇckovi´c † E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA Department of Physics, Stanford University, Stanford, California 94305, USA Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA Stanford Institute for Materials and Energy Sciences,SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA Department of Materials Science and Engineering,Stanford University, Stanford, California 94305, USA Department of Applied Physics, Stanford University, Stanford, California 94305, USA (Dated: February 24, 2021)The realization of quantum networks critically depends on establishing efficient, coherent light-matter interfaces. Optically active spins in diamond have emerged as promising quantum nodesbased on their spin-selective optical transitions, long-lived spin ground states, and potential forintegration with nanophotonics. Tin-vacancy (SnV - ) centers in diamond are of particular interestbecause they exhibit narrow-linewidth emission in nanostructures and possess long spin coherencetimes at temperatures above 1 K. However, a nanophotonic interface for SnV - centers has not yetbeen realized. Here, we report cavity enhancement of the emission of SnV - centers in diamond.We integrate SnV - centers into one-dimensional photonic crystal resonators and observe a 40-foldincrease in emission intensity. The Purcell factor of the coupled system is 25, resulting in channelingof the majority of photons (90%) into the cavity mode. Our results pave the way for the creationof efficient, scalable spin-photon interfaces based on SnV - centers in diamond. I. INTRODUCTION
The basis of scalable quantum networks are nodes withoptically accessible, long-lived quantum memories cou-pled to efficient photonic interfaces [1]. A critical mile-stone toward the implementation of a quantum networkis the development of an efficient, coherent light-matterinterface [2, 3]. Such an interface can be implementedby coupling optically active spin qubits to nanophotoniccavities. By strongly confining optical fields, nanocavi-ties enhance the coherent emission of embedded qubitsand channel the emitted photons into a single opticalmode. Diamond hosts a number of color centers that areexcellent optically interfaced spin qubit candidates [4–6]. The most established of these color centers is thenitrogen-vacancy (NV - ) center, which has enabled semi-nal experiments such as heralded long-distance entangle-ment [7] and entanglement distillation [8]. Unfortunately,fabrication-induced charge noise degrades the optical co-herence of NV - centers and thus precludes the integra-tion of NV - centers with nanophotonic cavities [9, 10].The entanglement rates in these experiments are there-fore limited by the small fraction of coherent emissioninto the NV - zero-phonon line (ZPL).Group-IV color centers in diamond are significantlyless sensitive to local charge fluctuations due totheir inversion-symmetric structure [11]. As a conse-quence, these color centers exhibit nearly lifetime-limitedlinewidths in nanostructures despite the use of invasive ∗ These authors contributed equally to this work. † [email protected] plasma etching techniques [12–14]. While silicon-vacancy(SiV - ) centers coupled to nanophotonic resonators haveenabled the demonstration of high-fidelity single-shotreadout and nuclear spin memory-enhanced quantumcommunication [15], SiV - centers suffer from low quan-tum efficiency, and their excellent spin properties are ac-cessible only under high strain [16] or at millikelvin tem-peratures [17]. Fortunately, other group-IV color centersin diamond are expected to share many of the favor-able properties of SiV - centers but at higher tempera-tures [18]. Tin-vacancy (SnV - ) centers [13, 19–21] standout because of their high quantum efficiency [19] and longspin coherence times at cryogenic temperatures above1 K [13]. Despite their promise as optically active spinqubit candidates, the critical step of incorporating