An Ideal Experiment to Determine the 'Past of a Particle' in the Nested Mach–Zehnder Interferometer

An ideal experiment is designed to determine the past of a particle in the nested Mach–Zehnder interferometer (MZI) by using standard quantum mechanics with quantum non-demolition measurements. We find that when the photon reaches the detector, it only follows one arm of the outer interferometer and leaves no trace in the inner MZI. When it goes through the inner MZI, it cannot reach the detector. Our result obtained from the standard quantum mechanics contradicts the statement based on two-state vector formulism, 'the photon did not enter the (inner) interferometer, the photon never left the interferometer, but it was there'. Therefore, the statement and also the overlapping claim are incorrect.