Quantum information is currently too fragile to travel the very long distances involved in the global internet. A team of researchers is proposing an unexpected alternative: moving quantum information around the globe via container ships.
The first transatlantic telegraph cable was laid in 1858, spanning the North Atlantic between Newfoundland and Ireland. Getting a message from Europe to North America by steamship had been a 10 day ordeal—still a blink compared to the two to four months required by sailing vessels—which the cable reduced to mere minutes.
Nowadays, information is passed under the Atlantic in milliseconds via self-healing fiber optic materials at rates of tens of terabits per second per cable, of which there are lots. The future, however, is less concerned with bits in the first place.
In a paper posted to the arVix preprint server, Simon Devitt of Tokyo's Ochanomizu University and his team describe a ship-based system of delivering quantum information. Rather than cables, quantum information could move by container; instead of messages written with ink and parchment, it would be housed in diamond-based drives. It makes more sense than it may seem.
Obviously, the catch is that it doesn't occur continuously—diamond-based or not, you'd still be shipping physical hard drives around the world—but this sort of transfer is more common than you might think. It's called the sneakernet, which is basically just the transfer of digital information manually.
Google has done this for very large data sets, such as the 120 terabytes collected by the Hubble Space Telescope. Likewise, SETI@home has shipped data stored on magnetic tape from the Arecibo Observatory's radio telescope to its facilities in Berkeley. Movie production companies use hard drives to send scanned film images to reduce bandwidth usage, and Motherboard itself ships hard drives overnight from time to time. There's a whole other internet in the back of a truck somewhere.
Building larger and longer quantum networks will require two challenging tasks: First, the building of quantum repeaters every 100 kilometers or so along a transmission line to boost the signal, much like repeaters in traditional telecom applications.
The second is the exploitation of a quantum property known as entanglement, by which two particles have the same properties, despite being separated by physical space. So if you change a property of one entangled particle, the other, separate particle displays an equal change (imagine a 1 flipping to a 0 in computer binary), thus providing a potential avenue for instant communication over the distance they're separated by.
The problem is, even with repeaters boosting the signal of entangled particles, distances remain extremely limited for this form of quantum communication.
"Quantum repeaters have been demonstrated over short distances, but an error-corrected repeater network with sufficient bandwidth over global distances will require new technology," Devitt writes. "In particular, no proposed hardware appears suitable for deployment along undersea cables, leaving the prospect of isolated metropolitan networks."
Short-distance quantum networks are becoming more feasible, but recreating an undersea cable with quantum particles is a much harder proposition. Remember: instead of a physical wire connecting two physical computers, a quantum network would consist of particles connected with each other by an entanglement force we don't fully understand—or know how to easily repair if it breaks. You can't just lay new quantum connections like you do a cable.
So to make a long-distance quantum network, you'd need to build a theoretical repeater node, and even with one, it would require a single particle system, some number of particles sharing the same state that can be flipped back and forth, to be maintained in extreme conditions.
Long-distance entanglement is a highly sketchy thing. The distance record is currently 300 kilometers, but that's only within a well-controlled experimental setup. Lining up repeaters along undersea cables thousands of miles long is a distant possibility at best.
While quantum storage doesn't actually exist yet in a real-world form, Devitt and his team calculated the space and energy required to ship quantum data via theoretical diamond-based drives and other proposed quantum drive technologies. The details of how data would actually flow to and from the ships are vague, but the actual storage costs/space can be calculated just based on current experimental setups.
"Quantum memories may be transported to locations where entanglement is required or to intermediate locations to facilitate entanglement swapping between traditional repeater networks," Devitt writes, "enabling a complete network structure without the full deployment of physical links."
The quantum sneakernet is the first architecture that could feasibly underpin an entanglement-based economy
So while it wouldn't be instantaneously connected, physically shipping quantum data is far more feasible than trying to build a quantum analog to the telecommunications grid.
If anything, the paper serves to illuminate the long road ahead for the would-be quantum internet. Figure that a single shipping container would be capable of storing only 125 bytes of data, mostly given the cooling and power requirements of maintaining quantum information.
Here's the interesting thing. Container ships are huge, capable of carrying 10,000 containers at a time. If you follow the math all the way to its conclusion, we can actually figure out a data transfer rate for our ship-based quantum internet. Given the most promising quantum storage technology under development, a single ship might transfer data at a rate of 1 terabyte per second, after you average out the total transit time.
"Eventually, once quantum computers are commonplace, entanglement will be the fungible resource that enables a vast range of distributed applications," Devitt concludes. "The quantum sneakernet is the first architecture that could feasibly underpin an entanglement-based economy of this kind, connecting users of local quantum networks to a global quantum internet."