Quantum discord, EPR steering and Bell-type correlations for secure CV quantum communications

Abstract

Quantum states can be correlated in ways beyond what is possible for classical states. These correlations are considered as the main resource for quantum computation and communication tasks. In this thesis, I present my studies on the different forms of Quantum Correlations known as "Quantum Discord", "Einstein-Podolsky-Rosen(EPR) Steering" and "Bel-type correlations" in the continuous-variable quantum states and investigate their practical applications for the secure quantum communication. While previously quantum entanglement was considered as the only form of quantum correlation, in the recent years a notion known as quantum discord which captures extra quantum correlations beyond entanglement was introduced by Ollivier and Zurek. This sort of non-classicality that can exist even in separable states, has raised so much aspiration for the potential applications, as they are less fragile than the entangled states. Therefore, of especial interest is to know if a bipartite quantum state is discordant or not. In this thesis I will describe the simple and efficient experimental technique that we have introduced and experimentally implemented to verify quantum discord in unknown Gaussian states and a certain class of non-Gaussian states. According to our method, the peak separation between the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurement conducted on the other subsystem is an indication of nonzero quantum discord. We implemented this method experimentally by preparing bipartite Gaussian and non-Gaussian states and proved nonzero quantum discord in all the prepared states. Though quantum key distribution has become a mature technology, the possibility of hacking the devices used in the quantum communications has motivated the scientists to develop the schemes where one or non of the devices used by the communicating parties need to be trusted. Quantum correlations are the key to develop these schemes. Particularly, EPR steering is connected to the one-sided-deviceindependent quantum key distribution in which devices of one party are solely trusted and Bell-type correlations to the fully device-independent quantum key distribution where non of the apparatuses of the communicating parties is trusted. Here, I will present the result of our theoretical and experimental research to develop one-sided-device-independent quantum key distribution in continuous variables. We identify all Gaussian protocols that can in principle be one-sided-device independent. This consists of 6 protocols out of 16 possible Gaussian protocols, which surprisingly includes the protocol that applies only coherent states. We experimentally implemented both the entanglement-based and coherent state protocols and manifested their loss tolerance. Our results open the door for the practical secure quantum communications, asserting the link between the EPR-steering andone-sided-device-independence. Due to the maturity of quantum information using continuous variables, it is important to develop a Bell-type inequality in this regime. Despite its fundamental importance, Bell-type correlation is linked to the device-independent quantum key distribution. I developed a computer modelling based on the proposal of ref [1, 2] to demonstrate continuous-variable Bell-type correlation. The results of my computer simulations that are presented in this thesis show the feasibility of these proposals, which makes the real-life implementation of continuous-variable device-independent quantum key distribution possible. Show more