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Is A Single Particle Always Entangled?

Shengwang Du

Published Apr 5, 2023• 0 citations

Abstract

We investigate the single-particle multi-mode "entanglement" and raise the questions: Is a single particle always entangled? Is entanglement a physical reality which is invariant under an arbitrary unitary transformation or choice of mode basis? We show that, if accepting the single-particle entanglement picture, a single particle can always be entangled by choosing a proper mode basis. This "mode entanglement", expressed as a superposition of single-particle mode basis, may have important applications in quantum information processing as it has some similar mathematical features as the multi-particle entanglement.

DOI: 10.1364/opticaopen.22557619.v1

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I give a simple argument that demonstrates that the state $\ensuremath{\mid}0⟩\ensuremath{\mid}1⟩+\ensuremath{\mid}1⟩\ensuremath{\mid}0⟩$, with $\ensuremath{\mid}0⟩$ denoting a state with 0 particles (or photons) and $\ensuremath{\mid}1⟩$ a one-particle state, is entangled in spite of recent claims to the contrary. I also discuss viewpoints on the old controversy about whether the above state can be said to display single-particle or single-photon nonlocality.

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