- The space-based quantum internet would substantially outperform ground-based networks of quantum repeaters.
- Building a global-scale quantum internet would require placing more than 400 satellites at an altitude of 1,900 miles.
The most useful application of quantum mechanics is the ability to perform secure communication through quantum-key distribution. The quantum internet — which is a dream for many technologists — would enable the execution of other quantum-information-processing tasks, including quantum clock synchronization, quantum teleportation, and distributed quantum metrology and sensing.
This means quantum internet can secure everything from private messages and contracts to financial transactions. And since the upcoming quantum computers will be able to crack existing encryption algorithms, this kind of security will become necessary.
But building a global-scale quantum internet is a tough experimental challenge. Recently, a team of researchers at Louisiana State University presented the most cost-efficient method to do this.
It involves creating a constellation of quantum-enabled satellites that can continuously broadcast entangled photons to the ground. The two most basic questions arise when considering such an approach are:
1) How many satellites are required to achieve global coverage that outperforms ground-based quantum-repeater configurations?
2) At what altitude these satellites should be placed?
First Some Background
The most important feature of such a network is quantum entanglement – a phenomenon where two quantum particles share the same existence, even when they are separated by a large distance. The entangled particles remain connected and actions performed on one affect the other.
Scientists mostly use pairs of photons (created at the same instant) to distribute entanglement. When you send photons to different locations, you can utilize their entanglement to send secure messages.
However, the entanglement (that connects photons) is extremely fragile and difficult to preserve. Even a tiny interaction between the photon and its environment could break the connection.
This is usually what happens while transmitting entangled photons directly through optical fibers or the atmosphere. These photons interact with other atoms in the glass or atmosphere. With existing technology, the entanglement can be shared only a few hundred miles.
So How To Build A Quantum Internet?
There are two options: the first one involves using devices called quantum repeaters that assess the quantum characteristics as soon as they arrive and transmit them to new photons that are sent on their way. Although it could preserve entanglement, the technique is prone to errors and may take several years to implement.
The second option involves creating entangled pairs of photons in space and broadcasting them to two ground-based stations at different locations. The stations will be able to swap information with perfect secrecy.
China has already conducted quantum experiments at space scale. In 2016, they launched a satellite named Micius to facilitate quantum optics experiments over long distances.
In such satellite-based scenarios, photons cover only the last 13 miles of their journey through the atmosphere. Thus they can travel much further (given the satellite isn’t too close to the horizon).
According to researchers, a similar network of satellites (if implemented on large scale) would create a more efficient global quantum internet. To send/receive information securely, two ground-based stations must communicate with the same satellite at the same instant so that both stations receive entangled photons from the satellite.
Constructing and launching satellites cost millions of dollars, which is why it’s important to keep the number of satellites as low as possible in the network without compromising the coverage.
Researchers modeled such a network and found that there are a few crucial trade-offs to consider. For instance, fewer satellites placed at higher altitudes can provide global coverage, but it could result in greater photon losses. Whereas, satellites at lower altitudes can only cover shorter distances between ground-based stations.
According to the research team, the best compromise would be a network of about 400 satellites flying at an altitude of 1,900 miles. To put this into perspective, GPS has 24 satellites operating at an altitude of 12,500 miles.
However, the maximum distance between two ground-based stations will still be limited to 4,700 miles. This means such a network could support secure communication between New York and London (3,459 miles apart), but not between Houston and London (4,846 miles apart).
Despite this major drawback, the space-based quantum internet would substantially outperform ground-based networks of quantum repeaters (where one repeater must be installed at every 120 miles).
Though hybrid networks interfacing space-based quantum communication platforms with ground-based quantum repeaters can turn this vision into reality.