Broadband waveguide quantum memory for entangled photons

Reversible mapping of quantum states, particularly entangled states, between light and matter is important for advanced applications of quantum information science. This mapping, i.e. operation of a quantum memory [1], is imperative for realizing quantum repeaters [2] and quantum networks [3]. Here we report the reversible transfer of photon–photon entanglement into entanglement between a photon and a collective atomic excitation in a solid-state device [4] (see also [5]). Specifically, we generate time-bin enangled pairs of photons [6] at the low-loss 795 nm (in free-space) and 1532 nm (in fibre) wavelengths. The 795 nm photons are sent into a thulium-doped lithium niobate waveguide cooled to 3K, absorbed by the Tm ions, and retrieved after 7 ns by means of a photon-echo quantum memory protocol employing an atomic frequency comb [7]. The acceptance bandwidth of the memory has been expanded to 5 GHz, more than one order of magnitude larger than the previous state-of-the-art [8], to match the spectral width of the filtered 795 nm photons. The entanglement-preserving nature of our storage device is assessed through quantum state tomography before and after storage. Within statistical error, we find a perfect mapping process. Furthermore, by violating the CHSH inequality [9], we directly verify the nonlocal nature of the generated and stored entangled photons. [1] A. Lvovsky, B. C. Sanders, and W. Tittel, Optical quantum memory, Nature Photonics 3, 706-71 (2009). [2] N. Sangouard et al., Quantum repeaters based on atomic ensembles and linear optics, Rev. Mod. Phys. 83, 33-80 (2011). [3] H. J. Kimble, The quantum internet, Nature 453, 1023-1030 (2008). [4] E. Saglamyurek et al., Broadband waveguide quantum memory for entangled photons, Nature 469, 512-515 (2011). [5] C. Clausen et al., Quantum storage of photonic entanglement in a crystal, Nature 469, 508-511 (2011). [6] I. Marcikic et al., Distribution of time-bin entangled qubits over 50 km of optical fiber, Phys. Rev. Lett. 93, 180502 (2004). [7] M. Afzelius et al., Multimode quantum memory based on atomic frequency combs, Phys. Rev. A 79, 052329 (2009). [8] I. Usmani et al., Mapping multiple photonic qubits into and out of one solid-state atomic ensemble, Nat. Comm. 1 (12), 1-7 (2010). [9] J. F. Clauser et al., Proposed experiment to test local hidden-variable theories, Phys. Rev. Lett. 23, 880-884 (1969).