Optical clock intercomparison with $6\times 10^{-19}$ precision in one hour (1902.02741v1)

Abstract: Improvements in atom-light coherence are foundational to progress in quantum information science, quantum optics, and precision metrology. Optical atomic clocks require local oscillators with exceptional optical coherence due to the challenge of performing spectroscopy on their ultra-narrow linewidth clock transitions. Advances in laser stabilization have thus enabled rapid progress in clock precision. A new class of ultrastable lasers based on cryogenic silicon reference cavities has recently demonstrated the longest optical coherence times to date. In this work we utilize such a local oscillator, along with a state-of-the-art frequency comb for coherence transfer, with two Sr optical lattice clocks to achieve an unprecedented level of clock stability. Through an anti-synchronous comparison, the fractional instability of both clocks is assessed to be $4.8\times 10^{-17}/\sqrt{\tau}$ for an averaging time $\tau$ in seconds. Synchronous interrogation reveals a quantum projection noise dominated instability of $3.5(2)\times10^{-17}/\sqrt{\tau}$, resulting in a precision of $5.8(3)\times 10^{-19}$ after a single hour of averaging. The ability to measure sub-$10^{-18}$ level frequency shifts in such short timescales will impact a wide range of applications for clocks in quantum sensing and fundamental physics. For example, this precision allows one to resolve the gravitational red shift from a 1 cm elevation change in only 20 minutes.

Optical Clock Intercomparison with High Precision

The paper "Optical clock intercomparison with $6\times 10^{-19}$ precision in one hour" presents an advanced paper in atomic clock precision, leveraging advances in optical coherence and frequency stability. The research utilizes cutting-edge technology, such as ultrastable lasers based on cryogenic silicon cavities and state-of-the-art frequency combs, to achieve unprecedented levels of clock stability and precision.

Key Contributions

• Precision Achieved: Through the application of synchronous and anti-synchronous comparisons between two strontium (Sr) optical lattice clocks, the project achieves a fractional instability of $3.5(2)\times 10^{-17}/\sqrt{\tau}$, pushing the boundaries of precision to $5.8(3)\times 10^{-19}$ after one hour of averaging. This significant achievement allows the resolution of frequency shifts at sub-$10^{-18}$ levels over short periods and has wide-ranging implications for quantum sensing and fundamental physics, such as the detection of gravitational waves or dark matter.
• Laser Stabilization: A major component of the experiment is the stabilization of laser frequencies to a cryogenic silicon cavity, operating at 124 K. This setup results in a world-record laser performance demonstrated by comparing identical systems, exhibiting sub-10 mHz linewidth and laser coherence times of 55 seconds. These developments lead to improved spectroscopic resolution and significant reduction in the Dick effect, which is a noise coupling phenomenon that typically limits the stability of optical lattice clocks.

Impact and Technical Implications

1. Applications: Optical clocks with improved precision have applications in relativistic geodesy, fundamental symmetry tests, and potential detection of transient phenomena like gravitational waves and dark matter. The ability to measure gravitational red shifts from minimal elevation changes showcases the clock's sensitivity.
2. Redefinition of the SI Second: Enhanced clock performance supports the eventual redefinition of the second in terms of optical transitions, potentially improving timekeeping standards globally.
3. Quantum Sensing: The usage of optical clocks in quantum sensing could drive innovation in fields requiring precise measurement capabilities.

Future Prospects

The paper opens pathways for future research in atomic coherence time improvement and laser stabilization, particularly aiming at overcoming current physical limitations such as thermal noise. Incorporating advanced techniques like spin squeezing could lead to surpassing standard quantum limits. Continued refinement in laser stability, potentially leveraging AlGaAs optical coatings, could approach the low $10^{-18}$ level, further extending the potential applicability and precision of optical clocks.

In conclusion, the paper presents significant advancements in optical clock stability and precision, through sophisticated experimental approaches and technology, paving the way for future developments in atomic clocks and their myriad applications in science and technology.