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arXiv:1805.02317 · Physical Review A

Stabilizing quantum coherence against pure dephasing in the presence of time-delayed coherent feedback at finite temperature

Nikolett Német, A. S. Parkins, Andreas Knorr, Alexander Carmele

Published May 7, 2019• 14 citations
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Abstract

We present a theoretical model of a system with time-delayed coherent feedback and show how this feedback can lead to the preservation of a substantial level of coherence for a two-level system embedded in a phononic cavity, even at a finite temperature. The exact nature of the solution obtained from the model enables transparent control of the polarization properties through appropriate choices of phase relations and initial conditions.

DOI
10.1103/physreva.99.053809

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We present a theoretical model of a system with time-delayed coherent feedback and show how this feedback can lead to the preservation of a substantial level of coherence for a two-level system embedded in a phononic cavity, even at a finite temperature. The exact nature of the solution obtained from the model enables transparent control of the polarization properties through appropriate choices of phase relations and initial conditions.
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Quantum coherent feedback control is a measurement-free control method fully preserving quantum coherence. In this paper we show how time-delayed quantum coherent feedback can be used to control the degree of squeezing in the output field of a cavity containing a degenerate parametric oscillator. We focus on the specific situation of Pyragas-type feedback control where time-delayed signals are fed back directly into the quantum system. Our results show how time-delayed feedback can enhance or decrease the degree of squeezing as a function of time delay and feedback strength.
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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.