Lifetime limited optical excitation lines of single nitrogen vacancy (NV) defect centers in diamond have been observed at liquid helium temperature. They display unprecedented spectral stability over many seconds and excitation cycles. Spectral tuning of the spin selective optical resonances was performed via the application of an external electric field (i.e. the Stark shift). A rich variety of Stark shifts were observed including linear as well as quadratic components. The ability to tune the excitation lines of single NV centers has potential applications in quantum information processing. 1Coupling between light and single spins in solids has attracted widespread attention particularly for applications in quantum computing and quantum communications 1 . The nitrogen-vacancy defect (NV) optical center in diamond is a particularly attractive solid state system for such applications. Its strong optical transition allows photoluminescence-based detection of single defect centers 2 . The potential of the NV center as a single photon source has been well recognized over the past few years 3,4 . Furthermore, because of its paramagnetic spin ground state, there are applications for quantum memory and quantum repeater systems 5 .In particular the long spin decoherence time (0.35ms), optical control of spin states 6-8 and the robustness of the spin coherence have enabled the demonstration of basic building block for quantum computing even at room temperature 9 .Recently it was demonstrated that the permanent magnetic dipole moment of the NV center can be exploited to couple defects for a separation distance of a few nm. Whilst this demonstrates the capability for the generation of correlated quantum states in defect center clusters, coupling based on this technique will be difficult to scale to many qubit systems.Other coupling schemes have recently been proposed which use instead their optical transition dipole moments and in some cases envisage coupling of the NV center to cavities. At the core of many such schemes is the underlying assumption that the optical transition can be tuned in resonance either with another NV center or with a cavity via an external applied field. Therefore, the ability to tune the frequency of spin-selective optical transitions of single NV centers is of crucial importance for any scalable architecture based on diamond NV centers.Externally controlled magnetic and electric fields are among the most prominent parameters that can be used for such control. Electric fields in particular allow for wide tuning of eigenstates. The electric field induced shift of the optical resonance lines has been observed for single atoms, ions in the gas phase 10 and single molecules 11 and quantum dots 12, 14 in the solid state. By contrast, for color centers in diamond, only a few bulk studies on electric field induced spectral line shifts have been carried out 15 . Usually these studies are difficult because the magnitude of the Stark effect is of the order of the inhomogeneous linewidth. Moreover, ...

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The Galileo probe showed that Jupiter's atmosphere is severely depleted in neon compared to protosolar values. We show, via ab initio simulations of the partitioning of neon between hydrogen and helium phases, that the observed depletion can be explained by the sequestration of neon into helium-rich droplets within the postulated hydrogen-helium immiscibility layer of the planets interior. We also demonstrate that this mechanism will not affect argon, explaining the observed lack of depletion of this gas. This provides strong indirect evidence for hydrogen-helium immiscibility in Jupiter.Jupiter is the most extensively probed and best understood of the giant planets, but many questions regarding its detailed composition, formation, and interior structure remain unanswered. One issue of major importance to structural models is the question of whether hydrogen and helium mix homogeneously throughout the planet or whether a layer of hydrogen-helium immiscibility exists deep within the interior [1][2][3]. In the immiscibility layer, helium would form dense droplets which would rain down into the deeper interior and redissolve, resulting in a gradual and ongoing transfer of helium from regions above the immiscibility layer to regions below. Such a layer almost certainly exists in Saturn, as evident from the observed depletion of helium from its upper atmosphere (compared to protosolar values) and the apparent excess luminosity of the planet [4]. For the hotter interior of Jupiter the case is less clear since there is no measurable excess luminosity and the observed helium depletion from the upper atmosphere is quite small (0.234 by mass compared to 0.274 in the protosolar nebula [5][6][7]). Theoretical attempts to determine the pressure/temperature range in which H and He are immiscible using successively more sophisticated levels of theory [3,[8][9][10][11][12][13][14] have produced quite different results, however recent work [13,14] provides a hydrogen-helium immiscibility line which is very close to the Jupiter isentrope in the 100-300 GPa region.