Optical Memory in a Microfabricated Rubidium Vapor Cell

Scalability presents a central platform challenge for the components of current quantum network implementations that can be addressed by microfabrication techniques. We demonstrate a high-bandwidth optical memory using a warm alkali atom ensemble in a microfabricated vapor cell compatible with wafer-scale fabrication. By applying an external tesla-order magnetic field, we explore a novel ground-state quantum memory scheme in the hyperfine Paschen-Back regime, where individual optical transitions can be addressed in a Doppler-broadened medium. Working on the ${}^{87}\mathrm{Rb}$ ${\mathrm{D}}_2$ line, where deterministic quantum dot single-photon sources are available, we demonstrate bandwidth-matching with hundreds of megahertz broad light pulses keeping such sources in mind. For a storage time of 80 ns we measure an end-to-end efficiency of ${\eta }_{e2e}^{80\mathrm{ns}}=3.12(17)\%$, corresponding to an internal efficiency of ${\eta }_{\mathrm{int}}^{0\mathrm{ns}}=24(3)\%$, while achieving a signal-to-noise ratio of $\mathrm{SNR}=7.9(8)$ with coherent pulses at the single-photon level.

Figure 1

Front (a) and side (b) view of the microfabricated vapor cell. Colored glass filters for absorbing the heating lasers sandwich the cell. (c) The ferromagnetic cores of the electromagnet limit the physical and optical access close to the cell. (d) Energy levels of ${}^{87}\mathrm{Rb}$ in the external field and $\mathrm{\Lambda }$ scheme used for the atomic memory. Energy splitting and decoupling of $\mathbf{J}$ and $\mathbf{I}$ yield a “clean” three-level system in the Doppler-broadened ${}^{87}\mathrm{Rb}$ ${\mathrm{D}}_2$ line. The signal and control fields are on two-photon resonance with a detuning $\mathrm{\Delta }=-2\pi \times 750\mathrm{MHz}$ with respect to $|e⟩$.

Figure 2

Sketch of the experimental setup. The dashed boxes represent the preparation stages for the various optical pulses involved in the memory protocol. DFB, distributed feedback (laser); EOM, electro-optic modulator; AWG, arbitrary waveform generator; DDG, digital delay generator; SOA, semiconductor optical amplifier; TA, tapered amplifier; IF, interference filter; $\lambda /2$, half wave plate; $\lambda /4$, quarter wave plate; $50:50$, beam splitter; SNSPD, superconducting nanowire single-photon detector. The labels S, C, P, and FP represent the fiber links of signal, control, pump, and forbidden pump, respectively.

Figure 3

Signal pulse shape. The blue trace shows the optical weak coherent pulse (WCP), measured as signal input (Fourier bandwidth 0.7 GHz FWHM). The target shape, consisting of the cross-correlation $g_{s,i}^{(2)}$ from the spontaneous parametric down-conversion (SPDC) source in [21], is shown in red. For comparison, the dashed yellow line shows the voltage pulse programmed in the AWG.

Figure 4

(a) Arrival-time histogram for storage and retrieval experiments and corresponding noise measurements. Zero time delay corresponds to the time at which signal photons leaking through the memory in failed storage attempts reach the detector. The shaded area corresponds to the 6.48-ns-wide ROI. The features between 20 and 40 ns constitute unintentional readout and are induced by insufficient suppression of the control EOM. (b) Memory lifetime measured as the decrease in efficiency. Each data point is integrated over 2 min and the data are scaled by a factor 1.13 to match the performance of the dataset shown in (a). A fitted exponential decay describes the data well. The shaded area corresponds to the 95% confidence interval of the fit.