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## Optomechanical interface between telecom photons and spin quantum memory - Nature Physics

Abstract . Quantum networks enable a broad range of practical and fundamental applications spanning from distributed quantum computing to sensing and metrology. A cornerstone of such networks is an interface between telecom photons and quantum memories, which has proven challenging for the case of spin-mechanical memories. Here we demonstrate a novel approach based on cavity optomechanics that utilizes the susceptibility of spin qubits to strain. We use it to control electronic spins of nitrogen vacancy centres in diamond with photons in the 1,550?nm telecommunication wavelength band. This method does not involve qubit optical transitions and is insensitive to spectral diffusion. Furthermore, our approach can be applied to solid-state qubits in a wide variety of materials, expanding the toolbox for quantum information processing. You have full access to this article via your institution. Download PDF Download PDF Main . Quantum technologies are evolving rapidly, driven by applications in quantum sensing 1 , communications 2 , computing 3 and networking 4 . Optically active defects in solids (colour centres) are a promising platform for implementing quantum technologies 5 . Their spin degrees of freedom serve as quantum memories that, in some cases, can operate at room temperature. When entangled with photons, they form quantum nodes, which are building blocks of a quantum network 6 . This has been achieved with microwave spin control and resonant optical excitation 7 , but is hindered by broadening of optical transitions from thermal phonons and spectral diffusion 8 , 9 . Furthermore, spin qubit optical transitions often lie outside telecommunication wavelength bands used for long-distance fibre-optic transmission. Harnessing coupling between mechanical motion and spins has emerged as an alternative route for controlling spin qubits 10 , 11 . However, connecting spin-mechanical systems to optical links has remained a challenge. Here we use a cavity optomechanical device 12 to create a spin–photon interface that does not depend on optical transitions and can be applied to a wide range of spin qubits. Acoustic waves in solids play a key role in practical devices such as modulators, electronic filters and sensors 13 . Mechanical degrees of freedom are also central to many quantum technologies, owing to their ability to couple to a wide range of fields (electrical, magnetic, electromagnetic and gravitational) through device engineering. For example, phonons mediate quantum gates between trapped ions in quantum computers 14 and can coherently connect superconducting qubits 15 . Experiments with spin qubits have demonstrated that piezoelectrically generated acoustic waves can control electronic spins of diamond and silicon carbide colour centres in bulk 16 , 17 , 18 , 19 , cantilever 20 , 21 , 22 and hybrid nanowire 23 , 24 mechanical resonators, as well as erbium ions in cantilevers 25 . Despite these advances in spin-mechanical devices, combining them with an interface for optically controlling the mechanical resonator has yet to be realized. This capability would enable optomechanical spin control but is challenging due to the weak interactions between mechanical resonators and photons. Cavity optomechanical devices 12 solve this challenge: by integrating mechanical resonators within an optical cavity, they increase the photon–phonon interaction time and the optomechanical coupling rate ( g om ). They offer a parametric enhancement of g om by increasing the number of intracavity photons N , creating a coherent optomechanical interface characterized by optomechanical cooperativity $${C}_{{{\mathrm{om}}}}=4N{g}_{{{\mathrm{om}}}}^{2}/\kappa {\gamma }_{{{\mathrm{m}}}} > 1$$ , where κ and γ m are the optical cavity and mechanical resonator dissipation rates, respectively. This regime has been realized in a variety of cavity optomechanical devices, including those fabricated from diamond 26 , 27 . When cooled to near their mechanical ground state, devices with C om ?