Here, we present a proof-of-principle high-dimensional quantum key distribution (QKD) protocol utilizing the position and momentum entanglement of photon pairs. The protocol exploits the fact that position and momentum form mutually unbiased bases, linked via a Fourier transform. One photon of the entangled pair is measured by the sender in a randomly chosen basis—either position or momentum—selected passively via a beam splitter. This projective measurement remotely prepares the partner photon in a corresponding spatial mode, which is sent to the receiver, who similarly performs a random measurement in one of the two bases. In this implementation, we achieve a photon information efficiency of 5.07 bits per photon using 90 spatial modes and a maximum bit rate of $0.9\mathrm{Kb}/\mathrm{s}$ with 361 modes. To assess the scalability of this spatial mode encoding scheme, we theoretically show that using a brighter entangled photon source along with next-generation single-photon cameras—featuring improved quantum efficiency, timing, and spatial resolution—this approach could achieve 9 bits per photon at 2000 spatial modes and a bit rate of over $700\mathrm{Mb}/\mathrm{s}$ at 4400 modes while accounting for finite-key effects. These results quantify the opportunities and performance bounds of spatially encoded, entanglement-based QKD and provide a benchmark for future high-dimensional quantum communication systems.

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• Entanglement detection
• Measures of continuous variables entanglement
• Optical quantum information processing
• Photonics
• Quantum correlations in quantum information
• Quantum cryptography
• Quantum entanglement
• Quantum information processing with continuous variables