Portable optical atomic clock

Context (TH): A study recently published in the journal Nature introduced a kind of portable optical atomic clock that can be used onboard ships.

Inaccurate mechanical clocks

The previous generation of clocks consisting of a quartz crystal oscillator, was performing efficiently.
But the best of such oscillators would be late by a nanosecond after an hour of efficient performance.
So, if this clock were used to gauge the position of a spacecraft, it would be very prone to error.
To improve on this, atomic clocks were invented.

Atomic clocks contain an element like caesium (Cs-133) or calcium and microwave radiation source.
When excited by a microwave, the electrons of caesium or calcium can absorb some of the incident radiation and get excited to a higher state.
For this, microwave radiation has to match the characteristic frequency of the caesium or calcium atom.
Tuning the microwave source and observing at what frequency the transition takes place calculates the exact value of the characteristic frequency, which can be used to measure time accurately.
Atomic clocks are the backbone of the Global Positioning System (GPS). The International Committee for Weights and Measures first used it in 1967 to define the duration of one second.

Frequency refers to the number of waves that cross a particular point in time in one unit of time.

The official definition of a second is the frequency needed for electrons to transition between two levels in a caesium atom.

The accuracy of atomic clocks comes from a feedback mechanism that detects any changes in the resonance frequency and adjusts the microwave radiation to maintain resonance.
Thus, a caesium atomic clock loses or gains a second every 1.4 million years. More advanced NASA’s Deep Space Atomic Clock misses a second once in 10 million years.
India also uses a Cs-133 atomic clock to define the second for timekeeping within its borders.

Cs-133 is a highly stable atom and is naturally found. Hence it is commonly used in atomic clocks.

Optical atomic clocks are even more accurate. While they have the same working principle, the resonance frequency here is in the optical range.
Radiation in this range includes visible light (to humans) and ultraviolet and infrared radiation.
As part of an optical atomic clock, researchers use lasers to stimulate atomic transitions.
Laser light is highly coherent: the emitted light waves all have the same frequency, and their wavelengths are related to each other in a way that doesn’t change, generating light with more precise properties and great stability.

A higher operating frequency is able to measure smaller increments of time more accurately.
Further, optical atomic clocks have much narrower line widths. The narrower the linewidth, the easier it is to tune the frequency of the optical light that produces the resonance, leading to better accuracy.

The linewidth is the range of frequencies over which the transition occurs.

Optical atomic clocks use strontium (Sr), which has narrow line widths and stable optical transitions.

Traditional optical atomic clocks are large and not easy to transport.
The new clock’s spectrometer, laser system, and frequency comb are miniaturised for portability.
The new optical atomic clock uses molecular iodine as the frequency standard.
The researchers also equipped the clock with a software control system that could autonomously initialise the clock from an ‘off’ state to a fully operational state.
The optical atomic clocks also had 10x lower long-term drift compared to rubidium atomic clocks.
This means that over long periods, the rate at which the clock’s frequency changes is much lower compared to changes in rubidium atomic clocks.

A frequency comb is a device that generates a series of equally spaced optical frequencies, providing a stable and accurate reference.

Researchers developed a few prototypes, including VIPER (sea applications), PICKLES and EPIC.
Despite the ship’s motion, a temperature fluctuation of 2-3 degrees C, and 4-5% changes in humidity, the clocks were nearly as stable.

Atomic clocks are prized for their accuracy, losing or gaining just one second over 300 million years.
Optical atomic clocks only lose or gain a second over 300 billion years.
The new iodine clock isn’t as accurate as an optical atomic clock in the laboratory, trading it off for mobility and robustness. Loses or gains a second only every 9.1 million years.