Nonlocal bunching of composite bosons

It was suggested that two entangled fermions can behave like a single boson and that the bosonic quality is proportional to the degree of entanglement between the two particles. The relation between bosonic quality and entanglement is quite natural if one takes into account the fact that entanglement appears in bound states of interacting systems. However, entanglement can still be present in spatially separated subsystems that no longer interact. These systems are often the subject of studies on quantum nonlocality and foundations of quantum physics. Here, we ask whether an entangled spatially separated fermionic pair can exhibit bosonic properties. We show that under certain conditions the answer to this question can be positive. In particular, we propose a nonlocal bunching scenario in which two such pairs form an analog of a two-partite bosonic Fock state.

Figure 1

Beam splitting of elementary and composite particles. (a) A single elementary particle can either go through or reflect from the BS. (b) Two noninteracting particles evolve independently and each of them can either go through or reflect from the BS, which leads to four possible outcomes. If the input state is considered to represent a single composite particle, then the evolution inevitably leads to its decay. (c) Two interacting particles stay together; they collectively go through or reflect from the BS, which allows us to treat them as a single particle.

Figure 2

Schematic of the nonlocal bunching of two cobosons. It is impossible to say which pairs of particles form a coboson once they are in the same mode.