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Physics package validation and system integration for a low-SWaP, portable cold multi-ytterbium ion 12.6 GHz microwave clock

M. A. Trigatzis, Ulas Sabahattin Gokay, Martin Knapp, Sean Mulholland, Peter Tsoulos, Gary I. Hockley, Chris Lucas, G. P. Barwood, Patrick Robert Gill

Published Jun 18, 2026• 0 citations
Publisher

Abstract

Abstract Compact atomic clocks provide the stable, traceable timing required for 5G/6G telecom networks, for resilient operation of critical infrastructure including power grids, data centres and financial systems, and for maintaining accurate synchronisation during GNSS interruptions or outages. Microwave clocks based on caesium beam tubes have been the industry-standard rack-mounted clock for many years but are now outperformed in short-to-midterm instability by emerging technologies. Vapour cell clocks suffer from long-term drift which must be calibrated out periodically and on startup. This limits their use for long-term holdover and in applications where the device needs to operate after a cold start. Additionally, optical clocks require a narrow-linewidth clock laser and optical frequency comb. The ytterbium ion microwave clock developed at the National Physical Laboratory (NPL) does not require these subsystems and combines this relative simplicity with the benefits of laser cooling (long-term stability, accuracy). In this paper, we describe an ion trap physics package design where its functionality has been evaluated in a benchtop clock and forms the basis of a new design for a compact, transportable clock. We describe the operation and performance of the benchtop clock, demonstrating a short-term fractional frequency instability of 1.4 × 10 − 12 /√τ. The design for a new transportable clock based on this new physics package is detailed, as are the validation tests of other key subsystems.

