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If black holes were able to clone quantum states, a number of paradoxes inblack hole physics would disappear. However, the linearity of quantum mechanicsforbids exact cloning of quantum states. Here we show that black holes indeedclone incoming quantum states with a fidelity that depends on the black hole'sabsorption coefficient, without violating the no-cloning theorem because theclones are only approximate. Perfectly reflecting black holes are optimaluniversal "quantum cloning machines" and operate on the principle of stimulatedemission, exactly as their quantum optical counterparts. In the limit ofperfect absorption, the fidelity of clones is equal to what can be obtained viaquantum state estimation methods. But for any absorption probability less thanone, the cloning fidelity is nearly optimal as long as $\omega/T\geq10$, acommon parameter for modest-sized black holes.Comment: 5 pages, 3 figures. To appear in J. Phys.

Black holes are almost optimal quantum cloners