>?1 can generate entanglement between photons and phonons 28 , 29 . These devices are promising for universal quantum transducers, for example, between optical photons and superconducting microwave resonators 30 , 31 , 32 , 33 . In this Article, we couple phonons to both light and electronic spins, creating a cavity optomechanical interface with spin qubits. Using telecommunication-wavelength photons and operating at room temperature, we manipulate an ensemble of spin qubits in a diamond microdisk cavity. This device and the spin-optomechanical interface’s operating principle are shown in Fig. 1 . Radiation pressure from photons in a microdisk whispering gallery mode coherently excites vibrations of a mechanical mode. This motion, approximately described by oscillation of the microdisk diameter, creates a microscopic stress field at the mechanical resonance frequency that interacts with spin qubits in the diamond. The optomechanical interaction can be tuned for reversible photon–phonon conversion and can operate at any wavelength resonant with the cavity. The resulting photon–spin interface does not rely on qubit optical transitions, and we show that it allows manipulation of diamond nitrogen vacancy (NV) spins 34 with telecommunication light. Moreover, it can be applied to other colour centres, including optically inactive qubits in solids 35 , and to manipulate quantum-dot single-photon sources 36 . Fig. 1: The device and its operating principle. The microdisk cavity optomechanical interface between spin quantum memories and telecommunication-wavelength photons. The insets show the transduction scheme (right) and a conceptual large-scale quantum network (left). Full size image Experiment . Tunable optomechanical phonon excitation . Nanophotonic devices such as microdisks are naturally suited for spin-optomechanical interfaces. Their small size supports GHz frequency mechanical modes that are resonant with a variety of qubits, can be cryogenically cooled to low phonon occupation 28 and whose small mechanical and optical mode volumes enhance spin–phonon 20 and photon–phonon coupling rates, respectively. The 5.3-μm-diameter microdisk used here minimizes mechanical mode volume while maintaining low optical loss needed for coherent optomechanics. It was patterned from a diamond chip (Element Six, optical grade) using quasi-isotropic plasma etching 37 . An optical mode at wavelength λ o ?=?1,564?nm with quality factor Q o ?=?1.1?×?10 5 is used to measure and drive the device’s mechanical radial breathing mode (RBM), whose displacement and stress distributions are shown in Fig. 2a . From thermomechanical spectroscopy (Supplementary Section II ), its frequency and quality factor are ω m /2π?=?2.09?GHz and Q m ?=?4,300, respectively. The optomechanical interaction between these modes has a phonon–photon coupling rate g om /2π?≈?25?kHz (refs. 27 , 37 ). Other optomechanical parameters are summarized in Supplementary Table 1 . The RBM creates stress predominantly along the microdisk’s radial, $$\hat{r}$$ , and tangential, $$\hat{\phi }$$ , unit vectors. Other stress tensor components are an order of magnitude smaller (Supplementary Section IV ). This stress field is concentrated at the microdisk centre, where a single phonon is predicted to produce p 0 ?≈?1?kPa of stress. While this is near the state of the art 11 , it is too weak for single-phonon driving of the NV ground-state spin qubits used here. Instead, we generate a large coherent phononic state using phonon lasing 38 . Fig. 2: Optomechanical characterization of the microdisk resonator. a , The displacement profile of the RBM and cross-sections for its radial σ rr and azimuthal σ ? ? stress tensor components produced by a single phonon. ‘Point of interest’ is the location of the confocal spot for the collection of NV photoluminescence. The inset shows a scanning electron micrograph image of the microdisk under study. b , Relative frequencies of the relevant optical fields and optical cavity mode (black Lorentzian) during phonon lasing with injection locking. c , Measured amplitude of the optomechanically amplified RBM for varying optical power input to the fibre taper waveguide. d , Power spectral density (PSD) spectrographs of the RBM displacement for free-running (top left) and injection-locked (bottom left) self-oscillations. Right panel shows the PSD for varying injection-locking frequency. The colour bar spans 90 dBm. e , Normalized PSD as a function of detuning between the mechanical resonator and the injection locking source δ m,i ?=? ω m ??? ω inj . The red lines are the PSD spectra for the injection-locked RBM at discretely varying δ m,i . Grey dots are the peak values of each PSD, which are fitted to a Lorentzian. Full size image Phonon lasing requires driving a sideband-resolved cavity ( ω m ?