Relativistically covariant state-dependent cloning of photons
Kamil Brádler, Rocío Jáuregui · Physical Review A · 2008
The influence of the relativistic covariance requirement on the optimality of the symmetric state-dependent $1\ensuremath{\rightarrow}2$ cloning machine is studied. Namely, given a photonic qubit whose basis is formed from the momentum-helicity eigenstates, the change to the optimal cloning fidelity is calculated taking into account the Lorentz covariance unitarily represented by Wigner's little group. Among other, we found states for which the optimal fidelity of the cloning process drops to 2/3 which corresponds to the fidelity of the optimal classical cloner. Also, an implication for the security of the Bennett-Brassard 1984 protocol is analyzed.