This post is a bit comprehensive.

In this article, I hope to hint at the Nobel Prize winning works of Arthur Ashkin, Claude Tannouji etc.

First off, Temperature of a body is the measure of Energy of a substance. For an atomic gas, it effectively measure the average energy of the atoms in the gas. So, cooling down atoms means slowing them down.

In macroscopic world, the act of slowing down the object seems rather intuitive. We need to offer some sort of resistance to the moving object in order to slow it down. Magnitude of resistance is proportional to the size of the object, its velocity and its acceleration.

How do we slow down atoms ? They are really small that, if we place a barricade in front of them, they hit it and fly back with same kinetic energy. We have not slowed it down, rather we have changed its trajectory.

This issue is not straightforward, I suppose. How would we deal with them if they were ions ? Then, we have another force in play, the Electro-magnetic force. We can apply an slowly varying Electric field opposite to the motion of a positive charge to slow it down. This is quite simple to do. But we can do a lot better with Magnetic field.

A single current carrying circular coil produces Magnetic field lines perpendicular to the plane of the coil that go through the coil. Current and the nature of field lines can be detected using Right Hand Thumb rule. Wrap your fingers around the direction of current in the coil and your thumb is the direction of field lines.

Now, what would happen if I place another loop parallel to the original loop but carrying current in the opposite direction ? One loop would give us Right moving lines and the other would give us Left moving lines. Exactly at the centre the field would be zero. To the left of centre the force would be towards right and vice versa. Such a configuration is known as anti-helmholtz configuration of coils.

Do you see what happens here ?

Any charge in the centre would have to remain in the centre. Small movement to the either side would result in a simple harmonic motion. Let me explain. A charge in the centre moves to right, the field in right force it to move back. But it overshoots the centre mark and again is forced to move back to centre by field on the left. It will oscillate continuously and will eventually end up in the centre.

Well, an atom has no charge, so we cannot implement this procedure right ? Not quite so. But before that we should talk about Laser cooling.

Laser cooling

Let’s entertain this idea in clear, naive semi-classical terms. Say an atom is moving towards x-axis with some momentum and Kinetic energy. Let a photon, moving opposite to atom with a different momentum collide with the atom. Unlike classical collision where they will bounce off of each other and move in trajectories different from their previous one, here the atom can observe the photon if the photon frequency matches an energy transition in the atom.

Now, the atom effectively slows down, because the photon momentum, once it’s been absorbed reduces the total momentum of the atom. The excited atom then emits a photon in a random direction and this process is repeated over and over again. The initial momentum and its direction is wiped out completely. The result is that the atom is effectively slowed down.

You would have seen laser being used to energize materials. Superman uses his laser vision to burn things down. It is amazing that laser can be used to cool down things, isn’t it !

This is the fundamental idea of Laser trapping. There are some physical concepts like photon red-shift and blue-shift frequencies and Kinetic gas distribution which are at play here. But the essence remains the same.

We have developed Optical tweezers which can hold atoms. The principle remains the same. We use two intersecting laser cross-sections. The atom received force from the lasers which hold it in place.

If I have a randomly diffusing cloud of gas molecules, particles will be emerging out at all directions. To combat that, we can make a 6 laser combination. A laser going towards x-axis and one towards -x axis. Similarly for y and z axes.

This is known as an Optical trap.

The next step would be to include Magnetic fields. But atoms are neutral. Doesn’t matter!

Let’s solicit some help from Dr.Zeeman. He discovered that light from atoms placed in a Magnetic field shows finer splitting. This means that, electrons in the atoms with their angular momentum form a current loop like object with a magnetic dipole moment which interacts with the external field and produces more energy levels varying with the angular momenta.

If we place an atom in the middle of a Anti-helmholtz configuration with 6 laser combination, the Magnetic field actually aides in the absorption of photon.

How does this work ?

A stationary atom absorbs photon which has a frequency value that matches its energy. However, a moving atom experiences a Doppler shift. It can no longer absorb the original photon because of this effect.

Remember the train analogy, train moving towards you will have a horn that has a shriller tone and the train moving away from you will have a horn that will have a plainer tone.

A photon which is moving towards you will seem to have higher frequency. This is called as Blue-shift. A photon away from you, Red-shift. Remember light has a spectrum of frequency. If we tune the light to have the appropriate frequency, we can make the moving atoms absorb too. This is what the magnetic field does. In addition to experimentally tuned laser frequency, the magnetic field increases your energy accordingly to facilitate the photon absorption.

The magnetic field produced in the Anti-helmholtz configuration is zero at the center and gets stronger at the ends. Consequently, the atoms at the end get a greater kick than the one at the middle.

This is the concept of Magneto-Optic Traps. When atoms come out of an oven they are few hundred kelvins hot and are moving at speeds of Hundreds of meters per second. After the laser cooling, their speeds are around few meters per second.

After the MOT, their speeds are few centimetres per second. Their temperature would be around few micro-kelvins.
If you want to cool them down further, you have to do what is called as evaporative cooling. It is basically letting hotter, more energetic ones escape and keeping the rest of them. This takes them down to few Nano-Kelvins.

What’s the use of all this?

When the atoms have been cooled to such temperatures, they show a lot of interesting quantum properties. These properties won’t show up at higher temperatures because the thermal fluctuations will be so rapid that they will kill any such emergent behaviour.

Super cooled atoms exhibit super-conductivity, long range correlation etc.

Their most basic quantum properties like spin and correlation emerge and remain untainted and viable for experiments. Single minute temperature shock will kill all these properties.

Quantum computing is done only by measuring and changing all these properties in a controlled fashion. That’s why the Quantum Computers all need super cold, vibration damped, safe and secure environment.

Presently, we have a Quantum Computer that utilizes 50 qubits to perform computations. We have seen in media that Quantum computers can break all internet security protocols because of its nature of operation. Traditional computers use 1, 0 states to perform operations. QC can use any real combination of the 1 and 0 states to perform operations.

Take one of those path finding puzzles. We can only pick one path at a time to check if that path actually solves the puzzle. A QC can take a linear combination of paths to find out the solutions. QC will be a lot faster than traditional computers in problems like these.

But we have nowhere reached the levels to break internet security protocols. We don’t have that much of Quantum computational power yet. You might say that we can add more atoms to increase computational power, right ?

Do you see why such idea would be wrong?

If we add more atoms to a cloud of correlated-cold atoms, we would increasing the agitations between the atoms and the system will get excited and the correlations will become unstable.

To make bigger QC, we need better ultra-cold atomic processes to synthesise and control these atoms.

Super-cooled bosonic atoms for Bose-Einstein Condensate. BEC exhibits super-fluidity. Super cooled fermionic atoms also show super-fluidity that arises due to a totally different mechanism. In fact, I might go ahead say that super-cooled metals allow electronic pairing due to the cooling down. In that case, super-cooled electrons flow without any resistance. i.e. the metal is super-conducting now.

These ultra cold atoms are the main components of Atomic clock accuracies and precisions.