  We propose a scheme for fault-tolerant long-range entanglement generation at
the ends of a rectangular array of qubits of length $R$ and a square cross
section of size $d\times d$ with $d=O(\log R)$. Up to an efficiently computable
Pauli correction, the scheme generates a maximally entangled state of two
qubits using a depth-$6$ circuit consisting of nearest-neighbor Clifford gates
and local measurements only. Compared with existing fault-tolerance schemes for
quantum communication, the protocol is distinguished by its low latency:
starting from a product state, the entangled state is prepared in a time
$O(t_{\textrm{local}})$ determined only by the local gate and measurement
operation time $t_{\textrm{local}}$. Furthermore, the requirements on local
repeater stations are minimal: Each repeater uses only $\Theta(\log^2 R)$
qubits with a lifetime of order $O(t_{\textrm{local}})$. We prove a converse
bound $\Omega(\log R)$ on the number of qubits per repeater among all
low-latency schemes for fault-tolerant quantum communication over distance $R$.
Furthermore, all operations within a repeater are local when the qubits are
arranged in a square lattice.
  The noise-resilience of our scheme relies on the fault-tolerance properties
of the underlying cluster state. We give a full error analysis, establishing a
fault-tolerance threshold against general (circuit-level) local stochastic
noise affecting preparation, entangling operations and measurements. This
includes, in particular, errors correlated in time and space. Our conservative
analytical estimates are surprisingly optimistic, suggesting that the scheme is
suited for long-range entanglement generation both in and between near-term
quantum computing devices.
