Spin gradient demagnetization cooling of ultracold atoms

A major goal of ultracold atomic physics is quantum simulation of spin Hamiltonians in optical lattices. Progress towards this goal requires the attainment of extremely low temperatures. Here we demonstrate a new cooling method which consists of applying a time-varying magnetic field gradient to a spin mixture of ultracold atoms. We have used this method to prepare isolated spin distributions at positive and negative spin temperatures of +/-50 picokelvin. The spin system can also be used to cool other degrees of freedom, and we have used this coupling to reduce the temperature of an apparently equilibrated sample of rubidium atoms in a Mott insulating state to 350 picokelvin. These are the lowest temperatures ever measured in any system.Comment: 4 pages, 4 figures; (v4) Shortened, added journal re

Improved constraints on non-Newtonian forces at 10 microns

Several recent theories suggest that light moduli or particles in "large" extra dimensions could mediate macroscopic forces exceeding gravitational strength at length scales below a millimeter. Such new forces can be parameterized as a Yukawa-type correction to the Newtonian potential of strength $\alpha$ relative to gravity and range $\lambda$. To extend the search for such new physics we have improved our apparatus utilizing cryogenic micro-cantilevers capable of measuring attonewton forces, which now includes a switchable magnetic force for calibration. Our most recent experimental constraints on Yukawa-type deviations from Newtonian gravity are more than three times as stringent as our previously published results, and represent the best bound in the range of 5 - 15 microns, with a 95 percent confidence exclusion of forces with $|\alpha| > 14,000$ at $\lambda$ = 10 microns.Comment: 12 pages, 9 figures, accepted for publication in PRD. Minor changes, replaced and corrected Figs 4,5,

Reconceptualising South Africa's international identity : post-apartheid foreign policy in a post-cold war world

Bibliography: leaves 74-78.With the ending of the apartheid regime and the transition to power of a government of national unity, South Africa is now a legitimate member of the international community. It has joined the Organisation of African Unity, the British Commonwealth, and the Southern African Development Community, and it is busily fostering trade links with Europe, North America, the Far East, and Latin America. Its diplomats have worked to mediate conflicts in Angola and Mozambique, and its president is widely seen as an international statesman and a moral leader of almost unprecedented repute. Yet the new· government continues to operate within South Africa's traditional international paradigm and has not yet developed a unique global role that reflects the country's internal "negotiated revolution". As a result, substantial challenges face efforts to forge a new south African approach to the world. From outside the country, forces unleashed by the fall of communism and the rise of a truly global marketplace mark a volatile and uncertain transition in world history. From the inside, political transition has sparked a redefinition of what it means to be South African, but this has not been reflected in new policies. The Foreign Ministry is widely recognised as a bastion of old-guard stalwarts; the ANC and NP have done little to reconcile their past international experiences; and. the information flow on international political and economic trends has barely improved since April 1994, leaving interest groups and private citizens in the new democracy generally uninformed and therefore unable to help pressure policy. The result is a foreign policy over the past year that has had little vision and few cohesive threads, and has left a score of unresolved issues. The 'new' South Africa's relations with Cuba and China, its policies on illegal immigration, and regional development plans are all issues that require visionary, decisive leadership but for which none has yet been provided. What energy or vision, for example, has South Africa brought to the Southern African Development Community (SADC) since it joined last August? In the global peacekeeping debate, and again with Cuba and China, South Africa has made little effort to recognise more pro-active roles for which it is well equipped. Why is it not asserting itself? Who actually is in charge of its foreign policy? Few thus would deny that a paralysis has settled in on South African foreign policy. A recent analysis in the Weekly Mail lamented, "We are not consistent. We have not formulated clear principles. The formulators of our foreign policy do not consult with the people. The new appointments to our foreign ministry complain of being sidelined. There is no clear break with the past". At the core of this inaction is the fact that policy makers have failed to reconceptualise the way international issues are seen and policy is made. The world has changed and South Africa has changed, both dramatically; yet Cold War debates still divide the policy framework, old style security thinking still dominates higher ranks, and most importantly, the growing inter linkages between domestic and foreign policies in a post-Cold War world have gone largely unheeded. It is thus appropriate to sound a note of urgency: change and uncertainty in the world and dramatic transformation at home combine to make this an inopportune, even dangerous, time to have a directionless foreign policy. The broad purpose· of this paper is to identify the salient external and internal factors that will drive a new South African approach to the world. The first chapter presents a synthesis of dominant global trends, and sets them against the backdrop of major structural changes in international relations. The second chapter discusses change in South Africa in relation to world changes, new state objectives and shifting interest groups, and considers these implications for three major foreign policy areas. The third chapter looks at the policy framework and the ability of policy makers to conceptualise these dual changes and to formulate effective policies. The final chapter offers a 'road map' of policy options towards a true postapartheid, post-Cold War foreign policy

Photoacoustic ultrasound sources from diffusion-limited aggregates

Metallic diffusion-limited aggregate (DLA) films are well-known to exhibit near-perfect broadband optical absorption. We demonstrate that such films also manifest a substantial and relatively material-independent photoacoustic response, as a consequence of their random nanostructure. We theoretically and experimentally analyze photoacoustic phenomena in DLA films, and show that they can be used to create broadband air- coupled acoustic sources. These sources are inexpensive and simple to fabricate, and work into the ultrasonic regime. We illustrate the device possibilities by building and testing an optically-addressed acoustic phased array capable of producing virtually arbitrary acoustic intensity patterns in air.Comment: 5 pages, 5 figure

Bragg Scattering as a Probe of Atomic Wavefunctions and Quantum Phase Transitions in Optical Lattices

We have observed Bragg scattering of photons from quantum degenerate $^{87}$Rb atoms in a three-dimensional optical lattice. Bragg scattered light directly probes the microscopic crystal structure and atomic wavefunction whose position and momentum width is Heisenberg-limited. The spatial coherence of the wavefunction leads to revivals in the Bragg scattered light due to the atomic Talbot effect. The decay of revivals across the superfluid to Mott insulator transition indicates the loss of superfluid coherence.Comment: 5 pages, 4 figure

Transport in Floquet-Bloch bands

We report Floquet band engineering of long-range transport and direct imaging of Floquet-Bloch bands in an amplitude-modulated optical lattice. In one variety of Floquet-Bloch band we observe tunable rapid long-range high-fidelity transport of a Bose condensate across thousands of lattice sites. Quenching into an opposite-parity Floquet-hybridized band allows Wannier-Stark localization to be controllably turned on and off using modulation. A central result of this work is the use of transport dynamics to demonstrate direct imaging of a Floquet-Bloch band structure. These results demonstrate that transport in dynamical Floquet-Bloch bands can be mapped to transport in quasi-static effective bands, opening a path to cold atom quantum emulation of ultrafast multi-band electronic dynamics.Comment: 5 pages, 4 figure