Superbroadcasting of conjugate quantum variables

We consider the problem of broadcasting arbitrary states of radiation modes from N to M > N copies by a map that preserves the average value of the field and optimally reduces the total noise in conjugate variables. For N >= 2 the broadcasting can be achieved perfectly, and for sufficiently noisy input states one can even purify the state while broadcasting-the so-called superbroadcasting. For purification (i.e. M <= N), the reduction of noise is independent of M. Similar results are proved for broadcasting with phase-conjugation. All the optimal maps can be implemented by linear optics and linear amplification.