postselected

Postselected refers to a process in quantum mechanics where a quantum system is prepared in an initial state and then, after evolving, a measurement is performed on a part of the system or an auxiliary system. The outcome of this measurement, the "postselection," determines whether the remaining part of the system is considered in a particular final state. This is distinct from standard measurements where the entire system is directly probed.

The concept of postselection is crucial in understanding phenomena like quantum entanglement and in certain experimental techniques. For instance, in experiments involving entangled photons, one photon might be measured, and if it yields a specific outcome, the other photon is considered to be in a correlated state, even if it wasn't directly measured at that moment. This "effective" state of the unmeasured photon is said to be postselected.

While postselection allows for the exploration of conditional quantum states, it's important to note that it does not enable faster-than-light communication. The information about the postselected state is only revealed after classical communication of the measurement outcome. Furthermore, the efficiency of postselection can be a limiting factor in experiments, as only a fraction of the prepared systems will satisfy the postselection criteria. Despite these limitations, postselection remains a powerful conceptual and experimental tool in quantum information science.