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The European Physical Journal Plus

Controlling qubit–photon entanglement, entanglement swapping and entropic uncertainty via frequency modulation

Mohadese Forozesh, Ali Mortezapour, Alireza Nourmandipour

Published Jul 1, 2021• 11 citations
Publisher
DOI
10.1140/epjp/s13360-021-01769-2

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In this paper, we examine the dynamics of Entropic Uncertainty (EU) and concurrence entanglement (CN) in a two-qubit superconducting system influenced by external parameters such as energy biases, coupling strength, and an oscillating external magnetic field. The Hamiltonian includes a Josephson junction coupling term along the [Formula: see text]-direction. Our results show an inverse relationship between EU and CN: stronger coupling reduces EU and enhances CN, while higher energy biases increase EU and decrease CN. The intrinsic magnetic field strength modulates EU and CN, with higher magnetic field reducing EU and enhancing CN at specific phase angles. Temperature variations significantly impact quantum measures, with higher temperatures increasing EU and reducing CN, indicating that lower temperatures favor entanglement and minimize uncertainty. These findings highlight the sensitivity of quantum properties to external parameters, providing optimization for the quantum state control in superconducting qubit systems.
For an open quantum system containing two qubits under homodyne-based feedback control, we investigate the dynamical behaviors of quantum-memory-assisted entropic uncertainty. Moreover, we analyze the influence of feedback modes and coefficients on the entropic uncertainty. Numerical investigations show that the memory qubit should be placed in a non-dissipative channel if the single dissipative channel condition can be chosen, which helps reduce the entropic uncertainty of the system. For the homodyne feedback control F = λ σ x (or F = λ σ y ) , due to different roles of the entangled qubits A and B, when they are subject to feedback control with different feedback coefficients λ, the exchange of feedback coefficients will cause variations of the entropic uncertainty. When the feedback coefficient corresponding to the memory qubit B is larger ( λ B > λ A ) , the steady value of the entropic uncertainty will be small, which is conducive to enhancing the robustness of the system. However, for the feedback control F = λ σ z , the difference between the feedback coefficients has no effect on the steady values of the entropic uncertainty.
In this paper, a dedicated feedback circuit module is designed for an experiment of entanglement swapping with frequency-nondegenerate photons. This experiment, aiming to entangle two frequency-nondegerate photons without prior interaction, is an advanced strategy for quantum communication. The key point of the experiment is to actively compensate a random phase shift originating from the frequency difference. The feedback circuit module has functions of measuring the time intervals, converting the time information to control level codes, and outputting control levels to an electro-optic modulator (EOM). A precision TDC is implemented in a field programmable gate array (FPGA) with 40 ps per LSB ( σRMS is less than 30 ps). A programmable look-up table is used as computing unit. The propagation delay time of the whole circuit module is less than 110 ns.