Optical quantum memory is an essential element for long distance quantum communication and photonic quantum computation protocols. The practical implementation of such protocols requires an efficient quantum memory with long coherence time. Beating the no-cloning limit, for example, requires efficiencies above 50%. An ideal optical fibre loop has a loss of 50% in 100 µs, and until now no universal quantum memory has beaten this time-efficiency limit. Here, we report results of a gradient echo memory (GEM) experiment in a cold atomic ensemble with a 1/e coherence time up to 1 ms and maximum efficiency up to 87 ± 2% for short storage times. Our experimental data demonstrates greater than 50% efficiency for storage times up to 0.6 ms. Quantum storage ability is verified beyond the ideal fibre limit using heterodyne tomography of small coherent states.

Single atoms or atom-like emitters are the purest source of single photons, they are intrinsically incapable of multi-photon emission. To demonstrate this degree of photon number-state purity we have realized a single-photon source using a single ion trapped at the common focus of high numerical aperture lenses. Our trapped-ion source produces single-photon pulses with = ´g 0 1.9 0.2 10 2 3 ( ) ( ) without any background subtraction. After subtracting detector dark counts the residual g 0 2 ( ) is less than 3×10 −4 (95% confidence interval). The multi-photon component of the source light field is low enough that we measure violation of a quantum non-Gaussian state witness, by this characterization the source output is indistinguishable from ideal attenuated single photons. In combination with efforts to enhance collection efficiency from single emitters, our results suggest that single trapped ions are not only ideal stationary qubits for quantum information processing, but promising sources of light for scalable optical quantum networks.

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