  Major obstacles against efficient long distance quantum communication are
photon losses during transmission and the probabilistic nature of Bell
measurement causing exponential scaling in time and resource with distance. To
overcome these difficulties, while conventional quantum repeaters require
matter-based operations with long-lived quantum memories, recent proposals have
employed encoded multiple photons in entanglement, providing an alternative way
for scalability. In pursuing scalable quantum communications, naturally arising
questions are thus whether any ultimate limit exists in all-optical
scalability, and whether and how it can be achieved. Motivated by these
questions, we derive the fundamental limits of the efficiency and
loss-tolerance of the Bell measurement with multiple photons, restricted not by
protocols but by the laws of physics, i.e. linear optics and no-cloning
theorem. We then propose a Bell measurement scheme with linear optics, which
enables one to reach both the fundamental limits: one by linear optics and the
other by the no-cloning theorem. The quantum repeater based on our scheme
allows one to achieve fast and efficient quantum communication over arbitrary
long distances, outperforming previous all-photonic and matter-based protocols.
Our work provides a fundamental building block for quantum networks within but
toward the ultimate limits of all-optical scalability.
