Cold Atom Cloud with High Optical Depth Measured with Large Duty Cycle^*

We present a cold atom system with a dark-line two-dimensional magneto-optical trap, to increase the atomic density by suppressing the atomic radiation pressure. Optical depth (OD) and duty cycle are used to evaluate the system performance. We demonstrate a 100% increase in OD with the dark line, and obtain an ultrahigh OD of 264 with 10% for the duty cycle. Also, with an efficient dark line region, the OD could maintain above 100 with duty cycle as high as 30%. The cold atomic ensemble with an ultrahigh OD with a 10%–30% duty cycle is particularly advantageous in quantum information processing and communication.