Atomic clocks often have a limited coherence time due to the interactions between the constituent atoms. While it is usually very easy to use fewer atoms to reduce the interactions, this leads to lower signal-to-noise and less precise measurements. This tension between strong interactions and noise seems unavoidable and limits the accuracy of the world's best cesium clocks, the keepers of international atomic time [1, 2] . As reported in a paper in Physical Review Letters, Christian Deutsch and

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... coworkers at three laboratories in Paris, France, have circumvented this seemingly unavoidable compromise by showing that a clock's coherence can actually be enhanced by strong atomic interactions [3] . Even better, strong atomic interactions might also reduce the clock's systematic frequency shifts. The authors demonstrated long coherence times with an "atom chip," but their key requirements can also be satisfied in optical lattice clocks [4]. The tick rate or frequency ν of an atomic clock is given by the energy difference between two atomic eigenstates, ν = ∆E/h. Clocks based on trapped atoms can suffer from a lack of coherence if the trap strength is different for the two atomic eigenstates. The tick rate of atoms in higher energy trap states can be faster than for those in lower energy trap states, limiting the coherence time. Researchers in this field often represent the coherence of a system of two-level atoms with a geometric tool called the Bloch sphere: vectors pointing to the north and south poles represent the pure states | ↑ and | ↓ , while vectors on the equator represent equal superpositions of | ↑ and | ↓ . In Fig. 1(a) , the dephasing of the atomic coherences is depicted as the red and blue arrows precessing in opposite directions on the equator of the Bloch sphere. Dephasing is a torque D acting in opposite directions on the fictitious spin vectors that represent the superposition of the two atomic eigenstates. The dephasing broadens the linewidth of FIG. 1: (a) Bloch sphere representation of two atomic coherences that are described by fictitious red and blue spins. The spins dephase by precessing in the horizontal plane with opposite dephasing torques D. (b) Triplet and singlet energy eigenstates for two interacting spins. In this picture, dephasing is a precession between the triplet state |t and the singlet |s . The precession is inhibited when the spins interact strongly, ω ex D, shifting the energy of the singlet state. (c) The interactions exert a torque ω ex on the spins about their mean spin. When ω ex D, the precession of the spins about the vector sum of the two torques ω ex and D, so they no longer precess about the equator, giving extraordinarily long coherence times for a clock. the clock and often degrades the clock's stability. Although there are techniques for some clocks that make the trap strengths nearly equal for the two eigenstates, such as magic wavelengths for optical lattice clocks [4], other effects, including atomic interactions, also limit the maximum coherence time. Even when traps are delicately tuned [5] , the coherence times are still limited, as this tuning is not very robust. The essential trick of Deutsch et al. is to increase