SnV - centers into cavities has yet to be realized.In this letter, we report the coupling of the ZPL emis-sion of SnV - centers to one-dimensional photonic crys-tal cavities. We fabricate a large array of devices on asingle chip using a fabrication technique based on thequasi-isotropic diamond undercut method [22–26]. Bytuning the cavity into resonance with the color center, wedemonstrate a 40-fold increase in SnV - center emissionintensity. Furthermore, we observe a 10-fold reduction inthe SnV - center excited-state lifetime, corresponding toa Purcell factor of 25. Because of this enhancement, themajority of photons are emitted into the cavity modevia the ZPL ( β = 90%). With their excellent opticalproperties [19] and competitive spin coherence times ac-cessible without a dilution refrigerator [13], SnV - centersintegrated with nanocavities constitute a promising plat-form for quantum networks. a r X i v : . [ phy s i c s . op ti c s ] F e b μ m xz y (b)(c)(d) (e) Q = 2135 ± 170 w P j |E| xy (a) Hole number j P j / P m i rr o r
615 616 617 618
Wavelength (nm) N o r m a li z ed T r an s m i ss i on FIG. 1. Design and simulation of the nanobeam photonic crystal cavity. (a)
Schematic of the proposed one-dimensionalphotonic crystal cavity. Two inverse-designed vertical couplers on either end of the beam enable the coupling of light intoand out of the device. (b)
Spacing between the holes for different hole numbers. A quadratic taper over the center 12 holescreates the cavity. (c)
FDTD simulation of electric field magnitude squared | E | showing confinement of light in the centerof the cavity for the circular-hole design. (d) Scanning electron microscope image of a fabricated device. (e)
Spectrum of acavity mode of a representative device obtained from a transmission measurement through the device. The quality factor ofthis device is Q = 2135 ± II. RESULTS
Our cavities are based on one-dimensional photoniccrystals where a periodic array of circular holes etchedinto a suspended diamond waveguide creates a pho-tonic bandgap [27]. As illustrated in Fig. 1(a), we im-plement inverse-designed vertical couplers on either endof the cavity to facilitate the in- and out-coupling ofthe light [26]. Fig. 1(b) shows the designed spacing be-tween the holes for a device with 20 holes on each side.We quadratically taper the spacing between the 12 cen-tral holes from P mirror in the mirror section to a re-duced period P center = 0 . P mirror in the center ofthe cavity to confine the light. We select the waveg-uide width w = 300 nm, thickness h = 200 nm, period P mirror = 190 nm, and hole radius r = 57 nm in orderto have a resonance around 620 nm of high quality fac-tor ( Q ) and small mode volume ( V mode ). We simulatethe performance of our design using three-dimensionalFinite-Difference Time-Domain method (FDTD; Lumer-ical). Fig. 1(c) illustrates the simulated | E | for theabove parameters with a simulated Q = 2 × and V mode = 0 .
56 ( λ/n ) .Next, we fabricate our photonic crystal cavities. Wefirst generate SnV - centers at a depth of 90 nm with ourrecently developed shallow ion implantation and growthmethod [28]. We then perform a fabrication routine basedon quasi-isotropic etching [22–26] to create a large matrix of devices. In order to account for shifts of the cavity res-onance wavelength caused by fabrication imperfections,we vary P mirror by ± ∼ - center so that we caneventually use gas tuning to tune the cavity mode intoresonance with the SnV - center ZPLs (see Methods). Wealso check the devices for SnV - center photoluminescence(PL) signal.To determine the quality factor and resonance wave-length of our cavities, we couple a supercontinuum laserthrough the vertical couplers and perform a broad-band transmission measurement. Fig. 1(e) shows themeasured transmission spectrum of a suitable device( P mirror = 228 nm and 14 holes), featuring a quality fac-tor of 2135 ±
170 at 616 .
