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Physics Today

Quantum entanglement reaches new heights

Ashley G. Smart

Published Aug 1, 2017• 4 citations
Publisher

Abstract

The satellite-based distribution of entangled photons to cities 1200 km apart bolsters prospects for a global quantum communication network.
DOI
10.1063/pt.3.3648

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Satellite-distributed entanglement

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Photon pairs generated in Earth orbit retain their quantum connection after being beamed down to ground stations located 1200 km apart.

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This paper proposes and analyzes the potential of a multi-photon tolerant quantum communication protocol to secure satellite communication. For securing satellite communication, quantum cryptography is the only known unconditionally secure method. A number of recent experiments have shown feasibility of satellite-aided global quantum key distribution (QKD) using different methods such as: Use of entangled photon pairs, decoy state methods, and entanglement swapping. The use of single photon in these methods restricts the distance and speed over which quantum cryptography can be applied. Contemporary quantum cryptography protocols like the BB84 and its variants suffer from the limitation of reaching the distances of only Low Earth Orbit (LEO) at the data rates of few kilobits per second. This makes it impossible to develop a general satellite-based secure global communication network using the existing protocols. The method proposed in this paper allows secure communication at the heights of the Medium Earth Orbit (MEO) and Geosynchronous Earth Orbit (GEO) satellites. The benefits of the proposed method are two-fold: First it enables the realization of a secure global communication network based on satellites and second it provides unconditional security for satellite networks at GEO heights. The multi-photon approach discussed in this paper ameliorates the distance and speed issues associated with quantum cryptography through the use of contemporary laser communication (lasercom) devices. This approach can be seen as a step ahead towards global quantum communication.

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Quantum Entanglement Scales to Cities, Sunlight, and Universal Circuit Laws — E8 Intelligence Research

Andrew Stewart Caldin

FINDING: Quantum entanglement extended to 1200 km (satellite), demonstrated across an entire city network, and created from ordinary sunlight — while MIT re-confirmed Bohr's complementarity with single photons; hybrid quantum circuits show universal statistical laws emerge from random ensembles. MATH: No new fundamental equations or constants are reported in these sources. The 1200 km result is an engineering scale-up of Bell-inequality violations (S > 2 for CHSH). The sunlight entanglement is a thermal-state correlation — no new ratio. The city-scale test is a network topology demonstration (graph connectivity, not new algebra). The hybrid circuits paper (arXiv:2111.08018) invokes random-matrix theory and statistical mechanics — universal eigenvalue statistics (Wigner surmise, level spacing ratio ~0.5307 for GOE, ~0.5996 for GUE) — but no new constants. CONNECTION: The 1200 km distance is not a harmonic ratio. However, the city-scale network and satellite links form lattice-like t Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com

•Oct 8, 2026•0 citations

Satellite quantum communication towards GEO distances

Giuseppe Vallone, Daniele Dequal, Marco Tomasin +7 more

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jan 1, 2016•0 citations

Global quantum network with ground-based single-atom memories in optical cavities and satellite links

Jia-Wei Ji, Shinichi Sunami, Seigo Kikura +2 more

The realization of a global quantum network holds the potential to enable groundbreaking applications such as secure quantum communication and blind quantum computing. However, building such a network remains a formidable challenge, primarily due to photon loss in optical fibers. In this work, we propose a quantum repeater architecture for distributing entanglement over intercontinental distances by leveraging low-Earth-orbit satellites equipped with spontaneous parametric down-conversion (SPDC) photon-pair sources and ground stations utilizing single-atom memories in optical cavities and single-photon detectors to implement the cavity-assisted photon scattering (CAPS) gates for high-fidelity entanglement mapping. The efficient entanglement swapping is achieved by performing high-fidelity Rydberg gates and readouts. We evaluate the entanglement distribution rates and fidelities by analyzing several key imperfections, including time-dependent two-photon transmission and time-dependent pair fidelity, for various satellite heights and ground station distances. We also investigate the impact of pair source fidelity and spin decoherence rate on the repeater performance. Furthermore, we introduce a spatial-frequency multiplexing strategy within this architecture to enhance the design's performance. Finally, we discuss in detail the practical implementation of this architecture. Our results show that this architecture enables entanglement distribution over intercontinental distances. For example, it can distribute over 10000 pairs per flyby over 10000 km with a fidelity above 90%, surpassing the capabilities of terrestrial quantum repeaters

