A simple internet with significant possibilities

Robust entanglement between quantum memory nodes connected by fiber optical infrastructure is a significant obstacle to realizing feasible quantum networks for long-distance quantum communication.

In a new study, Harvard physicists have demonstrated a two-node quantum network composed of multi-qubit registers based on silicon-vacancy (SiV) centers in nanophotonic diamond cavities integrated with a telecommunication fiber network. They used existing Boston-area telecommunication fiber to demonstrate the world’s longest distance between two quantum memory nodes.

The Harvard scientists created the functional foundations of the first quantum internet by entangling two quantum memory nodes separated by an optical fiber link placed along a roughly 22-mile loop via Cambridge, Somerville, Watertown, and Boston. The Laboratory for Integrated Science and Engineering at Harvard had the two nodes situated one level apart.

A vital element of an interconnected quantum computing future is quantum memory, which is comparable to classical computer memory and enables sophisticated network activities as well as the storing and retrieval of data. The Harvard team’s quantum network is the most extended fiber network between devices that can store, process, and transfer information, even though other quantum networks have previously been established.

Every node is a tiny quantum computer composed of a diamond slice with a defect known as a silicon-vacancy core in its atomic structure. Less than a hundredth of a human hair’s width in carved structures inside the diamond improves how light interacts with the silicon-vacancy center.

The silicon-vacancy center is home to two qubits or discrete quantum bits of information: one is a communication-useful electron spin, and the other is a longer-lived nuclear spin that serves as a memory qubit to store entanglement, the quantum-mechanical property that enables perfect correlation of information over any distance. Microwave pulses provide complete control over both spins. These tiny, millimeter-square diamond gadgets are kept in dilution refrigeration units, which can withstand temperatures as low as -459 Fahrenheit.

Harvard has been researching silicon-vacancy centers for single photons for some years. The method addresses a significant issue with the theoretical quantum internet: signal loss unabated by conventional means. Because it is impossible to copy arbitrary quantum information, regular optical fiber signal repeaters cannot be used in a quantum network. This makes the data secure and difficult to carry over long distances.

Network nodes based on silicon vacancy centers can detect, store, and entangle quantum bits of information while compensating for signal loss. Light passes through the first node once the nodes have cooled almost entirely to zero. Because of the atomic structure of the silicon vacancy core, light becomes entangled with it.

First author Can Knaut, a Kenneth C. Griffin Graduate School of Arts and Sciences student in Lukin’s lab, said“Since the light is already entangled with the first node, it can transfer this entanglement to the second node. We call this photon-mediated entanglement.”

For the past few years, the scientists have been renting optical fiber from a Boston-based company to conduct their experiments. They have installed their demonstration network on top of the existing fiber, suggesting that creating a quantum internet using comparable network lines would be feasible.

According to Lukin, the demonstration of quantum network node entanglement in a crowded metropolitan setting is a significant step toward the development of useful quantum computer networking.

This is just the start of a quantum network with two nodes. Scientists are putting a lot of effort into expanding the network’s performance by adding nodes and experimenting with additional networking protocols.

Journal Reference:

  1. Knaut, C.M., Suleymanzade, A., Wei, YC. et al. Entanglement of nanophotonic quantum memory nodes in a telecom network. Nature 629, 573–578 (2024). DOI: 10.1038/s41586-024-07252-z

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