Cryogenic sapphire oscillator: the world’s most precise clock

Over the past decade or more I have been involved in the development of the world’s most precise ‘clock’—the cryogenic sapphire oscillator (CSO). This is work that was pioneered by a group at the University of Western Australia and had been underway sometime when I joined them in 1996 to start a PhD in physics.

Sapphire small

Figure 1: Single crystal sapphire used as the frequency determining element of the ‘clock’.

DSCF2787

Figure 2: Liquid helium dewar based CSO at the French Space Agency (CNES) lab in Toulouse France as part of the preparations for the ESA Atomic Clock Ensemble in Space mission, due for launch in 2016.

I built several of these oscillators which used a large dewar of liquid helium to cool the single crystal of sapphire. (See Fig. 1) I worked with two Japanese government labs to develop these oscillators for their atomic clock programs. The CSO acts as a flywheel oscillator, which means it keeps the precise timing needed during the interrogation cycles of the atomic clock which might be about a 1 second period. I also worked with the European Space Agency, which used one of our CSO’s to ground test their PHARAO cold-atom clock that is now planned to be launched into space to be stationed on the International Space Station. See Fig. 2.

In about 2008 I started to look at introducing an ultra-low vibration pulse-tube cryocooler as the refrigeration source, doing away with the need for regular refills of liquid helium. The cryocooler uses a closed cycle system and consumes very little helium gas.

Cryostat

Figure 3: Cryostat that houses the cryogenic crystal.

Figure 3 shows the cryostat design. A litre of liquid helium is constant reliquified, which becomes a cold finger to keep the sapphire crystal cold, at about 6 K, or -267 degrees C. When the crystal is so cold everything is frozen and since the dimensions of the crystal sets the precision with which the clock ‘ticks’ this makes for a good clock. The resonance line width is about 10 Hz at 11.2 GHz. For the engineers this means a Q-factor of about 1 billion. The Q-factor sets the quality of the resonance. That is something like being equivalent to hitting a bell and it ringing for a million years.

My published results show that this ‘clock’ is the most precise in the universe. That means the stability of the signal frequency. It is analogous to an old grandfather clock with a pendulum arm 1000 kilometres long.  That provides an extremely regular tick tock. My clock would gain or lose a second in about 100 million years. That is more accurately expressed as a stability of 6 x10-16 at 1 s of averaging. Note: We do not call this type of clock accurate, as accuracy is defined according to a certain transition in a cesium atom, therefore only an atomic clock can be accurate.

Below in figs 4 and 5 are pictures of the new oscillators I have built in my lab at the University of Adelaide. I moved here in March of 2013 from the University of Western Australia.

Picture1 lab

Figure 5: 2013, University of Adelaide lab where I have built 3 new CSOs using cryocoolers.

Picture2 lab

Figure 4: A cryocooler is used to cool the sapphire in the new version of the CSO.

 

2010 W.G. Cady Award

For this work I was announced as the winner of the 2010 W.G. Cady award by IEEE Ultrasonics, Ferroelectrics and Frequency Control Society.

The W.G. Cady Award recognizes outstanding contributions in the fields of piezoelectric or other classical frequency control, selection and measurement and resonant sensor devices.

certThe citation reads: “for the construction of ultra-stable cryogenic sapphire dielectric resonator oscillators and promotion of their applications in the fields of frequency metrology and radio-astronomy.

The award was presented at the 2010 IEEE International Frequency Control Symposium at Newport Beach, California in June 2010.

See also It’s About Time

References

  1. C. Wang, J.G. Hartnett, “A Vibration Free Cryostat Using Pulse Tube Cryocooler,” Cryogenics, 50, 336-341, 2010
  2. J.G. Hartnett, N.R. Nand, C. Wang, J-M. Le Floch, “Cryogenic Sapphire Oscillator using a low-vibration design pulse-tube cryocooler: First results,” IEEE Trans on UFFC, 57, 5, 1034-1038, 2010; also available at http://arxiv.org/pdf/1004.0488v1.pdf
  3. J.G. Hartnett, N.R. Nand, “Ultra-low vibration pulse-tube cryocooler stabilized cryogenic sapphire oscillator with 10-16 fractional frequency stability,” IEEE Trans on MTT, 58, 12, 3580-3586, 2010; also available at http://arxiv.org/pdf/1004.2886v2.pdf
  4. N.R. Nand, J.G. Hartnett, E.N. Ivanov, G. Santarelli,“Ultra-stable very-low-phase-noise signal source for Very Long Baseline Interferometry using a cryocooled sapphire oscillator,” IEEE Trans on MTT, 59, 11, 2978 – 2986, 2011
  5. J.G. Hartnett, N.R. Nand and C. Lu, “Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators,” Appl. Phys. Lett., 100, 183501, 2012; also available at http://arxiv.org/pdf/1202.2206v2.pdf
  6. J.G. Hartnett, S.R. Parker, E.N. Ivanov, T. Povey, N.R. Nand and J.-M. le Floch, Radio frequency signals synthesized from independent cryogenic sapphire oscillators, Electronics Letters 50(4): 294-295, 2014

For additional references see my CV.