Figure 4

Schematic of the energy test at Bob’s side. Bob uses a beam splitter with transmissivity T to split the incoming signal into two parts. The transmission mode \({X}_{B}^{{\prime} }\) is used for generating Bob’s data and the reflection mode a′ is exploited to perform the energy test. a and b are two vacuum modes induced by beam splitters. Modes t₁ and t₂ are the output modes of the balanced beam splitter used for checking whether \(|{q}_{{t}_{1}}|\) and \(|{p}_{{t}_{2}}|\) are below a certain threshold.