**Tags**

double slit experiment, graduate school, probability amplitude, quantum mechanics, quantum world, spooky action at a distance

As you may have probably guessed, this diary is intended to be an account of my apprenticeship in the art of deciphering the quantum world. When I was in school, I wanted to be an astrophysicist, not exactly knowing what astrophysicists did, except that they studied cool things like space and stars and galaxies. I wrote to J.V. Narlikar, telling him I wanted to be an astrophysicist and asking what I should do in order that I may become one. The nice man replied and told me that I should first pass out of school (I was in tenth grade then) with good marks in physics, chemistry, and mathematics, and then join an undergraduate program in a science stream. He even sent me a booklet called “Careers in Astronomy and Astrophysics” from IUCAA. I was an innocent school kid then, unaware of the quantum world and its charms, and I worked at (classical) physics and maths in school so that I may grow up to become an astrophysicist.

I joined an undergraduate program in physics after school, but my astrophysics-related ambition withered away in second year of college when I went for an astronomy school, attended a lot of lectures on astronomy and astrophysics, and came away convinced that astrophysics wasn’t my cup of tea after all. Meanwhile, I had been introduced to quantum mechanics in college in a rather Feynmanesque style, and the double-slit experiment confounded and excited me. The phrase ‘probability amplitude’ entered my vocabulary and I was mystified by the appearance of these complex numbers that one needed to compute quantum probabilities. (Don’t worry if you do not know these concepts already, I will explain them when I use them in any of my diary entries.) Conversations with a particular friend, one year senior to me, led to increasing curiosity of my part regarding the quantum worldview. He would tell me about this thing called entanglement, some “spooky-action-at-a-distance” stuff that wasn’t really covered much in the college curriculum, and this other thing called “decoherence” which, he said, apparently tries to explain how the classical world we see around us comes about from the quantum world that underpins it. All in all, I was hooked to the quantum scheme of things, eager to catch a glimpse of what the world looks like through the quantum lens.

So, after college, I joined graduate school, hoping to unravel the quantum world and see it for what it is. The coursework took its toll and the questions that had so piqued my curiosity as an undergrad did not find enough space to be asked in the mostly “shut-up-and-calculate” courses. One avenue that I did find, however, for the basic questions I had was quantum information theory. The quantum information theorist’s way of looking at quantum theory at once appealed to me, compared to, say, the “traditional” particle physicist’s way of looking at quantum mechanics, or for that matter the condensed matter physicist’s view of quantum mechanics. The particle physicists would have me believe that quantum theory is all about scattering amplitudes and cross-sections and Feynman diagrams and the like. The condensed matter folks would have me believe its all about electrons in solids, superconductivity, and the like. Agreed, it was all cool and deep stuff at both ends, but it wasn’t what I was looking for. It did not really answer the questions I had. (I did not venture towards quantum gravity for the simple reason that I felt I had to sort out the ‘quantum’ first, before I even got to the ‘gravity’ bit, forget piecing together the quantum gravity puzzle.)

On the other hand, the quantum information theorist would begin with the simple things first. Things like qubits, or two-level quantum systems, which are the simplest ones that exist. And then go on to study them with a combination of detail and insight that I did not find in other fields that talked about quantum theory. Quantum theory made a lot more sense from the lens of quantum information; at some level, I felt I was finally beginning to understand *why *I did not relate so much to the other fields that used quantum theory — it was precisely because they *used *it like a mathematical blackbox and did not worry too much about how the blackbox worked or even what all one could really learn about the blackbox’s internal working if one asked it the right kind of questions. In short, the range of questions you could ask in those fields was limited mostly to phenomena rather than principles. And I figured I was more a man of principles than one of phenomena, for what I was really after was the question of what the theory implied about the world we lived in at the most basic level — at the level states, measurements, and transformations of *any* quantum system, rather than a particular kind of quantum system. I got a sense that perhaps quantum information would be the way to go to ask the questions that interested me, and perhaps find answers to some of them. And so I did.

This is where I begin my apprenticeship.