Signal detection without finite-energy limits to quantum resolution

We show that there are extremely simple signal detection schemes where the finiteness of energy resources places no limit on the resolution. On the contrary, larger resolution can be obtained with lower energy. To this end the generator of the signal-dependent transformation encoding the signal information on the probe state must be different from the energy. We show that the larger the deviation of the probe state from being the minimum-uncertainty state, the better the resolution.

► Finiteness of resources need not limit resolution in quantum metrology.
► Lower energy may favor larger resolution.
► Probe states deviating from minimum-uncertainty status provide better resolution.