Certification of a Nonprojective Qudit Measurement using Multiport Beamsplitters

Understand

Generalised quantum measurements go beyond the textbook concept of a projection onto an orthonormal basis in Hilbert space.

• They are not only of fundamental relevance but have also an important role in quantum information tasks.
• However, it is highly demanding to certify that a generalised measurement is indeed required to explain the results of a quantum experiment in which only the degrees of freedom are assumed to be known.
• Here, we use state-of-the-art multicore optical fiber technology to build multiport beamsplitters and faithfully implement a seven-outcome generalised measurement in a four-dimensional Hilbert space with a fidelity of $99.7\%$.