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Proceedings of the ... annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting/Proceedings of a meeting - Precise Time and Time Interval (PTTI) Systems and Applications Meeting

Measuring Multipole Moments of the CPT Density Matrix Under Optical Field Polarization-Modulation Conditions

Zachary Warren, James C. Camparo

Published Feb 14, 2022• 2 citations
Publisher

Abstract

Chip-scale atomic clocks (CSACs) based on Coherent Population Trapping (CPT) are at the forefront of next-generation timekeeping for diverse applications, including global navigation satellite systems (GNSS), satellite communications, cellphone networks, and hand-held GNSS receivers. Notwithstanding the potential ubiquity of this atomic device, a performancelimiting aspect of CSACs is the vapor-phase signal-to-noise ratio (SNR) of their ground-state (mF = 0 to mF = 0) atomic hyperfine resonance. Specifically, in commercially available devices angular-momentum optical pumping “pushes” atomic population towards high |mF| Zeeman sublevels at the expense of population in the 0-0 clock transition. Though mitigation strategies for this SNR limiting process have been proposed and demonstrated there has, to date, been little direct measurement of the population distribution among Zeeman sub-states for atoms undergoing CPT, and how that population distribution is altered by SNR-improving mitigation strategies. Here, we describe our initial studies examining this question.

DOI
10.33012/2022.18275

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Chip-scale atomic clocks (CSACs) based on Coherent Population Trapping (CPT) are at the forefront of next-generation timekeeping for diverse applications, including global navigation satellite systems (GNSS), satellite communications, cell-phone networks, and hand-held GNSS receivers. Notwithstanding the potential ubiquity of this atomic device, a performance-limiting aspect of CSACs is the vapor-phase signal-to-noise ratio (SNR) of their ground-state (mF = 0 to mF = 0) atomic hyperfine resonance. Specifically, in commercially available devices angular-momentum optical pumping “pushes” atomic population towards high |mF| Zeeman sublevels at the expense of population in the 0-0 clock transition. Though mitigation strategies for this SNR limiting process have been proposed and demonstrated there has, to date, been little direct measurement of the population distribution among Zeeman sub-states for atoms undergoing CPT, and how that population distribution is altered by SNR improving mitigation strategies. Here, we describe our initial studies examining this question.

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SummaryCurrent compact atomic clock technologies rely on coherent population trapping (CPT), which employs a microwave resonance to stabilize a crystal oscillator. Standard CPT, while a simple and easy-to-implement optical preparation scheme in alkali metal atoms, suffers from low signal-to-noise ratios (SNR). The primary contributor to the low signal amplitude is the collection of atoms at the extreme ends of the Zeeman sublevels in the hyperfine levels of the alkali ground state, so-called “trap states.” Solutions to the trap-state problem have been reported previously, though the redistribution of atomic population into the CPT signal states with these methods has not yet been carefully examined. This work aims to provide an experimental basis to evaluate trap-state recovery schemes.

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More and more, precise timekeeping is becoming a crucial element of space-system operations. Not only are atomic clocks required at mission control ground stations, but they are expected to fly on spacecraft ranging in size from the “large” to CubeSats. This, of course, requires that the size, weight, and power (SWaP) of the clock cannot be sacrificed for frequency stability, and that the frequency stability of the device cannot be sacrificed for SWaP. Traditional space clocks (e.g. Rb clocks, Cs clocks, and hydrogen masers) require microwave cavities, rf-discharge lamps, alkali ovens, hydrogen dissociators, filter vapor-cells, or state-selecting magnets. In contrast, Coherent-Population-Trapping (CPT) clocks can have very low SWaP and have led to the development of the chip-scale atomic-clock (CSAC) – perfect for CubeSat missions. In the standard CPT clock, a narrow atomic resonance is formed by a bichromatic optical field. Unfortunately, in the standard CPT clock design the atomic signal suffers a low signal-to-noise ratio (SNR), due to atoms becoming systemically stuck in “trap” states inaccessible to the laser light. At The Aerospace Corporation’s Physical Sciences Laboratories, we have constructed a CPT atomic clock testbed, with the goal of accelerating CSAC development for next-generation space missions. Specifically, our plan is to investigate atomic physics issues that influence CSAC performance and SWaP, and prioritizing issues not being addressed in academic or industrial laboratories. In this presentation, we will discuss our CPT testbed, and recent work investigating CPT-enhancement strategies for improved clock performance that recover atoms from trap states and improve the SNR. Additionally, we will discuss our studies of the physical processes taking place in these CPT-enhancement strategies, and how they can affect advanced-CSAC functioning and design.