76 nm.Next, we perform PL spectroscopy (see Methods) onthe device to verify the presence of SnV - centers in thedevice. Fig. 2(a) displays a heatmap of consecutive PLspectra. The four vertical lines apparent in Fig. 2(a) arethe ZPLs of two SnV - centers present in the cavity. At -40 -20 Delay (ns) C o i n c i den c e s
20 400 (a) (d) C transitionsD transitions (b) R e l a t i v e I n t en s i t y Pump Power ( W) I n t eg r a t ed C oun t s ( c p s ) on resonanceoff resonance (c) Counts/s/nm
FIG. 2. Intensity enhancement of SnV - centers. (a) Consecutive PL spectra acquired while gas tuning the cavity throughresonance with the ZPLs of the two SnV - centers. Two horizontal white dashed lines indicate locations of line cuts examinedin panel (b). (b) Two line cuts acquired when the cavity mode is 3.6 nm blue-detuned from the C transition (teal) and whenthe cavity mode is resonant with the C transition of interest at 619.6 nm (purple). (c)
Saturation of the SnV - center whencavity mode is 2.72 nm blue-detuned from the resonance (green diamonds) and when the cavity mode is resonant with the Ctransition at 619.6 nm (purple circles). In the resonant case the maximum achieved intensity of the C transition of the SnV - center is about 30 times greater than when the cavity is off resonance. (d) Second-order autocorrelation measurement of SnV - center when the cavity mode is resonant with the C transition. A fit (red) to the data (black) reveals a g (2) (0) = 0 . ± . - centers have two prominent ZPLscentered about 620 nm, known as the C and D transi-tions [19], where C transitions are higher in energy thanD transitions. For the remainder of this paper, we focuson the C transition located at 619.6 nm, which has thehighest count rate.We characterize the intensity of the C transition at dif-ferent detunings between the cavity mode and the C tran-sition. In Fig. 2(a), the cavity mode starts blue-detunedfrom the SnV - center ZPLs. The cavity mode is thentuned through the ZPLs of the SnV - centers by argongas condensation. We examine in Fig. 2(b) two spectrafrom Fig. 2(a) corresponding to the white dashed lines:one spectrum in which the cavity is 3.6 nm blue-detunedfrom the C transition of interest (PL number = 1) andanother in which the cavity mode is resonant with the Ctransition at 619.6 nm (PL number = 121). Between theoff- and on-resonance cases shown in Fig. 2(b), the inten-sity of the C transition increases by a factor of 40 ± - -cavity system has a large Pur-cell factor. The intensity enhancements of the other C transition at 619.3 nm and the D transitions at 620.4 nmand 621.0 nm are respectively 11 ±
2, 56 ±
11, and 42 ± ∼ - -cavity sys-tem under study has a large Purcell factor, which we willlater quantify.We confirm that the enhanced SnV - center is a single-photon source by performing a Hanbury-Brown-Twissexperiment with the cavity mode resonant with the Ctransition. The resulting second-order autocorrelation
618 620 622
Cavity wavelength (nm) ( / n s ) Time (ns) -2 -1 N o r m a li z ed C oun t s (a) (b) IRF ⎼ ⎼ ⎼ ⎼ ⎼ FIG. 3. Time-resolved PL measurements. (a)
Representative lifetime measurements at different cavity wavelengths. To fitthe data we convolve a single-exponential decay with the instrument response function (IRF) of our detectors (blue). (b)
PLrecombination rates of the studied SnV - center plotted as a function of cavity resonance wavelength (Γ( λ cav )). The data (circles)are fit with a Lorentzian model (red curve) to quantify the enhancement provided by the cavity. Filled circles correspond tolifetime data of the same color shown in (a). data, shown in Fig. 2(d), displays a distinct antibunch-ing dip. A fit to the data with a function of the form g (2) ( τ ) = 1 − (cid:0) − g (2) (0) (cid:1) e −| τ | /τ , where τ is the delaybetween detection events, reveals a τ = 1 . ± . g (2) (0) = 0 . ± .
08, indicating that the enhancedC transition is emission from a single quantum emitter.To quantify the Purcell enhancement of the system,we measure the excited-state lifetime of the SnV - centerfor different cavity resonance wavelengths ( λ cav ) throughtime-resolved PL measurements (see Methods). To tunethe cavity resonance wavelength, we inject Ar gas anddetermine λ cav using a reflectivity measurement prior toeach lifetime measurement. Fig. 3(a) displays represen-tative lifetime measurements at different cavity wave-lengths integrated for 100 s. To remove backgroundcontributions we subtract an off-resonant measurement( λ cav = 616 . λ cav ) from the respective lifetime fits.A comprehensive study of the effect of the cavity on thePL decay rates of the SnV - center is shown in Fig. 3(b).To get a better estimate for the non-resonant radiativedecay rate Γ off , we perform an additional measurementat λ cav = 617 . τ . = (6 . ± . - centersclose to the diamond bulk surface [21]. When the cavitymode is resonant with the C transition, the lifetime isstrongly reduced to τ . = (0 . ± . λ cav ) = Γ off + Γ cav δλ / (cid:0) λ cav − λ res ) + δλ (cid:1) . Here, the off-resonance decay rate is fixed toΓ off = Γ . = 0 .
