What is it about?

While most entanglement-based quantum applications consider a pure entangled state as an always-available resource, the generation of highly pure polarization entangled photons, so far, has been demonstrated on confined emission directions or over relatively narrow apertures. This article presents a principle and an experimental method that extends the high purity of polarization entanglement over all angles of emission. We found that the magnitude of the two-photon wavefunction is almost identical for the HH and VV possibilities but the profile of their relative-phase in momentum and frequency domains dominates the distinguishability. It is then all about the relative phase of the entangled state–which we have manipulated to make it constant in all photon’s degrees of freedom. We introduced an experimental approach to measure, in one step, the value of the relative phase. In the momentum domain, the relative phase over extended emission angles exhibits an obvious two-dimensional quadratic dependence, which has been equalized by using a high resolution PC-programed two-dimensional spatial light modulator (2D SLM) loaded by the inverted relative-phase function. In the frequency domain, the relative-phase gradient–that corresponds to the first order temporal effect: the delay–has been treated by the use of a unified birefringent element. As an experimental demonstration, we could extend the emission of high purity entangled state over one third of the spontaneous parametric downconversion (SPDC) cone. This article offers a near-perfect entangled photon source for a wide span of applications like quantum imaging, quantum holography, quantum metrology, quantum information, and optical quantum computation.

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Why is it important?

A formula for the relative phase function over all SPDC emission cone was derived. This function was inverted and applied to a spatial light modulator in an experiment which equalizes the relative phase over wide emission angles. This enables the production of highly pure entangled photons over all emission directions.

Perspectives

While the underlying nature of entanglement behavior against the local realism is not quite clear to the scientific community, it was my real excitement to invest what we know so far in modeling and modulating the entangled state of light to offer a nearly perfect source of polarization-entangled photons.

Salem Hegazy
Cairo University

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This page is a summary of: Extended source of indistinguishable polarization-entangled photons over wide angles of emission, Applied Physics Letters, December 2020, American Institute of Physics, DOI: 10.1063/5.0022646.
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