Dear Martin,

A little shameless self-promotion of our result on Fredkin gate and measuring purity/overlap:

Stárek, et al., npj Quantum Information 4, 35 (2018), https://www.nature.com/articles/s41534-018-0087-x

Generally speaking, nonlinear functions of a quantum state (density matrix) can be measured by interfering multiple copies of the state and applying parity detection:

Filip, PRA 65, 062320 (2002),

Ekert et al. PRL 88, 217901 (2002).

The same holds for entanglement characterization:

Horodecki, PRL 90, 167901 (2003),

Fiurášek & Cerf, PRL 93, 063601 (2004).

Experimental tests for bipartite states:

Walborn et al. Nature 440, 1022 (2006),

Islam et al. Nature 528, 77 (2015).

Best,

Miroslav

Dear Miroslav,

thank you very much!

Also, congratulation on Nature publication :)

Best,

Martin