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Physical Review Letters

Demonstration of a novel atomic beam splitter

John Lawall, Mara Goff Prentiss

Published Feb 14, 1994• 164 citations
Publisher

Abstract

We have demonstrated a novel beam splitter for atoms. The J=1 to J=0 system in metastable helium has two independent ``dark'' states which are radiatively stable. By preparing atoms in the two ``dark'' states of the ${2}^{3}$${\mathit{S}}_{1}$ level and passing them through a sequence of partially overlapping laser beams tuned to the ${2}^{3}$${\mathit{S}}_{1\mathrm{\ensuremath{-}}}$${2}^{2}$${\mathit{P}}_{0}$ transition, we cause the atoms in one ``dark'' state to be deflected by integral multiples of 2\ensuremath{\Elzxh}k, while the atoms in the other ``dark'' state are undeflected. The excited ${2}^{3}$${\mathit{P}}_{0}$ state is not populated, so the process is unaffected by spontaneous emission. We have observed up to 90% coherent momentum transfer with 4\ensuremath{\Elzxh}k deflection. We conclude by describing how one might construct an atom interferometer based on these ``dark state'' beam splitters.

DOI
10.1103/physrevlett.72.993

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We have demonstrated a novel beam splitter for atoms. The J=1 to J=0 system in metastable helium has two independent dark'' states which are radiatively stable. By preparing atoms in the two dark'' states of the {2}^{3}$${\mathit{S}}_{1} level and passing them through a sequence of partially overlapping laser beams tuned to the {2}^{3}$${\mathit{S}}_{1\mathrm{\ensuremath{-}}}$${2}^{2}$${\mathit{P}}_{0} transition, we cause the atoms in one dark'' state to be deflected by integral multiples of 2\ensuremath{\Elzxh}k, while the atoms in the other dark'' state are undeflected. The excited {2}^{3}$${\mathit{P}}_{0} state is not populated, so the process is unaffected by spontaneous emission. We have observed up to 90% coherent momentum transfer with 4\ensuremath{\Elzxh}k deflection. We conclude by describing how one might construct an atom interferometer based on these ``dark state'' beam splitters.
We have demonstrated a coherent beam splitter for metastable helium based on adiabatic passage. Light from a homemade LNA laser at 1.083 {mu} is used to excite the {sup 3}S{sub 1}-{sup 3}P{sub 0} transition in a highly collimated beam of metastable helium. The atoms are first optically pumped into the m=0 and m=-1 sublevels. These sublevels are two independent {open_quotes}dark{close_quotes} states from which no further absorption is possible. Population is transferred adiabatically from the m = -1 level to the m =+1 level by means of two partially overlapping laser beams. As well as absorbing 2{h_bar} of angular momentum from the radiation field, the atoms initially in the m = -1 sublevel acquire 2{h_bar}k of linear momentum via an absorption/stimulated emission process. The process can be repeated back to m = -1, and we have demonstrated 4{h_bar}k and 6{h_bar}k coherent momentum transfer using successive interactions regions. The population initially in the m = 0 sublevel is undeflected by the light.
We demonstrate a coherent atomic beam splitter for metastable helium atoms, based on the diffraction of atomic matter waves from a blazed phase grating. The beam splitter is created by driving the two transitions of a three-level V system with differentially detuned standing light waves that have a relative spatial phase shift of pi/2. The light f ields create a potential that is approximately triangular as a function of position in the laser field. Splittings of 38 times the photon momentum have been observed.
Abstract Atomic beam splitters are fundamental components of atom interferometers, with broad applications in high sensitivity metrology and integrated atom optics devices. In this work, we theoretically demonstrate coherent beam splitting of a metastable helium atomic beam using a pulsed bichromatic standing wave. Our key innovation is a fully self-consistent model coupling the optical Bloch equations - which include spontaneous emission via a Lindblad term - with the classical equations of atomic motion, enabling the simulation of real-time splitting dynamics. This allows us to simulate the real-time dynamics and identify optimal experimental parameters neglected in prior splitter studies. Crucially, we find that the optimal standing-wave phase difference is ϕ= 3π/4, not π/2, maximizing the distinction between the dynamical pathways for splitting the metastable helium beam (2 3 S 1 →3 3 P 2 transition). We propose and validate the use of optical pulses shorter than the spontaneous emission lifetime to suppress decoherence. Monte Carlo simulations predict a clean binary split with a tunable splitting angle to 56 mrad, corresponding to 56 ħk momentum transfer, and a splitting efficiency of 55 - 83% via detuning.
Two different research teams have taken a big step toward atom interferometry. They have succeeded in splitting atomic beams by using atoms in spin states that neither absorb nor reemit laser light. By proper adjustment of experimental conditions, atoms are changed from one spin state to another, without passing through the intermediary excited state. The atoms in essence absorb momentum from the laser photons, without absorption or emission of photons. The change in momentum deflects atoms in the proper spin state.
We perform an experimental and theoretical study of a novel distributed quasi-Bragg splitter for cold atoms propagating in crossed optical waveguides. The atoms are guided by horizontal red-detuned laser beams which cross with an angle of roughly 90°. The lattice formed by the interference between the two waveguides is used as a quasi-Bragg splitter to continuously deflect the atomic flux from one waveguide into the other. In the limit of strong waveguide confinement and depending on the velocity of the cloud, three main regimes are observed corresponding (1) to the absence of reflection, (2) to partial reflection and (3) to full reflection into the second waveguide. In view of the application to atom interferometry, the condition to split the cloud into mainly two equally-populated fragments is only met in the highest velocity regime, where the fraction of reflected and transmitted atoms can be controlled by tuning the lattice height. A diagnostic of the momentum distribution shows that a quasi-Bragg splitter with the occupation of mainly two momentum states is achieved in this regime. This behaviour can be understood by considering the band structure associated with the potential in the crossing region and agrees with numerical simulations of the atomic dynamics.
Two recent experiments demonstrating novel atomic lenses—one using a free standing Fresnel zone plate and the other using an optical standing wave with large (40 μm) period—will be discussed. In addition, an experiment demonstrating the splitting of an atomic beam by the optical Stern–Gerlach effect will be described. In this effect, a beam of (two-level) atoms is coherently split into two paths by an exactly resonant optical field gradient, each path containing atoms in one of the two dressed states. The field gradient was derived from a large period standing wave, produced by bouncing a laser beam from a glass surface at grazing incidence. The experiments were carried out with metastable He atoms interacting through the 23S1to 23P2 transition with an optical field at 1.083 μm. The interaction times were significantly less than the 23P natural lifetime of 100 ns, so spontaneous emission played no role during the interaction.