I've read De Broglie's book "La Physique Nouvelle Et Les Quanta" from 1936, which translates in "The New Physics and Quanta" but I haven't found the english version of the book (not sure it exists). This book looks over what you can learn during approximately the 2 first years of a bachelor in physics. So, close to the end of the book he's talking about this (translated by myself so sorry if some specific terms are not the right ones): "[...] one could say that the notion of physical individual is complementary, in the sense of Bohr, to the notion of system. The particle only has a well defined individuality when it is isolated. As soon as it interacts with other particles, its individuality is decreased. Perhaps had we not sufficiently noticed in the classical theories that the concept of potential energy of a system involves a weakening of individuality for the components of this system as a result of the pooling, under form of potential energy, of a part of the total energy. In cases envisaged by the new mechanics where particles of the same nature occupy, somehow simultaneously, the same region of space*, the individuality of these particles diminishes until it disappears. Passing gradually from the case of isolated particles without interactions to the case last mentioned, we see the concept of particles' individuality fade gradually as the system's individuality asserts itself. So it appears that the individual and the system are somehow complementary idealizations**. It may be an idea that should be studied more closely." *: Linked to what he calls "exchange energy" and says about it: "The particles are generally not localized in wave mechanics, they have a field of possible presence in space: there is an exchange energy when two particles of the same kind have areas of possible presence that impinge on one another, and in this case only." So like overlap in molecular orbitals. **: By this he means that this is of the same nature than the wave-particle duality, as he says in the first setence of this excerpt. So was this idea indeed studied more closely since he wrote that book? Did it lead somewhere? If it did, this is obviously somehow linked to quantum entanglement (but I don't know the theoretical part about it).