Fast quantum interferometry at the nanometer and attosecond scales with energy-entangled photons

In classical optical interferometry, loss and background complicate
achieving fast nanometer-resolution measurements with illumination at low
light levels. Conversely, quantum two-photon interference is unaffected by
loss and background, but nanometer-scale resolution is physically
difficult to realize. As a solution, we enhance two-photon interference
with highly non-degenerate energy entanglement featuring photon
frequencies separated by 177 THz. We observe measurement resolution at the
nanometer (attosecond) scale with only O(104) photon pairs, despite the
presence of background and loss. Our non-destructive thickness measurement
of a metallic thin film agrees with atomic force microscopy, which often
achieves better resolution via destructive means. With contactless,
non-destructive measurements in seconds or faster, our instrument enables
metrological studies in optically challenging contexts where background,
loss, or photosensitivity are factors.