Cryogenic Hydrogen Maser

We have developed a cryogenic hydrogen maser (CHM) that operates at 0.5 kelvin, and employs superfluid helium-coated walls to store the masing hydrogen atoms. See Figs. 1 and 2. The CHM may provide frequency stability that is one to three orders of magnitude better than a room temperature hydrogen maser because of greatly reduced thermal noise and larger signal power. Such exceptional frequency stability will be required for spacecraft tracking in future deep-space missions, for space-based tests of relativity and gravitation, and for local (i.e., flywheel) oscillators used with "next-generation" absolute frequency standards such as the laser-cooled cesium fountain and the linear ion trap. These new devices, which are under development at NIST and other laboratories, are passive high-resolution frequency discriminators. Alone, they cannot function as superior atomic clocks; their effective operation depends critically on being integrated with an active local oscillator with excellent short term stability--such as that expected from the CHM.

Fig. 1. SAO cryogenic hydrogen maser (CHM) operated inside a 3He refrigerator.

Fig. 2. Schematic diagram of the SAO cryogenic hydrogen maser.

To date, we have measured the CHM's frequency stability (Allan deviation) relative to a room temperature hydrogen maser to be approximately 1 × 10­13/t1/2, for averaging intervals, t, in the range of 10 sec < t < 300 sec. For intervals longer than a few minutes, the CHM's frequency stability is not yet as good as that of a room temperature hydrogen maser, because of large wall frequency shifts to the hydrogen hyperfine transition due to the sapphire substrate lying beneath the superfluid helium film. A typical set of data comparing CHM and room temperature hydrogen maser frequency stability is shown in Figure 3, along with the calculated limit to CHM performance set by thermal noise.

Fig. 3. Typical measured CHM and room temperature hydrogen maser frequency stabilities.

We are currently installing a thin-walled quartz bulb inside the sapphire resonant cavity to serve as the atomic hydrogen storage chamber, and thus to eliminate the large wall frequency shift that limits long-term CHM frequency stability. We are also installing a low temperature preamplifier to improve the CHM's short-term frequency stability to the thermal limit shown in Fig. 3.

In parallel we are using the CHM to study important effects in low temperature atomic physics, e.g., hyperfine-induced hydrogen-hydrogen spin-exchange collisions. These collisions depend sensitively upon: (i) details of the hydrogen-hydrogen interaction potential at long range that are otherwise experimentally inaccessible; (ii) non-adiabatic (i.e. non-Born-Oppenheimer) effects particular to cold atomic collisions; and (iii) inclusion of the intra-atomic hyperfine interaction (strength ~ 0.07 K) in addition to the electron exchange interaction (strength ~ 50,000 K), which fundamentally alters the rotational symmetry of the hydrogen-hydrogen collisional process. Furthermore, the calculated values for cold hydrogen-hydrogen collision cross-sections are sensitive to the theoretical techniques used in the calculations [9-11], making experimental comparisons particularly significant.