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Applied Physics Letters

Resolution of photon-recoil structure of the 6573-Å calcium line in an atomic beam with optical Ramsey fringes

R. L. Barger, Jonas Bergquist, Thomas C. English, David J. Glaze

Published Jun 15, 1979• 86 citations
Publisher

Abstract

The photon-recoil components of the Ca 6573-Å line have been resolved using the three-zone optical Ramsey interference technique with an atomic beam. Linewidths as narrow as 3 kHz HWHM (line Q of 8×1010) are reported for zone separations up to 7 cm. An indication of the light-shift-induced contraction of the recoil splitting predicted by C.J. Bordé is obtained. Techniques are discussed which should lead to an optical wavelength/frequency standard with an accuracy of better than 10−14.

DOI
10.1063/1.90690

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This dissertation details the developments of a calcium thermal beam atom interferometer designed to act as an ultra-stable frequency reference over a broad range of timescales. The obtained fractional frequency instabilities of the final apparatus reached ≤ 10−16 after 10's of seconds of averaging without the aid of vibration isolation or significant apparatus temperature stabilization. The vastly superior stability performance of this calcium system compared to other thermal atomic references was facilitated by the enormous atom numbers from the thermal beam, which can be efficiently addressed with Bord\'e-Ramsey spectroscopy on the well-studied (4s2)1S0-(4s4p)3P1 (m=0) clock transition in neutral 40Ca. This method yields interrogation laser frequency dependent interference fringes with a linewidth of 1.6 kHz. This measurement is further enhanced with an electron-shelving detection scheme used for state detection after the interferometry process, causing each excited state atom to emit many hundreds of photons in the detection region. The exceptional instability results from this atom interferometer highlight an underutilized realm of precision measurement science based on relatively simple thermal ensembles. An intuitive understanding of the Bordé-Ramsey process is presented and relevant experimental techniques explained such that this dissertation should prove useful to a broad readership.

•Jan 1, 2019•0 citations
The photon-recoil components of the Ca 6573-Å line have been resolved using the three-zone optical Ramsey interference technique with an atomic beam. Linewidths as narrow as 3 kHz HWHM (line Q of 8×1010) are reported for zone separations up to 7 cm. An indication of the light-shift-induced contraction of the recoil splitting predicted by C.J. Bordé is obtained. Techniques are discussed which should lead to an optical wavelength/frequency standard with an accuracy of better than 10−14.
A new method to suppress one of the two recoil components in saturation and Ramsey spectroscopy is presented. It is applied to the calcium intercombination line $^{1}S_{0}\ensuremath{-}^{3}P_{1}$ ($\ensuremath{\lambda}\ensuremath{\cong}675$ nm) by optical pumping of the calcium atoms from the metastable 3P1^{3}P_{1}3P1​ state through the 3S1^{3}S_{1}3S1​ state into the 3P0^{3}P_{0}3P0​ or the 3P2^{3}P_{2}3P2​ states. The suppression of the low-frequency recoil component could be deduced in saturated absorption from a narrowing and shift of the Lamb dip as well as observed directly in Ramsey fringe spectroscopy when the recoil doublet was resovled.
For a neutral (40)Ca-based optical frequency standard we report a fractional frequency instability of 4 x 10(-15) in 1 s, which represents a fivefold improvement over existing atomic frequency standards. Using the technique of optical Bordé-Ramsey spectroscopy with a sample of 10(7) trapped atoms, we have resolved linewidths as narrow as 200 Hz (FWHM). With colder atoms this system could potentially achieve an instability as low as 2 x 10(-16) in 1 s. Such low instabilities are important for frequency standards and precision tests of fundamental physics.
The 657 nm calcium intercombination line is an attractive optical frequency/length reference due to its high line Q (10/sup 12/) and insensitivity to external perturbations. In this paper we describe progress towards an all-diode-laser-based reference using laser-trapped Ca. To generate 35 mW of 423 nm trap light, we double the frequency of a 150 mW, 846 nm diode laser with a nonlinear crystal (KNbO/sub 3/) inside a resonant build-up cavity. We use this light to trap 2/spl times/10/sup 7/ atoms in a compact, magneto-optic trap. Spectroscopy of the narrow 657 nm transition is performed with a frequency-stabilized diode laser system. With the trapped atoms we have measured optical Ramsey fringes with a linewidth of 6 kHz. We discuss future improvements as well as potential performance which could achieve a fractional frequency instability of <3/spl times/10/sup -15/.
A calcium atomic beam excited in an optical Ramsey geometry was rotated about an axis perpendicular to the plane defined by the laser beams and the atomic beam. A frequency shift of the Ramsey fringes of several kHz has been measured which is proportional to the rotation frequency of the apparatus and to the distance between the laser beams. The results can be interpreted in three equivalent ways as the Sagnac effect in a calcium-atomic-beam interferometer: in the rotating frame of the laser beams either along straight paths or along the curved trajectories of the atoms, or in the inertial atomic frame.
This dissertation details the developments of a calcium thermal beam atom interferometer designed to act as an ultra-stable frequency reference over a broad range of timescales. The obtained fractional frequency instabilities of the final apparatus reached ≤ 10−16 after 10's of seconds of averaging without the aid of vibration isolation or significant apparatus temperature stabilization. The vastly superior stability performance of this calcium system compared to other thermal atomic references was facilitated by the enormous atom numbers from the thermal beam, which can be efficiently addressed with Bord\'e-Ramsey spectroscopy on the well-studied (4s2)1S0-(4s4p)3P1 (m=0) clock transition in neutral 40Ca. This method yields interrogation laser frequency dependent interference fringes with a linewidth of 1.6 kHz. This measurement is further enhanced with an electron-shelving detection scheme used for state detection after the interferometry process, causing each excited state atom to emit many hundreds of photons in the detection region. The exceptional instability results from this atom interferometer highlight an underutilized realm of precision measurement science based on relatively simple thermal ensembles. An intuitive understanding of the Bordé-Ramsey process is presented and relevant experimental techniques explained such that this dissertation should prove useful to a broad readership.