Shot-by-shot imaging of Hong–Ou–Mandel interference with an intensified sCMOS camera. M. Jachura and R. Chrapkiewicz in Opt. Lett. 40:1540 (2015). What the paper says!?
This great and seminal paper reports the first observation of the HOM effect in space, with a neat naked-eye observation of «the photon coalescence effect with high-spatial resolution» and «image [...] shot-by-shot the photon coalescence effect with high-spatial resolution»:
This prominent quantum optical effect has been studied so far only using area-integrating detectors.
The effect reduces to the usual HOM dip when using standard APDs.
They use an image intensifier that magnifies the input from a SPDC and image clicks («flashes») as 25-pixel Gaussian spots on the camera. The source generates two-photon pairs but being squeezed, it comes with accidental coincidences which are suppressed. Additional erasing is made on the source to make its photons further indistinguishable.
The spatial extension comes from a calcite displacer:
separated by means of the 30-mm-long calcite beam displacer whose rear surface is imaged onto the camera detector
This «defines two orthogonally polarized Gaussian-like modes separated by 3.2 mm.» This was then squished into an oblate form and
in 700px times 22px frames, each pixel «acting as a binary single-photon detector.»mapped onto the 6.5 μm × 6.5 μm pixel size sCMOS sensor with a magnification of M = 1.1 in horizontal direction
Since both modes are Gaussian, the indistinguishability is over the polarization degree of freedom:
In the experiment, we set the polarization angle of orthogonally polarized photon pair to 45° with respect to the basis defined by the calcite beam displacer, which then acts effectively as a balanced beam-splitter. Ideally, it leads to the perfect HOM interference where outgoing photons coalesce upon leaving the displacer together in one of the two available output modes [14].
They point to pioneering single-photon imaging experiments as: (both references are misquoted as PRA when they are really PRL)
methods for realizing spatially resolved coincidence measurements were based on scanning single-pixel detectors [12] or detector arrays reaching a dozen of pixels [13].
These previous experiments with photon detection, however, «operate in the regime of several to hundreds of photons per camera frame».
They also report interesting single-particle reconstruction of the single mode (their Fig. 5)