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By Sebastian Anthony
German scientists at the Max Planck Institute of Quantum Optics (MPQ) have created the first “universal quantum network” that could be feasibly scaled up to become a quantum internet. So far their quantum network only spans two labs spaced 21 meters apart, but the scientists stress that longer distances and multiple nodes are possible.
The network’s construction is ingenious. Each node is represented by a single rubidium atom, trapped inside a reflective optical cavity. These atoms communicate with each other by emitting a single photon over an optical fiber. Each atom is a quantum bit — a qubit — and the polarization of the photon emitted carries the quantum state of the qubit. The receiving qubit absorbs the photon and takes on the quantum state of the transmitter. Voila: A network of qubits that can send, receive, and store quantum information.
With this atom/photon setup, the scientists were able to perform a read/write operation between two labs, over a 60-meter run of optic fiber. There aren’t any photos of the equipment used, but I suspect we’re probably talking about very large machines to keep the rubidium atoms near absolute zero.
Historically the difficulty has been getting atoms and photons to interact — they’re both impossibly small, so getting them to collide is tricky. The reflective optical cavity solves this problem — the photon ricochets tens of thousands of times until it eventually collides — but even so, the MPQ scientists still only managed to successfully transfer quantum states 0.2% of the time.
In another, probably more exciting test, the emitted photons were actually used to entangle the rubidium atoms. Entangled particles exactly mirror the quantum state of their partner, instantaneously and over any distance. Entangled qubits might be able to form the basis of a quantum network with zero latency over any distance, which would make it rather useful for the intergalactic Galnet that will eventually succeed the internet. The researchers hope that entanglement could be used to mitigate the fickleness of single photons.
Back on Earth, though, the goal now is to improve on that 99.8% failure rate and to scale up the number of nodes. I wouldn’t get too excited just yet, but this advance from the Max Planck Institute could mean that quantum networks aren’t actually that far away. Like the internet, the first real quantum network will link up the world’s universities, laboratories, and military installations — and then, eventually, offices and homes.
Read more about applying Google’s PageRank to quantum networks, or about IBM’s latest quantum computing breakthroughs
