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Like Ross and Rachel, modern humans and Neanderthals had something of an on-again, off-again love affair. Yet while Friends may have gone extinct after 10 seasons, our ancient ancestors repeatedly reunited over hundreds of thousands of years, and new research may have pinpointed exactly where on the planet these romantic episodes occurred.

Previous studies have identified three waves of gene flow between the two human species, with the first interbreeding event taking place some 250,000 to 200,000 years ago. A second hook-up then happened around 120,000 to 100,000 years ago, followed by a final fling roughly 60,000 to 50,000 years ago.

Recently, it was demonstrated that the second of these get-togethers likely went down in the Zagros Mountains, which stretch across the Persian Plateau and were inhabited by both modern humans and Neanderthals at the time of this middle mating event. To learn more about the most recent admixture, the authors of an as yet un-peer-reviewed study analyzed all known archaeological sites showing evidence of occupation by the two lineages between 60 and 50 millennia ago.

They also analyzed paleoenvironmental data to identify regions capable of supporting large populations of both Homo sapiens and Neanderthals at the same time. Overall, they found that the most suitable habitats for ancient members of our own species were in southern Europe, northern and southern Africa, and large patches of Asia.

Neanderthals, meanwhile, are likely to have thrived along the coasts of the Mediterranean and the Black Sea. Seeking regions where the two hominids may have overlapped, the researchers identified the Iberian Peninsula and the Levant as the most likely settings for the third and final installment of our inter-species romance.

“The most suitable habitats of Neanderthals were located in Western Europe particularly the Iberian Peninsula at [60-50,000 years ago] where Neanderthals and modern humans live[d] alongside each other,” write the study authors. They therefore conclude that the region encompassing modern-day Spain and Portugal was “a highly probable area for the two species interbreeding.”

Around the same time, groups of Homo sapiens were flowing out of Africa and into Eurasia via a key corridor running through the Levant, which includes the Mediterranean shores of the Middle East. Though smaller than the Iberian Peninsula, this highly important region was something of a hominin melting pot during the Middle and Late Pleistocene, where populations of Homo sapiens, Neanderthals, and other human lineages lived side by side.

Putting all of this evidence together, the researchers say that these two areas were likely to have seen considerable species overlap at the time of the third interbreeding event, and that modern humans and Neanderthals may have done the dirty in either of these locations. Not wishing to sit on the fence, though, the authors put their money on the Levant, which they describe as the “main potential interbreeding area.”

The study is currently available as a preprint on bioRxiv.

Quantum scientists have long treated quantum entanglement as precious cargo, forging fresh links for every secure message or computation. A new theoretical study proposes a thriftier route, letting an existing pair pass portions of its entanglement down an extended chain.

Experts from the Harish-Chandra Research Institute (HRI) report that the hand-off could continue, in principle, without end, though each recipient would receive a smaller slice of the original connection.

Why quantum entanglement matters

Entanglement links the properties of two or more particles so tightly that measuring one instantly tells you the state of the other, regardless of separation. This non-local bond underpins quantum key distribution, distributed sensing, and proposals for a global quantum internet.

Because the link cannot be copied, every application has relied on fresh pairs produced on demand, often with delicate lasers or cryogenic circuits. Generating and storing those pairs remains an experimental bottleneck.

A strategy that lets devices recycle an existing pair would trim that overhead and simplify network design. It could also cut the number of fragile qubit memories required at future repeater stations.

Networks built from recycled links could therefore run longer without waiting for central stations to reset and re-synchronize. That endurance matters for satellites or remote sensors that see only brief contact windows with the ground.

One quantum link for many network users

The team models two theoretical participants (quantum systems) in the mathematical model they called Alice and Bob, who start with one entangled pair.

Two newcomers, who they called Charu and Debu, each hold a separate particle that can interact locally with Alice and Bob but not with each other.

After one withdrawal, Alice and Bob still retain a reduced connection. That leftover can be used to fund the next pair in line without requiring any fresh entanglement.

Math behind quantum network sharing

The authors prove that the protocol can be iterated indefinitely, as long as each new customer is satisfied with a smaller balance. Their proof relies on a measure called concurrence, which tracks how much two-particle entanglement survives each step.

Charu and Debu receive an amount that depends only on the local operation chosen at their step, not on the growing crowd waiting behind them. That independence lets the chain grow as long as the math permits.

The authors also derive an upper bound on the entanglement each newcomer can hope to gain, ensuring the protocol respects the monogamy rules that forbid one qubit from sharing full strength links with multiple partners. 

The calculations confirmed that no party can cheat the system by claiming more than its share.The algebra, however, kept refusing to impose a hard cutoff.

Real-world devices limit repeated sharing

Real hardware will impose limits long before the math does. Each interaction leaks photons, picks up phase noise, or generates detector errors that sap usable correlations.

Engineers must therefore weigh the cumulative losses against the cost of preparing fresh pairs. A hybrid architecture might recycle links only once or twice before moving on.

Numerical simulations with realistic loss and detector models suggest usefulness may fade after a dozen rounds at telecom wavelengths. Superconducting circuits, with higher gate fidelities, could squeeze a few more iterations before noise dominates.

Testing quantum networks in labs

The protocol needs just local interactions and classical coordination, tools already common in superconducting and photonic labs.

A tabletop demonstration could couple two microwave qubits to a shared resonator and then transfer part of their entanglement to two fresh qubits downstream.

Optical groups could pursue a version with time-bin entangled photons stored in fiber loops. Each loop could spin a photon back through a beam splitter network that hands off correlations to a new path without touching the partner loop.

Researchers will need a clear metric for “useful” entanglement in realistic settings. Measures linked to secret-key rate or teleportation fidelity would offer concrete benchmarks.

Cross-platform trials, in which a superconducting node hands off entanglement to a spin qubit or trapped ion, could test compatibility between disparate hardware families. Such demonstrations would mirror the heterogeneous nature of future metropolitan networks.

Future of quantum networks

The new model also raises new theoretical questions. How many independent customers can extract a fixed minimum entanglement from a single source before the well runs dry?

Future research could combine the sharing approach with entanglement distillation, replenishing the degraded entanglement after multiple uses.

Scientists might also investigate whether multipartite connections – like Greenberger-Horne-Zeilinger states – behave differently under repeated sharing.

Policymakers investing in quantum infrastructure will likely pay close attention to these findings. Greater utilization could mean quicker returns on costly hardware deployments.

The study is published in the journal Physical Review A.

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