Output state in multiple entanglement swapping
Aditi Sen, Ujjwal Sen, Marek Żukowski · Physical Review A · 2003
The technique of quantum repeaters is a promising candidate for sending quantum states over long distances through a lossy channel. The usual discussion of this technique deals with only a finite-dimensional Hilbert space. However the qubits, with which one implements this procedure, will ``ride'' on continuous degrees of freedom of the carrier particles. Here we analyze the action of quantum repeaters using a model based on pulsed parametric down-conversion entanglement swapping. Our model contains the basic traits of a real experiment. We show that the state created, after the use of any number of parametric down-converters in a series of entanglement swappings, is always an entangled (actually distillable) state, although of a different form than the one that is usually assumed. Furthermore, the output state always violates a Bell inequality.