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Operational path-phase complementarity in single-photon interferometry

Noam Erez A, Daniel Jacobs, Gershon Kurizki

Published Nov 20, 2012• 3 citations
OpenAlex

Abstract

Abstract. We examine two setups that reveal different operational implications of path-phase complementarity for single photons in a Mach-Zehnder interferometer (MZI). In both setups, the which-way (WW) information is recorded in the polarization state of the photon serving as a “flying which-way detector”. In the “predictive” variant, using a fixed initial state, one obtains duality relation between the probability to correctly predict the outcome of either a which-way (WW) or which-phase (WP) measurement (equivalent to the conventional path-distinguishibility-visibility). In this setup, only one or the other (WW or WP) prediction has operational meaning in a single experiment. In the second, “retrodictive ” protocol, the initial state is secretly selected for each photon by one party, Alice, among a set of initial states which may differ in the amplitudes and phases of the photon in each arm of the MZI. The goal of the other party, Bob, is to retrodict the initial state by measurements on the photon. Here, a similar duality relation between WP and WW probabilities, governs their simultaneous guesses in each experimental run.

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Optics Letters•Dec 20, 2007•6 citations
Abstract. We examine two setups that reveal different operational implications of path-phase complementarity for single photons in a Mach-Zehnder interferometer (MZI). In both setups, the which-way (WW) information is recorded in the polarization state of the photon serving as a “flying which-way detector”. In the “predictive” variant, using a fixed initial state, one obtains duality relation between the probability to correctly predict the outcome of either a which-way (WW) or which-phase (WP) measurement (equivalent to the conventional path-distinguishibility-visibility). In this setup, only one or the other (WW or WP) prediction has operational meaning in a single experiment. In the second, “retrodictive ” protocol, the initial state is secretly selected for each photon by one party, Alice, among a set of initial states which may differ in the amplitudes and phases of the photon in each arm of the MZI. The goal of the other party, Bob, is to retrodict the initial state by measurements on the photon. Here, a similar duality relation between WP and WW probabilities, governs their simultaneous guesses in each experimental run.
We examine two setups that reveal different operational implications of path-phase complementarity for single photons in a Mach-Zehnder interferometer (MZI). In both setups, the which-way (WW) information is recorded in the polarization state of the photon serving as a "flying which-way detector". In the "predictive" variant, using a {\em fixed} initial state, one obtains duality relation between the probability to correctly predict the outcome of either a which-way (WW) or which-phase (WP) measurement (equivalent to the conventional path-distinguishibility-visibility). In this setup, only one or the other (WW or WP) prediction has operational meaning in a single experiment. In the second, "retrodictive" protocol, the initial state is secretly selected {\em for each photon} by one party, Alice, among a set of initial states which may differ in the amplitudes and phases of the photon in each arm of the MZI. The goal of the other party, Bob, is to retrodict the initial state by measurements on the photon. Here, a similar duality relation between WP and WW probabilities, governs their {\em simultaneous} guesses in {\em each experimental run}.
Bohr’s principle of complementarity, after gaining contributions from several researchers, moved from a qualitative analysis to a quantitative analysis through what we call today complementarity relations (CR). This change made it possible to quantify aspects of path superposition and distinguishability (predictability and entanglement) in a single experimental apparatus, for example, the Mach-Zehnder interferometer (MZI), gaining historical appeal in the quantification of path superposition through interferometric visibility. Furthermore, CR of quantum coherence, predictability and entanglement functions that are dependent on the quantum state, have also gained relevance in this area of research. In this dissertation, based on other research, we were directed to carry out two works involving the IMZ and analysis through CR. In the first work, we build a version of the MZI and show that interferometric visibility is not always able to correctly quantify the path superposition within the MZI for two-path systems. We found that CR involving quantum coherence end up reflecting the expected behavior for the photon in this same experimental apparatus. In the second work, we performed an integral analysis, through the complete CR, of the entangled quantum eraser (EQE). In this experimental apparatus, we consider a variable partially-polarizing beam splitter (VPPBS) that can produce path superposition of the horizontal and vertical polarizations entering the MZI. Due to the inclusion of the VPPBS, part of the experimental apparatus of the standard quantum eraser was modified, so that we need to perform Bell’s basis measurements of the degree of freedom of the polarizations of the two entangled photons. Therefore, the analysis of the EQE, through the CR, ends up being much richer, producing situations where there are even partial erasures of the information available before the erasure process. In both papers, we verified our theoretical findings by implementing experiments on IBM’s quantum computers.
Wave-particle duality and entanglement are two fundamental characteristics of quantum mechanics. All previous works on experimental investigations in wave-particle properties of single photons (or single particles in general) showed that a well-defined interferometer setting determines a well-defined property of single photons. Here we take a conceptual step forward and control the wave-particle property of single photons with a Bell state. By doing so, we experimentally test the complementarity principle in a scenario in which the setting of the interferometer is not defined at any instance of the experiment, not even in principle. To achieve this goal, we establish the three-photon entangled state, i.e., the entanglement between a single photon and a two-photon Bell state, send the photon of interest SSS into a quantum Mach-Zehnder interferometer (MZI), in which the output beam splitter of the MZI is controlled by the quantum state of the second photon CCC, which is entangled with a third photon AAA. Therefore, the individual quantum state of photon CCC is undefined, which implements the undefined settings of the MZI for photon SSS. This is realized by using three cascaded phase-stable interferometers for three photons. There is typically no well-defined setting of the MZI and thus the very formulation of the wave-particle objectivity from local hidden variable models becomes internally inconsistent.
Which-path information of a quantum particle in interferometers is the key to infer the past of the quantum particle. It has led to many extensive discussions, including discussions of quantum complementarity and the path-visibility relation. The basics of these discussions are the description, detection, and control of which-path information. In this paper, we focus on the investigation of multidimensional which-path information in a nested Mach-Zehnder interferometer. A general expression of which-path information is given and can be partially extracted by different detection methods. Further analysis shows that the which-path information can be controlled by the phase differences and beam-splitting ratios between the arms of the nested Mach-Zehnder interferometer. Moreover, an alternative which-path information elimination phenomenon has been predicted and demonstrated experimentally. Our work can help us to understand the physics of quantum particles, with potential applications for quantum information processing and quantum metrology.