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4 March 2016 Creation and control of entanglement by time-delayed quantum-coherent feedback
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Quantum information science relies on the feature of distant quantum entities (mostly "qubits") to form non-local states. A main challenge consists of generating such non-local entangled states between qubits. We exploit the fact that for coupled qubits, the eigenstates of the coupled system are usually highly entangled, and of different excitation energies. This allows to address the different entangled eigenstates by frequency-dependent control schemes.

In our proposal, we present such a control mechanism, and demonstrate how it can be used to create entanglement from a fully separable initial state. The mechanism of our choice is time-delayed quantum-coherent feedback. If a qubit occupation decays via the emission of a photon, one can store this photon for a delay time τ and couple the radiation back into the qubit afterwards. Through the choice of τ, one can set the phase of the feedback, which will then lead to either an increased or decreased qubit decay. Since this phase depends on sin(ωτ), this effect strongly depends on the qubit frequency ω. In particular, it can be used to separate different entangled states in a quantum network by enhancing the decay of all entangled eigenstates except one.

We discuss this protocol on the example of two coupled qubits, and analyze in detail its effectiveness depending on the feedback delay time τ.
© (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Sven M. Hein, Alexander Carmele, and Andreas Knorr "Creation and control of entanglement by time-delayed quantum-coherent feedback", Proc. SPIE 9742, Physics and Simulation of Optoelectronic Devices XXIV, 97420X (4 March 2016);

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