We experimentally demonstrate the preservation of entanglement between an NV spin and a photon upon quantum frequency conversion to the Telecom band. This is a crucial step in realizing long-distance quantum networks.

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We report on the realization and verification of quantum entanglement between a nitrogen-vacancy electron spin qubit and a telecom-band photonic qubit. First we generate entanglement between the spin qubit and a 637 nm photonic time-bin qubit, followed by photonic quantum frequency conversion that transfers the entanglement to a 1588 nm photon. We characterize the resulting state by correlation measurements in different bases and find a lower bound to the Bell state fidelity of ≥0.77±0.03. This result presents an important step towards extending quantum networks via optical fiber infrastructure.

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We report on the conversion to telecommunication wavelengths of single photons emitted by a nitrogen-vacancy (NV) defect in diamond. By means of difference frequency generation, we convert spin-selective photons at 637 nm, associated with the coherent NV zero-phonon line, to the target wavelength of 1588 nm in the L-telecommunication band. The successful conversion is evidenced by time-resolved detection revealing a telecommunication-photon lifetime identical to that of the original 637-nm photon. Furthermore, we show by second-order correlation measurements that the single-photon statistics are preserved. The overall efficiency of this one-step conversion reaches 17% in our current setup, along with a signal-to-noise ratio of approximately 7 despite the low probability (<10-3) of an incident 637-nm photon. This result shows the potential for efficient telecommunication-photon-NV-center interfaces and marks an important step towards future long-range entanglement-based quantum networks.

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In July 2015 we observed the first Bell inequality violation with all experimental loopholes closed. We discuss this experiment as well as new results on the road towards a large-scale quantum network of diamond spins.@en

The recently reported violation of a Bell inequality using entangled electronic spins in diamonds (Hensen et al., Nature 526, 682–686) provided the first loophole-free evidence against local-realist theories of nature. Here we report on data from a second Bell experiment using the same experimental
setup with minor modifications. We find a violation of the CHSH-Bell inequality of 2.35 ± 0.18, in agreement with the first run, yielding an overall value of S = 2.38 ± 0.14. We calculate the resulting P-values of the second experiment and of the combined Bell tests. We provide an additional analysis of the distribution of settings choices recorded during the two tests, finding that the observed
distributions are consistent with uniform settings for both tests. Finally, we analytically study the effect of particular models of random number generator (RNG) imperfection on our hypothesis test. We find that the winning probability per trial in the CHSH game can be bounded knowing only the mean of the
RNG bias. This implies that our experimental result is robust for any model underlying the estimated average RNG bias, for random bits produced up to 690 ns too early by the random number generator.@en