We employ active feedback to stabilize the frequency of single photons emitted by two separate quantum dots to an atomic standard. The transmission of a single, rubidium-based Faraday filter serves as the error signal for frequency stabilization to less than 1.5% of the emission linewidth. Long-term stability is demonstrated by Hong-Ou-Mandel interference between photons from the two quantum dots. The observed visibility of $V_{mathrm{lock}}=(41 pm 5)$% is limited only by internal dephasing of the dots. Our approach facilitates quantum networks with indistinguishable photons from distributed emitters.
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