European scientists propose world's largest quantum network, between Earth ...

A group of European researchers has proposed the largest quantum network yet: Between Earth and the International Space Station. Such a network would see entangled photons transmitted over a distance of 250 miles â€" two or three times greater than previous quantum communication experiments. Not only will this be the first quantum experiment in space, but it will allow the scientists to see if entanglement really is instantaneous over long distances, and whether it’s affected by gravity.

In recent years, quantum physicists have successfully teleported entangled photons over a free-space distance of 143 kilometers (89 miles) using lasers, and 250 kilometers (155 miles) over optical fiber in the lab. In the past year we have also seen the first ground-to-air network, between a base station and an airplane flying 20 kilometers (12 miles) above. These were impressive feats, but to prove the possibility of a worldwide, satellite-based quantum network, larger distances are needed â€" something like the 400 kilometers (248 miles) to the ISS.

ESA’s NightPod (an Earth-based mockup)

The proposal, published by the Institute of Physics and the New Physics Journal today, is surprisingly simple, and exceptional because it requires very little in the way of modification to the ISS. Basically, the ISS is already equipped with a Nikon camera and 400mm lens (together called NightPod), pointed at the Earth through a 70cm window in the Cupola Module. The European physicists’ proposal would keep the lens in place, but replace the camera with a new, single-photon counting module. This module would be shipped to the ISS aboard a SpaceX Dragon or a Russian Soyuz capsule. Once the module is in place, a base station here on Earth on will entangle pairs of photons, with one half being kept on Earth, and the other half being transmitted to the ISS.

A diagram detailing the ground-to-ISS quantum experiments

The physicists propose two experiments. The first is a standard Bell-type experiment, which confirms that the entangled photons are indeed under the governance of quantum physics, rather than classical physics (which strictly doesn’t allow for these quantum entangled shenanigans). The second experiment would see the transmission of a quantum cryptography key, to see if it’s viable to secure conventional communications with space-based quantum key distribution (QKD). These experiments will be carried out as the ISS makes overhead passes of the optical ground station. ”During a few months a year, the ISS passes five to six times in a row in the correct orientation for us to do our experiments. We envision setting up the experiment for a whole week and therefore having more than enough links to the ISS available,” says Rupert Ursi, co-author of the proposal.

The results from this experiment will should tell us two things: Whether it’s possible to reliably transmit single, entangled photons over long distances, thus enabling the creation of a worldwide quantum network â€" and whether gravity has an affect on entanglement. The longer distance should also give us more accurate data about whether quantum entangled particles really do communicate their quantum state instantaneously, over infinite distances. As we recently reported, another research group recently showed this quantum channel to be at least 10,000 times faster than the speed of light.

The ISS’s Cupola, looking down at Earth

Moving forward, it is now a matter of whether the experiments are approved. Considering NightPod was built by the European Space Agency, the experiments will probably require its approval. With the growing interest and success of quantum entanglement experiments in recent years, we wouldn’t be surprised if the proposal is accepted in short order.

Now read: Qubits and binary data successfully squirted down the same fiber, at the same time: Here comes quantum cryptography

Research paper: doi:10.1088/1367-2630/15/4/043008 - “Quantum optics experiments using the International Space Station: a proposal”