A protocol to measure the relative arrival time between two photons with few-attosecond time resolution has been developed by researchers in the UK. Ashley Lyons and co-workers measure coincident event counts for photon pairs by Hong–Ou–Mandel (HOM) interferometry, but importantly, they use the estimated total Fisher information as a means to extract highly precise timing information. Photon pairs at the wavelength of 808 nm (generated by sending 130-fs pulses from a frequency-doubled Ti:sapphire laser to a nonlinear crystal) were sent to a dual-arm HOM interferometer. When the HOM coincidence dip was scanned as a function of the relative arrival time, two peaks asymptotically appeared in the estimated total Fisher information as the team theoretically predicted. By using a fitting calculation, the relative arrival time was obtained with accuracy of 0.5 as and precision of 4.7 as. With a small modification, HOM interferometry could be applied to quantum optical coherence tomography for biological specimens such as DNA.