High-Capacity Angularly Multiplexed Holographic Memory Operating at the Single-Photon Level

We experimentally demonstrate an angularly multiplexed holographic memory capable of intrinsic generation, storage, and retrieval of multiple photons, based on an off-resonant Raman interaction in warm rubidium-87 vapors. The memory capacity of up to 60 independent atomic spin-wave modes is evidenced by analyzing angular distributions of coincidences between Stokes and time-delayed anti-Stokes light, observed down to the level of single spin-wave excitation during the several-microsecond memory lifetime. We also propose how to practically enhance rates of single- and multiple-photon generation by combining our multimode emissive memory with existing fast optical switches.

Figure 1

(a) Idea of holographic memory generating, storing, and releasing on-demand angularly correlated photons. Photons are produced in $2\times M$ modes which are pairwise coupled. In each independent pair of modes (e.g., colored circles), anti-Stokes photons are heralded by Stokes photons generation, with microsecond-time delay advance. (b) The generation and retrieval of photons is performed in a Raman scattering process, using $\mathrm{\Lambda }$-system ${}^{87}\mathrm{Rb}$ energy levels. (c) The phase-matching condition relates wave vectors of driving laser beams, photons, and spin-wave excitation and determines the scattering angle of an anti-Stokes photon ${\mathit{\theta }}^{(\mathrm{AS})}\simeq -{\mathit{\theta }}^{(\mathrm{S})}$. (d) The predictability of the scattering angles of anti-Stokes photons can be utilized to match outgoing photons to an optical switch [16, 17] with one or multiple outputs (not shown). The switch architecture is reconfigured after the detection of the Stokes trigger photons so as to pass single or $N$ photons into the desired fiber links.

Figure 2

The simplified scheme presenting key parts of the experimental setup for the holographic memory. The generation and storage of photons are implemented in warm ${}^{87}\mathrm{Rb}$ vapors whose thermal motion is slowed down by an admixture of noble buffer gas. Inset: Experimental sequence. The triple filtering system [42] (polarization and two-step spectral; $\mathbf{B}$ denotes the magnetic field vector) passes scattered photons at different angles and blocks all laser beams detuned by 6.8 GHz. The detection of photons with a high angular resolution is performed with an I-sCMOS camera capable of resolving photon coincidences [33, 43].

Figure 3

(a) Two example cropped frames with Stokes and 500-ns-delayed anti-Stokes photons detected as bright spots on the I-sCMOS camera. Hundreds of angularly resolved photons from a high-gain Raman scattering form pairwise inverted random patterns. (b) A real-time software algorithm converts with subpixel resolution raw I-sCMOS images into sets of coordinates representing central positions of individual single-photon spots [43]. (c) An example processed frame with several photons scattered in a low-gain regime. We further accumulate histograms of photon coordinates $n_{\text{coinc}}({\theta }_x^{(\mathrm{S})},{\theta }_x^{(\mathrm{AS})})$ for coincidences where a detected anti-Stokes photon was preceded by a Stokes trigger found in a stripe fulfilling the phase-matching condition ${\theta }_y^{(\mathrm{S})}=-{\theta }_y^{(\mathrm{AS})}\pm \mathrm{\Delta }\theta$, where $\mathrm{\Delta }\theta =0.15$ or 0.3 mrad. Black crosses mark driving beam directions.

Figure 4

(a) Angular distributions of Stokes and anti-Stokes photon coincidences $n_{\text{coinc}}({\theta }_x^{(\mathrm{S})},{\theta }_x^{(\mathrm{AS})})$ for increasing storage times $\tau$ reveal angular correlations ${\theta }_x^{(\mathrm{AS})}\simeq -{\theta }_x^{(\mathrm{S})}$ between photons stored and retrieved from the holographic memory. A visible decay of high-scattering-angle events is determined by a diffusional decoherence [46]. (b) A doubled squared ratio between the antidiagonal and diagonal width of coincidence histograms is used to estimate the total number of retrieved angular modes $M<60\simeq \mathcal{F}$ versus the storage time in agreement with the diffusion model [33]. Tens of modes are retrieved within the first $2\mu \mathrm{s}$, which is sufficiently long to couple the holographic memory source with currently existing optical switches [16, 17] [cf. Fig. 1]. Inset: A few modes can be retrieved in up to tens of microseconds. (c) Coincidences are observable down to the single spin-wave excitation level.