>? κ ) with a laser blue-detuned by ω m from resonance (Fig. 2b ). Stokes scattering creates a photon in the optical cavity mode and a phonon in the mechanical mode, while the non-resonant anti-Stokes process is suppressed. When the drive field is strong enough for C om ≥ 1, the phonon generation rate exceeds its intrinsic dissipation rate γ m , generating mechanical self-oscillations. Figure 2c plots the measured RBM amplitude for varying blue-detuned drive laser power input to the fibre (Supplementary Section IV ), showing a self-oscillation threshold near 10.2?mW (~0.5?mW dropped into the cavity). The maximum displacement of 9?pm, measured in Fig. 2c by calibrating the self-oscillation signal with the thermomechanical signal at low laser power, corresponds to a stress p max ?=?20.8?MPa that is large enough to drive diamond NV spin qubits (Supplementary Section IV ). Efficient spin-optomechanical coupling also requires that mechanical oscillations are resonant with the desired electronic spin transition frequency ( ω s ). We coarsely tune ω s with a magnetic field, and then fine-tune the mechanical oscillation frequency using injection locking 39 (Supplementary Section II ). Phase modulating the drive laser at frequency ω inj modulates the intracavity radiation pressure and entrains the self-oscillations. Without injection locking, the self-oscillation frequency slowly drifts due to laser and environmental fluctuations (Fig. 2d , top left). With injection locking, the self-oscillation frequency is stable (Fig. 2d , bottom left). Importantly, we can tune the self-oscillation frequency by ±5 MHz by adjusting the detuning δ m,i ?=? ω m ??? ω inj between the intrinsic mechanical frequency and the injection tone (Fig. 2d , right). However, as shown in Fig. 2e , the oscillations’ power spectral density decreases with increasing ∣ δ m,i ∣ (ref. 40 ), following a Lorentzian profile with FWHM of 2π?×?380?kHz, close to the mechanical resonance’s intrinsic bandwidth γ m ?=? ω m / Q m ?≈?2π?×?490?kHz. Since the largest self-oscillation amplitude occurs when the injected tone and mechanical frequency are resonant ( δ m,i ?=?0), to maximize optomechanical spin driving, the spin-mechanics detuning δ s,m ?=? ω s ??? ω m should be zeroed via the magnetic field. The combination of optomechanical direct driving and cavity backaction used to control the mechanical resonator shares advantages of both: the tunability of direct driving and the relative simplicity of exciting stable large-amplitude self-oscillations. Spin-optomechanical driving . We next use this tunable phonon lasing to demonstrate a spin-optomechanical interface. The negatively charged NV centre has an electronic spin-triplet ground state $$\{\left0\right\rangle ,\left\pm 1\right\rangle \}$$ (Fig. 3a ) that can be optically initialized and read out at room temperature 41 . Mechanical coupling to NV spins arises from deformation of their molecular orbitals by crystal lattice strain that displaces carbon atoms surrounding the NV defect (Fig. 3b ) 10 , 42 . Coupling NV centres to mechanical resonators 16 , 17 , 21 , 43 , 44 has led to new spin manipulation capabilities 45 , 46 , techniques for suppressing decoherence 22 , 24 and tuning of NV emission 47 . However, the lack of optical cavities in these systems has prevented interfacing them with photons. Diamond microdisks provide this needed element, enabling optomechanical manipulation of the strain-coupled $$\left+1\right\rangle \to \left-1\right\rangle$$ transition. Each of these levels is split by interaction with the 14 N nuclear spin (Supplementary Fig.? 5 ). Since strain coupling preserves nuclear spin, we focus on one of these transitions 16 , choosing the zero nuclear spin hyperfine level $$(\left{0}_{\mathrm{I}}\right\rangle )$$ to simplify initialization and read out (Supplementary Section III ). Note that the other transitions are ±4 MHz detuned, where the injection-locked mechanical self-oscillation amplitude is too small to drive them efficiently. Fig. 3: NV centres in diamond and spin driving sequence. a , Atomic structure of the NV centre. Grey circles represent the displacement of carbon atoms under a stress along the y axis ( σ yy ). b , The energy level diagram of the NV ground-state spin triplet at a magnetic field B ?