  A simple method is proposed to prepare conveniently the effective pure state
|00...0><0...00| with any number of qubits in a quantum ensemble. The
preparation is based on the temporal averaging (Knill, Chuang, and Laflamme,
Phys.Rev.A 57, 3348(1998)). The quantum circuit to prepare the effective pure
state is designed in a unified and systematical form and is explicitly
decomposed completely into a product of a series of one-qubit quantum gates and
the two-qubit diagonal quantum gates. The preparation could be programmed and
implemented conveniently on an NMR quantum computer.
