Measuring faster with quantum mechanics

Entangled quantum states offer a route to enhanced precision measurements by reducing the quantum projection noise below the limit of classical, independent particles. This has now been demonstrated in an optical ion clock based on two entangled ⁴⁰Ca⁺ ions.

Experimentalists from Physikalisch-Technische Bundesanstalt and theorists from Leibniz Universität Hannover joined forces within the Collaborative Research Centre DQ-mat and the Cluster of Excellence QuantumFrontiers to realise the experiment, supported by theoretical modelling. The entangled calcium ion clock was directly compared to a strontium optical lattice clock at PTB whose large atom number provides superior short-term stability. This comparison enabled a direct test of the ion clock’s frequency stability.

Surpassing the standard quantum limit requires suppressing laser frequency noise, disturbances of the atomic energy levels, and achieving high detection contrast. By preparing the ions in an entangled decoherence-free state (enDFS), magnetic field noise - the dominant limitation in calcium ion clocks is strongly suppressed. This extends the interrogation times by a factor of more than 500, approaching the natural lifetime of the excited clock state without extensive magnetic field shielding. Using the enDFS protocol, a frequency instability inaccessible to classical protocols for identical interrogation times was demonstrated. The experiment further realizes the most stable 40Ca+ ion clock to date, enabled by the long coherence times, while simultaneously relaxing technical requirements on magnetic field control.

The results demonstrate that entanglement is not only a resource for quantum information processing but also provides a concrete and practical advantage for precision metrology, opening new perspectives for future multi-ion optical clocks.

Originalpublikation
K. Dietze et al.
Entanglement-enhanced optical ion clock 
Phys. Rev. Lett. 136, 073601 (2026)