We are developing optical atomic clocks based on two-photon transitions, for ultimate use in gravitational wave detectors.
Any good clock requires two parts: Something that oscillates or “ticks”, such as the pendulum on a grandfather clock, and something to count the oscillations. The faster that a clock ticks, the more precise it is – as long as the ticks can still be accurately counted. Clocks of very high precision are of interest because they can be used to detect gravitational waves.
Optical atomic clocks are the most precise clocks in existence today. The oscillations of the clock are provided by an electron jumping from one energy level to another and back, each time absorbing and releasing electromagnetic waves with a very precise frequency. These frequencies corresponding to the optical part of the spectrum -- the frequencies are 100’s of teraHertz, meaning that these clocks tick almost a quadrillion (one million trillion) times per second. Frequencies of this magnitude can only be measured with a laser. Lasers produce light waves that oscillate at similar frequencies.
A gravitational wave passing through a region of space changes the frequency of light waves traveling through the same region 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 in the energy jump that the calcium atoms make, they actually absorb two packets of light instead of one. 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 extreme temperatures, which simplifies the clock construction and makes it smaller.
A “clock” laser is then sent through the group of atoms. A laser that has a tunable frequency is used to count the ticks. The laser beam is sent through the calcium and as the frequency of the laser gets closer and closer to the calcium atoms’ transition frequency, the atoms absorb energy from the packets of light and jump to a higher energy level. By precisely measuring the laser frequency when the atoms are maximally excited, the ticks of the clock are counted.