We propose a method to improve transmission distance of continuous-variable quantum key distribution via a heralded hybrid linear amplifier (HLA), involving an ideal deterministic linear amplifier and a probabilistic noiseless linear amplifier. The phase-insensitive amplification of quantum light states may experience a degradation of signal-to-noise ratio (SNR) of 3dB which can be surmounted by the probabilistic amplification process. The new hybrid amplifier allows one to tune between the regimes of high gain or high noise reduction and control the trade-off between the amplification coefficient and a finite heralding probability flexibly. Our equipment could clone unknown coherent states with fidelity surpassing the no-cloning limit by abandoning determinism and using probabilistic methods. The setup has a better compatibility with continuous variable protocols as it only uses Gaussian elements and a post-selection algorithm. The hybrid linear amplifier enhances the SNR and hence the transmission distance of the CVQKD protocol is extended. Both the asymptotic secret key rate and the secret key rate in a finite-size scenario are analyzed in this paper.

This study investigates the implementation of measurement-device-independent multiparty quantum communication using continuous-variable Greenberger-Horne-Zeilinger (CV GHZ) states under the fast fading channel. The communication parties are connected through free space, and factors such as atmospheric turbulence cause beam drift, resulting in variations in channel transmittance according to specific probability distributions. We assume a worst-case scenario in which an eavesdropper has complete control over the channel, forcing the communicators to estimate the channel based on this probability distribution. The protocol employs CV GHZ states to achieve secure communication through quantum cryptographic conference and quantum secret sharing. For the aspect of security, independent entangling cloner attack and coherent attack are analyzed. Simulation results demonstrate that the protocol can withstand advantageous attacks from eavesdroppers.

A hybrid linear amplifier is inserted at the output of the source-in-the-middle distribution protocol to overcome the shortcomings of the transmission distance. The modified protocol aims to maintain a high key rate for long-distance transmission under high noise. It has the potential to significantly broaden the application range of the continuous variable quantum key distribution protocol. The effects of amplifier parameters and noise on the modified protocol are analyzed in detail with regard to applying it to a practical system. To make the simulation more realistic, the effect of finite size on the new protocol is taken into account. It will serve as a guideline for the future use of hybrid linear amplifiers. Different parameters can be adjusted to achieve the best performance for key rates of different quantum channels.