Siddhartha Santra

Entanglement swapping of two arbitrarily degraded entangled states

We consider entanglement swapping, a key component of quantum network operations and entanglement distribution. Pure entangled states, which are the desired input to the swapping protocol, are typically mixed by environmental interactions causing a reduction in their degree of entanglement. Thus an understanding of entanglement swapping with partially mixed states is of importance. Here we present a general analytical solution for entanglement swapping of arbitrary two-qubit states. Our result provides a comprehensive method for analyzing entanglement swapping in quantum networks. First, we show that the concurrence of a partially mixed state is conserved when this state is swapped with a Bell state. Then, we find upper and lower bounds on the concurrence of the state resulting from entanglement swapping for various classes of input states. Finally, we determine a general relationship between the ranks of the initial states and the rank of the final state after swapping.

Entanglement-enabled interferometry using telescopic arrays

We consider quantum enhancement of direct-detection interferometric measurements to increase telescope resolution. We propose a protocol of measuring interferometric visibility function using imperfectly entangled states shared between remote telescopes. We show how errors in visibility measurement, and in turn, errors in intensity distribution of a distant object depend on the degree of entanglement of the shared quantum resource. We determine that these errors are sufficiently small over a wide range of resource states which makes our technique feasible in practical environments.ABSTRACT A protocol of measuring interferometric visibility function using imperfectly entangled states shared between remote telescopes is proposed. We demonstrate how quantum entanglement can be utilized to increase the baseline size of telescopic arrays thereby providing substantial enhancement to the resolution of direct-detection interferometric measurements. We demonstrate, through a comprehensive analysis, how errors in visibility measurements and in the intensity distribution of a distant object show dependence on the entanglement degree of the shared quantum resource. We analyse the feasibility of the protocol using currently available technology and identify the nature of sources that can benefit most from it.

Entanglement swapping with two imperfect states

We present a formal description of entanglement swapping of any two, arbitrarily mixed, density matrices. Application of this result reveals bounds on the rank and concurrence of the final state for several classes of input states.

Quantum-scheme for improving interferometric visibility with imperfect distributed entangled-states

We analyze a quantum-scheme for measuring interferometric visibility for telescopic applications with imperfectly entangled states as a resource. The scheme overcomes photon loss thus permitting measurement with larger baseline leading to finer resolution of angular intensity distribution of remote sources of light.

Propagation of correlations in local random quantum circuits

We derive a dynamical bound on the propagation of correlations in local random quantum circuits—lattice spin systems where piecewise quantum operations—in space and time—occur with classical probabilities. Correlations are quantified by the Frobenius norm of the commutator of two positive operators acting on disjoint regions of a one-dimensional circular chain of length L. For a time

Quantum Repeaters Based on Two-Species Trapped Ions

t=O(L)

Quantum repeater architecture using two-species trapped ions

t=O(L), correlations spread ballistically to spatial distances