=?375?G applied along the NV symmetry axis ( z ); δ s,i is the detuning between the injection-locked mechanical self-oscillation frequency and the $$\left+1\right\rangle \to \left-1\right\rangle$$ spin transition. c , A photoluminescence scan over the microdisk, showing the position of the NV measurement spot. d , Pulse sequence used during measurements of optomechanical NV spin driving. Init., initialize e , Population of NV levels at each step of the pulse sequence in d . The diameter of each solid red (dashed black) circle represents the population after (before) each step. Full size image To study spin-optomechanical coupling, the device was mounted in a confocal microscope (0.8 numerical aperture (NA)) operating in ambient conditions (Extended Data Fig. 1 ). A 532 nm laser is used for NV initialization and read out. Figure 3c shows an image obtained from photoluminescence upon rastering the laser over the microdisk. A 375?G magnetic field from a permanent magnet, aligned along one of four possible NV crystallographic orientations, splits the $$\left\pm 1\right\rangle$$ levels of this subset of NVs close to resonance with ω m . A thin wire delivers microwave (MW) pulses for spin measurement sequences discussed below (Supplementary Section III ). For NVs at the point of maximum stress at the microdisk centre (Fig. 2a ), we predict that mechanical self-oscillations drive the spin transition at a spin-mechanics coupling rate $$\varOmega_{\mathrm{{m}}}^{\mathrm{max}} / 2 \uppi \approx p_0 g_{\mathrm{str}} \approx 395\, {\mathrm{kHz}}$$ ?, where g str ?≈?19?Hz kPa ?1 is the NV stress susceptibility 21 . Unfortunately, photoluminescence at the device centre is dominated by NVs in the pedestal (Fig. 3c ). These NVs are uncoupled to the RBM, and their emission degrades the signal from microdisk NVs. As a compromise, we study NVs offset by 0.7?μm from the microdisk centre (Figs. 1c and 3c ). Here, stress is reduced by <30%, and pedestal photoluminescence is sufficiently suppressed to observe spin-mechanical coupling. Taking into account the direction of the NV symmetry axis and the tensorial nature of the NV–stress interaction, we estimate Ω m /2π?≈?100?kHz at this location (Supplementary Section IV ). To measure stress-induced driving of NV spins, we apply the pulse sequence shown in Fig. 3d . First, NVs are pumped into $$\left0\right\rangle$$ with the 532 nm laser (Fig. 3e ). Then a MW π-pulse transfers spins from $$\left0\right\rangle$$ to $$\left+1\right\rangle$$ . We then mechanically drive $$\left+1\right\rangle \to \left-1\right\rangle$$ for 7 μs at a frequency set by the injection-locking tone ω inj . Finally, we measure the population remaining in $$\left+1\right\rangle$$ by transferring it back to $$\left0\right\rangle$$ with a second π-pulse, followed by reading out $$\left0\right\rangle$$ using a green laser pulse. During this sequence, the mechanical driving depletes the population in $$\left+1\right\rangle$$ by promoting it to $$\left-1\right\rangle$$ . Upon read out, this appears as missing population in $$\left0\right\rangle$$ . Note that the ω inj tone and mechanical drive amplitude are constant during this sequence. The scheme is then repeated with the MW π-pulses modified to read out the population in $$\left-1\right\rangle$$ mechanically transferred from spins initialized in $$\left+1\right\rangle$$ . As described above, the mechanical amplitude is maximum when δ m,i ?=?0 (Fig. 2e ). However, efficient spin–stress driving requires that the spin transition be resonant with the injection-locked oscillations: δ s,i ?=?0. Both conditions are met if δ s,m ?=?0 through magnetic field tuning. In our apparatus, the closest to this condition that we achieved during a measurement run was δ s,m ?=?2π?×?182?kHz due to imprecision in positioning the magnet. We then measured the population p ±1 in $$\left\pm 1,{0}_{I}\right\rangle$$ as a function of δ s,i by varying ω inj (Fig. 4a ) (Supplementary Sections III and V ). The coinciding dip in p +1 and peak in p ?1 , together with their dependence on δ s,i , verifies that spins are being optomechanically driven. As a control dataset, we repeated the same measurements for ω m far from the spin resonance, δ s,m /2π?=???769?kHz (Fig. 4b ). Setting the injection-locking detuning to compensate for the spin-mechanics detuning, δ m,i ?