Qubit vs observable resource trade-offs in measurement-based quantum computation

Since quantum measurement is universal for quantum computation (Nielsen [3]), the minimization of the resources required for measurement-based quantum computation is a crucial point ([3, 1, 4, 5, 6]). We have shown in [6] that a set of observables composed of one two-qubit measurement and three one-qubit measurements form a universal set for quantum computation if one ancillary qubit is available as an additional resource. This set is minimal in terms of two-qubit measurements, because at least one multi-qubit operation is needed to create entanglement.. and is also minimal in terms of ancillary qubits, because at least one ancillary qubit is required to simulate reversible transformations with projective measurements only. We discuss in this paper the required number of one-qubit measurements for a set of observables to be universal. A set composed of one two-qubit measurement and only two one-qubit measurements is proved universal. at the cost of a second ancillary qubit. Thus an additional ancillary qubit seems to be required to obtain a smaller universal set of measurements. One may conclude to a trade off between ancillary qubits and observables; in measurement-based quantum computation.