%e present the first realization of a measurement of a "weak value, " a concept recently introduced by Aharonov, Albert, and Vaidman (AAV). Our experiment uses a birefringent crystal to separate the two linear-polarization components of a laser beam by a distance small compared to the laser-beam waist. This "weak measurement" is followed by a strong measurement which translates the centroid of the beam by a distance far larger than the birefringence-induced separation. In addition, we present data corresponding to orthogonal initial and final states, for which the weak value is not defined. This interference eAect may have application in the amplification and detection of weak eff'ects.

We have obtained spectra of the high-lying vibrational levels of the 1 X state of Liz [5] and have been applied to ultracold photoassociation spectra [2,3]. However, in those investigations the details of the potential are not accounted for, and the accuracy of the resulting long-range coefficients is only a few percent. We are able to extract a more precise value for the long-range potential coefficient, and therefore, the atomic radiative lifetime by fitting a model of the full interaction potential to the observed photoassociation spectrum. [7].Unfortunately, the theoretical value differs from the experimental one by more than 0.7%, or more than four standard deviations. Clearly, the resolution of this discrepancy is a high priority.The experiment uses a six-beam magneto-optical trap (MOT) [8] to produce an ultracold lithium vapor [9]. A 500-mW probe laser beam with a Gaussian beam waist (1/e radius) of 500 p, m is directed through and retroreflected back through the trapped atom cloud to induce photoassociation. As the probe is tuned to the red of the 2S»2-2P»2 atomic transition, it can excite colliding groundstate atoms into various high-lying vibrational levels of the molecular excited-state manifold. These newly formed diatomic molecules decay into a bound ground state of the molecule or into a dissociative continuum of two groundstate atoms that may gain enough kinetic energy to escape the trap. These combined processes result in an observable reduction in trap-laser-induced fluorescence, which is monitored by a photodiode. Vibrational levels up to 3 THz (100 cm ) red of the 2S»2-2Pi/2 atomic transition have been observed in this manner (Fig. 1) [10].

Accurate measurements are presented of the rate of trap-loss-producing collisions between ultracold magneto-optically trapped Li atoms for a range of trap laser intensities and frequencies. Intensities from near the atomic saturation intensity to well above it are investigated. At low intensities, fine-structure-changing collisions cause trap loss with a rate constant of -10 ' cm /s. At sufficiently high intensity, the trap can be deep enough to effectively freeze out the dominant fine-structure-changing collisions as a loss mechanism, [11].This ability to separate the contributions of RE and FS greatly simplifies the interpretation of the experiment. In this paper, we present measurements of the RE and FS rates in I i for a range of trap laser intensities and detunings. The measured rates are compared with calculations that account for the variation of ET with laser intensity and detuning.The experimental apparatus and procedure are similar to other measurements of collisional loss in a MOT [2,4]. The MOT consists of six near-resonant laser beams along the three orthogonal axes that provide for dissipation of the atomic kinetic energy, and, when combined with an inhomogeneous magnetic field, also produce a restoring force [1].The six beams are produced by a dye laser that is frequency locked relative to a saturated absorption feature of Li in an absorption cell to provide long-term frequency stability and a relative frequency reference. The waist (1/e intensity radius) of the laser beams is 0.64 cm. A pair of anti-Helmholtz configured coils generates an axial magnetic-field gradient of 3 mT/cm and a radial gradient of 1.5 mT/cm. To ensure a near-spherically-symmetric cloud of trapped atoms, the intensities of the four radial laser beams are equal, while the intensity of an axial beam is -60% of the intensity of a radial beam. Transitions from both I' = 1 and 2 ground-state hyperfine levels to the excited states are driven with equal intensity using the upper and lower first-order sidebands produced by a standing-wave electro-optic modulator, modulated at 406.4 MHz. The lower first-order sideband is de-

Magnetism of Cu 2 Cd B 2 O 6 : Quantum spin system having a nearly singlet state and antiferromagnetic longrange order J. Appl. Phys. 99, 08H504 (2006); 10.1063/1.2176111Determination of long-range interactions from photoassociative spectroscopy of ultracold atoms AIP Conf.

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