Counterfactuality, back-action, and information gain in multi-path interferometers
arXiv:2404.16477 · doi:10.1088/2058-9565/ad63c7
Abstract
The presence of an absorber in one of the paths of an interferometer changes the output statistics of that interferometer in a fundamental manner. Since the individual quantum particles detected at any of the outputs of the interferometer have not been absorbed, any non-trivial effect of the absorber on the distribution of these particles over these paths is a counterfactual effect. Here, we quantify counterfactual effects by evaluating the information about the presence or absence of the absorber obtained from the output statistics, distinguishing between classical and quantum counterfactual effects. We identify the counterfactual gain which quantifies the advantage of quantum counterfactual protocols over classical counterfactual protocols, and show that this counterfactual gain can be separated into two terms: a semi-classical term related to the amplitude blocked by the absorber, and a Kirkwood-Dirac quasiprobability assigning a joint probability to the blocked path and the output port. A negative Kirkwood-Dirac term between a path and an output port indicates that inserting the absorber into that path will have a focussing effect, increasing the probability of particles arriving at that output port, resulting in a significant enhancement of the counterfactual gain. We show that the magnitude of quantum counterfactual effects cannot be explained by a simple removal of the absorbed particles, but originates instead from a well-defined back-action effect caused by the presence of the absorber in one path, on particles in other paths.
13 pages, 3 figures. Accepted for publication in Quantum Science and Technology; matches accepted version
References in corpus (13)
- Protocol for direct counterfactual quantum communication
- Quantum theory of successive projective measurements
- Quantum circuits for measuring weak values, Kirkwood--Dirac quasiprobability distributions, and state spectra
- Experimental observation of Hardy-like quantum contextuality
- Quantum Counterfactual Communication Without a Weak Trace
- Analysis of counterfactuality of counterfactual communication protocols
- What is nonlocal in counterfactual quantum communication?
- Coherence and contextuality in a Mach-Zehnder interferometer
- Three approaches for analyzing the counterfactuality of counterfactual protocols
- Characterization of the non-classical relation between measurement outcomes represented by non-orthogonal quantum states
- Sequential propagation of a single photon through five measurement contexts in a three-path interferometer
- Quantitative relations between different measurement contexts
- Tracing quantum correlations back to collective interferences