以光鑷與協同冷卻實現可擴展的離子阱量子電腦

Abstract

如何擴展離子阱量子電腦一直以來是一項挑戰，其困難主要來自穩定離子陣列以及克服背景雜訊造成的加熱。在實現量子邏輯閘的操控上，由於要求雷射光能準確照射在某個特定的離子量子位元而不影響鄰近位元，離子間的距離必須大於雷射光寬度，而這個間距的數量級大約為微米等級。在常用的無線電頻率（radio-frequency, RF）離子阱架構下，當離子的數目越來越多時，軸向的束縛頻率須越來越小，以保持離子間距。故當大型離子陣列用於量子計算時，首要困難就是軸向模態的頻率趨近於零，使得該方向振動難以冷卻。因此我們提出在大型離子陣列中加入光鑷來固定離子，借此引入了新的軸向振動頻率。我們計算了當離子處於背景加熱時在協同冷卻下達到穩定態時的位置不準量。我們發現光鑷增加了軸向的穩定度。此外，光鑷阻擋了徑向模態的熱傳導。這個特性保護了在不同區域的量子邏輯閘不受彼此的影響，由此可以實行平行運算。等效來說，我們可以將兩個光鑷間的離子視為一段「局部離子阱」。我們計算了局部離子阱的冷卻效率以及探討其弛豫動力學。

Scalability of quantum computing based on trapped ions in a linear radio-frequency trap has been a challenge due to instability of crystallization and heating. To construct a large-scale ion array with single qubit addressability, the ions’ spacing must be kept a few times larger than or at least comparable to the beam size, which is of the order of microns. This implies that the longitudinal confinement vanishes as the number of ions gets very large. Meanwhile, the collective motional modes of very low frequencies are easily thermally populated and hard to be cooled. We thus propose a scalable scheme combining the applications of optical tweezers and sympathetic cooling. We demonstrate that for a large-scale ion chain, the application of optical tweezers raises the lowest longitudinal frequency by effectively pinning the ions in space. We calculate the steady-state profile of ions’ position fluctuations given that the system exposes to heat and is also sympathetically cooled at the same time. We find that the optical tweezers can enhance the stability of the longitudinal arrangement. Also, it blocks heat propagation of the transverse motion, suggesting that the qubit gate operation based on transverse modes can be done in parallel and thus protected by optical tweezers. This allows us to deal with only a portion defined by two edge ions that are illuminated by tweezer beams. This segment of the ion array is confined by a “local trap” provided by two effectively pinned ions. We demonstrate the relevant cooling efficiency and discuss the relaxation dynamics.