prospects of proposals such as quantum-interferometric optical lithography. The method can be adapted to generate entangled states of arbitrarily large photon number. Because prior entanglement is not required, the procedure would work well with singlephoton-on-demand sources 29,30 , which promise to be more efficient and scalable than down-conversion sources. Scalability would also be enhanced by the use of photon-number-resolving detectors. The construction proceeds from spatially separated, unentangled photons to a maximally entangled state in a single spatial mode, a state suitable for Heisenberg-limited phase measurements.A

The Sahara Desert is the largest source of mineral dust in the world. Emissions of African dust increased sharply in the early 1970s (ref. 2), a change that has been attributed mainly to drought in the Sahara/Sahel region caused by changes in the global distribution of sea surface temperature. The human contribution to land degradation and dust mobilization in this region remains poorly understood, owing to the paucity of data that would allow the identification of long-term trends in desertification. Direct measurements of airborne African dust concentrations only became available in the mid-1960s from a station on Barbados and subsequently from satellite imagery since the late 1970s: they do not cover the onset of commercial agriculture in the Sahel region approximately 170 years ago. Here we construct a 3,200-year record of dust deposition off northwest Africa by investigating the chemistry and grain-size distribution of terrigenous sediments deposited at a marine site located directly under the West African dust plume. With the help of our dust record and a proxy record for West African precipitation we find that, on the century scale, dust deposition is related to precipitation in tropical West Africa until the seventeenth century. At the beginning of the nineteenth century, a sharp increase in dust deposition parallels the advent of commercial agriculture in the Sahel region. Our findings suggest that human-induced dust emissions from the Sahel region have contributed to the atmospheric dust load for about 200 years.

Supporting Material and MethodsCoral records. Core OGA-02-1 was drilled vertically from the top of a massive Porites colony at Miyanohama on the north coast of Chichijima in the Ogasawara Islands (Japan) on 27 October 2002, using a diver-operated pneumatic drill. The core is without gap over its total length of 1.74 m. Individual segments fit seamlessly. Microsampling,18 O and 13 C analyses, age model construction and bimonthly interpolation followed methods described previously (Felis et al., 2004). More than 7 samples/year on average were obtained. For chronology construction annual 18 O maxima were set to February/March; on average the coolest months. The maximum difference between annual 18 O maxima and those in Sr/Ca and U/Ca is one data point.A shorter core (OGA-02-3) was drilled horizontally from the colony's side, and analysed for stable isotopes following methods described previously (Suzuki et al., 2005).

Changes in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high‐resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near‐linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.

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