  Bell-state projections serve as a fundamental basis for most quantum
communication and computing protocols today. However, with current Bell-state
measurement schemes based on linear optics, only two of four Bell states can be
identified, which means that the maximum success probability of this vital step
cannot exceed $50\%$. Here, we experimentally demonstrate a scheme that amends
the original measurement with additional modes in the form of ancillary
photons, which leads to a more complex measurement pattern, and ultimately a
higher success probability of $62.5\%$. Experimentally, we achieve a success
probability of $(57.9 \pm 1.4)\%$, a significant improvement over the
conventional scheme. With the possibility of extending the protocol to a larger
number of ancillary photons, our work paves the way towards more efficient
realisations of quantum technologies based on Bell-state measurements.

Quantum Physics