19.1-19.2, due on November 19

1. (Difficult) I think understanding the physics behind quantum computing is difficult.  Fortunately, the authors try to explain the principles in very simple ways though.  Still, I have trouble understanding why measuring something causes its state to be fixed.  How do we know its state wasn't that way before it was measured!  Something else I found difficult was following the security guarantees of quantum key distribution (e.g. 25% chance of Bob measuring incorrectly after eavesdropping).
2. (Reflective) This relates to answer 1.  I wonder how much of the physics can be abstracted away when thinking about quantum algorithms.  It seems that understanding the principles behind quantum physics is essential to understanding quantum key distribution, although it seems once someone wrapped their minds around qubits, they could understand quantum key distribution while ignoring some of the physics details?  I will be interested to see if factoring also requires a lot of physics understanding.  Perhaps there are a few core principles that cryptographers, mathematicians, computer scientists, etc. could understand to create new and interesting quantum algorithms.  Perhaps, though, if quantum computing does become possible, advancements in algorithms will come from teams with experts from more than one field.