A cold-atom beam clock based on coherent population trapping

We present results from an atomic clock that employs a beam of cold ⁸⁷Rb atoms and spatially separated (Ramsey) coherent population trapping interrogation of the hyperfine clock transition at 6.834 GHz. The cold atomic beam is generated through the use of a 2D⁺-magnetooptical trap. The interrogation is performed on the D2 line of ⁸⁷Rb, and the optical fields use a counter-propagating <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>σ</mml:mi> <mml:mo>+</mml:mo></mml:msub> <mml:mo>-</mml:mo> <mml:msub><mml:mi>σ</mml:mi> <mml:mo>-</mml:mo></mml:msub> </mml:mrow> </mml:math> probing scheme. The use of cold atoms allows for relatively narrow Ramsey fringes even for a small spatial separation between the two interrogation zones(4.6 cm). The resulting clock has a short-term stability of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>3</mml:mn> <mml:mo>×</mml:mo> <mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>11</mml:mn></mml:mrow> </mml:msup> <mml:mo>/</mml:mo> <mml:msqrt><mml:mi>τ</mml:mi></mml:msqrt> </mml:mrow> </mml:math> .