=??? δ m,s , brings the mechanical oscillation frequency back to spin resonance, albeit at a significantly reduced amplitude (Fig. 2e ). Within the measurement’s signal-to-noise ratio (SNR), we were not able to reliably identify any peak or dip in this case, supporting the above claims. Next, we repeated these measurements for varying mechanical amplitude by changing δ s,m while keeping δ s,i constant via adjustment of ω inj (Fig. 4c ). This was possible owing to slow drift in δ s,m over the course of several measurement runs (Supplementary Section V ). As expected, the transferred spin populations increase monotonically with stress amplitude. Fig. 4: Optomechanical control of NV centres. a , Measured populations of $$\left\pm 1,{0}_{\mathrm{I}}\right\rangle$$ when the self-oscillation injection-locking frequency and hence δ s,i is varied, for 7?μs of mechanical drive and δ s,m /2π?=?182?kHz. The top panel shows the corresponding measured variation in optomechanically induced stress. Solid lines are fits to the model with r being a free parameter and $${T}_{2}^{ }=0.8\ \upmu{{\mathrm{s}}}$$ (Supplementary Sections IV and V ). b , Same as a for δ s,m /2π?=??769?kHz. Solid lines are theoretical predictions based on parameters from a . The inset shows the detunings between the NV spin transition, the intrinsic mechanical frequency and the injection-locking tone. Dashed vertical lines in a and b denote detunings where mechanical stress is maximum ( δ m,i ?=?0). c , Measured population of $$\left-1,{0}_{\mathrm{I}}\right\rangle$$ for varying optomechanically induced stress after 7?μs of mechanical drive when ∣ δ s,i ∣ /2π?=?263?kHz. d , Simulated FWHM ( Δ ) and population change for different spin–stress coupling strength Ω m , generated using the δ s,i -dependent stress amplitude from a for $${T}_{2}^{ }=0.5$$ and 0.8 μs, δ s,m /2π?=?182?kHz and r ?=?0. The horizontal dashed line denotes Δ /2π?=?0.54?MHz; the vertical grey region represents the estimated range of Ω m . Error bars are one standard deviation. Source data Full size image Estimating the spin-mechanics coupling rate . To extract the spin-mechanics coupling rate Ω m from the measurements, we compare them with predictions from a quantum master equation model (Supplementary Sections IV and V ). This model, which includes the spin transition dephasing ( $$1/\uppi {T}_{2}^{ }=400\ \,{{\mathrm{kHz}}}\,$$ ) and the injection-locking bandwidth Γ tune , generates the theoretical curves in Fig. 4 . Its fitting parameters are Ω m and the fraction r of photoluminescence from pedestal NVs uncoupled to the mechanical resonance (Fig. 3c and Supplementary Section V ). In our parameter regime, $${{{\varOmega }}}_{{{\mathrm{m}}}} < 2\uppi /{T}_{2}^{ }$$ , the width (FWHM Δ ) of the dip (peak) in p +1 ( p ?1 ) is determined predominantly by Γ tune and Ω m . Figure 4d shows the predicted dependence of Δ on Ω m . Given Δ /2π?≈?540 kHz observed in Fig. 4a , we infer that Ω m /2π?=?168?±?8 kHz (error due to uncertainty in $${T}_{2}^{ }$$ as discussed in Supplementary Section III ). For this coupling rate, Fig. 4d also shows that a 45% change in p ?1 is expected. This is significantly larger than the 10% change observed in Fig. 4a , and can be explained by pedestal NVs reducing the measured contrast. To account for this, we define corrected spin populations $${p}_{-1}^{\,{{\mathrm{corr}}}\,}={p}_{-1}/(1-r)$$ and $${p}_{+1}^{\,{{\mathrm{corr}}}}=1-{p}_{-1}^{{{\mathrm{corr}}}\,}$$ . For $${p}_{-1}^{\,{{\mathrm{corr}}}\,}$$ to agree with the 45% prediction, we estimate r ?≈?0.8. A secondary vertical axis for $${p}_{\pm 1}^{\,{{\mathrm{corr}}}\,}$$ obtained for this r is included in Figs. 4a,b . Note that a conservative lower bound on Ω m can be inferred by ignoring Δ and using the uncorrected ( r ?=?0) change of 10% in p ?1 , which Fig. 4d shows corresponds to $${{{\varOmega }}}_{\,{{\mathrm{m}}}}^{{{\mathrm{min}}}\,}/2\uppi \approx 50$$ ?kHz. The theoretically predicted p ±1 for the values of Ω m and r extracted above are plotted in Figs. 4a–c and agree with the measured data within our signal to noise. The far-detuned control data in Fig. 4b would ideally exhibit a small signal, but it is not resolvable due to noise. Note that we have assumed that the spins are distinguishable due to inhomogeneous broadening, and as such there is no multispin enhancement to the spin–phonon coupling in the experiment. In the future, more precise determination of Ω m could be achieved by measuring single NVs 21 in ultrapure diamond devices. This would eliminate the need for r and allow spatial imaging of Ω m (ref. 18 ). Measuring p ±1 for