Abstract
Measurements of an object's temperature are important in many disciplines,
from astronomy to engineering, as are estimates of an object's spatial
configuration. We present the quantum optimal estimator for the temperature of
a distant body based on the black body radiation received in the far-field. We
also show how to perform quantum optimal estimates of the spatial configuration
of a distant object, i.e. imaging. In doing so we necessarily deal with
multi-parameter quantum estimation of incompatible observables, a problem that
is poorly understood. We compare our optimal observables to the two mode
analogue of lensed imaging and find that the latter is far from optimal, even
when compared to measurements which are separable. To prove the optimality of
the estimators we show that they minimise the cost function weighted by the
quantum Fisher information-this is equivalent to maximising the average
fidelity between the actual state and the estimated one.
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