SUPERPOSITION-PRESERVING PHOTON-NUMBER AMPLIFIER

We analyze a pure-state photon-number amplifier (PNA), i.e., a device that performs the action \psi(in)] =Sigma(lcl)\l]-->\psi(out)]=Sigma(l cl)\Gl], where G is a positive integer. We derive expressions for the fidelity of a finite internal state PNA and demonstrate that the ampli fier fidelity, i.e., the similarity between the obtained and the desir ed output state can be made to approach unity. Finally we outline a sy stematic procedure to find the Hamiltonian for any finite Hilbert-spac e PNA and compute the Hamiltonian for a simple but nontrivial PNA. [S1 050-2947(98)07301-6].