The Hydrogen Maser

1. The Hydrogen Maser Almost The ideal clock. Almost…

2. Hydrogen MASER A hydrogen maser uses a population inversion in the hyperfine levels of the ground state of atomic hydrogen to amplify an input signal. MASER: Microwave Amplification by the Stimulated Emission of Radiation.

3. H Hyperfine Energy Levels High magnetic field to create polarization of the atomic state through a hexapole magnet (F,mF) (1,1) (1,0) (1,-1) (0,0) H mass= 1 amu Clock transition in the ground state (1,0)-(0,0) F=1 1.420 GHz H F=0 Energy levels converted in GHz

4. Types of Hydrogen maser • Active: the gain of an ensemble of polarized H atoms is high enough to start and maintain a strong oscillation. • Passive: the sharp amplification of the atomic ensemble (perhaps a few Hz wide at 1.4 GHz) is used as a frequency discriminator to lock an OCXO to the H resonance. A large number of H atoms can be used (up to 1012 per second) an so the passive H maser can be more stable in the short term than a cesium beam clock, although the H maser is subject to drift over periods of days and weeks.

5. Passive hydrogen maser

6. Passive hydrogen maser • Small package, about twice the size of a high performance caesium clock. • About the same price as a high performance caesium clock. • Better stability on short term, up to a few days • Long term stability limited by frequency drift. • Runs continuously for 5-10 years until the pump elements are saturated.

7. Active hydrogen maser Note that there no need for external feedback. The gain of the atomic ensemble is high enough to start the maser operation.

8. Active hydrogen maser • Very expensive • Very stable on short term to a few weeks. • Subject to a low level of frequency drift of about 10-16 per day. • Runs continuously for years (typically until the pump elements are saturated, 5-10 years)

9. Noise characteristics of a passive and active atomic clock Physics package short term noise:

10. Noise of passive and active clocks Caesium clocks Passive H maser Active masers

11. The ideal atomic clock • Hydrogen is the most simple atom • The hyperfine frequency is 1.4 GHz • The signal strength is good Why is it not the atomic definition of the second?

12. Why not Hydrogen? • It is difficult to confine the hydrogen atoms without strong interactions: teflon coating is perturbing the atoms at each collision. • Chemical and physical properties of the walls of container or microwave cavity change with time. The maser frequency will then drift.

13. Cavity tuning • The microwave cavity frequency will also change with time. • Auto tuning methods can compensate for this, often at the expense of the short term stability.

14. Hydrogen masers for VLBI • Hydrogen masers are very useful in Very Long Base Interferometry for radio-telescope. • Their low noise characteristics allow deconvolution of the recorded signals from the antennas maintaining the phase information from each station.

15. Hydrogen masers in time scales • The most performing time scales are using hydrogen masers to get the best short term stability: NIST, USNO. • To evaluate caesium fountains and optical clocks, hydrogen masers are the only way to keep a good time scale with excellent short term and long term stability.

16. Hydrogen masers in measurement • One hydrogen maser is enough to help local operation. • better noise measurement in the laboratory. • better GPS measurements.

17. Short Term Stability Ranges of Various Frequency Standards -9 Quartz -10 -11 Rubidium Log (y()) -12 -13 Cesium -14 Cesium fountain -15 Hydrogen Maser -16 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Log (), seconds 1 day 1 month

18. Conclusion • Hydrogen masers are expensive • Hydrogen masers are very useful for all low noise operation in a laboratory. • Even in the era of caesium fountains and optical clocks, they are needed to bridge the gaps.