varying interaction time would provide further characterization of Ω m . However, the relatively low SNR in our setup prevents observing smaller signals obtained for shorter interaction times. For longer interactions, the signal does not change significantly yet noise increases. In all measurements, drift in ω m and ω s prevented boosting SNR via additional data acquisition and averaging. Active temperature and magnetic field tuning could reduce these noise sources. Increasing Ω m offers a more favourable approach to boosting the SNR and would also allow observation of coherent oscillations of spin population 43 . At present, Ω m is limited by clamping of the self-oscillation amplitude by nonlinearities in the optical mode’s optomechanical transduction 48 . Direct optomechanical driving with a large-amplitude tone will be similarly limited by nonlinear effects. The nonlinearity can be reduced and the clamped amplitude increased by using a lower- Q o mode at the expense of requiring a higher drive laser power. Alternatively, creating devices incorporating spins with higher strain sensitivity and supporting more localized mechanical modes could increase Ω m , as discussed below. Discussion and future directions . Creating a single-photon interface . The device demonstrated here is a proof-of-principle optomechanical interface between classical light and spin qubits. Many of its quantum networking applications require operation in a regime where a single photon coherently and reversibly couples to a spin qubit. Reaching this regime requires that both C om and the spin-mechanical cooperativity ( C sm ) exceed unity 33 . As summarized in Fig. 5 , reaching this quantum regime is possible using already demonstrated diamond cavity optomechanical devices coupled to diamond spin states with higher stress sensitivity than the NV ground state. Fig. 5: Roadmap for realizing a single-photon spin-optomechanical interface. a , Using diamond silicon vacancy centres as a spin qubit will increase C sm above 0.1 owing to their ~10 5 -fold higher strain sensitivity. b , Replacing the microdisk with an optomechanical crystal will further increase C sm by more than 10 3 -fold, and a phononic shield will further boost both C sm and C om . Similarly high cooperativity can be realized using NV ground-state spins coupled through a phonon-assisted optical Raman process. Full size image Optomechanical C om ?>?1 is routinely demonstrated in diamond microdisks, as verified by the onset of self-oscillations in our current devices. Reversible photon–phonon conversion is achieved via optomechanically induced transparency, where the drive laser is red-detuned (in contrast to the blue detuning used above for generating self-oscillations). This regime has already been demonstrated and explored with diamond microdisks 49 . The outstanding technical challenge is performing coherent conversion in a device cryogenically cooled near its mechanical quantum ground state without heating it via material optical absorption. This has been demonstrated in silicon optomechanical quantum memories 28 , 29 . In diamond devices, this will be aided by the material’s low nonlinear absorption and excellent thermal properties. Realizing spin-mechanical $${C}_{{{\mathrm{sm}}}}={g}_{{{\mathrm{sm}}}}^{2}/{\gamma }_{{{\mathrm{m}}}}{\gamma }_{{{\mathrm{spin}}}} > 1$$ in our device is hindered by the NV ground state’s intrinsically weak spin–phonon coupling, resulting in C sm ?≈?10 ?9 . Fortunately, as detailed in Supplementary Section VI and Fig. 5a , realizing C sm ?>?1 is possible by coupling phonons to NV excited states, or to diamond silicon vacancy (SiV) ground states. Both of these systems are approximately 10 5 times more sensitive to stress than the NV ground state. However, they require low-temperature operation for good coherence, making them less convenient for the proof-of-concept spin-optomechanics experiment presented here. We focus primarily on the SiV since it has been successfully incorporated into nanophotonic devices while maintaining excellent spin and optical properties. The interaction between SiV spins and GHz frequency phonons has been well characterized 19 . We predict that a single phonon of the RBM will couple to an SiV spin with a rate g sm /2π?≈?0.1 MHz, using either a Raman phonon–MW coupling scheme 50 or direct coupling of a magnetically tuned qubit 51 . This corresponds to C sm ?≈?0.2 for γ spin /2π?