1. Turbulence and Transport within Nuclear Fusion Reactors 

Almost everyone I meet in random, be a cab driver, or a fellow passenger in train, or in parties, (and sometimes my relatives too), I always fall in awkward condition when they ask me - "So, what do you do?" Every time I uttered the word, "Nuclear-Fusion", I had to complement it with the follow-up sentence to bring their eyebrows down - "Oh, no, no, no, you got the absolutely wrong end. It has nothing to do with the nuclear fission bomb. Rather it is completely opposite." Interestingly, I have experienced this picture as globally true - be it America or Asia or Europe.

Nuclear fission is, breaking of atoms while, on the contrary, nuclear fusion is, clubbing or joining (or fusing) two atoms. Fusion also releases energy but is not quite suitable for making bombs. Here we make the nuclear fusion happen in a hot soup (also called plasma - imagine our Sun for example) that is confined within a virtual bottle of magnetic field. The energy produced from this fusion is often called as green-energy which can be used to generate electricity for the peaceful development of the civilizations! 

However, in the case of the Sun, the hot soup or plasma is confined due to its own gravity produced because of its own mass. In our Earth, we confine it by magnetic fields instead. Unfortunately, till now, we have not been able to make a full-proof 'magnetic bottle', both conceptually and experimentally. These defects in our 'magnetic bottle' cause the hot plasma to leak and spill outside causing several damages. We have to make the wall of our 'magnetic bottle' such; that it will selectively allow part of already fused plasma to come out and remove from the machine and the to-be-fused plasma keep confined within the bottle. Yup! I am one of the dreamers who believe, 'we shall overcome' this difficulty 'one day' and that too is not far! So, I am trying to draw a 'more accurate' geometry of the virtual bottle made up of magnetic fields! "Enough, I am leaving."  Hey, wait, I can explain...

2. Artificial Intelligence and Machine Learning 

I first encountered artificial intelligence (AI) as an extension of my research in nonlinear oscillators. I started reading about, something called, Genetic Programming (Book) and learned that the fundamental equations governing the physical systems can actually be "discovered" via machine learning algorithms! Princeton gave me the opportunity to interact with some the world's best experts in artificial intelligence. Bubbling within an AI community, I have recently started out experimenting with some new deep neural networks models, that can help reduce the computational cost of some computationally expensive fusion plasma simulations that I am currently involved into.

3. Astrophysical Plasma Turbulence and Dynamical Systems

Plasmas beyond our Earth, appear in a widely varying length scale. Though in the Introduction of my PhD Thesis, I wrote, "the underlying fundamental physical laws that govern the dynamics of plasma occurring in burning stars as well as laboratory experiments are identical" (I was trying to motivate, where the theory of magneto-hydrodynamics is applicable), nature of plasma flows and hence their dynamics are quite different in different plasmas. For example the plasma in the core of our Sun and the one in between our galaxy and Andromeda are pretty different. A considerable amount of effort during my graduate school I dedicated into understanding the generation of flows and magnetic fields in such plasmas. Is it possible to generate a flow that will perpetually add up the magnetic fields and we can solve the puzzle of cosmic magnetic fields? Further down the road I felt, this is closely related to Chaos theory, specifically the old works by Arnold! Well, there are few well-known fluid flows that are known to amplify the magnetic fields in plasmas. But can we show that these flows have a 'strange-attractor' sitting within their heart? Well, until now, I could not prove this. But I believe, one day I will!!!

4. Stirring, Mixing and Transport in HydroDynamics

Huh, I never knew, school of fishes can alter the ocean currents upto 30%. Fluid motion close to a rigid body is so interesting! Also in a static water the way an ink-drop slowly filaments out. It stretches, sometimes gets twisted and folded too! You know ideal magnetized plasma and two dimensional hydrodynamics posses a homology! Yes, if I ever get an opportunity to do controlled experiments, I will surely generate water flows, put an ink-drop and will see the way in which the magnetic fields in the inter-galactic-medium got generated! Well, forget about plasma. You know, this stretching, twisting and folding actually make the toffee/candies softer? Thanks to Prof Jean-Luc Thiffeault for offering a mesmerizing course!

