Public Release: 

Physics tip sheet #17 - June 12, 2002

American Physical Society

1) All-Silicon Quantum Computer
T. D. Ladd, J. R. Goldman, F. Yamaguchi, Y. Yamamoto, E. Abe, K. M. Itoh
Physical Review Letters (Print issue: July 1, 2002)

Current endeavors to build a silicon-based quantum computer offer scalability advantages over liquid NMR devices but suffer from the need to control individual nuclear spins in dopant atoms. A new proposal is based entirely on silicon controlled by magnetic fields with computation results read out using magnetic resonance force microscopy. Controlling the all-silicon computer is similar to controlling liquid NMR quantum computers, a technique shown to be successful. The qubits are formed by an isotope of silicon (Si-29) that comprises about 5% of natural silicon and is thus suitable for nuclear spin isotope engineering, in which structures are built on an atomic level using isotopes in different configurations. The qubit atoms lie in a chain bathed in a large magnetic field gradient that serves to identify the different qubits. Roughly 10,000 copies of this configuration can exist in the crystal, serving the role of the many identical molecules in a liquid NMR computer. The authors claim that, overall, this scheme will allow substantially more qubits than liquid NMR without requiring unrealistic advances in fabrication, measurement and control technologies.

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2) Measuring the speed of gravity
S. Kopeikin, E. Fomalont
6th European VLBI Network Symposium (To be held June 25-28, 2002, Bonn, Germany)

Einstein's general relativity predicts in theory that the speed of gravity is the same as the speed of light. However, there are ways that the speed of gravity can be tested experimentally. Such a test would provide an independent confirmation of Einstein's difficult-to-test theory. The authors propose using the passage of a particular quasar past Jupiter on September 8, 2002 to measure the speed of gravity. If signals at the Very Long Baseline Array (VLBA) radioastronomy facility are measured with a precision of a few picoseconds, the speed of gravity can be measured with a precision of about 10%.


3) Asteroid escape rates
C. Jaffé, S. D. Ross, M. W. Lo, J. Marsden, D. Farrelly, T. Uzer
Physical Review Letters (Print issue: July 1, 2002)

Ideas about transport of matter between planets, such as from Mars to Earth, are intriguing. Some theories suggest that an impact with Mars could send material into orbit around the planet. Then the dynamics of the solar system could allow that material to escape Mars and travel to Earth. A new study looks at how asteroids are temporarily trapped around planets and the rate at which they escape. The analysis shows how asteroids are confined to lie either inside or outside certain orbits with the only chance of crossing those orbits by passing through narrow bottlenecks near a planet and on to other parts of the solar system.

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4) Pyramids inside the atom
J. Dudek, A. Gozdz, N. Schunck, M. Miskiewicz
Physical Review Letters (Print issue: June 24, 2002)

Nuclei are usually thought of as nearly spherical. However, new calculations demonstrate that nuclei could be pyramidal and stable for certain numbers of neutrons and protons. Furthermore, these shapes could be extremely common throughout the periodic table. Such configurations would have characteristic gamma ray spectra when excited but could only be detected using next-generation detectors. Curiously, tetrahedral configurations in nuclei could show some distinctly classical behavior in what is usually a very quantum environment.

Physics News Update:
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5) BEC accelerator
J. H. Denschlag, J. E. Simsarian, H. Haeffner, C. McKenzie, A. Browaeys, D. Cho, K. Helmerson, S. L. Rolston, W. D. Phillips
arXiv preprint server (To appear in J. Phys. B)

A BEC has been created in an optical lattice. Techniques to transfer the BEC from one lattice site to another have evolved into a BEC accelerator. The group also uses the lattice to create a large-momentum-transfer coherent beamsplitter.


6) Wormholes from pure exotic radiation
S. A. Hayward
Physical Review D (Print issue: June 15, 2002)

The author proposes that wormholes through spacetime could be constructed from negative energy without requiring any matter. Turning off the negative energy would result in the wormhole collapsing to a black hole. The paper suggests that reversing the procedure - shining negative energy radiation on a black hole - might be a way to construct a wormhole.

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