Experimental observation of anomalous trajectories of single photons

Abstract

A century after its conception, quantum mechanics still hold surprises that contradict many "common sense" notions. The contradiction is especially sharp in case one consider trajectories of truly quantum objects such as single photons. From a classical point of view, trajectories are well defined for particles, but not for waves. The wave-particle duality forces a breakdown of this dichotomy and quantum mechanics resolves this in a remarkable way: Trajectories can be well defined, but they are utterly different from classical trajectories. Here, we give an operational definition to the trajectory of a single photon by introducing a novel technique to mark its path using its spectral composition. The method demonstrates that the frequency degree of freedom can be used as a bona fide quantum measurement device (meter). The analysis of a number of setups, using our operational definition, leads to anomalous trajectories which are non-continuous and in some cases do not even connect the source of the photon to where it is detected. We carried out an experimental demonstration of these anomalous trajectories using a nested interferometer. We show that the Two-state vector formalism provides a simple explanation for the results.

Quantum twisted double-slits experiments: confirming wavefunctions’ physical reality

“…The quantum conundrum unlocked in this work, on the one hand, directly confirms the reality of wavefunctions and supports previous works dedicating on this issue [31][32][33]; and on the other hand, clarifies that the phenomena (as well as the unexpected phenomena such as wave-particle superposition [19,20] and discontinuous trajectories [21,22]) obtained in collapse measurements do not represent photons' (or beams') real past in some form of M-Z interferometers.…”

“…Even more puzzling, the recently observed wave-particle superposition state has led researchers to question Bohr's principle of complementarity [19,20]. Moreover, a disconnected trace left by photons in a Mach-Zehnder interferometer, through which photons could not pass, was observed [21,22]. These surprising findings make the long-standing debate more intense, and the core of the problem is that if wavefunctions are not real, do the results measured at a given moment represent the photon's real behavior (physical reality) before the measurement?…”

When Photons Are Lying about Where They Have Been

“…Vaidman, together with his collaborators, performed an experiment demonstrating a surprising trace of the photons in nested interferometers [ 1 ] (see Figure 1 ). These results became the topic of a very large controversy [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ].…”

Past of a particle in an entangled state

“…In fact, as mentioned above, when the weak value of the projection operator is one or zero, the presence/absence can be verified by a strong measurement as well [19]. The predictions obtained from the weak trace criterion regarding the past of a particle were also verified in several experiments [22,23]. Furthermore, it was shown how to measure sequential weak values revealing the particle's past at several instances in time [24].…”