Department of Physics, Texas A & M University, College Station, Texas 77843, USA.
J Chem Phys. 2011 Mar 28;134(12):124107. doi: 10.1063/1.3567486.
In proposals for quantum computers using arrays of trapped ultracold polar molecules as qubits, a strong external field with appreciable gradient is imposed in order to prevent quenching of the dipole moments by rotation and to distinguish among the qubit sites. That field induces the molecular dipoles to undergo pendular oscillations, which markedly affect the qubit states and the dipole-dipole interaction. We evaluate entanglement of the pendular qubit states for two linear dipoles, characterized by pairwise concurrence, as a function of the molecular dipole moment and rotational constant, strengths of the external field and the dipole-dipole coupling, and ambient temperature. We also evaluate a key frequency shift, △ω, produced by the dipole-dipole interaction. Under conditions envisioned for the proposed quantum computers, both the concurrence and △ω become very small for the ground eigenstate. In principle, such weak entanglement can be sufficient for operation of logic gates, provided the resolution is high enough to detect the △ω shift unambiguously. In practice, however, for many candidate polar molecules it appears a challenging task to attain adequate resolution. Simple approximate formulas fitted to our numerical results are provided from which the concurrence and △ω shift can be obtained in terms of unitless reduced variables.
在使用被捕获的超冷极性分子阵列作为量子比特的量子计算机的提案中，为了防止偶极矩因旋转而猝灭并区分量子比特位，会施加一个具有可观梯度的强外部场。该场诱导分子偶极子经历摆动振荡，这显著影响量子比特状态和偶极子-偶极子相互作用。我们评估了两个线性偶极子的摆动量子比特状态的纠缠，由对纠缠来表征，作为分子偶极矩和旋转常数、外场和偶极子-偶极子耦合的强度以及环境温度的函数。我们还评估了由偶极子-偶极子相互作用产生的关键频移△ω。在拟议的量子计算机所设想的条件下，基态本征态的纠缠和△ω都变得非常小。原则上，只要分辨率足够高以明确检测到△ω的位移，这种弱纠缠就足以用于逻辑门的操作。然而，在实践中，对于许多候选极性分子来说，达到足够的分辨率似乎是一项具有挑战性的任务。提供了简单的近似公式，拟合到我们的数值结果，可以根据无量纲的约化变量来获得纠缠和△ω位移。
