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This work was conductivity than the outer core (inner to outer core conductivity supported by NSERC and NSF. ratios of 0.1–0.0001), and found that they do produce non-dipolar, non-axisymmetric magnetic fields, as predicted. Competing interests statement The authors declare that they have no competing financial interests. Future missions to Uranus and Neptune to study further these planets’ magnetic fields and internal properties could determine the Correspondence and requests for materials should be addressed to S.S. validity of our model. More accurate surface power spectra extend- ([email protected]). ing out to higher degree and order could be produced with magnetic-field measurements acquired closer to the planets with more global spatial distribution. These spectra could then be compared to our numerical model to assess the model’s validity. The evolution in time of the magnetic fields could also be compared .............................................................. to our numerical model by examining the changes in the fields since the Voyager II observations. Although no missions to Uranus Observation of entanglement and Neptune are currently planned, the upcoming data from MESSENGER and Cassini (on the magnetic fields of Mercury and between a single trapped atom Saturn, respectively) will provide tests of the implication of our results: convective-region geometry is an important factor in and a single photon determining magnetic-field morphology, albeit in a different con- text from this study. A B. B. Blinov, D. L. Moehring, L.- M. Duan & C. Monroe Received 9 October 2003; accepted 28 January 2004; doi:10.1038/nature02376. FOCUS Center and University of Michigan Department of Physics, Ann Arbor, 1. Ness, N. F. et al. Magnetic fields at Uranus. Science 233, 85–89 (1986). Michigan 48109-1120, USA 2. Ness, N. 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Interior structure of Neptune—comparison with Uranus. Science 253, 648–651 111 1 (1991). Cd ion—and an ideal quantum communication channel, 12. Nellis, W. J. et al. The nature of the interior of Uranus based on studies of planetary ices at high provided by a single photon that is emitted spontaneously dynamic pressure. Science 240, 779–781 (1988). from the ion. Appropriate coincidence measurements between 13. Kuang, W. & Bloxham, J. An earth-like numerical dynamo model. Nature 389, 371–374 (1997). 14. Kuang, W. J. & Bloxham, J. Numerical modelling of magnetohydrodynamic convection in a the quantum states of the photon polarization and the trapped rapidly rotating spherical shell: weak and strong field dynamo action. J. Comput. Phys. 153, 51–81 ion memory are used to verify their entanglement directly. Our (1999). direct observation of entanglement between stationary and ‘fly- 15. Glatzmaier, G. A. & Roberts, P. H. 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We envision that this source of entanglement may be 153 NATURE | VOL 428 | 11 MARCH 2004 | www.nature.com/nature © 2004 Nature Publishing Group letters to nature used for a variety of quantum communication protocols2,8 and for reminiscent of the production of entangled photon pairs through seeding large-scale entangled states of trapped ion qubits for spontaneous optical parametric down-conversion3,butinthe scalable quantum computing9. current system one of the two daughter qubits resides within a Atom–photon entanglement has been implicit in many previous trapped atomic ion—perhaps the most reliable of all qubit mem- experimental systems, from early measurements of Bell inequality ories18. When accompanied by conventional quantum gates violations in atomic cascade