Loading Classical Data Into a Quantum Computer

Quantum circuits and methods load N=2n classical bits into an entangled quantum output state using a gate depth of order O(n). Loading is accomplished by dividing the 2n input bits into data words and entangling these data words using ancilla qubits. The output of the circuit consists of one data word and one or several index qubits, drawn from the ancilla, to select between the input data words. Entanglement of the data words is performed in a single time slice (i.e. with a gate depth of 1), while the number of sequential gates needed to produce the appropriate pre-entanglement quantum state in the ancilla, and to disentangle the non-output ancilla, has the desired order O(n). Also disclosed is a circuit for disentangling qubits used to store non-output data words during processing.