The maximum separation between two entangled qubits achieved experimentally has varied depending on the specific experimental setup and platform used. Here are a few notable examples:
Photons: In experiments with entangled photons, long-distance entanglement distribution has been achieved over several hundreds of kilometers through techniques such as quantum teleportation or using satellite-based systems. Notable experiments include the Chinese satellite-based quantum communication project known as Micius, which achieved entanglement distribution between ground stations separated by distances of over 1,200 kilometers.
Trapped ions: Entangled qubits created with trapped ions have been demonstrated over relatively short distances within a few meters. However, ions can be individually manipulated and transported, allowing for the creation of entanglement between ions located at different locations. For instance, in 2011, researchers at the University of Innsbruck in Austria entangled two ions separated by a distance of about one meter.
Superconducting qubits: In the case of superconducting qubits, entanglement has been achieved between physically separated qubits within a single chip, typically over distances ranging from micrometers to millimeters. However, in terms of long-distance entanglement, the current state-of-the-art experiments with superconducting qubits have achieved entanglement over distances of a few meters.
It's important to note that the maximum separation achieved experimentally is subject to ongoing research and technological advancements. The field of quantum communication and long-distance entanglement distribution is rapidly evolving, and researchers are actively exploring various techniques and platforms to extend entanglement over longer distances.