143 ns − . We find a cavity-induced decay rate of Γ cav = (1 . ± .
16) ns − corresponding to an on-resonance decayrate of Γ on = Γ off + Γ cav = (1 . ± .
16) ns − .The width and center wavelength of theLorentzian fit are δλ cav = (0 . ± . λ res = (619 . ± . - center with the cavity isgiven by τ off /τ on = Γ on / Γ off = 10 . ± .
2. From thecavity-induced decay rate Γ cav = F expP ξ c Γ off we deter-mine the experimental Purcell factor F expP = 24 . ± . ξ c = 0 .
36 is a factor that corrects for the non-unity probability of the SnV - center relaxing radiativelyvia the C transition. ξ c is a product of quantum ef-ficiency (80% [19]), Debye-Waller factor (57% [21]), andbranching ratio into the C transition (80%, extractedfrom an off-resonance PL spectrum of this emitter). Thefigure of merit of our system, β , the resulting probabilityof an excited-state decaying through emission into thecavity mode via the C transition, is approaching unity: β = Γ cav / Γ on = (90 . ± . λ cav ) yields a cavity Q factor of 2384 ±
366 (Fig. 3(b))which is consistent with transmission measurement inFig. 1(e). The theoretical Purcell factor of our deviceis thus F theoP = 3 / (4 π ) Q/V mode ( λ/n ) = 302 ± . Accounting for the angular mismatch between cavity po-larization of (cid:104) (cid:105) and SnV - center symmetry axis along (cid:104) (cid:105) (cos φ = 2 / - ex-periences only (12 . ± . | E max | . This discrepancy hints at a displacement of thestudied emitter from the field maximum. III. CONCLUSION
We have demonstrated the Purcell enhancement of aSnV - center in a diamond photonic crystal cavity. Withour SnV - -cavity system, we can achieve a 40-fold increasein emission intensity into the C transition. The systemdisplays a lifetime reduction of 10 and a Purcell factorof 25. As a result, 90% of the PL emission is channeledthrough the ZPL into the cavity mode, enabling a ZPLphoton creation rate in excess of 1 GHz. The Purcell fac-tor could be further increased by improving the Q / V mode ratio of the cavities [23, 25]. Additionally, deterministicpositioning of the emitter can improve the yield of deviceswith large Purcell enhancement. Sub-10-nm placementaccuracy can be achieved by combining our shallow-ionimplantation and growth technique [28] with implanta-tion masks [30–32]. For their use in extended quantumnetworks, SnV - center ZPL photons need to be efficientlycoupled into an optical fiber network [33]. To that end,the main loss channel of the coupled system needs to betransmission into the waveguide mode. Waveguide-to-fiber coupling efficiencies of >
90% can be achieved usingfiber tapers [34]. As an alternative, optimized inverse-designed vertical couplers have the advantage of featuringa small footprint while potentially exhibiting similar cou-pling efficiencies >
85% [26]. Combining our SnV - -cavitysystems with on-chip photonic architectures via hybridintegration techniques would enable large-scale quantuminformation processing systems [12, 35, 36]. Our workpaves the way toward establishing a coherent and effi-cient spin-photon interface based on diamond color cen-ters without the need for dilution refrigerators. IV. METHODSA. Device fabrication
We fabricate our nanophotonic resonators from anelectronic-grade single-crystalline diamond plate (Ele-ment Six). The chip is first cleaned in a boiling tri-acid solution (1:1:1 sulfuric/nitric/perchloric acids). Wethen remove the top 500 nm of the chip with an oxygen(O ) plasma etch. By employing our recently developedshallow ion implantation and growth (SIIG) method [28]we create a δ -doped layer of high-quality SnV - centers.Here, Sn + ions are implanted shallowly using low im-plantation energies (1 keV) with a dose of 5 × cm − .Ion implantation was performed by CuttingEdge Ions. Athin film (90 nm) of high-quality diamond material is sub-sequently