Physics News Update, Number 499

Number 499, August 25, 2000 by Phillip F. Schewe and Ben Stein

RUBIDIUM FOUNTAIN CLOCK. Atomic clocks keep time by counting the cycles of light in a microwave cavity tuned to correspond to the internal energy transition of atoms launched from a trap. The atoms never absorb light at precisely the same frequency, so clock accuracy can be enhanced by averaging over larger samples of atoms, or by watching the atoms for longer periods by chilling the atoms to make them more uniform. High precision is manifested in a narrow linewidth in the spectrum of microwaves absorbed by the atoms. In the process of cooling the atoms with lasers, however, there is a drawback; the quantum wavelengths of the atoms themselves increase at lower temperatures, giving them a larger "cross section" for scattering from other atoms, which in turn corrupts the measurement process.

Physicists at Yale (Kurt Gibble, 203-432-6365, kurt.gibble@yale.edu) have succeeded in reducing the scattering problem by a factor of 30 by using rubidium atoms (instead of the more commonly used cesium atoms) in a "fountain" setup in which cooled atoms are put into an excited state in one microwave cavity (which is slightly detuned so as to cancel the effect of atom collisions) and then sent upwards until, when they are nearly at rest at the moment gravity starts to drag them back down, their transition energy is measured in a second cavity. With improved precision the Yale researchers expect to achieve an accuracy of 1 part in 10¹⁶, which would result in the best timekeeping ability yet attained. (Fertig and Gibble, Physical Review Letters, 21 August 2000; Select Articles; see figure at Physics News Graphics.)

SYNCHRONIZING CLOCKS USING ENTANGLED PARTICLES has been proposed by two independent teams, offering possible advantages over traditional techniques. In timekeeping, physicists already exploit the fact that an atom (or any other object at the quantum level) acts like a tiny clock. For example, an atom's nucleus in a magnetic field contains a "spin" analogous to an arrow or a second hand rotating at a precise frequency. But synchronizing two clocks has relied on more conventional approaches.

Now, one team (Jonathan Dowling, JPL/Caltech, 818-393-5343, Jonathan.P.Dowling@jpl.nasa.gov) proposes entangling two particles and sending them to the locations that need to be synchronized. Measuring one particle causes them both to start "ticking," so that when one spin begins at a "12:00 position," say, the other starts ticking at a complementary 6:00 position. Once this is set up, the two users can synchronize their clocks by communicating to each other when they measured their particle, and the direction that each person's spin was pointing.

In a separate scheme, Isaac Chuang of IBM (408-927-2845, ichuang@almaden.ibm.com) proposes that two users at remote locations exchange a series of particles that serve as "quantum wristwatches." Determining the average time it takes a wristwatch to complete its round trip, measuring the wristwatch at each stop to determine its elapsed time, and adjusting the clocks on each end can allow precise synchronization to occur in many fewer steps than with traditional approaches, Chuang shows, because of the subtle interplay of quantum interference at work in a quantum particle. These new schemes may circumvent traditional problems of synchronization, such as distortions in radio signals sent from a GPS satellite to a user. However, entanglement-based synchronization calls for technical advances that, if realized, would also improve more conventional techniques, according to MIT's Seth Lloyd (Chuang and Jozsa et al., Physical Review Letters, 28 August 2000, second paper.)

CORRECTIONS. The highest energy beamline is not Fermilab's 800-GeV beam (Update 495), but HERA's 820-GeV beam of protons. Lene Hau, who performs experiments with slowed light, is at Harvard but during much of her recent research (Updates 415, 472, 495) she has also been affiliated with the Rowland Institute for Science.