  The standard procedure for quantum process tomography (QPT) involves applying
the quantum process on a system initialized in each of a complete set of
orthonormal states. The corresponding outputs are then characterized by quantum
state tomography (QST), which itself requires the measurement of non-commuting
observables realized by independent experiments on identically prepared system
states. Thus QPT procedure demands a number of independent measurements, and
moreover, this number increases rapidly with the size of the system. However,
the total number of independent measurements can be greatly reduced with the
availability of ancilla qubits. Ancilla assisted process tomography (AAPT) has
earlier been shown to require a single QST of system-ancilla space. Ancilla
assisted quantum state tomography (AAQST) has also been shown to perform QST in
a single measurement. Here we combine AAPT with AAQST to realize a `single-shot
QPT' (SSPT), a procedure to characterize a general quantum process in a single
collective measurement of a set of commuting observables. We demonstrate
experimental SSPT by characterizing several single-qubit processes using a
three-qubit NMR quantum register. Furthermore, using the SSPT procedure we
experimentally characterize the twirling process and compare the results with
theory.