grown by microwave-plasma chemical vapor de-position (Seki Diamond Systems SDS 5010; 300 sccm H ,0.5 sccm CH , stage temperature of 650 ° C, microwavepower of 1100 W, and pressure of 23 Torr).We fabricate our photonic devices via the quasi-isotropic etching technique [22–26]. First, 200 nm ofSi x N y are grown via plasma-enhanced chemical vapordeposition. The structures are then patterned in hydro- gen silsesquioxane FOx-16 via electron-beam lithography.The Si x N y is then etched with SF , CH , and N reac-tive ion etch (RIE). We use the patterned Si x N y layeras an etch mask for the diamond substrate. The dia-mond is etched with an anisotropic O RIE. We thengrow 30 nm of Al O via atomic layer deposition. Thehorizontal planes of the Al O layer are removed witha Cl , BCl , and N RIE so that only the sidewalls ofthe diamond structures are covered by Al O . Usinga second anisotropic O RIE we expose bare diamondsidewalls. The quasi-isotropic etch [22–26] step is nowperformed to undercut the structures. This O plasmaetch step is performed at high temperature (300 ° C) withzero forward bias and high inductively coupled plasmapower [22–26]. This etch progresses preferentially alongthe { } planes [23]. Once the nanobeam waveguideshave been released and etched to the desired thickness,as validated by high-voltage SEM, the sample is soakedin hydrofluoric acid to remove the Si x N y and Al O etchmasks. B. Measurements
All measurements were conducted in a home-built con-focal microscope setup, with the sample cooled to ∼ ± - centers. For thePL data presented in Fig. 2(a), the excitation power wasset to 500 µ W. Before being coupled into a single-modefiber, the collected light is filtered by a 568-nm long-passfilter and a 532-nm notch filter. The excitation and col-lection spots are aligned to the same spot, on top of thecenter of the cavity.For the g (2) measurement, the excitation power wasset to 450 µ W. The light collected by the single-modefiber is filtered by a monochromator (Princeton Instru-ments Acton SP2750). The light is then sent via multi-mode fiber to a Hanbury-Brown-Twiss setup compris-ing a fiber-beamsplitter and two single-photon count-ing modules (SPCM; PerkinElmer SPCM-AQR-14-FC).A time-correlated single photon counting module (TC-SPC; PicoHarp 300) was used to construct the histogramshown in Fig. 2(d).To measure the lifetimes of the SnV - center in the cav-ity presented in Fig. 3, we used a supercontinuum laser(Fianium SC400) filtered with a 450-nm-long-pass filterand a 550-nm short-pass filter to excite the SnV - cen-ter. The emitted light was then filtered by a 568-nmlong-pass filter before being coupled into a fiber. Thecollected light was further filtered by a monochromator(Princeton Instruments Acton SP2750) before being cou-pled into a multi-mode fiber and sent to a SPCM. Thelifetime measurements were recorded using the TCSPC. V. ACKNOWLEDGMENTS
This work is financially supported by Army ResearchOffice (ARO) (award no. W911NF-13-1-0309); Na-tional Science Foundation (NSF) RAISE TAQS (awardno. 1838976); Air Force Office of Scientific Research(AFOSR) DURIP (award no. FA9550-16-1-0223). Stan-ford Institute for Materials and Energy Sciences (SIMES)research is supported by the Division of Materials Scienceand Engineering, Office of Basic Energy Sciences, DOE, and SLAC LDRD. A.E.R. acknowledges support fromthe National Defense Science and Engineering Gradu-ate (NDSEG) Fellowship Program, sponsored by theAir Force Research Laboratory (AFRL), the Office ofNaval Research (ONR) and the Army Research Office(ARO). S.A. acknowledges support from Bloch post-doctoral fellowship in quantum science and engineer-ing from Stanford Q-FARM. D.R. acknowledges supportfrom the Swiss National Science Foundation (ProjectP400P2 194424). 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