appropriate [*pre-*]{} and [*post-selection*]{} of the polarisation of the output light, tiny changes of the temporal delay $\tau$ can cause anomalously large changes of its spectrum, rendering in principle detectable very small temporal delays.

![Weak value amplification scheme aimed at detecting extremely small temporal delays. The input pulse polarisation state is selected to be left-circular by using a polariser, a quarter-wave plate (QWP) and a half-wave plate (HWP). A first polarising beam splitter (PBS$_1$) splits the input into two orthogonal linear polarisations that propagate along different arms of the interferometer. An additional QWP is introduced in each arm to rotate the beam polarisation by $90^{\circ}$ to allow the recombination of both beams, delayed by a temporal delay $\tau$, in a single beam by the same PBS. After PBS$_1$, the output polarisation state is selected with a liquid crystal variable retarder (LCVR) followed by a second polarising beam splitter (PBS$_2$). The variable retarder is used to set the parameter $\theta$ experimentally. Finally, the spectrum of each output beam is measured using an optical spectrum analyzer (OSA). ($\hat{x}$,$\hat{y}$) and ($\hat{u}$,$\hat{v}$) correspond to two sets of orthogonal polarisations. Figure drawn by one of the authors (Luis-Jose Salazar Serrano).[]{data-label="figure_scheme"}](figure1.eps){width="70.00000%"}

Let us be more specific 