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arXiv:1903.11231 · Physical Review Applied

Compact Optical Atomic Clock Based on a Two-Photon Transition in Rubidium

Kyle W. Martin, Gretchen R. Phelps, N. Lemke, Matthew S. Bigelow, Benjamin Stuhl, Michael Wojcik, Michael Holt, Ian R. Coddington, Michael W. Bishop, John H. Burke

Published Jan 18, 2018• 171 citations
arXivHTMLPDF

Abstract

Optical frequency standards surpass their microwave counterparts in both stability and accuracy, yet they are often bulky, power-hungry, and unable to operate outside of a well-controlled laboratory environment. Leveraging a two-photon transition in ${}^{87}$Rb vapor and recent advances in fiber frequency combs, the authors build an optical clock to beat the current portable standards, with an architecture that can be made compact and low-power. These results point the way to a real-world optical frequency standard of even higher stability, for applications such as satellite navigation.

DOI
10.1103/physrevapplied.9.014019

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Optical frequency standards surpass their microwave counterparts in both stability and accuracy, yet they are often bulky, power-hungry, and unable to operate outside of a well-controlled laboratory environment. Leveraging a two-photon transition in 87{}^{87}87Rb vapor and recent advances in fiber frequency combs, the authors build an optical clock to beat the current portable standards, with an architecture that can be made compact and low-power. These results point the way to a real-world optical frequency standard of even higher stability, for applications such as satellite navigation.
Portable atomic clocks are essential in a wide variety of applications, most notably in the operation of global navigation satellite systems. Existing portable atomic clocks utilizing microwave-based interrogation schemes are now routinely eclipsed by the next generation of atomic frequency standards based on optical interrogation. While optical frequency standards demonstrate greatly improved frequency stability, they have only recently reached a level of technical maturity required to demonstrate this improved performance outside of well curated laboratory environments. Here, we demonstrate a fully autonomous and portable optical frequency standard based on an efficient dual-color excitation of the 5S1/2→5D5/25S_{1/2}\rightarrow5D_{5/2}5S1/2​→5D5/2​ two-photon transition in 87^{87}87Rb. The standard utilizes a combination of robust, highly developed commercial-off-the-shelf telecommunications technologies and a fully integrated portable optical frequency comb, providing the optical and microwave outputs vital for interfacing with existing electronic systems and infrastructure. The system demonstrates a fractional frequency stability of 1.9×10−131.9\times10^{-13}1.9×10−13 at 1s of integration time, reaching 3.5×10−153.5\times10^{-15}3.5×10−15 at 8000s of integration time without the need for drift removal. This portable demonstrator unit marks a significant achievement in the development of Rb optical atomic frequency standards, and for the deployment of optical atomic frequency standards outside of the laboratory.
We demonstrated a compact 780 nm rubidium optical clock, which includes an optical frequency standard and an optical frequency comb, with an optical volume of 11.6 liters. Unlike the 778 nm rubidium atomic clocks based on two-photon transition, here, the laser frequency is stabilized to the Rb D2 transition, using modulation transfer spectroscopy. This approach effectively eliminates Doppler background and provides a high signal to noise ratio and high sensitivity. A nearly 300 MHz microwave signal, whose phase exactly tracks that of the optical frequency standard, is generated via the optical frequency comb, yielding a frequency instability of 1.91 E-13 @1 s and 5.29 E-14 @1000 s in the electronic domain. To the best of our knowledge, this is the most precise frequency stabilization result for the first-excited-state transition of alkali metal atoms to date and represents the first optical clock based on this transition. These results offer a promising approach for the development of portable optical clocks.
Optical clocks represent the hallmark of precise timekeeping. The most precise atomic clocks are laboratory-based ones that utilize transitions in the microwave/optical domain of neutral atoms or trapped ions. These clocks define an international time scales, confirm the consistency of physical constants and stimulate the search for new physics outside the Standard Model. The other area is the regime of portable atomic clocks based on warm atomic vapor, which is small, power-efficient and deployable in the field with a variety of civilian applications, including sensing, communication and navigation. Two-photon resonance and optical transition based field deployable clocks present a viable alternative that may outperform current commercial chip scale clocks by at least a factor of 10 in terms of both long-term and short-term stability. For portable clocks, two-photon transitions in alkaline earth metals are of special interest. One of the systematic inaccuracies in the optical reference is eliminated by using counter-propagating laser beams to create Doppler-free spectroscopy. At IIT Tirupati, we are engaged in developing the next generation of atomic vapor based portable frequency standard using two-photon transition in Rubidium (Rb) for atomic sensing and quantum positioning applications. This article discusses the physics behind the two-photon transition in Rubidium atoms suitable for a clock reference. Briefly, it discusses the experimental methodology for realizing an optical clock using warm rubidium vapor.
Frequency standards find wide use in navigation and timing for industry, civilian, and defense applications. Unfortunately, the very best frequency standards are bulky, power-hungry, and confined to well-controlled laboratory environments. Here the authors aim to deliver a compact, high-performance standard, the key challenge being the need to produce a strong, narrow atomic transition. They produce a superstrong signal by driving a two-photon transition in 87{}^{87}87Rb vapor that exploits a fortuitous third atomic level to greatly enhance the effective strength of the atom-light interaction, allowing short-term clock performance comparable to today's best commercial frequency standards.
Precision navigation and timing is an important focus in the Air Force Research Laboratory's experimental efforts. The development and testing of an optical frequency standard is suitable for an array of both terrestrial and space based applications. An optical atomic clock based upon a two-photon transition at 778 nm in rubidium, is an ideal candidate to meet the requirements of Global Satellite Navigation System (GNSS), while being a viable option for other applications. Here we demonstrate that a simple architecture, using off-the-shelf components and an external cavity diode laser system that is confined to less than 3 ft2 of breadboard, exhibits a frequency instability of 7 x 10-13/??, for times less than 100 seconds.
A compact rubidium optical clock is presented, incorporating an optical frequency standard, a fiber frequency comb, and associated control electronics. Modulation transfer spectroscopy, devoid of Doppler background and characterized by a high signal-to-noise ratio and sensitivity, is employed to lock the laser frequency to the hyperfine transitions of rubidium atomic lines. The clock exhibits a short-term stability of 4.10 × 10-13at a 1-second gate time, and the stability remains below 7.27 × 10-13up to 1000 seconds. These results offer a promising alternative approach for the development of portable optical clocks.