1. A quantum circuit adapted to perform quantum computation in a quantum computer to probabilistically achieve a chosen transformation of an initial n-qubit state, comprising: a unitary quantum operator operating on a qubit state and an ancilla state to obtain an evolved qubit state and an evolved ancilla state; and a measurement operator to measure the evolved ancilla state, the evolved ancilla state being adapted to assume a success condition or a failure condition, wherein when the evolved ancilla state assumes a success condition the chosen transformation is obtained, and when the evolved ancilla state assumes a failure condition the unitary quantum operator further operates on the ancilla state and the evolved qubit state, the unitary quantum operator further operating on the ancilla state and previously evolved qubit states until the evolved ancilla state assumes a success condition.