Cavity-Free Distributed Quantum Computing with Rydberg Ensembles via Collective Enhancement
We present a complete protocol for cavity-free quantum networking based on collective enhancement in Rydberg atom ensembles. The scheme combines Rydberg blockade, collectively enhanced light--matter coupling, and phase-matched directional emission to remove the need for optical cavities while retaining efficiencies comparable to cavity-assisted interfaces. The protocol proceeds in three steps: (i)~local control--ensemble entanglement generated by Rydberg blockade with gate fidelity $F_{\mathrm{gate}}\approx 99.93\%$; (ii)~atom--photon conversion through Raman emission from an oblate spheroidal ensemble, yielding directional emission efficiency $\eta_{\mathrm{dir}}\approx 73\%$ and single-node efficiency $\eta_{\mathrm{node}}\approx 40\%$; and (iii)~remote atom--atom entanglement via Hong--Ou--Mandel interference, producing Bell states with fidelity $F>97.5\%$. Incorporating quantum memories allows up to $M\approx 100$ retry attempts within a coherence time $T_2>100\,\mu\mathrm{s}$, enabling entanglement generation rates of approximately $4\,\mathrm{kHz}$ over a 20~km separation. Collectively enhanced Rydberg ensembles thus provide a practical, cavity-free interface for scalable distributed quantum computing and secure quantum communication.