Ultralow shot noise limited giant passive resonant gyroscope for Earth rotation measurement

Abstract: Optical gyroscopes directly measure the Earth's rotation and are promising instruments for real-time geophysical observations and Earth orientation parameter (EOP) determination requiring both high precision and high temporal resolution. Large-scale ring laser gyroscopes (RLGs) currently reach rotational resolutions around $10^{-11}\,\mathrm{(rad/s)/\sqrt{Hz}}$, but their quantum noise limits make it challenging to meet the requirements of future high-temporal-resolution EOP measurements. Passive resonant gyroscopes (PRGs), on the other hand, offer a potentially lower photon shot noise limit and more flexible power scaling, even if their demonstrated rotational resolutions are still about two orders of magnitude below those of leading RLGs. Here we demonstrate a $64\,\mathrm{m^{2}}$ giant passive resonant gyroscope HUST-2, and develop with an extremely low shot noise level. We experimentally obtain a shot noise limited of $5.7(1)\times10^{-13}\,\mathrm{(rad/s)/\sqrt{Hz}}$ at $1\,\mathrm{mW}$ incident optical power, following the characteristic $1/\sqrt{P}$ scaling. Through systematic suppression of dominant technical noise sources, HUST-2 further achieves a measured rotational resolution of $3\times10^{-11}\,\mathrm{(rad/s)/\sqrt{Hz}}$, bringing PRGs into the performance regime of leading large-scale RLGs for the first time. The gap between the present demonstrated rotational resolution and the shot noise limit indicates nearly two orders of magnitude further improvement potential. Reaching this limit would enable high-precision length-of-day (LOD) measurements with $10$-$100\,\mathrm{s}$ temporal resolution and lays the foundation for future large-scale gyroscope networks dedicated to real-time EOP determination.

• The paper introduces the HUST-2 PRG leveraging a 64 m² square-cavity with bidirectional interrogation to attain a shot-noise-limited sensitivity of 5.7×10⁻¹³ rad/s/√Hz.
• It implements double Pound-Drever-Hall stabilization and extensive noise suppression techniques that drastically reduce technical noise and environmental impacts.
• The achieved resolution of 3×10⁻¹¹ rad/s/√Hz and robust noise budgeting pave the way for real-time Earth rotation monitoring and advanced geophysical applications.

Ultralow Shot Noise Limited Giant Passive Resonant Gyroscope for Earth Rotation Measurement

Introduction and Context

The paper presents the HUST-2 giant passive resonant gyroscope (PRG), a $64\,\mathrm{m}^2$ square-cavity instrument designed for precision Earth rotation measurement. This work directly addresses the limitations inherent in both Earth Orientation Parameter (EOP) monitoring infrastructure and the current generation of large-scale ring laser gyroscopes (RLGs). RLGs underpin high-fidelity EOP sensing but are bounded by quantum noise and active gain medium constraints, which prevent scaling to the ultralow rotational resolutions required for real-time, sub-milliseconds LOD monitoring. PRGs, decoupling the gain medium from the sensing cavity, exhibit fundamentally lower photon shot noise limits and enhanced power-scaling flexibility, but previously demonstrated systems have not achieved competitive absolute sensitivity due to technical noise dominance.

HUST-2 Architecture and Shot Noise Limit

HUST-2 features an $8\,\mathrm{m}\times8\,\mathrm{m}$ vacuum ring cavity interrogated bidirectionally by a $532\,\mathrm{nm}$ injection-locked external laser. The cavity is constructed with ultralow-loss mirrors ($Q = 6.4\times10^{12}$) and is sited underground for maximal vibrational, acoustic, and thermal shielding.

