  Quantum information processing has been effectively demonstrated on a small
number of qubits by nuclear magnetic resonance. An important subroutine in any
computing is the readout of the output. ``Spectral implementation'' originally
suggested by Z.L. Madi, R. Bruschweiler and R.R. Ernst,
  [J. Chem. Phys. 109, 10603 (1999)], provides an elegant method of readout
with the use of an extra `observer' qubit. At the end of computation, detection
of the observer qubit provides the output via the multiplet structure of its
spectrum. In "spectral implementation" by two-dimensional experiment the
observer qubit retains the memory of input state during computation, thereby
providing correlated information on input and output, in the same spectrum.
"Spectral implementation" of Grover's search algorithm, approximate quantum
counting, a modified version of Berstein-Vazirani problem, and Hogg's algorithm
is demonstrated here in three and four-qubit systems.
