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npj Quantum Information

Many-body entanglement via ‘which-path’ information

Ron Ruimy, Offek Tziperman, Alexey A. Gorlach, Klaus Mølmer, Ido E. Kaminer

Published Nov 20, 2024• 10 citations
Publisher

Abstract

We propose a multi-particle ‘which-path’ gedanken experiment with a quantum detector. Contrary to conventional ‘which-path’ experiments, the detector maintains its quantum state during interactions with the particles. We show how such interactions can create an interference pattern that vanishes on average, as in conventional ‘which-path’ schemes, but contains hidden many-body quantum correlations. Measuring the state of the quantum detector projects the joint-particle wavefunction into highly entangled states, such as GHZ’s. Conversely, measuring the particles projects the detector wavefunction into desired states, such as Schrodinger-cat or GKP states for a harmonic-oscillator detector, e.g., a photonic cavity. Our work thus opens a new path to the creation and exploration of many-body quantum correlations in systems not often associated with these phenomena, such as atoms in waveguide QED and free electrons in transmission electron microscopy.

DOI
10.1038/s41534-024-00899-6

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We propose a multi-particle ‘which-path’ gedanken experiment with a quantum detector. Contrary to conventional ‘which-path’ experiments, the detector maintains its quantum state during interactions with the particles. We show how such interactions can create an interference pattern that vanishes on average, as in conventional ‘which-path’ schemes, but contains hidden many-body quantum correlations. Measuring the state of the quantum detector projects the joint-particle wavefunction into highly entangled states, such as GHZ’s. Conversely, measuring the particles projects the detector wavefunction into desired states, such as Schrodinger-cat or GKP states for a harmonic-oscillator detector, e.g., a photonic cavity. Our work thus opens a new path to the creation and exploration of many-body quantum correlations in systems not often associated with these phenomena, such as atoms in waveguide QED and free electrons in transmission electron microscopy.
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The possibility of nonlocal quantum communication is considered. We investigate three gedankenexperiments that have variable entanglement: (1) a 4-detector polarization-entangled system, (2) a 4-detector path-entangled system, and (3) a 3-detector path-entangled system that uses an innovative optical mixer to combine photon paths. A new quantum paradox is reviewed in which the presence or absence of an interference pattern in a path-entangled two photon system, controlled by measurement choice, is a potential nonlocal signal. We show that for the cases considered, even when interference patterns can be switched off and on, there is always a “signal” interference pattern and an “anti-signal” interference pattern that mask any observable interference when they are added, even when entanglement and coherence are simultaneously present. This behavior can be attributed to what in the literature has been called “the complementarity of one- and two-particle interference”.
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This work proposes a novel variation of the double-slit experiment in which two different types of quantum particles — for example, electrons and photons — pass simultaneously through a shared double-slit apparatus. The primary objective is to test the hypothesis that the act of measurement may not only collapse the wave function of the observed particle, but could also influence the interference pattern of an unmeasured, co-propagating particle of a different type. In the proposed configuration, photons and electrons are emitted in synchrony and directed through the same set of slits. Immediately after the slits, a path-detecting measurement apparatus is applied only to one type of particle (e.g., photons), while the interference pattern of the other type (e.g., electrons) is recorded on a distant screen. The experiment is designed to explore whether the destruction of coherence through measurement is strictly local, or whether it can propagate to other particles through a shared medium, entangled fields, or an unknown interaction. If the unmeasured particle continues to display interference, this would support the locality of measurement effects. Conversely, if interference vanishes for both, it could indicate a more global or field-level decoherence process. This investigation aims to contribute new insights into the nature of quantum measurement, the role of observation, and the interdependence of coherent systems involving distinct particle species.
The possibility of nonlocal quantum communication is considered. We investigate three gedankenexperimentsgedankenexperimentsgedankenexperiments that have variable entanglement: (1) a 4-detector polarization-entangled system, (2) a 4-detector path-entangled system, and (3) a 3-detector path-entangled system that uses an innovative optical mixer to combine photon paths. A new quantum paradox is reviewed in which the presence or absence of an interference pattern in a path-entangled two photon system, controlled by measurement choice, is a potential nonlocal signal. We show that for the cases considered, even when interference patterns can be switched off and on, there is always a "signal" interference pattern and an "anti-signal" interference pattern that mask any observable interference when they are added, even when entanglement and coherence are simultaneously present. This behavior can be attributed to what in the literature has been called "the complementarity of one- and two-particle interference".