Photons are critical to quantum technologies because they can be used for virtually all quantum information tasks, for example, in quantum metrology1, as the information carrier in photonic quantum computation2,3, as a mediator in hybrid systems4, and to establish long-distance networks5. The physical characteristics of photons in these applications differ drastically; spectral bandwidths span 12 orders of magnitude from 50 THz (ref. 6) for quantum-optical coherence tomography7 to 50 Hz for certain quantum memories8. Combining these technologies requires coherent interfaces that reversibly map centre frequencies and bandwidths of photons to avoid excessive loss. Here, we demonstrate bandwidth compression of single photons by a factor of 40 as well as tunability over a range 70 times that bandwidth via sum-frequency generation with chirped laser pulses. This constitutes a time-to-frequency interface for light capable of converting time-bin to colour entanglement9, and enables ultrafast timing measurements. It is a step towards arbitrary waveform generation10 for single
and entangled photons.