Satyabrata Adhikari

Entanglement witness operator for quantum teleportation.

The ability of entangled states to act as a resource for teleportation is linked to a property of the fully entangled fraction. We show that the set of states with their fully entangled fraction bounded by a threshold value required for performing teleportation is both convex and compact. This feature enables the existence of Hermitian witness operators, the measurement of which could distinguish unknown states useful for performing teleportation. We present an example of such a witness operator illustrating it for different classes of states.

Operational meaning of discord in terms of teleportation fidelity

Quantum discord is a prominent measure of quantum correlations, playing an important role in expanding its horizon beyond entanglement. Here we provide an operational meaning of (geometric) discord, which quantifies the amount of non-classical correlation of an arbitrary quantum system in terms of its minimal distance from the set of classical states, in terms of teleportation fidelity for general two qubit and

Swapping path-spin intraparticle entanglement onto spin-spin interparticle entanglement

d \otimes d

Information transfer using a single particle path-spin hybrid entangled state

dimensional isotropic and Werner states. A critical value of the discord is found beyond which the two qubit state must violate the Bell inequality. This is illustrated by an open system model of a dissipative two qubit. For the

Optimal quantum communication using multiparticle partially entangled states

d \otimes d

Quantum teleportation using non-orthogonal entangled channels

dimensional states the lower bound of discord is shown to be obtainable from an experimentally measurable witness operator.

Teleportation of two-mode squeezed states

Based on a scheme that produces an entanglement between the spin and the path variables of a single spin-(1/2) particle (qubit) using a beam-splitter and a spin-flipper, we formulate a procedure for transferring this intraparticle hybrid entanglement to an interparticle entanglement between the spin variables of two other spatially separated spin-(1/2) particles which never interact with each other during the entire process. This procedure of entanglement swapping is accomplished by a Mach-Zehnder setup in conjunction with the Stern-Gerlach measuring device and by using suitable unitary operations. The proposed protocol, thus, enables the use of intraparticle entanglement as a resource —a feature that has remained unexplored.

Broadcasting of continuous variable entanglement

The path-spin entangled state of a single spin-1/2 particle is considered which is generated by using a beam-spitter and a spin-flipper. Using this hybrid entanglement at the level of a single particle as a resource, we formulate a protocol for transferring of the state of an unknown qubit to a distant location. Our scheme is implemented by a sequence of unitary operations along with suitable spin-measurements, as well as by using classical communication between the two spatially separated parties. This protocol, thus, demonstrates the possibility of using intraparticle entanglement as a physical resource for performing information theoretic tasks.

Quantum cloning, Bell's inequality and teleportation

We propose a three-qubit partially entangled set of states as a shared resource for optimal and faithful quantum information processing. We show that our states always violate the Svetlichny inequality, which is a Bell-type inequality whose violation is a sufficient condition for the confirmation of genuine three-qubit nonlocality. Although our states can be physically realized from the generalized Greenberger-Horne-Zeilinger (GGHZ) states using a simple quantum circuit, the nonlocal properties of the set are quite different from the GGHZ states. Instead, they are similar to the maximal slice (MS) states, even though our states are not locally equivalent to the MS states. Unlike other two- and three-qubit partially entangled states, quantum teleportation using our states results in faithful transmission of information with unit probability and unit fidelity by performing only standard measurements for the sender, controller, and receiver. We further demonstrate that dense coding also leads to the deterministic transfer of a maximum number of bits from the sender to the receiver. We also introduce witness operators able to experimentally detect the family of states introduced. This work highlights the importance of both the local as well as nonlocal aspects of quantum correlations in multiqubit systems.

Hybrid Quantum Cloning Machine

We study quantum teleportation with the resource of non-orthogonal qubit states. We first extend the standard teleportation protocol to the case of such states. We investigate how the loss of teleportation fidelity resulting from the use of non-orthogonal states compares to a similar loss of fidelity when noisy or non-maximally entangled states are used as the teleportation resource. Our analysis leads to some interesting results on the teleportation efficiency of both pure and mixed non-orthogonal states compared to that of non-maximally entangled and mixed states.