Quantum Interference of Electromechanically Stabilized Emitters in Nanophotonic Devices

Photon-mediated coupling between distant matter qubits may enable secure communication over long distances, the implementation of distributed quantum computing schemes, and the exploration of new regimes of many-body quantum dynamics. Solid-state quantum emitters coupled to nanophotonic devices represent a promising approach towards these goals, as they combine strong light-matter interaction and high photon collection efficiencies. However, nanostructured environments introduce mismatch and diffusion in optical transition frequencies of emitters, making reliable photon-mediated entanglement generation infeasible. Here we address this long-standing challenge by employing silicon-vacancy color centers embedded in electromechanically deflectable nanophotonic waveguides. This electromechanical strain control enables control and stabilization of optical resonance between two silicon-vacancy centers on the hour timescale. Using this platform, we observe the signature of an entangled, superradiant state arising from quantum interference between two spatially separated emitters in a waveguide. This demonstration and the developed platform constitute a crucial step towards a scalable quantum network with solid-state quantum emitters.

Popular Summary

Techniques for rapid and reliable distribution of quantum information will enable new computation, communication, and sensing technologies. In recent years, light-emitting defects inside solids have emerged as promising candidates for these technological applications, because they can be integrated easily into photonic and electronic devices. Unfortunately, these emitters suffer from variations in their optical properties induced by fluctuations in their environments, which makes it difficult to reliably share quantum information between them. Here, we present a “tuning knob” that allows for dynamic corrections of emitter optical properties.

For many promising quantum emitters, stretching the environment around the emitter is the only feasible technique for controlling optical behavior. We experimentally demonstrate that diamond waveguides that can be electrostatically deflected can be used to completely overcome the variations in the optical response of embedded silicon-vacancy (SiV) color centers. Using these devices, we create a situation in which quantum information is shared between two remote SiV centers and observe an entangled state.

Our demonstration indicates that using strain to tune SiV centers can enable reliable information sharing between distant quantum memories and removes one of the last barriers to their application in quantum networks.

Figure 1

Schematic of diamond nanophotonics device. (a) Diamond waveguides (gray) are implanted with color centers (purple and orange) at desired locations on the device. Electrodes (gold) are used to define a capacitor between plates located on the device (negative terminal) and below the device (positive terminal). Applying bias voltage between the plates causes the deflection of the doubly clamped cantilever. This tunes color centers between the plates and the first clamp (orange circle) without perturbing color centers beyond the clamp (purple circle). (b) Scanning electron micrograph (SEM) of the photonic devices. (c) Capacitor plates located on and below the devices. (d) Diamond tapers used to extract photons from waveguides. This enables the extraction efficiency of more than 85% from the diamond waveguide into the fiber.

Figure 2

Characterization of dc and ac voltage response of the devices. Voltage bias is applied to the capacitor plates, resulting in strain fields that tune the optical transitions of SiV1 in the deflected portion of the device (orange) compared to the optical transitions of SiV2 in the stationary regions (purple). The tuning range of the SiV color center greatly exceeds the inhomogeneous distribution for 50% (75%) of SiV color centers observed in this experiment shown in dark (light) blue shading. The right-hand inset shows the photoluminescence excitation spectra of two color centers at a voltage near the overlap. The left-hand inset shows the ac response of the cantilever system, measured by observing SiV optical transitions while modulating the ac driving frequency. Driving the cantilever resonance with its mechanical mode results in linewidth broadening of the color center optical transitions.

Figure 3

Reduction of spectral diffusion using nanomechanical strain. (a) Top: Spectral diffusion of SiV center optical transition, measured over a 5-h period. Bottom: Time-averaged spectrum over the same period. (b) Top: Measurement of the SiV spectral diffusion with 20-sec pulsed feedback over a 5-h period. Bottom: Time-averaged spectra over a 5-h period (blue dots) show the reduction of total linewidth down to 500 MHz. The average of single-scan SiV linewidths is 350 MHz (green line). (c) Power spectral density of measured SiV count fluctuations with (without) feedback is presented by the green (orange) line, showing reduction of noise by an average of 8 dB. Blue line indicates statistical noise limit set by finite photon emission and collection rate.

Figure 4

Observation of the superradiant entangled state using strain control. (a) Level structure for two SiV centers. Strain tuning of one SiV color center changes the detuning ($\mathrm{\Delta }$) between their $|e⟩$ to $|c⟩$ transitions. Separate lasers are applied to excite the $|u⟩$ to $|e⟩$ transition of each emitter. (b) A single photon emitted from the two excited emitters with distinguishable transitions ($\mathrm{\Delta }\ne 0$) projects the system to a statistical mixture of $|ec⟩$ and $|ce⟩$ states (blue decay path). When the emitters are indistinguishable ($\mathrm{\Delta }=0$), the emission of one photon projects the system into a superradiant bright state (purple decay path) that decays at a rate 2 times faster than that of the statistical mixture. (c) The second-order photon correlation function is measured for a single emitter (left-hand panel, gray data points) and for two spectrally distinguishable emitters (left-hand panel, blue data points). Measured $g^{(2)}(0)$ is 0.13(4) and 0.50(7) for the single emitter and distinguishable cases, respectively. Exponential curves fit to the data are plotted and mirrored onto the right half of the plot (gray and blue lines). For two emitters tuned into resonance, we observe the generation of a superradiant entangled state, signified by the peak in the photon correlation (right-hand panel, purple data points). Data are overlaid with a simulated model with a single fit parameter (see Appendix pp4). For indistinguishable emitters, we measure $g^{(2)}(0)=0.88(8)$, limited primarily by detector jitter.

Figure 5

Schematic of nanofabrication process. Diamond waveguides are fabricated using top-down and angled etching technique into the diamond substrate [22, 23, 31]. This was followed by patterning apertures on top of the fabricated structures through which $\mathrm{Si}+\mathrm{ions}$ can reach the diamond substrate at desired locations [21]. Finally, we deposit gold electrodes on top of and below the diamond devices [11, 32], which serve to deflect fabricated waveguides when a voltage is applied between them.

Figure 6

Overview of the setup used in this experiment.

Figure 7

Simulated photon correlation function for two emitters on resonance without (a) and with (b) inclusion of electronic jitter measured in the experiment.