Universal cloning of continuous quantum variables

The cloning of quantum variables with continuous spectra is analyzed. A universal - or Gaussian - quantum cloning machine is exhibited that copies equally well the states of two conjugate variables such as position x and momentum p. It also duplicates all coherent states with a fidelity of 2/3. More generally, the copies are shown to obey an Heisenberg-like uncertainty relation implying that the product of the x-error variance on the first copy times the p-error variance on the second remains bounded by $(\hbar/2)^2$.