(a) Input signal photons stored in the diamond by the strong write pulse can be retrieved with modified spectral properties upon output. The output spectrum is controlled by the spectrum of the read pulse. (b) Photons are Raman-absorbed to create optical phonons (|1〉), 40 THz above the ground state (|0〉). A read pulse of tunable wavelength and bandwidth retrieves the photon, determining its spectrum. Here, Δ is the detuning from the conduction band (|2〉), δ is the input photon bandwidth, Δω is the detuning between input and output frequencies. (c) The master laser (red) is split between the write field and photon source. In the photon source, frequency-doubled laser light pumps SPDC and heralded input signal photons (green) are generated. Signal photons are Raman-absorbed into the optical phonon modes in the diamond via the write field. The slave laser (orange) emits the read field which retrieves the photon from the diamond after time . The output signal photon (blue) spectrum is measured on a monochromator. The SFG of read and write pulses triggers the experiment. Coincident detections of output, herald photons and SFG events are measured by a coincidence logic unit. SFG, second-harmonic generation; SPDC, spontaneous parametric downconversion; APD, avalanche photodiode; PD, photodiode; SFG, sum-frequency generation; BBO, β-barium borate; PBS, polarizing beamsplitter.