Dork Scratchings

I'm still a bit obsessed with the GHZ experiment.  This is the one where you prepare a GHZ state $\lvert 000 \rangle + \lvert 111 \rangle$ and send one qubit to each of 3 protagonists: Alice, Bob, and Charlie.  (I will be ignoring shared normalizer constants throughout this post, as I find they don't add anything to the understanding.) If you do the maths it turns out that when all three choose to measure in the $\lvert + \rangle$, $\lvert - \rangle$ basis (shorthand for $\lvert 0 \rangle + \lvert 1\rangle$ and $\lvert 0 \rangle - \lvert 1\rangle$) then they are guarranteed to get a parity zero result.  On the other hand if only one measures in this basis and the other two measure in the $\lvert +i\rangle$, $\lvert -i\rangle$ basis (shorthand for $\lvert 0 \rangle + i\lvert 1\rangle$ and $\lvert 0 \rangle - i\lvert 1\rangle$) they are guarranteed to get a parity one result.  As I showed in an earlier post this appears to be incompatible with the outcomes being prede...

In 1964 John Stewart Bell proposed an experiment to determine whether the results of quantum measurements were truly random, or governed by hidden variables, i.e. state that exists prior to the measurement, but which we don’t have access to. The experiment involved creating a large number of EPR pairs, and firing them at two observers, Alice and Bob, who measure their photon’s polarisation, choosing the $\updownarrow$ direction or the $\nearrow\llap\swarrow$ direction at random. Determining the result of the experiment involves doing a complex statistical calculation to see if something called Bell’s inequality is satisfied or violated. The Bell experiment was first performed by in 1982 by Alain Aspect, and the result, as most commonly interpreted, is that hidden variables can only exist if Quantum Mechanics is non-local, i.e. if it supports faster-than-light causality! Some time after Bell proposed his experiment, Greenberger, Horne, and Zeilinger suggested an alternativ...

I was looking up David Deutsch when I stumbled across this amazing lecture by the gloriously dorky Sidney Coleman.  It's a grainy recording of a talk he gave to the American Physical Society in 1994.  It's full of witty repartee as he talks about his philosophical struggles with QM prior to embracing Many Worlds.  He wonders, for example, who or what exactly can collapse a wavefunction.  The Copenhagen Interpretation doesn't - cannot - answer this.  Coleman describes going to a colleague and sharing his concern that maybe solipsism is the only consistent interpretation - that maybe he is the only one capable of collapsing wavefunctions.  The colleague puffs on his pipe and says: tell me, before you were born, was your father able to collapse wavefunctions?   Wonderful! Quantum Physics meets Psychoanalysis. After watching the video I made some notes on my favorite app. I'd wanted to understand Bell's Inequality/Bell's Theorem for some time, bu...