We demonstrate direct coupling between phonons and diamond nitrogen-vacancy (NV) center spins by driving spin transitions with mechanically generated harmonic strain at room temperature. The amplitude of the mechanically driven spin signal varies with the spatial periodicity of the stress standing wave within the diamond substrate, verifying that we drive NV center spins mechanically. These spin-phonon interactions could offer a route to quantum spin control of magnetically forbidden transitions, which would enhance NV-based quantum metrology, grant access to direct transitions between all of the spin-1 quantum states of the NV center, and provide a platform to study spin-phonon interactions at the level of a few interacting spins.

The recently discovered spin defects in hexagonal boron nitride (hBN), a layered van der Waals material, have great potential in quantum sensing. However, the photoluminescence and the contrast of the optically detected magnetic resonance (ODMR) of hBN spin defects are relatively low so far, which limits their sensitivity. Here we report a record-high ODMR contrast of 46% at room temperature, and simultaneous enhancement of the photoluminescence of hBN spin defects by up to 17-fold by the surface plasmon of a gold-film microwave waveguide. Our results are obtained with shallow boron vacancy spin defects in hBN nanosheets created by low-energy He + ion implantation, and a gold-film microwave waveguide fabricated by photolithography. We also explore the effects of microwave and laser powers on the ODMR, and improve the sensitivity of hBN spin defects for magnetic field detection. Our results support the promising potential of hBN spin defects for nanoscale quantum sensing.

were charged to identify the scientific and community needs, opportunities, and significant challenges for quantum interconnects over the next 2-5 years.

Coherent control of the nitrogen-vacancy (NV) center in diamond's triplet spin state has traditionally been accomplished with resonant ac magnetic fields under the constraint of the magnetic dipole selection rule, which forbids direct control of the |−1 ↔ |+1 spin transition. We show that high-frequency stress resonant with the spin state splitting can coherently control NV center spins within this subspace. Using a bulk-mode mechanical microresonator fabricated from single-crystal diamond, we apply intense ac stress to the diamond substrate and observe mechanically driven Rabi oscillations between the |−1 and |+1 states of an NV center spin ensemble. Additionally, we measure the inhomogeneous spin dephasing time (T * 2 ) of the spin ensemble using a mechanical Ramsey sequence and compare it to the dephasing times measured with a magnetic Ramsey sequence for each of the three spin qubit combinations available within the NV center ground state.These results demonstrate coherent spin driving with a mechanical resonator and could enable the creation of a phase-sensitive ∆-system within the NV center ground state. Here we use a mechanical microresonator to apply a large amplitude ac stress to a single crystal diamond. Building on recent spectroscopy experiments [8], we tune the frequency of this stress wave into resonance with the |(m s =) − 1 ↔ |+1 spin transition to mechanically drive Rabi oscillations of an NV center spin ensemble. Using this capability, we measure the inhomogeneous dephasing time for an ensemble of mechanically controlled NV center spin qubits to be T * 2 = 0.45±0.05 µs and compare this result to T * 2 for magnetically driven qubits constructed from the same NV center ensemble. We find that the mechanically driven {−1, +1} qubit coherence is similar to that of a magnetically driven {−1, +1} qubit, and these {−1, +1} qubits dephase twice as quickly as magnetically driven {0, −1} or {+1, 0} qubits.NV centers couple to mechanical stress (σ ⊥ and σ ) and magnetic fields (B ⊥ and B ) 2 through their ground-state spin Hamiltonian (shown schematically in Fig. 1a)where D 0 /2π = 2.87 GHz is the zero-field splitting, γ N V /2π = 2.8 MHz/G is the gyromagnetic ratio, ⊥ /2π = 0.015 MHz/MPa and /2π = 0.012 MHz/MPa are the perpendicular and axial stress coupling constants [10,14], P/2π = −4.945 MHz and A /2π = −2.166 MHz are the hyperfine parameters [15][16][17], and S x , S y , S z (I x , I y , I z ) are the x, y, and z components of the electronic (nuclear) spin-1 operator. The NV center symmetry axis defines the z-axis of our coordinate system as depicted in Fig. 1b In this work, we use two devices, both fabricated from type IIa, 100 "optical grade" diamonds purchased from Element Six. These samples are specified to contain fewer than 1 ppm nitrogen impurities, and each contained a native NV ensemble as received. The first sample, Sample A, has an NV center density of ∼ 110 NVs/µm 3 , while Sample B has a density of ∼ 120 NVs/µm 3 . To generate the large amplitude, high-frequency stress waves neede...

Silicon nitride (Si3N4) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore understanding all nonlinear optical properties of Si3N4 is important. So far, only stimulated Brillouin scattering (SBS) has not been reported. Here we observe, for the first time, backward SBS in fully cladded Si3N4 waveguides. The Brillouin gain spectrum exhibits an unusual multi-peak structure resulting from hybridization with high-overtone bulk acoustic resonances (HBARs) of the silica cladding. The reported intrinsic Si3N4 Brillouin gain at 25 GHz is estimated as 7×10 −13 m/W. Moreover, the magnitude of the Si3N4 photoelastic constant is estimated as |p12| = 0.047 ± 0.004. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of Si3N4 to handle high optical power, central for integrated nonlinear photonics. arXiv:1908.09815v1 [physics.optics]