Proceedings of the ... annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting/Proceedings of a meeting - Precise Time and Time Interval (PTTI) Systems and Applications Meeting•Feb 23, 2021•0 citations
Chip-scale atomic clocks (CSACs) based on Coherent Population Trapping (CPT) are at the forefront of next-generation timekeeping for diverse applications, including global navigation satellite systems (GNSS), satellite communications, cellphone networks, and hand-held GNSS receivers. Notwithstanding the potential ubiquity of this atomic device, a performancelimiting aspect of CSACs is the vapor-phase signal-to-noise ratio (SNR) of their ground-state (mF = 0 to mF = 0) atomic hyperfine resonance. Specifically, in commercially available devices angular-momentum optical pumping “pushes” atomic population towards high |mF| Zeeman sublevels at the expense of population in the 0-0 clock transition. Though mitigation strategies for this SNR limiting process have been proposed and demonstrated there has, to date, been little direct measurement of the population distribution among Zeeman sub-states for atoms undergoing CPT, and how that population distribution is altered by SNR-improving mitigation strategies. Here, we describe our initial studies examining this question.
Chip-scale atomic clocks (CSACs) based on Coherent Population Trapping (CPT) are at the forefront of next-generation timekeeping for diverse applications, including global navigation satellite systems (GNSS), satellite communications, cell-phone networks, and hand-held GNSS receivers. Notwithstanding the potential ubiquity of this atomic device, a performance-limiting aspect of CSACs is the vapor-phase signal-to-noise ratio (SNR) of their ground-state (mF = 0 to mF = 0) atomic hyperfine resonance. Specifically, in commercially available devices angular-momentum optical pumping “pushes” atomic population towards high |mF| Zeeman sublevels at the expense of population in the 0-0 clock transition. Though mitigation strategies for this SNR limiting process have been proposed and demonstrated there has, to date, been little direct measurement of the population distribution among Zeeman sub-states for atoms undergoing CPT, and how that population distribution is altered by SNR improving mitigation strategies. Here, we describe our initial studies examining this question.
The traditional Coherent-Population-Trapping (CPT) approach to atomic signal generation in chip-scale atomic clocks (CSACs) requires circularly polarized light, which creates spin polarization in the vapor and leads to reduced CSAC signal-to-noise ratios. Though various strategies have been developed to mitigate this problem, there is only a superficial and/or empirical understanding of these strategies' efficacies due to the difficulty of measuring ground-state population distributions among hyperfine Zeeman sublevels. Here, we demonstrate an optical technique to measure the first three (density-matrix) state multipole moments of a single ground-state hyperfine manifold. Using this technique, we have discovered that ground-state angular momentum alignment (i.e., the quadrupole moment) can play an important role in CPT signal-to-noise ratios.
SummaryCurrent compact atomic clock technologies rely on coherent population trapping (CPT), which employs a microwave resonance to stabilize a crystal oscillator. Standard CPT, while a simple and easy-to-implement optical preparation scheme in alkali metal atoms, suffers from low signal-to-noise ratios (SNR). The primary contributor to the low signal amplitude is the collection of atoms at the extreme ends of the Zeeman sublevels in the hyperfine levels of the alkali ground state, so-called “trap states.” Solutions to the trap-state problem have been reported previously, though the redistribution of atomic population into the CPT signal states with these methods has not yet been carefully examined. This work aims to provide an experimental basis to evaluate trap-state recovery schemes.
SummaryChip-scale atomic clocks (CSAC) are the products of a burgeoning field of miniature precise timing efforts in support of low-SWaP space-qualified timing sources. Most CSACs are based on the coherent population trapping (CPT) effect, which uses laser light to selectively pump atoms into dark states. Because of the multiple accessible and inaccessible hyperfine Zeeman states, many atoms don’t contribute to the signal leading to a low signal-to-noise ratio of the CPT resonance. In turn, clock stability is limited. While solutions to this problem have been studied previously, the examination of the redistribution of atoms through the novel pumping solutions has not been thoroughly studied. This work provides continued exploration of an experimental effort to evaluate Zeeman end-state recovery schemes.
This article reports on the characterization of a compact coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique. This method allows to detect high-contrast CPT resonances on the 0-0 magnetic-field insensitive clock transition. The clock short-term fractional frequency stability is measured to be 3×10−13 τ−1/2 up to 100 s averaging time, in correct agreement with the signal-to-noise ratio limit. These results are among the best performances ever reported for a CPT-based clock. The noise budget is detailed. A preliminary study is reported to evaluate limitations to the clock mid-term frequency stability. The clock performances are found to be presently mainly limited in the mid-term by laser power effects.
More and more, precise timekeeping is becoming a crucial element of space-system operations. Not only are atomic clocks required at mission control ground stations, but they are expected to fly on spacecraft ranging in size from the “large” to CubeSats. This, of course, requires that the size, weight, and power (SWaP) of the clock cannot be sacrificed for frequency stability, and that the frequency stability of the device cannot be sacrificed for SWaP. Traditional space clocks (e.g. Rb clocks, Cs clocks, and hydrogen masers) require microwave cavities, rf-discharge lamps, alkali ovens, hydrogen dissociators, filter vapor-cells, or state-selecting magnets. In contrast, Coherent-Population-Trapping (CPT) clocks can have very low SWaP and have led to the development of the chip-scale atomic-clock (CSAC) – perfect for CubeSat missions. In the standard CPT clock, a narrow atomic resonance is formed by a bichromatic optical field. Unfortunately, in the standard CPT clock design the atomic signal suffers a low signal-to-noise ratio (SNR), due to atoms becoming systemically stuck in “trap” states inaccessible to the laser light. At The Aerospace Corporation’s Physical Sciences Laboratories, we have constructed a CPT atomic clock testbed, with the goal of accelerating CSAC development for next-generation space missions. Specifically, our plan is to investigate atomic physics issues that influence CSAC performance and SWaP, and prioritizing issues not being addressed in academic or industrial laboratories. In this presentation, we will discuss our CPT testbed, and recent work investigating CPT-enhancement strategies for improved clock performance that recover atoms from trap states and improve the SNR. Additionally, we will discuss our studies of the physical processes taking place in these CPT-enhancement strategies, and how they can affect advanced-CSAC functioning and design.