  A gate is called entangler if it transforms some (pure) product states to
entangled states. A universal entangler is a gate which transforms all product
states to entangled states. In practice, a universal entangler is a very
powerful device for generating entanglements, and thus provides important
physical resources for accomplishing many tasks in quantum computing and
quantum information. This Letter demonstrates that a universal entangler always
exists except for a degenerated case. Nevertheless, the problem how to find a
universal entangler remains open.
