Efficiently loading 40Ca+-27Al+ ion crystal using sympathetic cooling and pulsed laser ablation.

We demonstrate a method to efficiently load a pair of 40Ca+-27Al+ ion crystals with sympathetic cooling and pulsed laser ablation, serving as a starting step for the 27Al+ clock. We achieved a technique to rapidly detect the loading of hot ions by monitoring the 2S1/2 → 2D5/2 narrow transition of 40Ca+ that couples to the shared motional modes between the two ions. The sympathetic cooling time of the 40Ca+-27Al+ ion pair is measured. Two traps are employed to compare the loading time from two directions and it was found that the loading from the axial direction takes much shorter time than loading from the radial direction of the trap. With the help of adaptively controlled trap potential, our method reduced the average loading time of a 40Ca+-27Al+ pair from 26 to 1 min. This research is an important step for increasing the uptime ratio of the 27Al+ optical clock and is useful for other mixed-species ion crystals based on sympathetic cooling.

Scalable Quantum Logic Spectroscopy.

In quantum logic spectroscopy (QLS), one species of trapped ion is used as a sensor to detect the state of an otherwise inaccessible ion species. This extends precision measurements to a broader class of atomic and molecular systems for applications like atomic clocks and tests of fundamental physics. Here, we develop a new technique based on a Schrödinger cat interferometer to address the problem of scaling QLS to larger ion numbers. We demonstrate the basic features of this method using various combinations of ^{25}Mg^{+} logic ions and ^{27}Al^{+} spectroscopy ions. We observe higher detection efficiency by increasing the number of ^{25}Mg^{+} ions. Applied to multiple ^{27}Al^{+}, this method will improve the stability of high-accuracy optical clocks and could enable Heisenberg-limited QLS.

Chloroplast and Nuclear Genetic Diversity Explain the Limited Distribution of Endangered and Endemic Thuja sutchuenensis in China.

Narrow-ranged species face challenges from natural disasters and human activities, and to address why species distributes only in a limited region is of great significance. Here we investigated the genetic diversity, gene flow, and genetic differentiation in six wild and three cultivated populations of Thuja sutchuenensis, a species that survive only in the Daba mountain chain, using chloroplast simple sequence repeats (cpSSR) and nuclear restriction site-associated DNA sequencing (nRAD-seq). Wild T. sutchuenensis populations were from a common ancestral population at 203 ka, indicating they reached the Daba mountain chain before the start of population contraction at the Last Interglacial (LIG, ∼120-140 ka). T. sutchuenensis populations showed relatively high chloroplast but low nuclear genetic diversity. The genetic differentiation of nRAD-seq in any pairwise comparisons were low, while the cpSSR genetic differentiation values varied with pairwise comparisons of populations. High gene flow and low genetic differentiation resulted in a weak isolation-by-distance effect. The genetic diversity and differentiation of T. sutchuenensis explained its survival in the Daba mountain chain, while its narrow ecological niche from the relatively isolated and unique environment in the "refugia" limited its distribution.

Lifetime-Limited Interrogation of Two Independent

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 ^{1}S_{0}↔^{3}P_{0} transition in ^{27}Al^{+}. 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 1.12×10^{15} Hz. The observed contrast in the correlation spectroscopy signal is consistent with the 20.6 s ^{3}P_{0} state lifetime and represents a measurement instability of (1.8±0.5)×10^{-16}/sqrt[τ/s] for averaging periods longer than the probe duration when dead time is negligible.