Entangled states hold the promise of improving the precision and accuracy of quantum sensors. We experimentally demonstrate that spectroscopy of an optical clock transition using entangled states can outperform its classical counterpart. Two ${{}^{40}\mathrm{Ca}}^{+}$ ions are entangled in a quantum state with vanishing first-order magnetic field sensitivity, extending the coherence time of the atoms and enabling near-lifetime-limited probe times of up to 550 ms. In our protocol, entangled ions reach the same instability as uncorrelated ions, but at half the probe time, enabling faster cycle times of the clock. We run two entangled ${{}^{40}\mathrm{Ca}}^{+}$ ions as an optical clock and compare its frequency instability with a ${}^{87}\mathrm{Sr}$ lattice clock. The instability of the entangled ion clock is below a clock operated with classically correlated states for all probe times. We observe instabilities below the theoretically expected quantum projection noise limit of two uncorrelated ions for interrogation times below 100 ms. The lowest fractional frequency instability of $7\times {10}^{-16}/\sqrt{\tau /1\mathrm{s}}$ is reached for 250 ms probe time, limited by residual phase noise of the probe laser. This represents the lowest instability reported to date for a ${{}^{40}\mathrm{Ca}}^{+}$ ion clock.

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• Atomic, optical & lattice clocks
• Coherent control
• Open quantum systems & decoherence
• Quantum metrology
• Trapped ions