🚀 go-pugleaf

You would think that a single photon could be amplified, for example
in a laser-like structure to produce additional copies.  Measuring one
of the copies in one polarization, and another in the orthogonal
polarization, would give the complete answer to the question of the
state of the original particle, destroying the cryptanalytic basis of
quantum cryptography.  Unfortunately (or fortunately if you are a
cryptographer), there is intrinsic noise associated with the
amplification process, in just the amount required to prevent this
process from succeeding.  If you want to look at the details of the
quantum mechanics, look at e.g. "Quantum noise in linear amplifiers"
H.A. Haus and J.A. Muller, Phys. Rev. Vol 128 pp 2407-2413 (1962).  It
is perhaps most easily accesible in the excellent collection "Quantum
Theory and Measurement" edited by Wheeler and Zurek, Princeton U.
Press, 1983.