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Springer eBooks

Eliminating frequency and space-time entanglement in multi-photon states

Warren P. Grice, Alfred B. U’Ren, Ian A. Walmsley

Published Jan 1, 2003• 1 citation
Publisher
DOI
10.1007/978-1-4419-8907-9_148

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Optics Express•Dec 7, 2020•22 citations
We demonstrate a novel, passive method for processing frequency-bin encoded quantum information using linear interferometry and time-resolved detection, eliminating the need for electro-optic modulation or nonlinear optics. This approach is compatible with multimode propagation, a crucial step for free-space quantum communication, and favorable for size, weight, and power (SWaP) requirements. Using a high-brightness multi-resonator source, we generate frequency-bin entangled photons and show the ability to perform arbitrary projective measurements in both single- and multi-mode propagation scenarios. For the first time, we measure the joint temporal amplitude of the generated state, directly linking its entangled nature to the temporal oscillation of non-local correlations. We leverage this to certify entanglement with a Bell curve visibility exceeding 95%. Our technique enables full quantum state tomography, yielding a state fidelity of up to 91% for a maximally entangled state. This work paves the way for robust entanglement deployment over free-space and satellite-based links.
Abstract Frequency-bin entangled photons can be efficiently produced on-chip which offers a scalable, robust and low-footprint platform for quantum communication, particularly well-suited for resource-constrained settings such as mobile or satellite-based systems. However, analyzing such entangled states typically requires active and lossy components, limiting scalability and multi-mode compatibility. We demonstrate a novel technique for processing frequency-encoded photons using linear interferometry and time-resolved detection. Our approach is fully passive and compatible with spatially multi-mode light, making it suitable for free-space and satellite-to-ground applications. As a proof-of-concept, we utilize frequency-bin entangled photons generated from a high-brightness multi-resonator source integrated on-chip to show the ability to perform arbitrary projective measurements over both single- and multi-mode channels. We report the first measurement of the joint temporal intensity between frequency-bin entangled photons, which allows us to certify entanglement by violating the Clauser-Horne-Shimony-Holt (CHSH) inequality, with a measured value of ∣ S ∣ = 2.32 ± 0.05 over multi-mode fiber. By combining time-resolved detection with energy-correlation measurements, we perform full quantum state tomography, yielding a state fidelity of up to 91%. We further assess our ability to produce non-classical states via a violation of time-energy entropic uncertainty relations and investigate the feasibility of a quantum key distribution protocol. Our work establishes a resource-efficient and scalable approach toward the deployment of robust frequency-bin entanglement over free-space and satellite-based links.
High quality time-bin entanglement is widely exploited to achieve the purposes of fundamental tests of physics and the implementation of quantum communication protocols both in free space and optical fiber propagation. However, the imperfect approaches of generating time-bin entangled state will degrade its quality and limit its practical application. Entanglement purification is to distill high quality entangled states from low quality entangled states. In this paper, we present the first entanglement purification protocol (EPP) for time-bin entanglement. We first explain this EPP for two-photon time-bin entangled state and then extend it to the system of multi-photon time-bin entangled state. We also design a possible realization of this EPP with practical spontaneous parametric down conversion (SPDC) source. Differ from the conventional EPPs, this EPP does not require the sophisticated controlled-not (CNOT) gate or similar operations, and it uses the feasible sum-frequency generation (SFG) to perform the purification. Moreover, the double-pair noise emitted from the SPDC source can be eliminated automatically which is the other advantage of this EPP. If we combine with the faithful entanglement swapping, this EPP may have potential to be a part of full quantum repeaters.