arXiv:2507.02333•Physical Review Applied•Jan 13, 2026•2 citations
This paper proposes and analyzes the potential of a multi-photon tolerant quantum communication protocol to secure satellite communication. For securing satellite communication, quantum cryptography is the only known unconditionally secure method. A number of recent experiments have shown feasibility of satellite-aided global quantum key distribution (QKD) using different methods such as: Use of entangled photon pairs, decoy state methods, and entanglement swapping. The use of single photon in these methods restricts the distance and speed over which quantum cryptography can be applied. Contemporary quantum cryptography protocols like the BB84 and its variants suffer from the limitation of reaching the distances of only Low Earth Orbit (LEO) at the data rates of few kilobits per second. This makes it impossible to develop a general satellite-based secure global communication network using the existing protocols. The method proposed in this paper allows secure communication at the heights of the Medium Earth Orbit (MEO) and Geosynchronous Earth Orbit (GEO) satellites. The benefits of the proposed method are two-fold: First it enables the realization of a secure global communication network based on satellites and second it provides unconditional security for satellite networks at GEO heights. The multi-photon approach discussed in this paper ameliorates the distance and speed issues associated with quantum cryptography through the use of contemporary laser communication (lasercom) devices. This approach can be seen as a step ahead towards global quantum communication.
FINDING: Quantum entanglement extended to 1200 km (satellite), demonstrated across an entire city network, and created from ordinary sunlight — while MIT re-confirmed Bohr's complementarity with single photons; hybrid quantum circuits show universal statistical laws emerge from random ensembles. MATH: No new fundamental equations or constants are reported in these sources. The 1200 km result is an engineering scale-up of Bell-inequality violations (S > 2 for CHSH). The sunlight entanglement is a thermal-state correlation — no new ratio. The city-scale test is a network topology demonstration (graph connectivity, not new algebra). The hybrid circuits paper (arXiv:2111.08018) invokes random-matrix theory and statistical mechanics — universal eigenvalue statistics (Wigner surmise, level spacing ratio ~0.5307 for GOE, ~0.5996 for GUE) — but no new constants. CONNECTION: The 1200 km distance is not a harmonic ratio. However, the city-scale network and satellite links form lattice-like t Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com
The realization of a global quantum network holds the potential to enable groundbreaking applications such as secure quantum communication and blind quantum computing. However, building such a network remains a formidable challenge, primarily due to photon loss in optical fibers. In this work, we propose a quantum repeater architecture for distributing entanglement over intercontinental distances by leveraging low-Earth-orbit satellites equipped with spontaneous parametric down-conversion (SPDC) photon-pair sources and ground stations utilizing single-atom memories in optical cavities and single-photon detectors to implement the cavity-assisted photon scattering (CAPS) gates for high-fidelity entanglement mapping. The efficient entanglement swapping is achieved by performing high-fidelity Rydberg gates and readouts. We evaluate the entanglement distribution rates and fidelities by analyzing several key imperfections, including time-dependent two-photon transmission and time-dependent pair fidelity, for various satellite heights and ground station distances. We also investigate the impact of pair source fidelity and spin decoherence rate on the repeater performance. Furthermore, we introduce a spatial-frequency multiplexing strategy within this architecture to enhance the design's performance. Finally, we discuss in detail the practical implementation of this architecture. Our results show that this architecture enables entanglement distribution over intercontinental distances. For example, it can distribute over 10000 pairs per flyby over 10000 km with a fidelity above 90%, surpassing the capabilities of terrestrial quantum repeaters