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arXiv:1407.2605

Is a Single Photon Always Circularly Polarized? A Proposed Experiment using a Superconducting Microcalorimeter Photon Detector

Alan M. Kadin, Steven B. Kaplan

Published Jul 9, 2014• 4 citations
arXivHTMLPDF

Abstract

A single photon is well known to have spin S = hbar, which would correspond to circular polarization, and all quantum transitions with photon absorption or emission correspond to DeltaS = +/-hbar. However, it is also widely believed that a single photon may be linearly polarized, which would correspond to a state with S = 0. Indeed, linearly polarized single photons are central to most quantum entanglement experiments. On the contrary, it has recently been suggested (based on a realistic spin-quantized wave picture of quantum states) that a linearly polarized photon state must be a superposition of a pair of circularly polarized photons, each with S = +/-hbar. This question cannot be resolved using a conventional photon detector, which generally cannot distinguish one photon from two simultaneous photons. However, it can be addressed using a superconducting microcalorimeter detector with sub-eV energy resolution and high quantum efficiency (QE). A careful experiment demonstrating this photon pairing could place in question some of the paradoxical central foundations of modern quantum theory, including quantum entanglement and nonlocality.

DOI
10.48550/arxiv.1407.2605

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A single photon is well known to have spin S = hbar, which would correspond to circular polarization, and all quantum transitions with photon absorption or emission correspond to DeltaS = +/-hbar. However, it is also widely believed that a single photon may be linearly polarized, which would correspond to a state with S = 0. Indeed, linearly polarized single photons are central to most quantum entanglement experiments. On the contrary, it has recently been suggested (based on a realistic spin-quantized wave picture of quantum states) that a linearly polarized photon state must be a superposition of a pair of circularly polarized photons, each with S = +/-hbar. This question cannot be resolved using a conventional photon detector, which generally cannot distinguish one photon from two simultaneous photons. However, it can be addressed using a superconducting microcalorimeter detector with sub-eV energy resolution and high quantum efficiency (QE). A careful experiment demonstrating this photon pairing could place in question some of the paradoxical central foundations of modern quantum theory, including quantum entanglement and nonlocality.
This paper continues to explore a possible physical interpretation of the wavefunction but with a focus on the wavefunction(s) of a single photon in the Mach-Zehnder experiment. It focuses, in particular, on how one might visualize linear and circular polarization states for photon waves, and how beam splitters may or may not split a circular polarization state into two independent linear polarization states or – vice versa – recombine two linear polarization states into one circular state. As such, it attempts to provide a more refined approach to the rather crude hidden-variable theory for explaining quantum-mechanical interference that was presented in a previous paper (http://vixra.org/pdf/1811.0005v1.pdf). The outcome is the same, however: the theory does not work. Hence, this paper again shows the limit of such physical interpretations, thereby confirming the intuition behind Bell’s Theorem.
The correlations of the linear and circular polarizations in the system of two photons have been theoretically investigated. The polarization of a two-photon state is described by the one-photon Stokes parameters and by the components of the correlation “tensor” in the Stokes space. It is shown that in the case of two-photon decays π0 → 2γ, η → 2γ, K 0 → 2γ, K 0 → 2γ and the cascade process |0〉 → |1〉 + γ → |0〉 + 2γ(|0〉 and |1〉 are states with the spin 0 and 1, respectively) the final two-photon state represents a characteristic example of the entangled (nonfactorizable) state, and the correlations between the Stokes parameters in all these decays have the purely quantum character: the incoherence inequalities of the Bell type for the components of the correlation “tensor”, established previously for the case of classical “mixtures”, are violated. The general analysis of the registration procedure for two correlated photons by two one-photon detectors is performed.
Nonlocality as a fundamental aspect of quantum mechanics is witnessed by violation of Bell inequality or its variants, for which all relevant studies assume some correlations exhibited by local realistic theories. The strategy of Bell's theorem is to establish some criteria to distinguish local realistic theories from quantum mechanics with respect to the nonlocal nature of entangled systems. Here we propose a model independent study of nonlocality that needs not to assume any local theory since observation of the expected nonlocal effect is straightforward quantum mechanically. Our proposal involves a bipartite polarization-entangled system in which one photon immediately reduces into a circular-polarization (CP) state when its partner at a space-like distance is detected in another CP state. The state reduction of the photon can be mechanically monitored because a CP photon carries angular momentum and exerts a torque on a half-wave plate whose mechanical motion is measurable, which is well described by quantum mechanics and independent of any local realistic assumption.
Polarization correlation measurement of two-photon state generated in type-II spontaneous parametric downconversion (SPDC) has been performed in a single input port beam splitter configuration. Two orthogonally polarized entangled photons with degenerate k vectors were directed to the single input port 50%-50% beam splitter. Polarization were measured with linear and circular polarization entangled state by the use of two polarization analyzers followed by two photon counting detectors. A strong correlation of polarization has been observed. The type of two-photon state generated in Type-II SPDC was tested. A strong dependence of the polarization correlation upon the spectral bandwidth of the detector spectral filter was found and a different correlation between linear and circular polarization entangled state were observed. It seems that the standard theoretical models for SPDC need to be improved in order to provide a comprehensive treatment for the result of the experiment.
Light polarization is a primary degree of freedom for encoding quantum information. The scaling up of photonic quantum networks and computer architecture depends crucially on its precise characterization. This is typically achieved by placing external waveplates, polarizers, moving mounts, and recently metasurfaces, on top of the detectors. All these solutions complicate integration and scaling. Here we break convention with traditional architecture and present a monolithic, self-aligned metal-superconductor nanowire single photon detector (M-SNSPD) possessing intrinsic full polarization selectivity. Gold nanowires, co-fabricated atop NbTiN superconducting nanowires within the same lithographic footprint, act as polarization-selective plasmonic metamaterials inducing resonant absorption in the NbTiN. U-shaped wires provide linear polarization selectivity, while S-shaped meanders distinguish circular polarization, while retaining the high-count rates and low dark count rates of conventional SNSPDs. By arranging them into a four-pixel array we realize simultaneous projection onto four polarizations and demonstrate continuous polarization state tomography with an ensemble average fidelity exceeding 98%. Our approach opens new avenues towards scalable detector arrays with integrated plasmonic functionalities, for single photon polarimetry, imaging and spectroscopy.
A slightly modified two-photon-cascade experiment of the Einstein–Podolsky–Rosen type is discussed. It allows one of two observers to select photons either according to linear or to circular polarization properties. Hence one and the same photon can be integrated, at will, either in an ensemble with linear polarization or an ensemble with circular polarization. This shows that polarization properties cannot generally be ascribed to individual photons in an unambiguous manner, in contradiction to the classical reality concept.