A versatile source of polarization-entangled photons
 

We propose a method for the generation of a large variety of entangled states, encoded in the polarization degrees of freedom of N photons, within the same experimental setup. Starting with uncorrelated photons, emitted from N arbitrary single-photon sources, and using linear optical tools only, we demonstrate the creation of all symmetric states (e.g., GHZ and W states), as well as all symmetric and nonsymmetric total angular momentum eigenstates of the N-qubit compound. [1]

 [1] A. Maser, R. Wiegner, U. Schilling, C. Thiel, J. von Zanthier, Phys. Rev. A 81, 053842 (2010) 

 

 Heralded entanglement of arbitrary degree in remote qubits
 

Incoherent scattering of photons off two remote atoms with a Λ-level structure is used as a basic Young-type interferometer to herald long-lived entanglement of an arbitrary degree. The degree of entanglement, as measured by the concurrence, is found to be tunable by two easily accessible experimental parameters. Fixing one of them to certain values unveils an analog to the Malus’ law. An estimate of the variation in the degree of entanglement due to uncertainties in an experimental realization is given. [1]

 [1] U. Schilling, C. Thiel, E. Solano, T. Bastin, J. von Zanthier, Phys. Rev. A 80, 022312 (2009) 

 

 Generation of Total Angular Momentum Eigenstates in Remote Qubits
 

We propose a scheme enabling the universal coupling of angular momentum of N remote noninteracting qubits using linear optical tools only. Our system consists of N single-photon emitters in a Λ configuration that are entangled among their long-lived ground-state qubits through suitably designed measurements of the emitted photons. In this manner, we present an experimentally feasible algorithm that is able to generate any of the 2^N symmetric and nonsymmetric total angular momentum eigenstates spanning the Hilbert space of the N-qubit compound. [1]

 [1] A. Maser, U. Schilling, T. Bastin, E. Solano, C. Thiel, J. von Zanthier, Phys. Rev. A 79, 033833 (2009) 

 

 Operational Determination of Multiqubit Entanglement Classes via Tuning of Local Operations
 

We present a physical setup with which it is possible to produce arbitrary symmetric long-lived multiqubit entangled states in the internal ground levels of photon emitters, including the paradigmatic Greenberger-Horne-Zeilinger and W states. In the case of three emitters, where each tripartite entangled state belongs to one of two well-defined entanglement classes, we prove a one-to-one correspondence between well-defined sets of experimental parameters, i.e., locally tunable polarizer orientations, and multiqubit entanglement classes inside the symmetric subspace. [1]

 [1] T. Bastin, C. Thiel, J. von Zanthier, L. Lamata, E. Solano, G. S. Agarwal, Phys. Rev. Lett. 102, 053601 (2009) 

 

 Generation of Symmetric Dicke states of Remote Qubits with Linear Optics
 

We propose a method for generating all symmetric Dicke states, either in the long-lived internal levels of N massive particles or in the polarization degrees of freedom of photonic qubits, using linear optical tools only. By means of a suitable multiphoton detection technique, erasing Welcher-Weg information, our proposed scheme allows the generation and measurement of an important class of entangled multiqubit states. [1]

 [1] C. Thiel, J. von Zanthier, T. Bastin, E. Solano, G. S. Agarwal, Phys. Rev. Lett. 99, 193602 (2007)