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Physical Review Applied

Integrated pulsed optically pumped Rb atomic clock with frequency stability of 10 − 15

Qiang Hao, Shaojie Yang, Jun Ruan, Peter Yun, Shougang Zhang

Published Feb 1, 2024• 14 citations
Publisher

Abstract

Pulsed optically pumped (POP) Rb atomic clock is considered to be a powerful technique for the Rb atomic clock due to the capability of light-shift mitigation and atomic spectroscopy narrowing. Relevant investigations are conducted over two decades, however, no integrated prototype of such a kind of clock has been reported yet. Here, we present an integrated prototype of the POP Rb atomic clock and quantitatively characterize its physical properties. The atomic clock shows a short-term fractional frequency stability of $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}/\sqrt{\ensuremath{\tau}}$ (where $\ensuremath{\tau}$ is the averaging time), and comes to $2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ with 40 000 s averaging (drift removed) under atmospheric conditions. Physical effects contributing to the long-term frequency stability are carefully analyzed and cavity-pulling shift is identified as the leading limit. The techniques presented here enable realizing a low ${10}^{\ensuremath{-}15}$ level vapor-cell atomic clock under atmospheric conditions, which could benefit a wide range of terrestrial applications, such as quantum communication, decimeter-level terrestrial positioning.

DOI
10.1103/physrevapplied.21.024003

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Pulsed optically pumped (POP) Rb atomic clock is considered to be a powerful technique for the Rb atomic clock due to the capability of light-shift mitigation and atomic spectroscopy narrowing. Relevant investigations are conducted over two decades, however, no integrated prototype of such a kind of clock has been reported yet. Here, we present an integrated prototype of the POP Rb atomic clock and quantitatively characterize its physical properties. The atomic clock shows a short-term fractional frequency stability of 2.6\ifmmode×\else\texttimes\fi10\ensuremath−13/\ensuremathτ2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}/\sqrt{\ensuremath{\tau}}2.6\ifmmode×\else\texttimes\fi10\ensuremath−13/\ensuremathτ​ (where \ensuremathτ\ensuremath{\tau}\ensuremathτ is the averaging time), and comes to 2.3\ifmmode×\else\texttimes\fi10\ensuremath−152.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}2.3\ifmmode×\else\texttimes\fi10\ensuremath−15 with 40 000 s averaging (drift removed) under atmospheric conditions. Physical effects contributing to the long-term frequency stability are carefully analyzed and cavity-pulling shift is identified as the leading limit. The techniques presented here enable realizing a low 10\ensuremath−15{10}^{\ensuremath{-}15}10\ensuremath−15 level vapor-cell atomic clock under atmospheric conditions, which could benefit a wide range of terrestrial applications, such as quantum communication, decimeter-level terrestrial positioning.
In this paper we report the results we obtained with a prototype of pulsed optically pumped (POP) Rb clock. In terms of frequency stability, an Allan deviation σy(τ)=1.7 ×10-13τ-1/2has been measured, where τ is the integration time. In addition, the long term frequency drift results below 10-14/day. In the paper we also describe a technique to stabilize the amplitude of the microwave signal using the atoms themselves as discriminator. Moreover, an anomalously large temperature sensitivity of the clock frequency and a technique to keep it under control are also reported. Finally, we describe some non linear effects that modify the shape of the clock signal represented by the Ramsey central fringe.
Rb atomic clock refers to the microwave transition of87Rb atoms, generally considered as a low-cost and low-performance atomic frequency standard. The maturity of semiconductor laser offers us a new opportunity to improve the Rb atomic clock. However, there are also many challenges in terms of electronic system and optical bench. Despite the performance superiority of the pulsed optically pumped (POP) Rb atomic clock has been demonstrated in the laboratory, there still no integrated prototype emerges to date. Here, we report the design and characteristics of an integrated POP Rb atomic clock. The primary test shows short-term stability of 3.2×10-13τ-1/2(1-1000s) and 4000s frequency stability of 8.5×10-15in air. The size and power consumption are 23 L and 45 W, respectively.
We present theoretical and experimental studies concerning the realization of a rubidium vapor cell frequency standard based on the pulsed optical pumping (POP) technique. This technique is extremely powerful since it avoids in principle the light shift effect and the noise conversions from the laser to the clock signal. The detection of the clock transition in the microwave domain (the so called POP maser) has been extensively investigated at INRIM and a frequency stability of 1.1 × 10-12τ-1/2 has been measured until integration times of 105 s, reaching the level of 5 × 10-15 (drift removed). The optical detection is still under evaluation but this technique may lead in effect to some advantages with respect to the microwave detection, such as a lower or negligible cavity pulling, a lower operation temperature (which implies a lower spin exchange contribution) and a higher signal-to-noise ratio. All these features may lead to a better frequency stability performance than that previously reached. Read More: http://www.worldscientific.com/doi/abs/10.1142/9789812838223_0040
We present a prospective experimental study on double-resonance Ramsey spectroscopy in a micro fabricated Rb vapor cell, in view of future miniature atomic clocks with improved stability performance. Ramsey signals with a narrow central fringe width of 1.5 kHz combined with a competitive contrast of > 5 % are observed, resulting in a clock short-term stability of ≈ 1×10-11τ−1/2. The employed pulsed optical pumping (POP) scheme bears promise for reduced light-shift effects and thus for improved clock stability on medium- to long-term timescales.
The pulsed optically pumped(POP) 87Rb vapor-cell frequency standard was studied based on the detection of free induction microwave signal or optical absorption signal.In POP operation the pumping,interrogation,and detection phases are separated in time,avoiding the coupling between microwave and optical coherence.The theoretical results show that an oscillation period doubled with respect to the classical Ramsey method which leads to a doubled atomic quality factor Q,and the reduction of optical shift and cavity pulling shift will be obtained,when the atoms are submitted to π/2 Ramsey microwave pulse.In experiment Ramsey fringes were obtained with a clock prototype based on 87Rb in buffer gas,with bandwidth of 65 Hz and 150 Hz for microwave and optical signal respectively.The frequency stability achieved was 1.2×10-12 for measurement times up to 100 s.
We present a multilevel theoretical approach to describe the behavior of a pulsed optically pumped (POP) 87Rb^{87}\mathrm{Rb}87Rb frequency standard based on a microwave cavity-vapor cell arrangement. The full Zeeman manifold of the ground-state hyperfine levels is then considered, and the dynamics induced among them by relaxation processes (buffer gas, spin exchange, and cell-walls collisions) is taken into account. The model includes as well the absorption of the pumping laser along the cell and the cavity feedback on the atoms; the effects related to a nonuniform excitation of the atomic sample are also discussed. Theoretical predictions are proven with a laboratory prototype of POP passive maser in which the clock transition is excited by means of the Ramsey technique. The agreement between theory and experiment is very satisfactory from both a quantitative and qualitative point of view, in terms of the shape of the Ramsey fringes, microwave power delivered by the atoms, and short-term frequency stability. In particular, a frequency stability of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at one second has been measured, in very good agreement with the expected value. On this basis, the model has been extended to predict the short-term frequency stability of a POP frequency standard operating with optical detection. A shot-noise limited frequency stability (Allan deviation) of ${\ensuremath{\sigma}}_{y}(\ensuremath{\tau})\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\ensuremath{\tau}}^{\ensuremath{-}1∕2}$ is foreseen. This value can be degraded by microwave phase noise and laser fluctuations so that the overall predicted clock stability is ${\ensuremath{\sigma}}_{y}(\ensuremath{\tau})\ensuremath{\approx}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1∕2}$.