=?1?MHz, typical for SiV spin qubits, and γ m /2π?=?200?kHz, as measured for nominally identical diamond microdisks in ref.? 49 . As shown in Fig. 5b , device improvements will further enhance both C sm and C om . A diamond optomechanical crystal cavity 26 will increase C sm to beyond 100 owing to its smaller mechanical mode volume and correspondingly larger g sm /2π?≈?2?3?MHz. These devices would similarly increase g om /2π to?≈?200?kHz, resulting in C om ?≈?10, as demonstrated in ref.? 26 . A phononic shield could dramatically reduce γ m , increasing both C sm and C om by several orders of magnitude; Q m ?>?10 9 has been observed in phononic crystal shielded silicon optomechanical devices 52 , enabling C om ?>?1 for single ( N ≤ 1) drive photons. Comparable C sm could be realized with NV ground-state spins coupled to a strain-sensitive NV excited state using an optical Raman process 17 . However, the spectral diffusion characteristic of optical transitions of NVs in nanophotonic devices would pose a challenge 9 . Finally, dynamically modulating the mechanical frequency through the optomechanical spring effect can offer exponential enhancement of g sm (ref. 53 ), providing a complementary approach to increasing C sm . Conclusion . The spin-optomechanical interface demonstrated here allows operation at telecommunication wavelengths regardless of the qubit’s resonant transitions, offers protection from spectral diffusion 9 and can be used with qubits lacking optical transitions 35 . It is an important step towards achieving single-photon coherent optomechanical coupling to spins, which will lead to universal coupling of telecom photons to hybrid quantum nodes built on spins and superconducting qubits 50 . It will also enable nonlinear optomechanics via spin-induced anharmonicity of the mechanical oscillator, in analogy with superconducting qubits in MW resonators, potentially leading to hybrid acoustic–quantum computers 54 . Other applications include spin cooling of mechanical resonators 55 , optomechanical control and detection of single photons 56 , 57 and phonons 52 and creating quantum transducers 33 . Methods . Measurements were performed using a home-built confocal microscope (Extended Data Fig. 1a,b ). A 532?nm laser (CL532-025-SO, maximum power 25?mW) and a fibre-coupled acousto-optic modulator (AOM, AAoptoelectronic MT200-BG18) generate laser pulses to prepare and read out NV spins. For imaging, we use a ×100 objective lens (Nikon TU Plan ELWD, NA 0.8) that is scanned across the sample with a 3D piezo nanopositioner (Tritor 101). NV photoluminescence (PL) is separated from the 532?nm excitation with a dichroic beamsplitter, spectrally filtered (550–800?nm) and sent to a single-photon counting module (SPCM). During spin-mechanics measurements, the position of the confocal spot on the microdisk was actively stabilized based on frequently acquired PL raster scans. To deliver MW pulses, we mount the diamond sample on a custom printed circuit board chip. A thin copper wire (diameter?~20?μm) is placed over the sample and soldered to the printed circuit board chip such that the wire is 30?μm from the microdisk. MW pulses are generated by an arbitrary waveform generator (AWG, Tektronix AWG70002A) and amplified using Mini-Circuits ZHL-25W-63+. AWG marker outputs synchronize a delay generator (Stanford Research Systems DG535) used to pulse the AOM and a time-tagger (PicoQuant TimehHarp 260), which builds a time-resolved histogram of photon counts from the SPCM. A direct-current magnetic field is produced by a permanent magnet mounted on a manual 3D translation stage. Nanopositioners (Attocube) are used to position the device and an optical fibre taper (Extended Data Fig. 1 a,b). The sample is positioned on an XYZ stack of slip–stick stages, while the fibre taper is mounted on an XYZ scanner stage. These stages are located within a chamber that is continuously purged with nitrogen. To drive the mechanics in the diamond microdisk, we use a tunable diode laser (NewFocus-6700 1,520–1,570?nm) amplified with an erbium-doped fibre amplifier (Pritel LNHPFA-30) to ~70 mW power. This drive laser was phase-modulated with an EOM (EOSpace PM-5S5-20-PFA-PFA-UV-UL) driven by a signal generator (Agilent N5171B) to injection-lock the mechanics. Another laser (NewFocus-6700 1,470–1,545?nm) is used to independently probe the mechanics during measurements where the injection-locking tone is turned on. Both lasers are