Quantum correlations between the light and kilogram-mass mirrors of LIGO

Understand

Measurement of minuscule forces and displacements with ever greater precision encounters a limit imposed by a pillar of quantum mechanics: the Heisenberg uncertainty principle.

• A limit to the precision with which the position of an object can be measured continuously is known as the standard quantum limit (SQL).
• When light is used as the probe, the SQL arises from the balance between the uncertainties of photon radiation pressure imposed on the object and of the photon number in the photoelectric detection.
• The only possibility surpassing the SQL is via correlations within the position/momentum uncertainty of the object and the photon number/phase uncertainty of the light it reflects.