5. Strongly Correlated Complex Plasma

I never took statistical mechanics quite seriously until I was very pleasantly motivated to think about phase-transition in my early graduate school. I probably deliberately ignored statistical mechanics because of some crazy ideas by which I obscured my own thoughts. I spent two nights without sleep (and they were our Durga-puja nights in 2015) trying to explain it churning my favorite quantum mechanics and field theory - but all failed. Finally I was guided to write a simple classical Monte-Carlo code (hardly two hundred lines in Fortran) from scratch by myself and I discovered, it is showing phase-transition. Being utterly puzzled, I entered into the arena of strongly correlated complex plasma or popularly called dusty plasma.  I developed another molecular dynamics code from scratch where I solved only Newton's equations and cross checked results with my earlier code. I never understood (even now), why did my simulation allow the formation of dust-crystals, though have detected the melting points of the dust crystals in presence of magnetic fields and devised new ways to measure it's temperature and even determined it's equation of state. How crazy I am!

6. Classical Mechanics and Nonlinear Dynamics

• Planetary and satellite orbits: 

Classical mechanics has become one of the most favorite subjects since my early university days. The book that I loved most is, 'Mechanics' by Landau (specifically Motion of a Rigid Body and The Canonical Equations). Arnold entered my world two years later! Unfortunately, I never could pursue research on the foundations of classical mechanics (I wish, I could!). My research works were mostly motivated by one of my college-teachers who lured me into the rich variety of central force motion! I am interested in finding the criteria by which a particle under a central force executes a bound orbit. When I say 'bound', I allow it to precess indefinitely and hence it may or may not be a 'closed' orbit. Landau says, "There are only two types of central field in which all finite motions take place in closed paths. They are those in which the potential energy of the particle varies as 1/r or as r^2." [Chap 3, Sec 14, Page 32, (1969)]. Huh, we found a plethora of exceptions! Yeyyy... But why? What happens to Bertrand's theorem? Isn't is a contradiction? Well, there are some nice works showing for certain cases, the Laplace–Runge–Lenz vector need not be conserved over the whole trajectory. 

• Brain dynamics and ageing: 

Another aspect which I greatly neglected in my early classes of classical mechanics is, collective effects. I was sitting in a seminar where people were discussing about synchronization. I realized, my knowledge of classical mechanics has very very poorly failed to explain why does my heart beat. I probably had an ego within myself that, I understand classical mechanics, at least, to some extent. I literally got insane when I realized my weakness and kept searching the answer. And then someone introduced me with a partial answer, and raised another follow-up question, how do I sleep. I entered into the arena of nonlinear coupled oscillators and slowly drifted to networks and artificial intelligence. Though I am not doing any active research in networks, I am happy that now I understand, though very crudely, how does my sleep gets affected as I grow old. After realizing this effect of age on sleep, I started thinking on the question - can I resurrect? Well, this I am still not sure!

7. Quantum Information and Quantum Computation

"Is the moon there when nobody looks?" - Physics Today; April, 1985. 

Einstein's dialogue with Niels Bohr regarding the foundation of quantum theory is surely an asset. And I am lucky, that I got hold of the detailed discussions (with elaborate footnotes) within a hardcover book in our university library. The 'spooky action at a distance' literally made me worry when I was a masters student. Interestingly, lately though, such questions are now being put to experimental tests. In Princeton, my colleagues are trying to build robust qubits. Countries are holding quantum encrypted video calling with satellites. And most amazingly, we are now able to teleport quantum states over 1400 KM. As a computational physicist, what I am interested, is building computer algorithms that can run on actual quantum computers. Today, I write codes in a classical computer, submit my code to run using a classical internet system. And I submit it into a real quantum computer by IBM. When the run finishes, I get the results back using the same classical internet in my classical computer. One of the dream areas of my future research is, simulating fusion plasmas using quantum algorithms. It is new and booming. Interestingly, at least, as of today, I am aware of only one such article, written by few of my department colleagues, in this area! So hopefully, mine will be the next one!!!

8. General Relativity, Black Hole and Cosmology

Yes, this is what I loved the most, until I encountered collective effects in my graduate school. The only person to blame for this is, one of my university teachers who drove me crazy! I loved the mathematical formalism of the subject and somehow was dragged into thinking about energy extraction from rotating black holes. Finally nothing came out. But my love deepened. While I came in graduate school, I indulged myself into continuing the works, mostly related to the crazy questions - what will happen if some higher curvature terms were present in the Hilbert-Einstein action. Can we explain our today's anisotropic (in scales smaller than cosmological scales) universe better? Yeah, we can, but I am not very satisfied with the answer we found. I am still passionately curious to understand, what will happen, if I could 'fall' into a black hole! Will I shrink in size and my hairs will turn gray? The funniest part is, I can't see someone else hitting the black hole singularity, I have to go myself, if I want to know. I may take a spaceship and orbit around it, to get some feelings and then come out! Now I know where to safely put my spaceship for that!