Quantum entanglement and cryptography - How does it work?

[314159]

I realise this a is little bit of a basic question, but how does quantum cryptography from entanglement *work*?

 

I mean, I know the general idea. Sharing a key without someone observing it, the two quanta to be observed having opposite but undetermined states, but what I mean is, in practice, how are the various steps carried out?

 

Breaking it down:

1/ How does Alice actually go about creating her two entangled particles?

2/ How does she send one of them without them being observed?

3/ When sent (and this is the part I just don't get) what do she and bob actually do *exactly* to transmit the info? What would you measure in a real system, and what info would she send to bob on a classical channel?

[bascule]

You might see my previous post on this matter, but here's a basic description:

 

When blue light is pumped into a nonlinear crystal, entangled photon pairs (imaged here as a red beam with the aid of a diode laser) emerge at an angle of 3° to the blue beam, and the beams are sent into single-mode fibers to be detected. Because the entangled photons “know” each other, any interference will result in a mismatch when the two beams are compared.

 

However, I've been quite curious about using a Bell test setup to produce a shared, streaming random pad. Once you have that you can use whatever OTP-based encryption methodology you like, and thus have provably unbreakable encryption.

[314159]

I'd read your post previously, but I'm still left a litle confused. So you create entangled pairs of photons by passing the light through a non-linear crystal, but how does that actually create an entangled pair?

I'm still left none the wiser on points 2/ and 3/, as to how the observations are carried out, and how they prevent any collapse of the states enroute.