Quantum steering without inputs

Quantum networks with multiple sources allow the observation of quantum nonlocality without inputs. Consequently, the incompatibility of measurements is not a necessity for observing quantum nonlocality when one has access to multiple quantum sources. In this talk, I will present the minimal input-free network where such correlations arise: a two-party, two-source configuration in which nonlocality appears as a new form of quantum steering, which we call swap-steering. Remarkably, this effect occurs even when the sources may be classically correlated, as long as one party is trusted to perform a fixed, known measurement. Interestingly, one cannot observe Bell-type correlations in this scenario; thus, a scenario exists where quantum steering can be observed but Bell non-locality cannot. I will introduce a simple linear witness to detect swap-steering and show that every entangled state and every entangled measurement gives rise to swap-steerable correlations—demonstrating that all entangled resources are fundamentally “non-classical” in the sense that they can steer a far-away system without signalling. As an application to practical problems, one of the proposed witnesses enables self-testing of the quantum states generated by the sources and the local measurement of the untrusted party. This, in turn, allows certifying two bits of randomness that can be obtained from the measurement outcomes of the untrusted device without the requirement of initially feeding the device with randomness. This is the first example of a scheme where secure randomness can be extracted without feeding in initial randomness into the scheme.
For curiosity, refer to arXiv:2307.08797, arXiv:2406.11994, arXiv:2502.06986.