Quantum free-fall motion and quantum violation of the weak equivalence principle

Abstract

The weak equivalence principle (WEP) in the quantum regime has been the subject of many studies with a broad range of approach to the problem. Here we tackle the problem anew through the time of arrival (TOA) operator approach by constructing the time of arrival operator for a non-relativistic and structureless particle that is projected upward in a uniform gravitational field with an intended arrival point below the classical turning point. The TOA-operator is constructed under the constraint that the inertial and gravitational masses are equivalent, and that Galilean invariance is preserved. These constraints are implemented by Weyl-quantization of the corresponding classical time of arrival function for the projectile. The expectation value of the TOA-operator is explicitly shown to be equal to the classical time of arrival plus mass-dependent quantum correction terms, implying incompatibility of the weak equivalence principle with quantum mechanics. The full extent of the violation of the WEP is shown through the mass dependence of time of arrival distribution for the projectile.