  We demonstrate an experimental method for measuring energy-time entanglement
over almost 80 nm spectral bandwidth in a single shot with a quantum bit error
rate below 0.5%. Our scheme is extremely cost-effective and efficient in terms
of resources as it employs only one source of entangled photons and one fixed
unbalanced interferometer per phase-coded analysis basis. We show that the
maximum analysis spectral bandwidth is obtained when the analysis
interferometers are properly unbalanced, a strategy which can be
straightforwardly applied to most of today's experiments based on energy-time
and time-bin entanglement. Our scheme has therefore a great potential for
boosting bit rates and reducing the resource overhead of future
entanglement-based quantum key distribution systems.
