Dork Scratchings

It has occurred to me that the cartoon I drew two days ago will appear to most people as absurd: Alice measures one half of an entangled pair and instantly splits into two versions of herself!  Bob measures the other half and also splits, but each Alice is paired with one of the Bobs in such a way that, should they meet later to discuss their results, everything remains consistent. Yet if you replace Alice and Bob with two qubits - to get a quantum circuit like the one below - no quantum computing expert would find this outcome absurd. The Einstein Podolsky Rosen experiment in circuit form with non-classical Alice and Bob Alice is qubit q0 and Bob is q3 .  Qubits q1 and q2 are entangled into the Bell state $(\lvert 01 \rangle + \lvert 10 \rangle)/\sqrt 2$ and then q1 is shared with Alice, and q2 with Bob.  In the standard (collapse-based, non-multiversal) description Alice and Bob measure q1 and q2 , collapse the q1q2 wavefunction, and find that one gets 0 while the...

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...

Why opening the box breaks the spell Quantum computers can answer questions like "what are the factors of this number?"   If the numbers are large enough these are questions that classical computers could never have the resources to answer.  For example you could ask it "what numbers divide 621405631250025693248096949484035695232" and it might respond "1932840132984031923 divides 621405631250025693248096949484035695232" as this is one of the divisors. Except not yet.  At the moment quantum computers are pretty puny and only have a few qubits each.  A quantum computer with 4 qubits might be able to tell you what the divisors of 15 are, but it couldn't go any higher without needing more qubits.  And the problem that is making it difficult to build bigger, better , quantum computers is decoherence . Before describing decoherence, let's look at how a quantum computer works.  In the following diagram points on the paper represent orthoganal stat...