A theory for a quantum cloning machine able to produce several copies of the quantum state of a particle has been proposed by Chinese researchers at the Henan University and the Chinese Academy of Sciences.

The researchers  demonstrated that it is theoretically possible to create four approximate copies of an initial quantum state, in a process called asymmetric cloning. They also demonstrated that their quantum cloning machine has the advantage of being universal, so it can work with any quantum state, ranging from a photon to an atom.

Quantum cloning is difficult because quantum mechanics laws only allow for an approximate copy — not an exact copy — of an original quantum state to be made, since measuring such a state prior to its cloning would alter it.

Asymmetric quantum cloning has applications in analyzing the security of message encryption systems based on shared secret quantum keys.

Ref.: Ren XJ, Xiang Y and Fan H, Optimal asymmetric 1 → 4 quantum cloning in arbitrary dimension, European Physical Journal D, 2011 [DOI: 10.1140/epjd/e2011-20370-2]