Person:

Colombo, Simone

State-of-the-art atomic clocks are based on the precise detection of the energy difference between two atomic levels, which is measured in terms of the quantum phase accumulated over a given time interval [1–4]. The stability of optical-lattice clocks (OLCs) is limited both by the interrupted interrogation of the atomic system by the local-oscillator (LO) laser (Dick noise [5]), and by the16standard quantum limit (SQL) that arises from the quantum noise associated with discrete measurement outcomes. While schemes for removing the Dick noise have been recently proposed and implemented [4, 6–8], performance beyond the SQL by engineering quantum correlations (entanglement) between the atoms [9–20] has been demonstrated only in proof-of-principle experiments with microwave clocks of limited stability. The generation of entanglement on an optical-clock transition and operation of an OLC beyond the SQL represent major goals in quantum metrology that have never been demonstrated [16]. Here we report the creation of a many-atom entangled state on an OLC transition, and use it to demonstrate a Ramsey sequence with an Allan deviation below the SQL after subtraction of the LO noise. We report a metrological gain of 4.4+0.6−0.4dB over the SQL using an ensemble consisting of a few hundred171Yb atoms, corresponding to a reduction of the averaging time by a factor of 2.8±0.3. Our results are currently limited by the phase noise of the LO and Dick noise, but demonstrate the available performance improvement in state-of-the-art OLCs [1–4] through the use of entanglement. This will enable further advances in timekeeping precision and accuracy, with many scientific and technological applications, including precision tests of the fundamental laws of physics [21–23], geodesy [24–26], or gravitational wave detection [27].