MEASUREMENT

I recently had this debate about measurement and decoherence with one of my physics professors. The issue came up when I was trying to explain (pretty pathetically I must say...) what I did in my research in quantum information. It turned out I used a phrase that he took objection to. It was the following: "Decoherence is the phenomenon of making unwanted measurements". Immediately he said I must back off because Decoherence is inevitable and unintentional whereas measurement is intentional (my italics). I replied with the Dirac-Feynman definition model of physics which says that whatever we define in Physics must not contain us or any human emotion because nature does not give a hoot about what we think we are doing. To be more precise, I will quote a line from Dirac's book:

"This is the test quote"

ENTANGLEMENT

This entry briefly outlines the history and physics behind Entanglement. Note I said briefly, I mean it; so, I also provide various references at the end for further research.

Well, entanglement is probably one of the weirdest things in quantum physics that I have seen. It was discussed at length at the World Science Festival held at NYU this year. During a discussion on quantum reality and the interpretations of quantum mechanics, one of the scientists tried to explain this concept to us by means of the following example: Take a pair of gloves (labeled L and R for obvious reasons), then close your eyes and mix them up in a random fashion. Then, pick one glove at random and put it in a box and the other one in another box (with your eyes closed). Now, close both boxes and open your eyes. If I ask you what is in each box? You'll reply that each box has either L or R with equal probabilities. Thus, the state of the system is:

1/√2 (| L > + | R >).

where 1/√2 is called the amplitude, whose square gives the probability associated with | L > or | R >. It is important to note that this is more than just a convention. But, one might argue that there is nothing "quantum" about this experiment. It is just plain classical probability with some extra notation. But, the idea of entanglement goes beyond classical physics. To illustrate this, one is directed to the paper of Einstein, Podolsky and Rosen which more or less says: Based on the existence of something as absurd as entanglement, one can say that the quantum theory is itself absurd. Then, a big debate issued over what exactly one means by "absurd"?, what is "reality"?, now what do you think you mean when you say something is "really absurd"? and all the other difficult questions which I cannot answer yet. Moreover, one must also note that the glove-example (that's how I'll refer to it in future posts as well if need be) is only an analog. It is well known that one cannot "picture" the quantum world using classical props. Thus, the said scientist was only trying to get the notion of entanglement into my brain as opposed to explaining what entanglement is. So, now the question arises: Can we then, hope to "understand" this concept? Of course! People have done it, people are doing it. All we need is a little background. So, I'll be learning it and in the process, posting whatever I learn here.

References: Of course, Wikipedia, SIDNEY LECTURE: QUANTUM MECHANICS IN YOUR FACE!, MIT lecture notes (this is better understood if the concept of state is clear I think), more to come later...

Motivation

I have always been meaning to post something coherent and meaningful on one of the most beautiful and enigmatic theories of physics viz. Quantum Physics (as opposed to Quantum Mechanics since the present theory covers electrodynamic properties of matter too). I was first introduced to this topic via the Feynman Lectures, I was hooked, no doubt. Then, at some point, someone mentioned Dirac's book. I read the first two chapters and I understood, partly, the meaning of the following paraphrased words of Neils Bohr, "Anyone who is not baffled by quantum mechanics has not fully understood it!". I was completely lost, one of the main reasons being that there was no quantum analogue of a "Picture" that I could use to aid my understanding of the theory. Here was a physical theory, that explained concrete (tangible) things in real life using mathematics that was very "pure" or abstract. After reading quite a few books that popularize physics (i.e. suck out all the mathematics rendering physics incomprehensible) and those that give the reader a good training on the subject, I have come to a conclusion that classical analogues like gloves in a box, cats in boxes etc., though suitable for expositions of the superposition of quantum states, are inaccurate in describing quantum phenomena since there exist classical probabilistic theories. In other words, there might not be any need for quantum mechanics to explain classical probability. So, how far can we take such analogues?