All clocks are based on stable oscillators, whether they are grandfather clocks using pendulum oscillators, or sundial clocks that rely on the steady rotation of the earth. In an atomic clock, the steady oscillator is a laser, which is regulated using the quantum oscillations of atoms. Optical atomic clocks are the most precise clocks in existence today. The lasers in these clocks have frequencies of 100s of terahertz, meaning that these clocks tick about a quadrillion (one million trillion) times per second.
Precise and stable atomic clocks are interesting because they can be used to search for evidence of dark matter and dark energy. Such clocks can also be used to build a gravitational wave telescope. Gravitational waves passing through a region of space change the frequency of light waves traveling through the same region, although very slightly. If light is sent from an optical clock on one satellite to another optical clock on a nearby satellite, the clocks can detect this slight change in light frequency and sense the effect of the gravitational wave.
The most precise optical clocks that exist today are not ideal for this purpose, due to their size and complexity. Our group is working to develop an optical atomic clock that is both precise enough to detect gravitational waves, and compact and portable enough that it could be put aboard a satellite. Our clock design makes use of a “two-photon transition” in calcium atoms. While optical clocks can use other atoms, such as ytterbium, strontium, or aluminum, calcium is chosen because of a convenient atomic transition: in the energy jump that the calcium atoms make during this transition, they simultaneously absorb two packets of light. Without this special transition, the atoms would need to be tightly confined and cooled to a temperature near absolute zero in order to ensure that they are moving very slowly, and their frequencies do not get perturbed by the Doppler effect. Since two-photon transitions are not affected by Doppler shifts, calcium does not need to be cooled to extremely low temperatures, which simplifies the clock construction and makes it smaller.
In our clock, a stabilized clock laser is sent through the cloud of calcium atoms. As the frequency of the laser gets close to the atoms’ resonance frequency, the atoms absorb energy from the laser and jump to a higher energy level. By controlling the laser frequency so that it is always kept on resonance with the atoms, the laser's electromagnetic oscillations are synchronized with the quantum oscillations of the calcium atoms.