An apparatus for measuring an input signal, the apparatus comprising: a first light source configured to output a sequence of pulses of light, wherein there is a random relationship between the phase of the pulses; a beam splitter having first and second inputs and first and second outputs, said first input being arranged to receive light pulses from said first light source and the second input being arranged to receive said input signal; a differencing circuit adapted to subtract signals obtained from the first and second outputs from each other; and output a value; and a processing circuit adapted to estimate the minimum entropy of said input signal, from the output of the differencing circuit corresponding to a sequence of said light pulses from the first light source.