  We consider a device which can be programmed using coherent states of light
to approximate a given projective measurement on an input coherent state. We
provide and discuss three practical implementations of this programmable
projective measurement device with linear optics, involving only balanced beam
splitters and single photon threshold detectors. The three schemes optimally
approximate any projective measurement onto a program coherent state in a
non-destructive fashion. We further extend these to the case where there are no
assumptions on the input state. In this setting, we show that our scheme
enables an efficient verification of an unbounded untrusted source with only
local coherent states, balanced beam splitters, and threshold detectors.
Exploiting the link between programmable measurements and generalised swap
test, we show as a direct application that our schemes provide an
asymptotically quadratic improvement in existing quantum fingerprinting
protocol to approximate the Euclidean distance between two unit vectors.
