Passive Hydrogen Maser (PHM)

Author: Tianjiang Says

Reference: Li Y et al. 2026 Chin. Phys. Lett. 43 031101

Website: https://cislunarspace.cn

Definition

A Passive Hydrogen Maser (PHM) is a high-precision atomic frequency standard that uses the hydrogen atom hyperfine transition (~1.42 GHz, corresponding to the 21 cm line) as a reference. Unlike active hydrogen masers (which are self-oscillating), PHMs use an external signal source to lock their oscillation frequency, offering better miniaturization potential for space applications.

Working Principle

The core of a PHM is a quartz bulb (storage bulb) coated with PTFE inner walls for storing hydrogen gas. The basic workflow:

  1. Hydrogen preparation: Hydrogen molecules are dissociated into high-energy hydrogen atoms via radio frequency discharge
  2. State selection: A magnetic gradient selectively directs high-energy hydrogen atoms (F=1,mF=0F=1, m_F=0) into the storage bulb
  3. Stimulated emission: Hydrogen atoms in the storage bulb interact with the microwave cavity, producing stimulated emission
  4. Frequency locking: The atomic transition signal in the storage bulb is detected and used to lock an external oscillator via a phase-locked loop (PLL)

Key performance specifications:

  • Frequency accuracy: Space-grade PHM can reach 5×10−125 \times 10^{-12}
  • Frequency stability: ADEV at 1000s averaging time can reach 10−1410^{-14}
  • Size/weight: DRO-A satellite PHM weighs only 8.5 kg, with dimensions ~39 cm

Space Environmental Sensitivity

Space environment effects on PHM include:

Environmental FactorSensitivityImpact
Temperature<2×10−14/∘C< 2 \times 10^{-14}/^\circ CFrequency drift < 2×10−142 \times 10^{-14} for ±1°C variation
Magnetic field2×10−13/G2 \times 10^{-13}/GFrequency drift < 1×10−141 \times 10^{-14} for <0.05 G variation
Cosmic radiation-Requires shielding protection

Application in DRO-A Satellite

The April 2025 DRO-A gravitational redshift experiment was the world's first deployment of a PHM in a lunar DRO for fundamental physics experiments. The experiment validated in-orbit performance of the compact space PHM:

  • Frequency accuracy: ~ 5×10−125 \times 10^{-12}
  • ADEV at 1000s: ~ 4.5×10−144.5 \times 10^{-14}
  • ADEV at 10000s: ~ 1.5×10−141.5 \times 10^{-14}

Results show space PHM performance is maintained within 10−1410^{-14} level under space environment effects.

Relation to Gravitational Redshift Measurement

Gravitational redshift measurement precision is limited by clock frequency accuracy rather than stability. In the DRO-A experiment:

  • Clock stability (10−1410^{-14} level) outperforms accuracy (10−1210^{-12} level) by two orders of magnitude
  • Therefore, accuracy—rather than stability—is the primary limiting factor
  • Achieving higher precision gravitational redshift tests (e.g., 10−610^{-6} level) requires clocks with 10−1610^{-16} accuracy

References

  • Li Y, Liu T et al. 2026 Chin. Phys. Lett. 43 031101
  • Vessot R F C et al. 1980 Phys. Rev. Lett. 45 2081
  • Cacciapuoti L et al. 2007 Nucl. Phys. B Proc. Suppl. 166 303