Ancilla-free optimal quantum adder

Abstract

We construct an exact $n$-bit quantum adder with $O(n)$ gates and $O(\log n)$ depth that requires no ancillary qubits. The circuit uses only $X$, CNOT, and Toffoli gates under all-to-all connectivity and is classically reversible. Our construction accelerates the controlled ladder underlying addition. Grouping the data into blocks and use existing control qubits as conditional workspace gives a recursive construction with $O(n\log\log n)$ gates and $O(\log n)$ depth. We then coordinate inter-block propagation via two short-path forest operations to cut the per-block workspace requirement, yielding a nonrecursive implementation with $O(n)$ gates. Both the gate count and circuit depth are asymptotically optimal.