Recently, a basis-invariant measure of coherence known as the intrinsic degree of coherence has been established for classical and single-particle quantum states [J. Opt. Soc. Am. B 36, 2765 (2019) [CrossRef] ]. In this article, we use the same mathematical construction to define the intrinsic degree of coherence of two-qubit states and demonstrate its usefulness in quantifying two-particle quantum correlations and entanglement. In this context, we first show that the intrinsic degree of coherence of a two-qubit state puts an upper bound on the violations of Bell inequalities that can be achieved with the state and that a two-qubit state with an intrinsic degree of coherence less than $ 1/\sqrt 3 $ cannot violate Bell inequalities. We then show that the quantum discord of a two-qubit state, which quantifies the amount of quantum correlations available in the two-qubit state for certain tasks, is bounded from above by the intrinsic degree of coherence of the state. Next, in the context of two-particle entanglement, we show that the range of values that the concurrence of a two-qubit state can take is decided by the intrinsic degree of coherence of the two-qubit state together with that of the individual qubits. Finally, for the polarization two-qubit states generated by the parametric down-conversion of a pump photon, we propose an experimental scheme to measure the intrinsic degree of coherence of two-qubit states. We also present our theoretical study that shows how the intrinsic degree of coherence of a pump photon dictates the maximum intrinsic degree of coherence of the generated two-qubit state.

中文翻译：

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二量子位态的本征相干度和二粒子量子相关性的度量

最近，对于经典和单粒子量子态，已经建立了一种基本不变的相干性度量，称为相干固有度[J. 选择。Soc。上午。乙36，2765（2019）[交叉引用] ]。在本文中，我们使用相同的数学构造来定义两个量子位态的内在相干度，并证明其在量化两个粒子量子相关性和纠缠度方面的有用性。在这种情况下，我们首先表明，两个量子位状态的内在相干度为该状态可以实现的违反贝尔不等式的行为设定了上限，并且具有固有相干度的二量子位状态的相干性较小比$ 1 / \ sqrt 3 $不能违反贝尔不等式。然后，我们显示了一个二量子位态的量子不和谐，它量化了在二量子位态下可用于某些任务的量子相关性的数量，从上方受该态的内在相干度限制。接下来，在两粒子纠缠的情况下，我们表明，两个量子位状态的并发性可以取的值的范围由两个量子位状态与各个量子位的内在相干度决定。 。最后，对于由泵浦光子的参数降频转换产生的极化两个量子位态，我们提出了一种实验方案来测量两个量子位态的内在相干度。