DOI
10.21203/rs.3.rs-9865578/v1

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Abstract Compact atomic clocks provide the stable, traceable timing required for 5G/6G telecom networks, for resilient operation of critical infrastructure including power grids, data centres and financial systems, and for maintaining accurate synchronisation during GNSS interruptions or outages. Microwave clocks based on caesium beam tubes have been the industry-standard rack-mounted clock for many years but are now outperformed in short-to-midterm instability by emerging technologies. Vapour cell clocks suffer from long-term drift which must be calibrated out periodically and on startup. This limits their use for long-term holdover and in applications where the device needs to operate after a cold start. Additionally, optical clocks require a narrow-linewidth clock laser and optical frequency comb. The ytterbium ion microwave clock developed at the National Physical Laboratory (NPL) does not require these subsystems and combines this relative simplicity with the benefits of laser cooling (long-term stability, accuracy). In this paper, we describe an ion trap physics package design where its functionality has been evaluated in a benchtop clock and forms the basis of a new design for a compact, transportable clock. We describe the operation and performance of the benchtop clock, demonstrating a short-term fractional frequency instability of 1.4 × 10 − 12 /√τ. The design for a new transportable clock based on this new physics package is detailed, as are the validation tests of other key subsystems.
High performance transportable microwave atomic clocks are enablers for resilient GNSS-free timescales. We present progress towards a transportable laser-cooled ytterbium ion microwave clock. This system incorporates our recently developed multi-ion trap chamber and redesigned electronics and optics systems, offering reductions in size, weight and power, with improved performance over our previous system. The benchtop prototype version of this clock uses the new physics package design and reaches a short-term instability of 1.2 × 10−12/√τ. We present the transportable clock design as well as subsystem test results, which so far indicate that a similar performance to the benchtop version is achievable in a 3U rack-mounted package.
Stable and accurate timekeeping is essential in today’s commercial and military applications. For decades, the 5071A Cesium beam clock has been the mainstay for long-term holdover in strategic applications for its unmatched performance. However, is too large, power hungry and cumbersome for most mobile platforms. To enable improved performance in lower size, weight and power applications, a new physics solution is required over traditional clock technology. In this paper we will present our Ytterbium microwave ion clock progress as a small form factor, high performance clock for use in military and commercial applications where portability is critical. Our device targets maximum performance improvement over traditional methods while minimizing technical risk, leveraging existing manufacturing, and targeting the lowest possible size, power and cost. The Physics Package includes a linear quadrupole trap, an electron gun (e-gun) ionizer, a passive getter pump and a Yb oven. We use electron impact ionization to load Ytterbium-171 ions which are cooled down to room temperature using buffer gas. Interaction-time limited Rabi spectroscopy using microwave radiation at 12.6 GHz is performed. The resulting microwave Rabi linewidth is used for closed-loop clock operation. Concurrent work is underway to develop a 369 nm Distributed Bragg Reflector laser which is essential for robust operation of the clock. We will discuss the specifications, the detailed design of our Ytterbium Microwave Ion Clock, the first assembled Physics Package and related preliminary test results.
Abstract The primary technology for high accuracy position, navigation and timing (PNT) capability worldwide is currently provided by global navigation satellite systems (GNSS). The vulnerability of GNSS to intentional interference, clock component failure or extreme space weather is increasingly pointing to the need for alternative independent navigation and timing solutions to maintain critical national infrastructures, defence, security, commerce and logistics. The availability of low size, weight and power (SWaP) portable atomic clock technology with resilience and extended holdover times for use both in space and terrestrially is now increasingly urgent. In this paper, we report on progress in the development of compact optical clocks that offer the capability to out-perform existing commercial microwave atomic clock systems. We discuss designs for a compact transportable cold trapped strontium ion optical clock with both a low SWaP design and high accuracy that can address extended holdover for future navigation and timing requirements. We describe how key sub-systems of an optical atomic clock, namely the single ion trap physics package and multi-wavelength dual-axis cubic cavity clock control unit have been redesigned to reduce SWaP. The optical cavity is needed to frequency stabilise the lasers for laser cooling and state preparation as well as the clock laser for interrogating the ion’s clock transition. We describe the results of finite element analysis (FEA) to model the electrical and thermal behaviour of a compact ion trap design. We also consider the design space for small cubic optical cavities, investigating the possible limits for achieving a minimal SWaP for the system.
We report on the development of a portable, trapped-ion, microwave frequency standard based on the 12.6 GHz hyperfine transition in laser-cooled ytterbium-171 ions. The entire system fits into a 6U 19-inch rack unit (51×49×2851\times49\times2851×49×28 cm) and comprises laser, electronics and physics package subsystems. The performance of this development system is evaluated; the fractional frequency instability was measured to be 3.6×10−12/√τ3.6\times10^{-12}/\surd\tau3.6×10−12/√τ for averaging times between 30 s and 1500 s.
A portable microwave clock incorporating laser-cooling of171Yb+promises to bring significantly improved performance compared to existing thermal commercial devices. We report on progress in the development of a compact microwave frequency standard designed to fit in a 19-inch rack enclosure that comprises laser, electronics and physics package subsystems. Recent results in which a local oscillator was disciplined by laser-cooled trapped171Yb+ions are presented. Preliminary measurements have demonstrated a fractional frequency instability of 1 × 10-13at 10,000 s. Potential applications include telecommunications and financial operations time reference systems as well as navigation system integrity.
Optical atomic clocks demonstrate a better stability and lower systematic uncertainty than the highest performance microwave atomic clocks. However, the best performing optical clocks have a large footprint in a laboratory environment and require specialist skills to maintain continuous operation. Growing and evolving needs across several sectors are increasing the demand for compact robust and portable devices at this capability level. In this paper we discuss the design of a physics package for a compact laser-cooled 88 Sr + optical clock that would, with further development, be suitable for space deployment. We review the design parameters to target a relative frequency uncertainty at the low parts in 10 18 with this system. We then explain the results of finite-element modelling to simulate the response of the ion trap and vacuum chamber to vibration, shock and thermal conditions expected during launch and space deployment. Additionally, an electrostatic model has been developed to investigate the relationship between the ion trap geometrical tolerances and the trapping efficiency. We present the results from these analyses that have led to the design of a more robust prototype ready for experimental testing.