We show that the phase of a field can be determined by incoherent detection of the population of one state of a two-level system if the Rabi frequency is comparable to the Bohr frequency so that the rotating wave approximation is inappropriate. This implies that a process employing the measurement of population is not a square-law detector in this limit. We discuss how the sensitivity of the degree of excitation to the phase of the field may pose severe constraints on precise rotations of quantum bits involving low-frequency transitions. We present a scheme for observing this effect in an atomic beam, despite the spread in the interaction time.
Incoherent Imaging with Spatially Structured Quantum Probes
Joint Center for Quantum Information and Computer Science, NIST/University of Maryland（量子信息与计算联合中心，美国国家标准与技术研究院/马里兰大学）；Joint Quantum Institute, NIST/University of Maryland（量子信息联合研究所，美国国家标准与技术研究院/马里兰大学）；Department of Electrical and Computer Engineering, University of Maryland（马里兰大学电气与计算机工程系）
我们提出了一种新型的传感器，它使用含有光学活性氮空位（NV $^-$）中心的金刚石作为激光介质。磁力计可以在室温下工作，产生的光可以很容易地进行光纤耦合，从而可以用于工业应用和遥感。通过将激光泵浦与射频拉比驱动场相结合，外部磁场改变了NV $^-$中心的荧光。我们利用这种荧光水平的变化将激光推到阈值以上，用外部磁场控制的强度打开它，这提供了具有非常高对比度的读出信号的相干放大。这种机制与传统的基于NV $^-$的磁力计有质的不同，后者使用基于非相干光子发射的荧光测量。我们称我们的方法激光阈值磁强计（LTM）。我们预测，体积为1 mm $^3$的NV $^-$型激光阈值磁强计可以实现散粒噪声限制的直流~ 1.86 fT $/\sqrt{\rm{Hz}}$和a.c.~灵敏度为3.97 fT $/\sqrt{\rm{Hz}}$。
