Exited atoms may relax to the ground state by radiative decay, a process which is usually very fast (of order nanoseconds). However, quantum-mechanical selection rules can prevent such rapid decay, in which case these "metastable" states can have lifetimes of order seconds or longer. In this Letter, we determine experimentally the lifetime of the longest-lived neutral atomic state-the first excited state of helium (the 2(3)S1 metastable state)-to the highest accuracy yet measured. We use laser cooling and magnetic trapping to isolate a cloud of metastable helium (He*) atoms from their surrounding environment, and measure the decay rate to the ground 1(1)S0 state via extreme ultraviolet (XUV) photon emission. This is the first measurement using a virtually unperturbed ensemble of isolated helium atoms, and yields a value of 7870(510) seconds, in excellent agreement with the predictions of quantum electrodynamic theory.
A method to create paired atom laser beams from a metastable helium atom laser via fourwave mixing is demonstrated. Radio frequency outcoupling is used to extract atoms from a Bose Einstein condensate near the center of the condensate and initiate scattering between trapped and untrapped atoms. The unequal strengths of the interactions for different internal states allows an energy-momentum resonance which leads to the creation of pairs of atoms scattered from the zerovelocity condensate. The resulting scattered beams are well separated from the main atom laser in the 2-dimensional transverse atom laser profile. Numerical simulations of the system are in good agreement with the observed atom laser spatial profiles, and indicate that the scattered beams are generated by a four-wave mixing process, suggesting that the beams are correlated.PACS numbers: 03.75. Pp,03.75.Nt,67.85.Jk Sources of matter waves gained a dramatic improvement with the achievement of Bose-Einstein condensation (BEC) in dilute gases and the development of the atom laser [1,2]. Like optical lasers before them, atom lasers can produce Heisenberg-limited beam profiles [3,4] and promise high spectral density through their dramatically lower linewidth [5]. Another exciting possibility resulting from having such a coherent source of atoms is the generation of non-classical matter waves through entangled beams. Such entangled beams are useful for tests of quantum mechanics, and are required to perform Heisenberg-limited interferometry [6,7]. In this Letter, we show that the asymmetric scattering rates between internal states of metastable helium (He*) cause welldefined peaks in the output of an atom laser. These peaks are due to a four-wave mixing (FWM) process, and are experimentally demonstrated.A nonlinear process is required to produce entanglement, and one of the advantages of atomic systems over optical systems is that there are strong inherent nonlinearities due to the atomic interactions, although these interactions can also lead to complications. These nonlinearities allow certain analogs of nonlinear optical experiments such as FWM and Kerr squeezing to be performed directly in the atomic sample [8]. Both of these produce entanglement in optical systems. FWM in a trapped BEC has been demonstrated experimentally in configurations where three distinct momentum states generated a fourth [9], and where two momentum states generated a pair of correlated atomic beams [10]. These experiments demonstrated that the output was phase coherent, but the correlation properties were not measured. More recently the pair correlations in a spontaneous scattering of two colliding condensates were measured using the single atom detectors available for He* atoms [11].Using these existing sources of entangled pairs of atoms for interferometric experiments will be complicated by the high densities of the sources, where the nonlinearities that generated the correlations ultimately degrade the long term coherence of the sample. While recent experiments have inc...
A method of active field cancellation is described, which greatly reduces the stray magnetic field within the trap region of a Bose-Einstein condensation experiment. An array of six single-axis magnetic sensors is used to interpolate the field at the trap center, thus avoiding the impractical requirement of placing the sensor within the trap. The system actively suppresses all frequencies from dc to approximately 3000 Hz, and the performance is superior to conventional active Helmholtz cancellation systems. A method of reducing the field gradient, by driving the six Helmholtz coils independently, is also investigated.
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