Direct telecom network between atomic and solid-state quantum nodes

Abstract

Future quantum networks will interconnect quantum systems with distinct functionalities, ideally over long distances via low-loss telecom optical fibers. Here, we realize a two-node hybrid network that directly connects an atomic single photon source to a solid-state quantum memory in the telecom C-band without the need of frequency conversion and external filtering. Both nodes exhibit state-of-the-art performance at 1530 nm: the source achieves a heralded auto-$g^{(2)}(0)$ = 0.031 at a photon rate of 46 kcps, and the memory a storage efficiency of 10.6% with high multimode capacity. We leverage the intrinsic tunability of both nodes to optimize spectral matching, enabling direct networking between the two: single-photon storage and retrieval for 1 $μ$s over up to 37 temporal modes across extended fibers of 10.6 km (metropolitan) and 49.2 km (laboratory) while preserving non-classicality. These results define a high-bandwidth source-memory link that operates natively in the telecom band, introducing a new paradigm for the design and scaling of hybrid quantum networks.

Figures (20)

• Figure 1: A hybrid two-node telecom quantum network. (A) An atomic single photon source at node A and a solid-state quantum memory at node B are connected through matched telecom photonic interfaces. This architecture forms a building block for a larger quantum network. (B) Energy levels of $^{87}$Rb used for spontaneous four-wave mixing (4WM). The $|4D_{3/2}\rangle\rightarrow|5P_{3/2}\rangle$ telecom transition (maroon) is at 1530 nm, and the $|5P_{3/2}\rangle\rightarrow|5S_{1/2}\rangle$ transition (yellow) is at 780 nm. (C) Coincidence histogram between the 780-nm heralding photon and 1530-nm signal photon, integrated over 20 s with a 50 ps time bin. (D) Histogram of the storage and retrieval of a weak coherent pulse from an $^{166}$Er$^{3+}$:YVO$_{4}$ atomic frequency comb (AFC) quantum memory. The three peaks in green are the transmitted input pulse, the first-order, and the second-order photon echoes, respectively. (E) Energy levels of $^{166}$Er$^{3+}$:YVO$_{4}$. Two crystal field levels are split into four Zeeman levels under an external magnetic field. The absorption line of the$|\downarrow_{g}\rangle\rightarrow|\downarrow_{e}\rangle$ transition at 1530 nm is used for AFC storage. (F) $^{166}$Er$^{3+}$:YVO$_{4}$ absorption spectrum with respect to the hyperfine transitions of Rb. Spectrum at 40 mT shows all four Zeeman transitions, which are degenerate at zero magnetic field. The $|\downarrow_{g}\rangle\rightarrow|\downarrow_{e}\rangle$ transition (depicted by solid green lines) red-shifts with increasing magnetic fields. At around 1 T, it matches with the $^{87}$Rb hyperfine transitions 2009_Moon_Rb4Dhyperfine. Dotted lines represent other optical transitions between the $|\downarrow_{g}\rangle, |\uparrow_{g}\rangle, |\downarrow_{e}\rangle, |\uparrow_{e}\rangle$ levels.
• Figure 2: Matching telecom photonic interfaces. (A) Single-photon spectrometer. The 1530-nm photon from the source is directed through a scanning Fabry-Pérot cavity and to an SNSPD. Spectra without and with crystal absorption as a notch filter are shown in (B) and (C), respectively. (B) Source node 1530-nm photon spectrum with blue-detuned two-photon pumping. The hyperfine levels involved are shown on the right. (C) 1530-nm photon spectra with crystal filtering. At various magnetic fields, the crystal transmission (solid lines) and corresponding transmitted photon spectra (translucent green shades) are overlaid. At 1000 mT (blue), the target spectral feature (red) is absorbed by the crystal, indicating the spectral match of the two systems. All spectra are frequency-referenced to a calibrated wavemeter. (D) Absorption spectrum of a 100-MHz wide atomic frequency comb (AFC) at the matching frequency identified in (C). (E) Zoom-in spectrum near the center of the AFC, showing a 1-MHz comb spacing. (F) Super-hyperfine levels involved in the spectral hole-burning. Arrows in (E) indicate the population transfer responsible for spectral holes.
• Figure 3: Source - Memory networking: Single photon storage and retrieval. (A) Hanbury-Brown-Twiss (HBT) interferometry: coincidence histogram between 1530-nm photons, heralded by a 780-nm photon detection, integrated for 600 seconds. For $\Delta n =$ 0, the normalized heralded auto-correlation $g^{(2)}_{s,s|h}(0)=$ 0.031(1). (B) Coincidence histogram between the heralding and signal photons after storage and retrieval from the memory. The inset plots the direct transmission peak (orange) at 0 $\mu$s, and the outset shows the echo (blue) at 1.01 $\mu$s. The background noise at echo retrieval is minimized with a temporal gating scheme. Counts are integrated for 3 hours with a 0.5 ns time bin. (C) Trade-off between heralded rate (top) and cross-correlation (bottom) of source photon (red) and echo photon (blue) with changing two-photon pump detunings $\Delta_{2}$ from 500 MHz to 1.2 GHz. The values and uncertainties are extracted from fitting. The vertical line corresponds to $\Delta_{2} =+$903 MHz used in (A) and (B) where the heralded echo rate is $R_{h,e}=$ 1.5(1) cps and the cross-correlation $[g^{(2)}_{h,e}]_{\mathrm{max}} =$ 4.94(4). The highest echo rate $R_{h,e}=$ 4.3(1) cps occurs at $\Delta_{2} = +$703 MHz with a $[g^{(2)}_{h,e}]_{\mathrm{max}} =$ 2.78(2).
• Figure 4: Multimode networking and metropolitan deployment. (A) Multiplexing scheme with a stream of continuously generated photon pairs. The maximal number of temporal modes for storage and retrieval increases with memory acceptance window (green). (B) Heralded echo rate and cross-correlation $[g^{(2)}_{h,e}]_{\mathrm{max}}$ with varying mode numbers. 37 temporal modes are demonstrated with a memory acceptance window of 0.74 $\mu$s, without any degradation of the the cross-correlation. (C) Map of the deployed 10.6-km fiber loop routed to and back from an off-campus location (Harper Court) in Hyde Park, Chicago. (D) Cross-correlation $[g^{(2)}_{h,e}]_{\mathrm{max}}$ and (E) heralded echo rate with extended fibers up to 49.2 km (laboratory) and 10.6 km (metropolitan) inserted between the source and the memory node. Expected rates (navy) in (D) are inferred from the measured rate with no extra fiber and the independently measured fiber losses. Dashed line shows the expected loss with distance. The inset of (D) shows the zoomed-in echo coincidence for the 10.6-km deployed fiber loop; the corresponding data points are highlighted in red boxes in (D) and (E).
• Figure S1: Experimental setup. (A) Laser module. (B), (C) Two quantum nodes are located in two labs separated by 35m. (D), (E), (F) Three detector modules for different measurements, all located in the same lab as node A. (G) Main devices as legends.