Spectrally resolved four-photon interference of time-frequency-entangled photons

In this paper and an accompanying submission to $P\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ $R\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}w$ $L\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}s$, the authors provide a demonstration of entanglement swapping extended into the multimode regime of ultrafast frequency-mode entanglement. The most salient feature of the method, which requires no source engineering, is the heralding of several mutually orthogonal Bell states derived from identical multimode entangled photon pairs.