Quantum computers go multi-dimensional with four-state photon gate

Researchers from TU Wien and a Chinese team have developed a novel quantum logic gate enabling quantum computations on pairs of photons each existing in four distinct quantum states, marking a significant advancement toward multi-dimensional quantum computing. Unlike traditional quantum experiments that rely on photon polarization (which offers two states), this new approach exploits the spatial waveforms of photons, specifically their orbital angular momentum, which can exist in infinitely many states. This allows photons to be manipulated in more complex superpositions and entangled states, enhancing the computational capacity of quantum systems. The team overcame the challenge of limited direct photon interaction in linear media by devising a protocol to realize a controlled phase-flip gate for two photonic qudits in arbitrary dimensions. Experimentally demonstrated and published in Nature Photonics, the four-dimensional qudit–qudit gate uses orbital angular momentum encoding combined with a newly developed high-precision phase-locking technology to stabilize the gate, achieving process fidelities between 0.71 and 0.85. This breakthrough represents