Interfacing fundamentally different quantum systems is key to building future hybrid quantum networks1. Such heterogeneous networks offer capabilities superior to those of their homogeneous counterparts, as they merge the individual advantages of disparate quantum nodes in a single network architecture2. However, few investigations of optical hybrid interconnections have been carried out, owing to fundamental and technological challenges such as wavelength and bandwidth matching of the interfacing photons. Here we report optical quantum interconnection of two disparate matter quantum systems with photon storage capabilities. We show that a quantum state can be transferred faithfully between a cold atomic ensemble3,4 and a rare-earth-doped crystal5,6,7,8 by means of a single photon at 1,552 nanometre telecommunication wavelength, using cascaded quantum frequency conversion. We demonstrate that quantum correlations between a photon and a single collective spin excitation in the cold atomic ensemble can be transferred to the solid-state system. We also show that single-photon time-bin qubits generated in the cold atomic ensemble can be converted, stored and retrieved from the crystal with a conditional qubit fidelity of more than 85 per cent. Our results open up the prospect of optically connecting quantum nodes with different capabilities and represent an important step towards the realization of large-scale hybrid quantum networks.
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This work was supported by the ERC starting grant QuLIMA, by the Spanish Ministry of Economy and Competitiveness (MINECO) and the Fondo Europeo de Desarrollo Regional (FEDER) through grant FIS2015-69535-R, by MINECO Severo Ochoa through grant SEV-2015-0522, by AGAUR via 2014 SGR 1554, by Fundació Privada Cellex and by the CERCA programme of the Generalitat de Catalunya. P.F. acknowledges the International PhD-fellowship program “la Caixa”-Severo Ochoa @ ICFO. G.H. acknowledges support by the ICFOnest international postdoctoral fellowship program.
The authors declare no competing financial interests.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
On the left, a cold 87Rb quantum memory is operated to generate non-classically correlated photon pairs. In the centre, the single read photons are frequency-converted from 780 nm to 1,550 nm in QFCD1 and afterwards to 606 nm in QFCD2. On the right, the converted read photons are stored and analysed in a crystal before being retrieved and detected. AOM, acousto-optic modulator; BF, bandpass filter; DM, dichroic mirror; Et, etalon; FS, fibre switch; Gr, diffraction grating; λ/2 (λ/4) half (quarter)-wave plate; PBS, polarizing beam splitter; PC, polarization controller; PP, anamorphic prism pair; QM, quantum memory; S, mechanical shutter.
Conditional histogram of a time-bin read photon, taken at pe = 5% after the MOT at site A.
Absorption spectra of AFCs with different periodicities Δ are shown on the left side. Input pulses (200 ns FHWM) derived from the 606 nm preparation laser are sent to the different AFC structures at 0 μs, and their corresponding echoes are shown on the right side. a, . b, . c, Double periodicity with and leading to a double echo at 2 μs and 2.5 μs. OD, optical depth.
Relative storage efficiency of classical light (derived from the 606 nm preparation laser) versus its frequency detuning δ with respect to the centre of the prepared AFC. For the blue dots, the input is a single Gaussian shaped pulse stored in a single AFC. For the green squares, the input is a doubly peaked pulse (mimicking a time-bin input photon) stored on two superimposed AFCs.
First, the AFC in the crystal is prepared (bottom row), before the main experiment involving the cold atomic quantum memory (top row) and the conversion interface (centre row) starts. Eventual detections of write photons at D1 and converted, stored and restored read photons at D2 are indicated by stars.
a, Signal-to-noise ratio of the echo retrieved from the crystal, if a weak coherent state is frequency-converted in the QFCDs and stored in the memory depending on the mean input photon number per pulse before the interface. The green line is a fit with the expected linear behaviour. b, Visibility of interfering weak coherent time-bin pulses depending on their mean input photon number , after the pulses were frequency converted and stored for and in the AFC memory. The green line is the predicted behaviour of the visibility, taking into account the measured signal-to-noise ratio. The inset shows as an example the interference fringe taken at .
Calculated visibility V0 as a function of the laser linewidth FWHM (2.35σ). The shaded area shows the typical operating range of the experiment.
a, Total storage efficiency and b, normalized cross-correlation function of the initial write photon at site A and the converted, stored and retrieved read photon at site B depending on the storage time τB in the crystal, taken at pe ≈ 10%.
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Maring, N., Farrera, P., Kutluer, K. et al. Photonic quantum state transfer between a cold atomic gas and a crystal. Nature 551, 485–488 (2017). https://doi.org/10.1038/nature24468
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