# Quantum Cryptography

First question was a little bit ambiguous.

Photons are passed through a linear polarizer that is oriented $\theta$ degrees again the photon passes through another linear polarizer that also have a $\theta$ degree orientation.

Now my question is that why the second polarizer will effect the spin axis angle of the photon.

Actually I am not from a physics background and new to Quantum Cryptography.Please. help.

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Do you mean "polarization" instead of "spin" by any chance? – ptomato Mar 4 '12 at 20:11
The ambiguity between polarisation and spin probably comes from the fact that the two most discussed realization of the qubit are a spin-1/2 and a polarised single photon. And since the polarisation is the manifestation of the spin of the photon, I'm not even sure the confusion is a real error. – Frédéric Grosshans Mar 5 '12 at 9:57

If I understood the question correctly, the two polarisers have the same orientation $\theta$. In that case, the second polariser has no effect at all :
• or the photon passes through the first polariser. In that case, it is polarized along $\theta$ and passes through the second polariser.
If the cryptographic protocol uses $n$ qubits, this probability decreases exponentially as $2^{-n}$, i.e. at the same speed as random guessing the encrypted bits. – Frédéric Grosshans Mar 5 '12 at 13:52