雷射頻率穩定於高精密度腔體用於銫原子的雷德堡激發

Abstract

將中性原子激發到雷德堡態是實現量子計算的具潛力的眾多平台之一。因為雷德堡態強偶極-偶極相互作用，有效範圍可達數微米，並且可以由雷射控制的此交互作用，因此能夠個別單獨對每個量子位進行操作，來實現快速控制的量子閘。由於雷德堡態的長生命期，其自然線寬相當窄，僅在亞千赫茲的範圍內。因此，我們需要一台亞千赫茲、線寬窄且能夠精確鎖定頻率的雷射來實現高保真度和長相干時間的雙量子邏輯閘。我們使用Pound-Drever-Hall技術將1039nm和918nm波長的外腔二極體雷射器精確鎖定到一個與超低膨脹玻璃組成的高精度法布里-珀羅腔。1039nm和459nm雷射器均用於激發銫原子至雷德堡態。459nm雷射是由918nm雷射通過二倍頻晶體產生的。得到1039 (918) nm雷射的反饋頻寬高達760kHz（1MHz）。另外，分析光通過共振腔體的強度擾動，我們估計在40毫秒的時間內，頻率的擾動範圍為350Hz（621Hz）。這說明我們成功地抑制了雷射的相位噪聲。此外，我們利用穿透共振腔的穩頻雷射進行光功率放大，窄線寬的共振腔有效防止伺服凸點成為額外的噪聲源。最後，為了確定腔體的零交叉溫度，我們採用無都普勒飽和光譜技術量測零交叉溫度，並在此溫度附近觀察約50天，平均每天頻率漂移7kHz，這使我們能夠根據觀察到的結果校正雷射的長期頻率頻移，並使用此雷射系統將原子激發到不同雷德堡態。

Neutral atoms excited to Rydberg states are one potential platform to realize quantum computing. The laser-controllable, strong dipole-dipole interactions which are effective at several micrometer range allow the implementation of fast and individual-addressable quantum gates. Nevertheless, owing to the long lifetime of Rydberg states, the natural linewidth is at the sub-kilohertz level. This demands the use of a sub-kHz, narrow linewidth laser that is tightly locked to achieve two-qubit gates with high fidelity and long coherence time. We lock an external-cavity diode laser (ECDL) at both 1039nm and 918nm wavelengths to a high finesse Fabry-Pérot cavity spaced with an ultra-low expansion glass (ULE) using the Pound-Drever-Hall (PDH) technique. Furthermore, the 459nm laser is generated by passing the 918nm laser through a second harmonic generation crystal, resulting in frequency doubling. Both the 1039nm and 459nm lasers are utilized as excitation lasers to drive cesium atoms to Rydberg states. The feedback bandwidth of the 1039nm (918nm) laser is as high as 760kHz (1MHz), allowing for precise control of the laser frequency. By analyzing the fluctuations in the transmitted light through the cavity, we estimate a frequency disturbance of 350Hz (621Hz) within a 40ms time interval, demonstrating the effective suppression of laser phase noise. Furthermore, we utilize the transmitted light through the cavity to amplify the optical power, effectively preventing servo bumps from becoming additional sources of noise. To determine the zero-crossing temperature of the cavity, we utilize Doppler-free saturation spectroscopy to accurately measure the frequency drift. The cavity is set near the zero-crossing temperature, and over a period of 50 days, we observe an average frequency drift of 7kHz per day. Using trap-loss spectroscopy, we have excited atoms to Rydberg states with this laser system.