An $Θ(\sqrt{n})$-depth Quantum Adder on a 2D NTC Quantum Computer Architecture

Abstract: In this work, we propose an adder for the 2D NTC architecture, designed to match the architectural constraints of many quantum computing technologies. The chosen architecture allows the layout of logical qubits in two dimensions and the concurrent execution of one- and two-qubit gates with nearest-neighbor interaction only. The proposed adder works in three phases. In the first phase, the first column generates the summation output and the other columns do the carry-lookahead operations. In the second phase, these intermediate values are propagated from column to column, preparing for computation of the final carry for each register position. In the last phase, each column, except the first one, generates the summation output using this column-level carry. The depth and the number of qubits of the proposed adder are $\Theta(\sqrt{n})$ and O(n), respectively. The proposed adder executes faster than the adders designed for the 1D NTC architecture when the length of the input registers $n$ is larger than 58.