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We demonstrated entanglement distribution between two remote quantum nodes located 3 meters apart. This distribution involves the asynchronous preparation of two pairs of atomic memories and the coherent mapping of stored atomic states into light fields in an effective state of near-maximum polarization entanglement. Entanglement is verified by way of the measured violation of a Bell inequality, and it can be used for communication protocols such as quantum cryptography. The demonstrated quantum nodes and channels can be used as segments of a quantum repeater, providing an essential tool for robust long-distance quantum communication. [ABSTRACT FROM AUTHOR]

We present and analyze four frequency measurements designed to characterize the performance of an optical frequency reference based on spectral hole burning in Eu3+:Y2SiO5. The first frequency comparison, between a single unperturbed spectral hole and a hydrogen maser, demonstrates a fractional frequency drift rate of 5 ⨰ 10-18 s-1. Optical frequency comparisons between a pattern of spectral holes, a Fabry-Pérot cavity, and an Al+ optical atomic clock show a short-term fractional frequency stability of 1 × 10-15τ-1/2that averages down to 2.51 1.1 -0.5 × 10-6 at τ = 540 s (with linear frequency drift removed). Finally, spectral-hole patterns in two different Eu3+:Y2SiO5 crystals located in the same cryogenic vessel are compared, yielding a short-term stability of 7 × 10-16τ-1/2 that averages down to 5.5+18-0.9 × 10-17 at τ = 204 s (with quadratic frequency drift removed). [ABSTRACT FROM AUTHOR]

Laser decoherence limits the stability of optical clocks by broadening the observable resonance linewidths and adding noise during the dead time between clock probes. Correlation spectroscopy avoids these limitations by measuring correlated atomic transitions between two ensembles, which provides a frequency difference measurement independent of laser noise. Here, we apply this technique to perform stability measurements between two independent clocks based on the ¹S0↔³P0 transition in 27Al+. By stabilizing the dominant sources of differential phase noise between the two clocks, we observe coherence between them during synchronous Ramsey interrogations as long as 8 s at a frequency of 1.12×1015 Hz. The observed contrast in the correlation spectroscopy signal is consistent with the 20.6 s ³P0 state lifetime and represents a measurement instability of (1.8±0.5)×10-16/√τ/s for averaging periods longer than the probe duration when dead time is negligible. [ABSTRACT FROM AUTHOR]