Controlled Dense Coding Research Articles

Controlled Dense Coding between Multi-Parties

Controlled dense coding via multi-particles GHZ state and multi-particles GHZ-class state are exploited in this letter. The quantum channel and the amount of information between the senders and the receivers are controlled by the supervisor via his local measurement. The amount of information is determined by Charlie’s measurement in the former case of GHZ state, and also by the coefficients of the original GHZ-class state in the latter case.

Experimental demonstration of tripartite entanglement and controlled dense coding for continuous variables.

A tripartite entangled state of bright optical field is experimentally produced using an Einstein-Podolsky-Rosen entangled state for continuous variables and linear optics. The controlled dense coding among a sender, a receiver, and a controller is demonstrated by exploiting the tripartite entanglement. The obtained three-mode "position" correlation and relative "momentum" correlation between the sender and the receiver, and thus the improvements of the measured signal to noise ratios of amplitude and phase signals with respect to the shot noise limit are 3.28 and 3.18 dB, respectively. If the mean photon number n equals 11 the channel capacity can be controllably inverted between 2.91 and 3.14. When n is larger than 1.0 and 10.52, the channel capacity of the controlled dense coding is predicted to exceed the ideal single channel capacity of coherent and squeezed state light communication, respectively.

Controlled dense coding for continuous variables using three-particle entangled states

In the development of theoretical and experimental studies of quantum information, the quantum teleportation that is the disembodied transport of an unknown quantum state from a sender to a remote receiver @1# and the dense coding in which the single bit sent from a sender to a receiver can successfully carry two bits of classical information @2#, have attracted extensive interests. The nonlocal quantum entanglement plays a determinant role in the quantum information processing. Towards possible applications in quantum communication, both theoretical and experimental investigations increasingly focus on quantum states of continuous variables in an infinite-dimensional Hilbert space, since the EinsteinPodolsky-Rosen ~EPR! entangled state can be efficiently generated using squeezed light and beam splitters, for instance, the entangled EPR pairs resulting from two-mode squeezed vacuum state have been successfully employed in demonstrating unconditional quantum teleportation @3#. Later, the schemes realizing highly efficient dense coding for continuous variables are theoretically proposed, in which the two-mode squeezed-state entanglement is utilized to achieve unconditional signal transmission @4–6#. The bright EPR beams have been experimentally produced with a nondegenerate optical parametric amplifier ~NOPA! @7# and the dense coding for continuous variables based on bright EPR beam has been demonstrated initially @8#. Loock and Braunstein have given that the superposition of more than independently squeezed states can yield multipartite entanglement for continuous variables and presented the scheme of quantum teleportation using entangled three-mode state @9#. The fidelity in this scheme depends on the measurement of the third particle. Controlled dense coding for discrete variables was proposed recently using the Greenberger-Horne-Zeilinger state ~GHZ! @10#. Inspired by the similarity and difference between dense coding and quantum teleportation, in this paper we study dense coding using the tripartite entangled state. It is shown that when using the tripartite entangled state, the information transmission capacity of dense coding is controlled by the measurement of the third particle. We introduce a simple, experimentally realizable, controlled dense coding protocol for continuous variables by exploring nondegenerate optical parametric amplifier. Due to adopting the