Continuous Real-Time Tracking of a Quantum Phase Below the Standard Quantum Limit

Area of Science:

• Quantum physics
• Atomic physics
• Quantum optics

Background:

• Accurate measurement of quantum phase is crucial for quantum information processing.
• Traditional methods like Ramsey sequences have limitations in real-time tracking and precision.
• Collective spin systems offer enhanced sensitivity but require precise phase measurement.

Purpose of the Study:

• To propose a novel scheme for continuous quantum phase measurement of collective spins.
• To enable real-time tracking of time-varying signals.
• To achieve phase estimation precision surpassing the standard quantum limit.

Main Methods:

• Utilizing quantum nondemolition measurements of a lossy cavity mode.
• Interacting the cavity mode with an atomic ensemble.
• Probing the collective atomic spin phase directly without population conversion.

Main Results:

• The proposed scheme allows continuous measurement of the evolving quantum phase.
• Real-time tracking of time-varying signals is achieved.
• Naturally developed spin-squeezed states provide phase estimation precision beyond the standard quantum limit (Δϕ_{SQL}=1/sqrt[N] rad).

Conclusions:

• This method offers a significant advancement in measuring quantum phase.
• The scheme provides a powerful tool for real-time monitoring of quantum systems.
• It opens new possibilities for high-precision quantum sensing and metrology.