A clock is something that ticks. A good clock ticks at a very stable rate. To get a clock to tick at a stable rate, it is slaved to a natural frequency. For example, sundial clocks rely on the steady rotation of the earth; grandfather clocks rely on the regular oscillations of a pendulum; quartz clocks use the natural vibrations of quartz crystals.
The stability of an optical atomic clock relies on the quantum oscillations within atoms. The thing that “ticks” is a laser, which is an oscillating electromagnetic field – this is the “optical” part of optical atomic clocks. The atoms absorb photons from the laser beam in order to jump from one energy level to another. Only when the frequency of the laser is tuned just right will the atoms be able to absorb the photons. This is how the laser frequency is slaved to the natural atomic frequency.
What’s so special about optical atomic clocks? These natural oscillations within atoms occur at frequencies in the 100s of terahertz, meaning that these clocks tick about a quadrillion (one million billion) times per second! This is what makes these the most precise clocks in existence today. These clocks only gain or lose a second once in 10 billions years!
What does anyone need all this precision for? 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. They can also be used to perform precise surveys of the earth’s gravitational field, yielding new insight into what lies below the surface of the earth and providing input into models used to study and forecast the effects of climate change.
The most precise optical clocks that exist today are not ideal for field applications, due to their size and complexity. We’re working to develop an optical atomic clock that is both precise enough for these exciting applications, and compact and portable enough that it could be put on a satellite or transported from location to location.
Our clock design makes use of a “two-photon transition”: in the energy jump that the atoms make, they simultaneously absorb two photons. This is important because usually when an atom absorbs a photon it gets physically kicked because the photon has momentum. This kick can mess up the clock, but if the atom absorbs two photons from two opposite directions, the kicks cancel out! Clocks that don’t use a special two-photon transition have to use complicated (and bulky) methods to correct for the kick. Since our clock doesn’t have a kick problem, we can build it in a smaller, simpler way – small enough to take into the field, put aboard a stratospheric balloon, and even put on a satellite!