Testing how weird nature really is

According to quantum theory, two quantum objects can be "entangled" in a way that only joint properties are sharply defined, while the individual properties of each object are lost. For instance, two "quantum arrows" (spins) can be such that they surely point in opposite directions (sharp joint property) but no arrow points in a definite direction (individual properties are lost). This is clearly at odds with our everyday experience, in which joint properties appear only as consequences of individual ones.
23 January 2008

But is it really the case that individual properties are lost? Or is this just an artificial feature of the mathematical formalism, with no counterpart in nature? Remarkably, experiments can be designed to settle the question.

Early in 2007, based on a proposal by Nobel Laureate Tony Leggett, Zeilinger’s group in Vienna performed the first experiment aimed at proving that the usual individual properties cannot indeed be ascribed to two entangled photons. A few months later, some researchers in CQT, together with colleagues from the University of Geneva, discovered a stronger criterion than the ones previously available, and could perform a new experiment that confirmed the previous conclusion while removing some unnecessary assumptions [1].

More recently, improvements have been presented as a follow-up of that work [2]. In particular, it has been proved that the observation of quantum entanglement is incompatible with the presence, not only of the usual individual properties, but of any individual property whatsoever. In an entangled state, individual particles really lose their own properties and keep only strong inter-particle relations.


  • C. Branciard, A. Ling, N. Gisin, C. Kurtsiefer, A. Lamas-Linares and V. Scarani, Physical Review Letters 99, 210407 (2007); preprint available
  • C. Branciard, N. Brunner, N. Gisin, C. Kurtsiefer, A. Lamas-Linares, A. Ling and V. Scarani, Nature Physics