Past of a particle in an entangled state

Abstract

Vaidman has proposed a controversial criterion for determining the past of a single quantum particle based on the "weak trace" it leaves. We here consider more general examples of entangled systems and analyze the past of single, as well as pairs of entangled pre- and postselected particles. Systems with non-trivial time evolution are also analyzed. We argue that in these cases, examining only the single-particle weak trace provides information which is insufficient for understanding the system as a whole. We therefore suggest to examine, alongside with the past of single particles, also the past of pairs, triplets and eventually the entire system, including higher-order, multipartite traces in the analysis. This resonates with a recently proposed top-down approach by Aharonov, Cohen and Tollaksen for understanding the structure of correlations in pre- and postselected systems.Comment: Added one reference and corrected a typo. Accepted to Int. J. Quantum In

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 ].…”

Can future observation of the living partner post-tag the past decayed state in entangled neutral K mesons?

“…This is a question that, for different scenarios, is being debated in the literature-see, for example, Refs. [66][67][68][69]. In our case, any state linearly independent to Eq.…”

Do neutrons disagree with photons about where they have been inside an interferometer?

“…The faint trace will be on all parts of this line, but also in a separate loop disconnected from the line. The prediction of this closed loop, which does not start at the source and does not end at the detector, generated a hot discussion [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] which only intensified [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] after the first experiment with photons [22] confirmed discontinuous trajectories.…”