光移不平衡引起的最低阶以上集体激发的阻塞
Light-Shift Imbalance Induced Blockade of Collective Excitations Beyond the Lowest Order
摘要：
目前针对中性原子的量子计算方案主要基于使用单个原子作为量子比特（qubit），而使用腔诱导耦合或偶极-偶极相互作用进行双量子比特操作。另一种方法是使用原子系综作为量子位。然而，当一个原子系综被激发时，例如被一束与二能级跃迁（或拉曼跃迁）相匹配的激光束激发时，随着越来越多的光子被激发，它会导致许多态的级联^[1]。为了利用系综作为量子比特，有必要破坏这种级联，并将激发限制为仅吸收（和发射）单个光子。在这里，我们展示了如何通过使用一种新型的封锁机制，基于光移不平衡（LSI）在拉曼过渡。我们首先描述了一个简单的例子，说明了光移不平衡诱导封锁（LSIIB）的概念，在一个单一的原子使用多层次的结构，并显示验证的分析预测，使用数值模拟。然后，我们扩展这个模型，以显示如何封锁可以实现通过使用LSI在激发的合奏。具体来说，我们展示了如何LSIIB过程使人们能够将系综视为两个集体量子态之间的完全确定性拉比振荡的两个能级原子，同时抑制高阶集体态的激发。
英文摘要：
Current proposals focusing on neutral atoms for quantum computing are mostly based on using single atoms as quantum bits (qubits), while using cavity induced coupling or dipole-dipole interaction for two-qubit operations. An alternative approach is to use atomic ensembles as qubits. However, when an atomic ensemble is excited, by a laser beam matched to a two-level transition (or a Raman transition) for example, it leads to a cascade of many states as more and more photons are absorbed^1. In order to make use of an ensemble as a qubit, it is necessary to disrupt this cascade, and restrict the excitation to the absorption (and emission) of a single photon only. Here, we show how this can be achieved by using a new type of blockade mechanism, based on the light-shift imbalance (LSI) in a Raman transition. We describe first a simple example illustrating the concept of light shift imbalanced induced blockade (LSIIB) using a multi-level structure in a single atom, and show verifications of the analytic prediction using numerical simulations. We then extend this model to show how a blockade can be realized by using LSI in the excitation of an ensemble. Specifically, we show how the LSIIB process enables one to treat the ensemble as a two level atom that undergoes fully deterministic Rabi oscillations between two collective quantum states, while suppressing excitations of higher order collective states.
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