  Quantum state preparation plays an equally important role with quantum
operations and measurements in quantum information processing. The previous
methods of preparing initial state for bulk quantum computation all have
inevitable disadvantages, such as, requiring multiple experiments, causing loss
of signals, or requiring molecules with restrictive structure. In this work,
three kinds of quantum circuits are introduced to prepare the pseudo-pure
states of ($n-1$) qubits in the Hilbert space of $n$ coupled spins which merely
need the assist of one ancilla spin and two experiments independent of $n$.
Being without gradient fields effectively avoids the reduction of the signals.
Our methods have no special requirements on the structure of the used
molecules. To test these methods more comprehensively, we experimentally
demonstrate the preparation of the labeled pseudo-pure states using
heteronuclear 2-qubit and homonuclear 4-qubit nuclear magnetic resonance
quantum information processor.
