原子磁力计能够探测亚 fT 量级(1 fT=10-15 T，地磁场大小约为 10-4 T)的磁场信号[6]。随着系统集成度的提升、功耗的降低、稳定性的增强[7]，原子磁力计已成为最灵敏的磁场探测手段之一，在基础物理[8]、量子信息[9]、医疗检测[10]、材料检测[11]等诸多领域都有广泛应用。原子磁力计通过对磁子能级之间的频率间隔的测量来实现对磁场大小的测量[1]：特定的原子能级在磁场作用下分裂形成的原子能级称作磁子能级，磁子能级之间的频率间隔与外磁场的幅值具有一定的比例关系。早期的原子磁力计采用原子气体放电灯作为系统光源[2][3]，随着激光技术的发展，气体放电灯逐渐被激光所取代，在提升原子磁力计探测灵敏度的同时[4]，也促进了原子磁力计小型化的技术实现[5]。目前的小型化原子磁力计主要基于无自旋交换碰撞弛豫(SERF)机制，即当碱金属原子弛豫速率大于原子能级的塞曼分裂时, 自旋交换碰撞弛豫展宽将消失，磁共振谱线将窄至不能被分辨，磁共振谱线越窄，原子磁力计的测量灵敏度越高。普林斯顿大学的Happer等通过理论计算证明当原子密度足够大且外磁场足够小时，原子之间的自旋交换碰撞弛豫速率就可以超过原子能级分裂的拉莫尔频率。通过增大原子密度，减小外磁场的方法可以在实验上获得更窄的磁共振谱线[12]，该原子谱线压窄方法尤其适用于零场下的小型化原子磁力计。
SERF机制在原子磁力计上的应用最早开始于2002年。由华盛顿大学的 Allred 团队与普林斯顿大学的Romalis团队组成的合作小组[13]，在一个直径2.5cm并充有氦气与氮气混合气体(作为缓冲气体)的热钾原子气室中，探测到了线偏振探测光在弱磁场下与钾原子相互作用产生的磁致偏转。为了尽量压窄信号线宽, Allred团队进一步采用在原子气室内充一定压力的缓冲气体的方法, 通过与钾原子频繁碰撞而阻止其向玻璃泡壁的扩散，延长了钾原子与激光相互作用的时间，而且缓冲气体与钾原子的无辐射碰撞还可以淬灭钾原子的自发辐射荧光，消除光噪声；其次，由钾与钾原子自旋交换碰撞速率较高，超过了原子在弱磁场下的拉莫尔进动频率，使得自旋交换碰撞对原子横向弛豫的一阶(即一次方)贡献消失，仅剩下较小的二阶(即二次方)贡献。通过这些措施，Allred团队实现了1.2Hz的共振线宽，结合使用5层嵌套磁屏蔽来降低磁噪声，使该磁场测量装置的灵敏度达到了10fT/ Hz1/2。在上述小型化磁力计基础上, Romalis小组又开发出了多通道的小型化磁梯度计，其磁探测灵敏度达到0.54 fT/Hz1/2(频率范围从 28-45Hz时)，空间分辨率为2mm[14]。 小型化磁力计也可用于液体样品的NMR探测。 2005年, Romalis小组利用其研制的钾原子小型化磁力计,测量磁化纯水在0.14T 磁场预极化之后的自由感应衰减(Free Induction Decay, FID)信号与129Xe原子的 NMR信号[15]，并提出将小型化磁力计用于样品的三维成像[16]。 Romalis小组研发的小型化磁力计主要通过测量探测光偏振方向的偏转角度来确定磁场强度。
目前商用小型化原子磁力计主要通过直接检测抽运光的透射强度来确定磁场强度，该方法被称为零场能级交叉(zero field level crossing)共振，也即 Hanle效应。早在1969年，Dupont-Roc等就利用了Rb87原子基态的 Hanle效应实现了高灵敏的原子磁力计[17]。基于同样原理，2007年美国国家标准局的Kitching小组利用了微电机系统(MEMS)工艺制造的原子气室,实现了基于透射光强检测的小型化磁力计。该原子气泡尺寸只有3*2*1 mm3, 共振线宽为83 nT (对Rb87原子,也即580 Hz),且其线宽的主要限制来源于自旋破坏弛豫[18]。另外，采用一束激光同时作为泵浦光及探测光, 在简化结构的同时，还降低了系统功耗。美国的Quspin公司商业化了基于SERF的原子磁力计产品[19]，该产品已被应用于零场核磁共振测量。美国的Kernel公司[20]开发了高密度原子磁力计阵列（Kernel Flux)，并尝试了在脑机接口方面开展应用。近年来，国内与国外的差距也在不断缩小，已有北京大学[21]、北京航空航天大学[22]、中国科学院[23-25]、东南大学[26]和浙江工业大学[27]等机构投入了小型化原子磁力计的研发，总体说来，国内的原子磁力计在研究领域和性能水平上已跻身国际第一梯队。
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