combined using a 90:10 fibre beamsplitter and evanescently coupled into the microdisk via the dimpled optical fibre taper. The fibre taper was created by manually pulling a standard SMF28 single-mode fibre while heating it using a hydrogen torch. A dimple is created in the fibre taper so that it can be positioned in the optical near field of the microdisk without interfering with the unpatterned diamond substrate. For all of the measurements presented in the paper, the dimpled fibre taper was ‘parked’ on the shoulders of the etched circular trench that defines and surrounds the microdisk, without touching the microdisk. This ensures the stability of the tapered fibre’s position, which was not adjusted over the course of any of the measurements. The output of the fibre taper is spectrally filtered to separate the drive and probe light using a wavelength-division multiplexer (WDM, Montclair MFT-MC-51-30 AFC/AFC-1) and is monitored by a high-bandwidth photodetector (NewFocus 1554-B). The mechanical displacement of the microdisk is read out by monitoring time-dependent transmission fluctuations of drive or probe light with a real-time spectrum analyser (Tektronix RSA5106A). 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This work was supported by the Alberta Innovates Strategic Research Project (G2018000888), the Canada Foundation for Innovation (CGI Project 36130), the National Research Council Nanotechnology Research Centre, the NSERC Discovery Grant (RGPIN/04535-2016), Strategic Partnership Grant (STPGP/521536-2018, STPGP/493807-2016), Accelerator, CREATE and RTI programmes. We thank H. Jayakumar, J. P. Hadden, T. Masuda and B. Khanaliloo for contributions to the initial setup of the experimental apparatus. Author information . Author notes These authors contributed equally: Prasoon K. Shandilya, David P. Lake. Affiliations . Institute for Quantum Science and Technology, University of Calgary, Calgary, Alberta, Canada Prasoon K. Shandilya,?David P. Lake,?Matthew J. Mitchell,?Denis D. Sukachev?&?Paul E. Barclay Authors Prasoon K. Shandilya View author publications You can also search for this author in PubMed ? Google Scholar David P. Lake View author publications You can also search for this author in PubMed ? Google Scholar Matthew J. Mitchell View author publications You can also search for this author in PubMed ? Google Scholar Denis D. Sukachev View author publications You can also search for this author in PubMed ? Google Scholar Paul E. Barclay View author publications You can also search for this author in PubMed ? Google Scholar Contributions . P.K.S., D.P.L. and D.D.S. set up and optimized the experiment. P.K.S. and D.P.L. measured the experimental data. P.K.S. and D.D.S. analysed the data and independently simulated the results in discussion with P.E.B. P.K.S., D.P.L. and M.J.M. prepared the sample for the experiment. P.E.B. was responsible for experimental infrastructure. P.K.S., D.D.S. and P.E.B. wrote the manuscript with input from and discussion with the co-authors. P.E.B. supervised the overall project and its direction. All the authors critically read the manuscript and approved it. Corresponding author . Correspondence to Paul E. Barclay . Ethics declarations . Competing interests . The authors declare no competing interests. Additional information . Peer review information Nature Physics thanks Lilian Childress and other, anonymous, reviewer(s) for their contribution to the peer review of this work. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Extended data . Extended Data Fig. 1 Experimental setup. . a) Image of the inside of the sample chamber. b) Widefield optical image of fiber taper coupled microdisk. c) Schematic of the basic setup. Supplementary information . Supplementary Information . Supplementary information. Source data . Source Data Fig. 4 . All the final experimental data plotted in Fig. 4. Rights and permissions . Reprints and Permissions About this article . Cite this article . Shandilya, P.K., Lake, D.P., Mitchell, M.J. et al. Optomechanical interface between telecom photons and spin quantum memory. Nat. Phys. (2021). https://doi.org/10.1038/s41567-021-01364-3 Download citation Received : 08 February 2021 Accepted : 17 August 2021 Published : 14 October 2021 DOI : https://doi.org/10.1038/s41567-021-01364-3 Share this article . Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative .

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