Talking entanglement

Vladyslav C. Usenko, Olena Kovalenko, Radim Filip

We address distribution of two-mode continuous-variable entanglement in the presence of linear cross-talk and channel imperfections. We show how cross-talk degrades entanglement and leads to necessity of optimization of the initial entangled resource. We suggest a method for compensating the cross-talk by a phase-flipping beamsplitter, which can optimally remove the negative effect of cross-talk once the modes undergo almost the same channel attenuation.

Olena Kovalenko, Vladyslav C. Usenko, Radim Filip

Two-mode squeezed states are scalable and robust entanglement resources for continuous-variable and hybrid quantum information protocols that are realized at a distance. We consider the effect of a linear cross talk in the multimode distribution of two-mode squeezed states propagating through parallel similar channels. First, to reduce degradation of the distributed Gaussian entanglement, we show that the initial two-mode squeezing entering the channel should be optimized already in the presence of a small cross talk. Second, we suggest simultaneous optimization of relative phase between the modes and their linear coupling on a receiver side prior to the use of entanglement, which can fully compensate the cross talk once the channel transmittance is the same for all the modes. For the realistic channels with similar transmittance values for either of the modes, the cross talk can be still largely compensated. This method relying on the mode interference overcomes an alternative method of entanglement localization in one pair of modes using measurement on another pair and feed-forward control. Our theoretical results pave the way to more efficient use of multimode continuous-variable photonic entanglement in scalable quantum networks with cross talk.

Vladyslav C. Usenko, Olena Kovalenko, Radim Filip

We analyze degradation of multimode entanglement and quantum key distribution in the presence of cross talk. As countermeasures we suggest optimization of initial entangled resource or cross talk compensation by additional coupling between the modes.