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Frontiers in Optics

Quantum delay metrology with complete frequency-bin Bell basis synthesizer

Suparna Seshadri, Hsuan‐Hao Lu, Daniel E. Leaird, Andrew M. Weiner, Joseph M. Lukens

Published Jan 1, 2022• 0 citations
Publisher

Abstract

We generate all four frequency-bin Bell states via single and dual spectral-line pumping of spontaneous parametric down-conversion and demonstrate their sensitivity to the sum and difference of entangled-photon delays using spectral interferometry.
DOI
10.1364/fio.2022.jtu4a.30

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We report the experimental generation of all four frequency-bin Bell states in a single versatile setup via successive pumping of spontaneous parametric down-conversion with single and dual spectral lines. Our scheme utilizes intensity modulation to control the pump configuration and offers turn-key generation of any desired Bell state using only off-the-shelf telecommunication equipment. We employ Bayesian inference to reconstruct the density matrices of the generated Bell states, finding fidelities ≥97% for all cases. Additionally, we demonstrate the sensitivity of the frequency-bin Bell states to common-mode and differential-mode temporal delays traversed by the photons comprising the state-presenting the potential for either enhanced resolution or nonlocal sensing enabled by our complete Bell basis synthesizer.
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A monochromatic laser pumping a parametric down-conversion crystal generates frequency-entangled photon pairs. We study this experimentally by addressing such frequency-entangled photons at telecommunication wavelengths (around 155015501550 nm) with fiber-optics components such as electro-optic phase modulators and narrow-band frequency filters. The theory underlying our approach uses the notion of frequency-bin entanglement. Our results show that the phase modulators address coherently up to eleven frequency bins, leading to an interference pattern which can violate by more than five standard deviations a Bell inequality adapted to our setup.
In the process of Spontaneous Parametric Down Conversion (SPDC), a pump photon spontaneously splits inside a quadratic nonlinear crystal to signal and idler photons. By spatially modulating the nonlinear coefficient, we form a bulk nonlinear metamaterial that can shape the first and second order correlations of the down-converted signal and idler photons in either the spatial domain or the spectral domains. These degrees of freedom span a multi-dimensional Hilbert space, which is beneficial for quantum information applications. Recently, we have designed and fabricated, using electric field poling, bulk nonlinear metamaterials in KTiOPO4, for generating spatially entangled signal-idler pairs. This includes a bi-photon Bell state in the Hermite-Gauss basis, or a state with 3 dominant pairs of coincidences, approximating a bi-photon qutrit. As for the spectral domain, we generated a variety of quantum states: high purity frequency uncorrelated states, frequency-bin Bell states and frequency entangled bi-photon qudit states. By increasing the pump power, we reach the regime of bright squeezed vacuum sources, enabling the generation of a square cluster state in the frequency domain.