Schematic showing the secure communication protocol, via photons, between Alice and Bob. Alice uses a spontaneous parametric down-converter (SPDC) to prepare two entangled photons, one of which she sends to Bob, the other of which she keeps. Bob encodes a signal on the photon he receives and sends it back, but he breaks the initial entanglement with a noise amplifier. The eavesdropper, Eve, is assumed to have access to all the transmission losses, but because she doesn’t have access to how the photons were first entangled, she can’t decode the signal as reliably as Alice. The surprising feature of this protocol is that Alice’s advantage over Eve holds despite the broken entanglement.