Simultaneous tracking of spin angle and amplitude beyond classical limits.

Area of Science:

• Quantum physics
• Metrology
• Spectroscopy

Background:

• Spin precession measurement is crucial for various scientific fields, including physics, geophysics, chemistry, nanotechnology, and neuroscience.
• Current indirect measurement methods for spin precession are limited by quantum measurement back-action, introducing errors and reducing sensitivity.

Purpose of the Study:

• To develop a method for reducing quantum measurement back-action below the classical N^1/2 limit.
• To enable simultaneous precise knowledge of spin angle and spin amplitude, overcoming limitations of non-commuting observables.

Main Methods:

• Directing quantum measurement back-action into an unmeasured spin component to create a planar squeezed state.
• Utilizing high-dynamic-range optical quantum non-demolition measurements on a precessing magnetic spin ensemble.

Main Results:

• Demonstrated spin tracking with angular sensitivity 2.9 dB below the standard quantum limit.
• Achieved amplitude sensitivity 7.0 dB below the Poissonian variance, surpassing classical limits for independent particles.
• Significantly reduced disturbance in spin projectors, enabling enhanced measurement precision.

Conclusions:

• The developed method surpasses standard quantum and Poissonian limits for non-commuting observables.
• This approach offers orders-of-magnitude improvements in sensitivity for advanced sensing and spectroscopy applications.