  Time-resolved photon detection can be used to generate entanglement between
distinguishable photons. This technique can be extended to entangle quantum
memories that emit photons with different frequencies and identical temporal
profiles without the loss of entanglement rate or fidelity. We experimentally
realize this process using remotely trapped $^{171}$Yb$^+$ ions where heralded
entanglement is generated by interfering distinguishable photons. This
technique may be necessary for future modular quantum systems and networks that
are composed of heterogeneous qubits.
