Time has always had a special status in physics because of its fundamental role in specifying the regularities of nature and because of the extraordinary precision with which it can be measured. This precision enables tests of fundamental physics and cosmology, as well as practical applications such as satellite navigation. Recently, a regime of operation for atomic clocks based on optical transitions has become possible, promising even higher performance. We report the frequency ratio of two optical atomic clocks with a fractional uncertainty of 5.2 x 10(-17). The ratio of aluminum and mercury single- ion optical clock frequencies nu(Al+)/nu(Hg+) is 1.052871833148990438( 55), where the uncertainty comprises a statistical measurement uncertainty of 4.3 x 10(-17), and systematic uncertainties of 1.9 x 10(-17) and 2.3 x 10(-17) in the mercury and aluminum frequency standards, respectively. Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine- structure constant alpha of alpha/alpha = (-1.6 +/- 2. 3) x 10(-17)/year.
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机译：时间在物理学中一直具有特殊的地位，这是因为时间在规定自然规律方面起着基本作用，并且由于可以精确地测量时间。这种精度可以测试基本的物理和宇宙学，以及实际应用，例如卫星导航。近来，基于光学跃迁的原子钟的工作方式已经成为可能，并有望实现更高的性能。我们报告了两个光学原子钟的频率比，不确定度为5.2 x 10（-17）。铝和汞单离子光学时钟频率nu（Al +）/ nu（Hg +）之比为1.052871833148990438（55），其中不确定度包括统计测量不确定度4.3 x 10（-17）和系统不确定度1.9 x汞和铝频率标准分别为10（-17）和2.3 x 10（-17）。在过去的一年中重复测量产生了对精细结构常数alpha / alpha =（-1.6 +/- 2. 3）x 10（-17）/年的时间变化的初步约束。
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