Entanglement of macroscopically distinct states of light

Abstract

Schrödinger's famous Gedankenexperiment has inspired multiple generations of physicists to think about apparent paradoxes that arise when the logic of quantum physics is applied to macroscopic objects. The development of quantum technologies enabled us to produce physical analogues of Schrödinger's cats, such as superpositions of macroscopically distinct states as well as entangled states of microscopic and macroscopic entities. Here we take one step further and prepare an optical state which, in Schrödinger's language, is equivalent to a superposition of two cats, one of which is dead and the other alive, but it is not known in which state each individual cat is. Specifically, the alive and dead states are, respectively, the displaced single photon and displaced vacuum (coherent state), with the magnitude of displacement being on a scale of 10 8 photons. These two states have significantly different photon statistics and are therefore macroscopically distinguishable.

Coherent Superpositions of Photon Creation Operations and Their Application to Multimode States of Light

“…Differently from other recent experimental approaches [56][57][58], in our experiment, the two-mode entangled state is always fully detected by the homodyne detector in its complete macroscopic form, without resorting to displacing it back to the limited Fock space spanned by the vacuum and single-photon components before measurement. Therefore, in our case, one can use the full quadrature measurements (comprising both the quadrature mean values and their fluctuations) in the two modes for extracting other important parameters of the state, such as some entanglement witness or some particular joint statistical properties.…”

“…Such a state, characterized by a degree of entanglement independent of the size of the entangled partners and surprisingly robust against losses [52], is of very high interest to investigate the resilience and detectability of entanglement for states of growing macroscopicity as well as for fundamental investigations concerning the very definition of macroscopic quantumness and entanglement [53][54][55]. Related experiments involving micro-macro [56,57] and macromacro [58] entanglement have recently explored these issues, with different approaches.…”

Entangling Macroscopic Light States by Delocalized Photon Addition

“…Moreover, entanglement is finally verified by 'un-doing' the macroscopicity and optically displacing the state back to the (|0 , |1 ) Fock subspace before measurements, as recently done in Ref. [21].…”

Two-mode light states before and after delocalized single-photon addition

“…[34][35][36]. When z 1 = 0 and z 1 = α, will degenerate to the discussions in other literature [37,40].…”