Quantum half-adder

Implementation of a linear optics quantum multibit half-adder that could show potential scalability is discussed. Parametric down-conversion sources are assumed that are pumped by a mode carrying orbital angular momentum $l$. A single photon from a pair is prepared by detection of its twin. This preparation of a single photon defines two bits to be summed and the starting carry control bit. This hyperentangled single photon enters a sequence of optical gates that acts on this photon and ancilla photons. At the adder output, each single photon detected gives the sum operation plus carry. The photons belong to a hyperentangled system with a ${2}^{3}$ Hilbert space describing each photon. Conditions to have a scalable system for quantum computation define the requirements for construction of this photon source.

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