Why Quantum "Clippers" Will Distribute Entanglement Across The Oceans

One possible future for communication is to create a quantum version of the internet that will have the ability, among other things, to send information with perfect security. This network will use entangled photons to transmit information from one locations to another without it passing through the space in between, hence the security.

Photons can only travel hundred kilometres or so through optical fibres before being absorbed. Conventional optical networks get around this with repeaters that boost the optical signal as it passes by.

This is more difficult with a quantum network but physicists have already tested many of the building blocks necessary to make quantum repeaters work. Nevertheless, quantum repeaters will be delicate pieces of kit, operating close to absolute zero with all the necessary cooling and power that is also required.

That should be relatively straightforward for quantum networks that stretch across land. But it is entirely unsuitable for undersea cables where conditions are far more hostile and the absence of infrastructure is a potential showstopper.

In fact, nobody is quite sure how it will be possible to operate the number of quantum repeaters necessary to carry one half of an entangled photon pair across the Atlantic or the Pacific. And without any technology even on the horizon that can do this job, there is a very real possibility that the quantum Internet might only ever consist of isolated quantum islands on different continents.

Today, Simon Devitt from Ochanomizu University in Japan and a few pals have come up with a way to solve this problem. Their idea is to transport the quantum bits or qubits across the ocean on a containership, a kind of quantum Clipper, that will shuttle back and forth across the seas with a ghostly quantum load.

Devitt and co say the idea is the quantum equivalent of the sneakernet, an informal name for the practice of moving classical information stored on memory sticks or other physical media. One of the big disadvantages of the sneakernet is the time it takes to transport information from one location to another, the so-called latency of the system.

But Devitt and co point out that this will not be a problem for the quantum Internet because information will not be carried on board the containership. Instead, the resource in transit is entanglement, the strange quantum phenomena in which two quantum objects share the same existence even when they are vast distances apart.

The information transport occurs later when each half of the entangled pair is on opposite sides of the ocean. Then, a measurement on one particle immediately determines the state of the other on the other side of the planet, a phenomena that physicists can use to transmit information with perfect security.

So by shipping large quantities of qubits, Devitt and co say it will be possible to send information at bandwidths measured in teraahertz. That’s faster than a quantum internet connected together with quantum routers. “Bandwidth in excess of one terahertz is feasible under realistic physical assumptions, exceeding even the fastest proposals for traditional repeater networks,” they conclude.

That’s the theory. The devil will be in the detail. For a start, Devitt and co will need a way of storing qubits reliably for months at a time. That requires them to maintain a quantum object in a superposition of states for at least the time it takes to travel across an ocean–20 days from Japan to the US, for example.

One technology that looks capable of achieving this in the not-too-distant future involves diamond crystals in which one carbon atom has been replaced with a nitrogen atom. This creates an electron vacancy that can absorb a photon and store its quantum state. Because these vacancies are well insulated from the environment, they can store these quantum states for lengthy periods, potentially for months at a temperature of 4 Kelvin.

What’s more, a single diamond crystal can hold huge numbers of these so-called nitrogen vacancy centres, which can be packed densely into the crystal structure and then accessed using lasers.

Of course, a certain percentage of these qubits will always decay and care will have to be taken to ensure that this does not introduce errors at an unacceptably high rate. “Extrapolating to large arrays, we find that quantum memories of 1600 qubits enable storage of logical qubits for two months with a logical error rate of 10^-10,” say Devitt and co.

They go on to describe the additional equipment needed to store these qubits. This will all need to fit in a standard shipping container which has an internal volume of 40 cubic metres. Devitt and co assume that the quantum memories will take up just one cubic metre with the rest reserved for power, refrigeration and the control infrastructure.

Each of these shipping containers will then be the quantum equivalent of a memory stick, to be carried around the world and used as necessary.

When the qubits have been used, the memory stick is recharged with new qubits and then sent back across the ocean as part of a conveyor belt of quantum memory sticks that will provide the fundamental resource of entanglement for the quantum internet.

At first glance, this is an idea that looks to have come way out of left field. But on closer inspection, it seems a thoroughly sensible approach to a thorny problem. Clearly, the technology that will allow quantum routers to operate over thousands of kilometres of undersea cables is a distant dream. But while this technology is being developed, quantum Clippers can distribute entanglement efficiently.

That’s a fascinating idea that raises the possibility of ocean-going quantum Clippers carrying the cargo necessary to enable a new kind of entanglement-based information economy.

Ref: arxiv.org/abs/1410.3224 : High-Speed Quantum Networking By Ship