Quantum Nondemolition Measurement Enables Macroscopic Leggett-Garg Tests

Area of Science:

• Quantum mechanics
• Macroscopic realism
• Atomic physics

Background:

• Testing macroscopic realism is crucial for understanding quantum mechanics.
• Leggett-Garg inequalities (LGIs) provide a framework for such tests.
• Previous tests were limited by experimental challenges.

Purpose of the Study:

• To demonstrate a feasible test of macroscopic realism using LGIs.
• To investigate quantum nondemolition (QND) measurements in atomic ensembles.
• To distinguish quantum effects from classical measurement invasivity.

Main Methods:

• Utilized a continuous-variable approach with atomic ensembles.
• Applied magnetically driven coherent oscillations.
• Employed Gaussian states and quantum nondemolition (QND) measurements.
• Analyzed realistic experimental parameters and imperfections.

Main Results:

• Achieved a significant violation of Leggett-Garg inequalities (LGIs).
• Demonstrated that the violation stems from quantum effects.
• Showed that classical invasivity of measurements does not explain the results.
• Identified QND measurements that tighten the 'clumsiness loophole'.

Conclusions:

• Macroscopic realism can be tested in realistic experimental settings.
• Quantum nondemolition measurements are key to probing macroscopic quantum phenomena.
• The results challenge macrorealist interpretations by highlighting quantum backaction.