How are qubits entangled?
I understand the basics of entanglement but what I do not get is how it occurs in nature or in the lab. What causes entanglement to occur or what is done to the particle to make it occur.
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Entanglement is created when two systems interact, in particular when they undergo a joint unitary evolution (generated by a Hamiltonian of the joint system that includes an interaction term). As they interact correlations develop between the states of the two systems, these correlations are essentially the defining characteristic of entanglement.
For a simple example, imagine taking two spin 1/2 particles. Suppose we measure their spins and that they point in different directions. Now if we bring them close to one another the spin-spin interaction wants to put the particles into a state where the spins are aligned (regardless of what their mutual direction is). This tendency to align represents a correlation between the individual states of the two particles. The longer we keep the particles close to each other the more pronounced this effect gets, until after some relaxation time when the spins of the two particles are completely correlated, that is in a state where the spins are definitely aligned. Now if we measure the spin of one of the particles, we know for certain that the other particle will have it's spin in that same direction.
