Witnessing quantum entanglement in ensembles of nitrogen-vacancy centers coupled to a superconducting resonator

Quasi-probability information in an coupled two-qubit system interacting non-linearly with a coherent cavity under intrinsic decoherence

We explore the phase space quantum effects, quantum coherence and non-classicality, for two coupled identical qubits with intrinsic decoherence. The two qubits are in a nonlinear interaction with a quantum field via an intensity-dependent coupling. We investigate the non-classicality via the Wigner functions. We also study the phase space information and the quantum coherence via the Q-function, Wehrl density, and Wehrl entropy. It is found that the robustness of the non-classicality for the superposition of coherent states, is highly sensitive to the coupling constants. The phase space quantum information and the matter-light quantum coherence can be controlled by the two-qubit coupling, initial cavity-field and the intrinsic decoherence.

Stereographic geometry of coherence and which-path information

Recently, it was shown that quantum entanglement is an indispensable part of the duality behavior of light. Here we report a surprisingly intimate connection between the stereographic projection and the duality-entanglement nature of a single photon. We show that the duality-entanglement relation [Optica5, 942 (2018)OPTIC82334-253610.1364/OPTICA.5.000942] naturally emerges from the stereographic projection geometry. We demonstrate that this geometry is complementarity sensitive, in the sense that it is sensitive to the particle nature, wave nature, and entanglement nature of a single photon.

Linear entropy of multiqutrit nonorthogonal states

We derive a closed-form analytical expression for the linear entropy of a multipartite qutrit state, providing a quantitative measure for quantum entanglement within the class of n-mode nonorthogonal qutrit states with any n. Conditions for enhanced and maximum quantum entanglement of multipartite qutrit states are identified. The usefulness of the introduced multipartite qutrit states as quantum communication channel resources is analyzed. The Hamiltonians allowing for the generation of multipartite qutrit states can be attained by combining optomechanical cavities with sequences of tunable beam splitters.

Macroscopically distinct superposition in a spin ensemble coupled to superconducting flux-qubits

Large optical nonlinearities can create fancy physics, such as big Schrödinger-cat states and quadrature squeezing. We present the possibility to practically generate macroscopic Schrödinger-cat states, based on a giant Kerr nonlinearity, in a diamond nitrogen-vacancy ensemble interacting with two coupled flux-qubits. The nonlinearity comes from a four-level N-type configuration formed by two coupled flux-qubits under the appropriately driving fields. We discuss the experimental feasibility in the presence of system dissipations using current laboratory technology and our scheme can be easily extended to other ensemble systems.