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arXiv:1907.06774 · Physical Review Letters

Ramsey-Bordé Matter-Wave Interferometry for Laser Frequency Stabilization at 10 − 16 Frequency Instability and Below

Judith Olson, Richard W. Fox, Tara M. Fortier, Todd F. Sheerin, Roger C. Brown, Holly Leopardi, Richard E. Stoner, Chris W. Oates, Andrew D. Ludlow

Published Aug 13, 2019• 61 citations
arXivHTMLPDF

Abstract

We demonstrate Ramsey-Bordé (RB) atom interferometry for high performance laser stabilization with fractional frequency instability <2×10^{-16} for timescales between 10 and 1000s. The RB spectroscopy laser interrogates two counterpropagating ^{40}Ca beams on the ^{1}S_{0}-^{3}P_{1} transition at 657 nm, yielding 1.6 kHz linewidth interference fringes. Fluorescence detection of the excited state population is performed on the (4s4p) ^{3}P_{1}-(4p^{2}) ^{3}P_{0} transition at 431 nm. Minimal thermal shielding and no vibration isolation are used. These stability results surpass performance from other thermal atomic or molecular systems by 1 to 2 orders of magnitude, and further improvements look feasible.

DOI
10.1103/physrevlett.123.073202

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We demonstrate Ramsey-Bordé (RB) atom interferometry for high performance laser stabilization with fractional frequency instability <2×10^{-16} for timescales between 10 and 1000s. The RB spectroscopy laser interrogates two counterpropagating ^{40}Ca beams on the ^{1}S_{0}-^{3}P_{1} transition at 657 nm, yielding 1.6 kHz linewidth interference fringes. Fluorescence detection of the excited state population is performed on the (4s4p) ^{3}P_{1}-(4p^{2}) ^{3}P_{0} transition at 431 nm. Minimal thermal shielding and no vibration isolation are used. These stability results surpass performance from other thermal atomic or molecular systems by 1 to 2 orders of magnitude, and further improvements look feasible.
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.
We demonstrate laser frequency stabilization at the 10-16fractional frequency level in one second through spectroscopy on a 650 °C calcium atomic beam. The calcium beam is interrogated with a cavity stabilized 657 nm laser arranged in a Ramsey-Borde configuration. A second laser at 431 nm reads off the final atomic state populations while allowing for high signal-to-noise. Limitations of this method are being explored and, and, based on current noise analysis, there is potential to achieve fractional frequency stabilities in the 10-17range or below.
Abstract Compact optical atomic clocks have become increasingly important in field applications and clock networks. Systems based on Ramsey-Bordé interferometry (RBI) with a thermal atomic beam seem promising to fill a technology gap in optical atomic clocks, as they offer higher stability than optical vapour cell clocks while being less complex than cold atomic clocks. Here, we demonstrate RBI with strontium atoms, utilizing the narrow "Equation missing" intercombination line at 689 nm, yielding a 60 kHz broad spectral feature. The obtained Ramsey fringes for varying laser power are analyzed and compared with a numerical model. The "Equation missing" transition at 461 nm is used for fluorescence detection. Analyzing the slope of the RBI signal and the fluorescence detection noise yields an estimated short-term stability of <4×10−14/τ/1 s<4\times 10^{-14} / \sqrt{\tau / {1~s}}<4×10−14/τ/1 s​ < 4 × 10 − 14 / τ / 1 s . We present our experimental setup in detail, including the atomic beam source, frequency-modulation spectroscopy to lock the 461 nm laser, laser power stabilization and the high-finesse cavity pre-stabilization of the 689 nm laser. Our system serves as a ground testbed for future clock systems in mobile and space applications.
We demonstrate a means of increasing the signal-to-noise ratio in a Ramsey-Bordé interferometer with spatially separated oscillatory fields on a thermal atomic beam. The S01↔P13 intercombination line in neutral Ca40 is used as a frequency discriminator, with an extended cavity diode laser at 423 nm probing the ground state population after a Ramsey-Bordé sequence of 657 nm light-field interactions with the atoms. Evaluation of the instability of the Ca frequency reference is carried out by comparison with (i) a hydrogen-maser and (ii) a cryogenic sapphire oscillator. In the latter case the Ca reference exhibits a square-root Λ variance of 9.2×10−14 at 1 s and 2.0×10−14 at 64 s. This is an order-of-magnitude improvement for optical beam frequency references, to our knowledge. The shot noise of the readout fluorescence produces a limiting square-root Λ variance of 7×10−14/√τ, highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with 10−14 range frequency instability.
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 authors report on a thermal beam optical frequency reference with a fractional frequency instability of 9.2×10−14 at 1 s reducing to 2.0×10−14 at 64 s before slowly rising. The 1S0↔3P1 intercombination line in neutral 40Ca is used as a frequency discriminator. A diode laser at 423 nm probes the ground state population after a Ramsey–Bordé sequence of 657 nm light-field interactions on the atoms. The measured fractional frequency instability is an order of magnitude improvement on previously reported thermal beam optical clocks. The photon shot-noise of the read-out produces a limiting square root Λ-variance of 7×10−14/.