Abstract
AbstractBell states are the most prominent maximally entangled photon states. In a typical four‐level emitter, like a semiconductor quantum dot, the photon states exhibit only one type of Bell state entanglement. By adding an external driving to the emitter system, also other types of Bell state entanglement are reachable without changing the polarization basis. In this work, it is shown under which conditions the different types of entanglement occur and analytical equations are given to explain these findings. Furthermore, special points are identified, where the concurrence, being a measure for the degree of entanglement, drops to zero, while the coherences between the two‐photon states stay strong. Results of this work pave the way to achieve a controlled manipulation of the entanglement type in practical devices.
Highlights
We study under which driving conditions, a fourlevel emitter (FLE) placed in a microcavity produces entangled photons being either in a Φ Bell state (ΦBS) or ΨBS
We demonstrate that a constantly driven FLE undergoes a sharp transition between regions of high ΦBS and ΨBS entanglement for a certain twophoton resonance
We find that at the special point the generated two-photon state is essentially the superposition of the two density matrices created by each transition individually with with the coefficients given in Appendix A.3
Summary
Entanglement of quantum states is one of the most remarkable and interesting physical effects that separate the quantum mechanical from the classical world.[1,2] Entanglement can be used to test quantum mechanical principles on a fundamental level, for example, by revealing violations of Bell inequalities.[2,3] many fascinating and innovative applications, for example, in quantum cryptography,[4,5] quantum communication,[6,7] or quantum information processing and computing,[8,9,10,11] rely on entangled photon pairs. To create maximally entangled states, one of the best established routes is via the cascaded relaxation in few-level systems like atoms, semiconductor quantum dots or F-centers.[12,13,14,15]. We study under which driving conditions, a fourlevel emitter (FLE) placed in a microcavity produces entangled photons being either in a ΦBS or ΨBS. We demonstrate that a constantly driven FLE undergoes a sharp transition between regions of high ΦBS and ΨBS entanglement for a certain twophoton resonance. At the transition the degree of entanglement drops to zero at a special point, because the quantum state of the system becomes factorizable. We will further study all twophoton resonances revealing a rich variety of different scenarios with or without switching the type of entanglement and with or without special points of zero concurrence
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