"In April 1831, a brigade of soldiers marched in step across England's Broughton Suspension Bridge. According to accounts of the time, the bridge broke apart beneath the soldiers, throwing dozens of men into the water.

After this happened, the British Army reportedly sent new orders: Soldiers crossing a long bridge must "break stride," or not march in unison, to stop such a situation from occurring again.

Structures like bridges and buildings, although they appear to be solid and immovable, have a natural frequency of vibration within them. A force that's applied to an object at the same frequency as the object's natural frequency will amplify the vibration of the object in an occurrence called mechanical resonance.

If soldiers march in unison across the structure, they apply a force at the frequency of their step.

If their frequency is closely matched to the bridge's frequency, the soldiers' rhythmic marching will amplify the vibrational frequency of the bridge. If the mechanical resonance is strong enough, the bridge can vibrate until it collapses from the movement''.

"Time keeping before atomic clocks

I will not go into a very long history of time keeping but will surely try to put it in some other post. So lets take a step backward and start our journey of keeping time using a Quartz Crystal.

Let me tell you in brief how it worked. Quartz Crystal has a special property - it is piezoelectric, which means that:

1. When you apply pressure to it it generates a tiny electric current.
2. When you pass electricity through it, it vibrates at a precise frequency.

Because of this piezoelectric property quartz is widely used in building clocks. It vibrates at a precise frequency of 32,768 Hz, so by applying a counter we can precisely know when time lapsed by 1 second. But there is a big drawback of this…

Issue with battery operated Quartz Crystal

As quartz crystal vibrates, it loses energy and it slows down and loses time. Hence it requires an electric pulse to reconfigure its vibrations again at its natural frequency. Can we solve this? Yes!

 A FEEDBACK LOOP.

But we need to create this feedback loop very very accurately. Atomic clocks do this very neatly and with amazing accuracy. Lets see how.

Lets get some basics right

Before we start digging into the working principles of an Atomic clock and eventually clearing our minds of myths, we first need to address some basics and some fundamental concepts

Atomic structure of Cesium

Atomic number of cesium is 55, hence it has in all 55 electrons distributed in 6 orbits. Distribution of electrons in shells is something like this 2-8-18-18-8-1, having 1 electron in its outermost shell.

Hyperfine interaction

The interaction between nucleus and its surrounding environment is known as hyperfine interaction. The magnitude of these interactions are very very small but enough to shift energy levels.

Consider the representation above, the interaction between nucleus and electron is called a hyperfine interaction. This interaction is a combination of the following

1. electrostatic force of attraction,
2. magnetic field of spinning nucleus and
3. gravity pull between the two.

The energies of the hyperfine interaction occur in the radio and microwave region. The resultant force acts as a spring with a minute amount of tension and hence the electron, while moving around the nucleus, oscillates slightly.

Resonant frequency of atoms

Every atomic system or molecular system has a resonant frequency. In simple terms, when a electromagnetic radiation is incident on a atomic or molecular system it interacts with the system in certain ways. The radiation consists in waves and each wave has a frequency. If the frequency of the radiation matches with the resonant frequency of the system, then the system vibrates with maximum amplitude and the energy is absorbed by the atoms and molecules of the system.

This oscillatory behavior is used to explain for various phenomenon like light absorption, light dispersion and radiation and is responsible for the change of energy states in an atom. You can read more about resonance here.

Special case of cesium atom

Cesium atoms can be in two energy states: Low energy states and high energy states. In the case of the cesium atom, the resonant frequency is 9,192,631,770 cycles per second, so if radiation of the same frequency is applied then the atom will resonate and start moving from one energy state into another.

Working

Basics done! let’s jump into the workings of it.

An atomic clock consists of a traditional Quartz Crystal which is used for marking mechanical pulses. The main advancement required was in creating a feedback loop across a Quartz Crystal so that whenever the crystal loses it energy it will be shot by an electric pulse so that the crystal regains its energy and thus always maintains a period.

Properties of Cesium atoms we care about:

1. Cesium atoms can be present in two states: Low energy and High energy.
2. They can be separated by applying a magnetic field.
3. Low energy cesium atoms can be converted into high energy by applying external radiation.

First, the cesium chloride is heated in an oven, this creates a gaseous stream of cesium ions. This stream contains both the low and high energy ions. A magnetic filter is applied to this stream. The application of a magnetic field to this stream bifurcates the it into two sub-streams. One containing low energy cesium and other containing high energy cesium.

Now the low energy cesium stream is passed through a radiation chamber in which it is bombarded with radiation with a frequency 9,192,631,770 cycles per second. This will make the low energy cesium to go into high energy state.

Emerging from the radiation chamber we have a stream of high energy cesium ions; but not all are converted to high energy state if there is an offset in radiation frequency. Hence to know how many cesium atoms jumped into high energy state another magnetic filter is applied. Hence we deduce that, the closer the radiation frequency is to the resonant frequency of cesium, the higher the conversion rate of low energy cesium to high energy cesium will be.

The stream is again divided into two and now the sub-stream of high energy cesium ions is redirected to a detector.

Now comes the catch: The purpose of the detector is to convert high energy cesium ions into electricity. The higher the number of ions incident on it the higher the current generated will be .

This current is passed to Quartz Oscillator with which we mark one pulse. This oscillator is responsible for controlling wavelength (and in turn the frequency) of incident radiation.

Suppose the quartz oscillator loses its energy. As soon as this happens, the radiation in the chamber weakens. Hence the number of cesium atoms jumping into high a energy state drops. This tells another electric circuit to zap the oscillator and correct the period of oscillation. Thus locking its frequency in a very narrow range. This locked frequency is then divided by 9,192,631,770 which results in the familiar one pulse per second.

Now we see how we overcame the drawback of traditional Quartz Clocks and created a much much more accurate atomic clock which loses 1 second in 138 million years''.