A critical contribution is the comprehensive shot noise model for the double Pound-Drever-Hall (PDH) stabilization chains that independently lock the CW and CCW modes to the cavity. The authors derive and validate the scaling of the rotation-equivalent shot noise floor as $$\sqrt{S_\Omega^{\mathrm{sn}}} \propto (1/\sqrt{P_{\mathrm{in}}})$$, demonstrating experimentally a shot-noise-limited sensitivity of $$5.7(1)\times10^{-13}\,\mathrm{rad/s}/\sqrt{\mathrm{Hz}}$$ at $1\,\mathrm{mW}$ injected power. This fundamentally low noise floor exceeds the performance of all contemporary PRGs and projects nearly two orders of magnitude beneath the best RLGs, such as the G-ring and ROMY.

Technical Noise Suppression and Achieved Resolution

Achieving this sensitivity presupposes aggressive suppression and quantitative modeling of technical noise sources. HUST-2's design incorporates:

• Extensive underground siting, multi-layer vacuum/thermal shielding, and independent vibration isolation for each mirror chamber.
• Reciprocal geometric stabilization with single-millimeter cavity deviation minimization and a parts-per-billion Sagnac scale factor error.
• Large PDH error slopes and optimized optoelectronics for minimum conversion of voltage noise to frequency error.

Methodical technical noise subtraction procedures separate electronic noise, residual amplitude modulation (RAM), and backscattering, enabling precise extraction of the optical shot noise component from the error signal. After subtraction, the non-shot-noise floor is dominated at sub-Hz frequencies by environmental cavity length fluctuations and RAM, with direct accelerometric and tiltmeter correlation confirming non-instrumental microseismic resonance features.

The experimentally demonstrated rotation resolution is $3\times10^{-11}\,\mathrm{rad/s}/\sqrt{\mathrm{Hz}}$, matching the performance echelon of contemporary large-scale active RLGs for the first time in a PRG architecture. The authors' quantitative noise budget shows their total measured noise is in substantive agreement with the sum of identified technical and environmental sources.

Implications for High-Precision Earth Rotation and Fundamental Physics

Reaching and quantifying the shot noise floor with the HUST-2 system marks a pivotal advancement in PRG technology, directly challenging the prevailing RLG dominance in ultrastable rotation sensing (2606.06822). The combination of validated power-scaling, noise budgeting, and performance benchmarking demonstrates a clear technical path to $10^{-13}\,\mathrm{rad/s}/\sqrt{\mathrm{Hz}}$ regimes, and eventual extension toward $10^{-14}\,\mathrm{rad/s}/\sqrt{\mathrm{Hz}}$ with marginal incremental improvements (e.g., further RAM suppression, active cavity length stabilization, environment decorrelation). Given that sub-centimeter GNSS orbit determination and sub-0.1 ms LOD fluctuation detection require these sensitivities at timescales of $8\,\mathrm{m}\times8\,\mathrm{m}$0–$8\,\mathrm{m}\times8\,\mathrm{m}$1, HUST-2's shot noise-limited operation would enable not only real-time EOP and LOD monitoring, but also new platforms for terrestrial tests of relativistic rotational phenomena—e.g., high-frequency general relativistic geodetic effects [schreiber_how_2011], and emerging geophysical events such as microseismic and polar motion oscillations [igel_romy_2021, schreiber_variations_2023].

Furthermore, scaling up PRGs and integrating them into orthogonally oriented or geographically distributed networks would support full vectorial determination of Earth rotation, enabling independent, continuous augmentation or replacement of VLBI-dependent EOP infrastructure. HUST-2 and its counterpart at Sun Yat-sen University constitute the experimental foundation for such future arrays.

Conclusion

HUST-2 establishes a new PRG performance regime, with noise characterization, mitigation, and validation methodologies sufficient to approach the photon shot noise limit at $8\,\mathrm{m}\times8\,\mathrm{m}$2, thus rivaling and soon surpassing traditional RLGs for the first time. The implications extend from immediate geodetic and EOP applications to precision tests of fundamental physics and next-generation rotational seismology. Future developments in active suppression of technical noise, advanced cavity feedback, and multi-instrument network integration are projected to cement PRGs as a central pillar in Earth rotation metrology and experimental gravitation.