Clock Precision beyond the Standard Quantum Limit at 10^{-18} Level.

Area of Science:

• Atomic Physics
• Quantum Metrology
• Precision Measurement

Background:

• Optical atomic clocks are crucial for precision measurement and fundamental physics.
• The standard quantum limit (SQL) restricts clock precision due to uncorrelated atomic noise.
• Scaling atomic ensembles in optical lattice clocks is challenged by density-dependent frequency shifts.

Purpose of the Study:

• To surpass the standard quantum limit (SQL) in optical atomic clocks using spin squeezing.
• To demonstrate quantum advantage from entanglement at state-of-the-art clock stability levels.
• To establish a new benchmark for entanglement-enhanced atomic clocks.

Main Methods:

• Preparation of two spin-squeezed strontium atom ensembles (approx. 30,000 atoms each) in a 2D optical lattice.
• Utilizing cavity-based quantum nondemolition measurements for spin squeezing.
• Synchronous clock comparison with a 61 ms interrogation time.

Main Results:

• Achieved a fractional frequency precision of 1.1×10⁻¹⁸ for a single spin-squeezed clock.
• Demonstrated a metrological improvement of 2.0(2) dB beyond the SQL.
• Corrected for state preparation and measurement errors to validate results.

Conclusions:

• This work presents the most precise entanglement-enhanced optical atomic clock to date.
• The results show clock performance beyond the SQL, enabled by spin squeezing.
• The developed platform facilitates exploration of quantum entanglement effects on gravity.