Computing-heightened low-cost high-dimensional controlled-SUM gates

Abstract: Qudit-based quantum gates offer several advantages over qubit-based counterparts, such as higher information density, the ability to address more complex problems, and richer quantum operations. In this paper, we present three realistic protocols for implementing a 4$\times$4-dimensional (16D) two-qudit controlled-SUM (CSUM) gate, where the 4D control qudit and 4D target qudit are encoded in the polarization degree of freedom (DoF) and spatial DoF of two photons, respectively. The first protocol is implemented exclusively using linear optical elements without auxiliary resources, making it feasible with current optical technologies and achieving an efficiency of 1/9. The second protocol utilizes photon scattering by a microcavity-quantum-dot system, enabling the 16D CSUM gate to operate deterministically without postselection. The third protocol introduces an error-heralded mechanism based on the second protocol, theoretically achieving unity fidelity. Moreover, all protocols operate without ancillary photons, offering the advantages of compact circuits and low cost while further promoting the development of high-dimensional quantum computation.