Droplet’s Adventure in Quantumland

Note: This post is an attempt at describing part of my PhD work to the public. Now that I am close to finishing my PhD, I think such a post was long due. This essay was also my submission to 'AWSAR 2019'.

Pitter patter, pitter patter, listen to the rain.
Pitter patter, pitter patter, on the window pane.

Oh, you’ve all seen the beautiful raindrops sliding against the glass window panes and, out comes the mobile phones from our pockets! Click-click, from this angle; snap-snap, this one will go to Instagram; and definitely don’t forget the lone droplet about to fall from the window frame. Increase the brightness, tweak the contrast and voila! ‘50 likes’ and ’10 comments’ later, soon forgotten are the droplets and cursed is the rain for daring to trap us indoors.

But scientists can’t forget. We fixate, dissect, calculate until everything there is to be known are known and then some! Those beautiful and round rain droplets are no exception. So next time your nerdy little cousin catches you off-guard asking, ‘How do the droplets form?’, you can be ready.

Let’s take a single water droplet. It obviously has more molecules in it than a number you can imagine[1]. These water molecules like to remain closer to other water molecules or in other words, they attract each other. Most of them are surrounded by each other, cocooned in their watery environment. But there’re those water molecules which are at the surface of the droplets, always being touched by the air or precisely, the air molecules. Even the air molecules and water molecules would like to remain close. There emerges a continuous tug-of-war between the inner water molecules and the outer air molecules to attract the surface water molecule towards them. Nonetheless, there’s the one winner- the inner water molecules, which hold its surface brethren with much more strength and love than the foreign air molecules. So, the surface water molecules behave as if a thin elastic cover holding together the water droplet, accurately playing out the motto ‘United we stand, divided we fall’.

But wait, why are you only talking about water drops?
I am sure you have seen oil drops sticking to the cooking pan too.

What about the drop of blood oozing out on your finger after the pinch of a doctor’s needle?
Gory, but yes, they work the same way too.

How small can a drop become[2]?
How large[3]?
Are all drops liquid?
Can we have droplets of anything else?

Hold your horses!

Take a deep breath and prepare your mind to be blown away. Yes, there exists droplets of gas too! What? Yes. How? Strongly dipolar bosonic gases in ultracold temperature can form quantum droplets. Wow. I didn’t understand anything but it sounds ultra-cool!

Don’t worry, we shall decipher the complex sentence word by word. Before we do that, you pause and think, why formation of a gas droplet is surprising at all. Imagine a liquid droplet vs a gas droplet. Left alone, the liquid droplet- which has to be a denser object- will remain a droplet because of its surface tension, while the gas atoms- with its lower density- will simply disperse away. This is the key point, how to balance this dispersion of the gas so that the gas droplet continues to remain as a droplet, especially without any container to hold it?

Let’s start simple. Stripping the already mentioned complicated statement of all adjectives we have:

“Gases in ultracold temperature can form droplets.”
Ultracold temperature is range of temperature below tens of micro-Kelvin. It’s barely short of -273 degrees Celsius. For comparison the coldest recorded natural temperature on Earth is approximately -90 degree Celsius. But after rapid technical advancements in cooling methods during the 19080s, in several laboratories, including some in our country, it has become possible to cool gases of atoms to such ultra-low temperatures. What it really means is that the atoms become awfully slow, because more jittering around represents more energy and thereby higher temperature. Unsurprisingly, it has opened a whole new world for scientists to fixate on. And one such fixation has led to the discovery of droplets of gas. But there’re a lot of nuances to it; let’s dig in.

“Bosonic gases in ultracold temperature can form quantum droplets.”
The water droplets we talked about earlier were large in size with unimaginably high number of molecules. These are classical droplets that we encounter with naked eyes. In contrast, the quantum droplets are microscopically tiny consisting of a few thousand atoms in it. Anything ‘quantum’ in physics is often accompanied by tiny substances and this intriguing phenomenon where it becomes extremely hard to identify between whether it is a wave or a particle. An atom becomes a fuzzy combination of a particle[4] and also a wave[5] carrying both of their nature. The atoms no more resemble point like particles but become more expanded like waves. The cooler they are, the larger is their expansion and soon all atoms are overlapping each other. Despite it being a gas[6] of atoms, none of them can be distinguished separately inside this blob of gas and they all have the same energy. Whatever behaviour we see now becomes a collective property of this blob. In scientific community we call the blob a ‘condensation’. Particles which can undergo such condensation are ‘bosons.’ So far, we understand, the condensation of bosonic particles can behave like a quantum droplet under certain conditions. What are they?

“Strongly dipolar bosonic gases in ultracold temperature can form quantum droplets.”
Unlike the water molecules in the classical gas, one would assume the gas atoms do not like to remain close because we know that they disperse. But in special types of atoms such as Erbium and Dysprosium, physicists have managed to find strong attractions between atoms. This attraction stems from the nature of some atoms to behave like tiny magnets. Such atoms are termed as dipolar atoms and a gas of such atoms is a dipolar gas. If this attraction is strong enough it can pull the atoms together against dispersion, which may be caused by other repulsive forces between atoms, quantum fluctuations due to slightly energetic atoms[7] and slow motion of the atoms in the gas.

There now, we have a quantum droplet of dipolar gas. The crucial factor is at ultralow temperature, the atoms are slow and less energetic enough for the dipolar attraction to hold them together which would not have been possible in a classical gas with huge number of energetic atoms.

In our group as well as with collaborators from Germany, we have theoretically studied the properties of quantum droplets of dipolar bosonic gases. This novel phenomenon is a newly emerged breakthrough in the field of ultracold gases, discovered in the ultracold experimental lab in Stuttgart. One quirky detail regarding these droplets are that, unlike classical liquid droplets, they are not spherical. They are often elongated like a cylinder. It is because of the way the atoms attract each other is asymmetric in all direction, just like how tiny magnets attract each other only one side. In our recent work we showed how to change the droplets from cigar shaped to pancake shaped simply by tweaking the way the atoms attract. That’s right, we can control how atoms attract or repel by applying electric or magnetic fields, similar to how you can control magnets using magnetic fields.

Just when you think we know everything about the fundamentals of Physics, discoveries like these have surprised us to our core. It goes to remind us why to never stop questioning and to always rage against the darkness of ignorance.

Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.

Reference: Self-bound Doubly-dipolar Bose-Einstein Condensate

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[1] Fun fact: There are approximately more than 1 sextillion water molecules in a single droplet. Guess what, it is still less than the number of drops in the oceans on our planet!

[2] Droplets can be as small as micrometres.

[3] Water drops bigger than a few millimetres will break apart into smaller drops.

[4] We can exactly know the location of a particle but not its momentum

[5] We can know the momentum of a wave very precisely but lose out on pinpointing its location

[6] A gas is much less dense than a liquid

[7] These atoms are not at the lowest energy possible.