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NEWS & VIEWS Corrected: Publisher Correction −19 ATOMIC CLOCKS figures (9.5 × 10 fractional uncertainty) by carefully evaluating the effects of numerous perturbations in their clock. Accurate and stable timekeeping Another challenge is the process of tuning (locking) the laser to the atomic transition Andrei Derevianko frequency. Because the measurement process is inherently noisy, locking requires averaging Atomic clocks reached a record-breaking uncertainty of less than 10−18 and over repeated measurements. The averaging the time needed to compare clocks is now less than an hour. These results error scales down as the inverse square root push towards a possible redefinition of the second from a microwave to an of the number of measurements, or as 1/√τ for a total integration time τ. For example, in optical standard. Al+ the reported fractional averaging uncer­ tainty or stability is 1.2 × 10−15/√τ, where τ is in Refers to Brewer, S. M. et al. 27Al+ quantum-logic clock with a systematic uncertainty below 10−18. Phys. Rev. Lett. 123, 033201 (2019) | Oelker E. et al. Demonstration of 4.8 × 10−17 stability at 1 s for two independent optical clocks. Nat. Photon. seconds. This means that reaching a frac­ −18 https://doi.org/10.1038/s41566-019-0493-4 (2019). tional uncertainty of 10 requires a 2­week experimental campaign. There are two aspects that any precision mea­ The appearance of an atom is far from being This brings us to the important advance surement must address: systematic errors and cameo in such clocks. Laser cavities and there­ in stability announced by Oelker et al. This statistical uncertainties. Two groups report fore laser frequencies drift over time, whereas collaboration demonstrated averaging to a advances on both these fronts1,2. Writing in atomic frequencies do not. Also, because fraction of 10−18 in 1 hour. What is the reason Physical Review Letters, Samuel Brewer and atoms are universal and identical objects, it for such an apparent disparity in integration co­workers report progress in characterizing is guaranteed that atomic clocks at different times for the two experiments? and reducing systematic errors for the alu­ locations measure the flow of time in the same The primary reason for the disparity is minium ion clock operated by the principal fashion. This is a powerful idea and over the that the Al+ clock uses a single ion, whereas USA metrological laboratory — the National past half­century, precision time­keeping has Oelker et al. have employed another class of Institute of Standards and Technology (NIST) been carried out with atomic clocks. atomic clock: optical lattice clocks3, which in Boulder, Colorado1. The NIST Al clock is There are two challenges in implement­ allow simultaneous probing of an ensemble now the world’s most accurate timepiece, ing an atomic clock. The first is to make sure of thousands of strontium atoms. In other guaranteed to neither gain nor lose 400 ms that the quantum oscillator (atom) is well words, optical lattice clocks enable thousands over the age of the Universe1. As for statisti­ protected from external perturbations. This of single quantum oscillator experiments to cal uncertainty, writing in Nature Photonics, is where the NIST aluminium ion clock has be run in parallel. Eric Oelker and co­workers report a paral­ excelled. Brewer et al. have determined the fre­ The most fundamental source of noise in lel advance in faster averaging of the noise quency of the Al+ transition to 18 significant both experiments is the projective nature of inherent to atomic clocks2. Measuring time requires observation of a stable periodic process. The elapsed time is simply a product of the number of counted periods and the duration of each period. I will refer to this simple, yet fundamen­ tal, equation as the timekeeper’s formula. A grandfather clock is a mechanical reali­ zation of the timekeeper’s formula — each swing of the pendulum is counted by the escapement mechanism, which converts the pen dulum motion into circular motion that advances the clock’s hands. Optical atomic clocks (pictured) realize the same formula in a rather different way. An atomic transition is resonantly excited by a tuneable laser. Once the laser is tuned to the atomic transi tion frequency, the timekeeper’s for­ mula applies: the oscillations of the laser electromagnetic wave are counted at the source, and the atomic transition frequency determines the period. Marti Edward Credit: 478 | AUGUST 2019 | VOLUME 1 www.nature.com/natrevphys NEWS & VIEWS a quantum measurement. Initially, the atoms care about even more accurate clocks? First, Finally, the SI unit of time, the second, is are in the ground state; a laser pulse then pushing quantum technology to its limits can defined in terms of the output of an atomic drives the initial state into a coherent super­ have far­reaching implications. For example, clock based on microwave transition in position of the ground and excited states. The ion clocks such as Al+ served as a platform caesium­133 atoms. Currently, optical clocks5 resulting excited state population depends on for pioneering demonstrations of quantum with their substantially higher frequencies the difference (detuning) between the laser information processing. In fact, the Al+ clock have outperformed the microwave Cs standard and atomic frequencies. Determination of the uses quantum logic gate for the population by two orders of magnitude in accuracy. The population is subject to quantum mechanical readout. Second, if the conventional physics reported advances in atomic timekeeping are uncertainty inherent to projective measure­ perturbations, such as the influence of stray anticipated to lead to an eventual redefinition ments. To paraphrase Einstein, God does electrical fields and black­body radiation, of the SI second. play dice, but the die is loaded, weighted by are well characterized and controlled, atomic Andrei Derevianko the laser detuning. Repeated measurements clocks can be used to search for exotic phys­ Physics Department, University of Nevada, Reno, are needed to infer that weight and thus the ics, such as gravitational waves, dark matter or NV, USA. 4 detuning of the clock laser. In optical lat­ variation of fundamental constants . e-mail: [email protected] tice clocks the projective measurements are Measuring more quickly and to exquisite https://doi.org/10.1038/s42254-019-0089-4 carried out simultaneously on many atoms, levels of accuracy has indisputable merits too. drastically enhancing the rate of inference. Of course, waiting for months to get a single 1. Brewer, S. M. et al. 27Al+ quantum-logic clock with a Another source of noise in atomic time­ measurement is an exercise in patience and systematic uncertainty below 10−18. Phys. Rev. Lett. 123, 033201 (2019). keeping is the noise of the laser cavity. To reduce speeding things up frees time. In particular, 2. Oelker E. et al. Demonstration of 4.8 × 10−17 stability this technical noise, Oelker et al. used a new it means that the evaluation of the system­ at 1 s for two independent optical clocks. Nat. Photon. https://doi.org/10.1038/s41566-019-0493-4 (2019). cryogenically cooled silicon laser cavity. This atic errors can be performed more quickly. 3. Derevianko, A. & Katori, H. Colloquium: Physics of design reduces Brownian noise in cavity mir­ Moreover, exotic physics signals, such as optical lattice clocks. Rev. Mod. Phys. 83, 331–348 (2011). rors and produces light that remains perfectly from gravitational waves or lumpy dark 4. Safronova, M. S. et al. Search for new physics with harmonic (coherent) over nearly a minute. matter, come as short transients. Therefore, atoms and molecules. Rev. Mod. Phys. 90, 025008 (2018). Atomic clocks are arguably the most accu­ better clock stability translates into a broader 5. Ludlow, A. D. et al. Optical atomic clocks. Rev. Mod. rate instruments ever built. Why would one discovery reach. Phys. 87, 637–701 (2015). NATURE REVIEWS | PHYSICS VOLUME 1 | AUGUST 2019 | 479.

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