Motivated by the question, which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping.There, entanglement can be "produced a posteriori, after the entangled particles have been measured and may no longer exist." In this work we report the first realization of Peres' gedanken experiment. Using four photons, we can actively delay the choice of measurement -implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator -on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as "quantum steering into the past".In the entanglement swapping 1-3 procedure, two pairs of entangled photons are produced, and one photon from each pair is sent to Victor. The two other photons from each pair are sent to Alice and Bob, respectively. If Victor projects his two photons onto an entangled state, Alice's and Bob's photons are entangled although they have never interacted or shared any common past. What might be considered as even more puzzling is Peres' idea of "delayed-choice for entanglement swapping" 4,5 . In this gedanken experiment, Victor is free to choose either to project his two photons onto an entangled state and thus project Alice's and Bob's photons onto an entangled state, or to measure them individually and then project Alice's and Bob's photons onto a separable state. If Alice and Bob measure their photons' polarization states before Victor makes his choice and projects his two photons either onto an entangled state or onto a separable state, it implies that whether their two photons are entangled (showing quantum correlations) or separable (showing classical correlations) can be defined after they have been measured.In order to experimentally realize Peres' gedanken experiment, we place Victor's choice and measurement in the time-like future of Alice's and Bob's measurements, providing a "delayed-choice" configuration in any and all reference frames. This is accomplished by (1) proper optical delays for Victor's photons and (2) a high-speed tunable bipartite state analyzer, which (3) is controlled in real time by a quantum random number generator (QRNG) 6 . Both delay and randomness are needed to avoid the possibility that the photon pairs can "know" in advance which setting will be implemented after they are registered and can behave accordingly by producing , illustrated by a Mach-Zehnder interferometer. In Bohr's gedanken experiment, he illustrated the complementarity principle, one of the most basic principles of quantum mechanics, with a double-slit apparatus. If both slits are open, the input quantum system exhibits "wave-like" behavior and shows interference on the...