Whatever the message or the medium, the essence of communication is the transfer of data between two points separated in space. The distance can be small (between components inside a computer chip, or people sitting across a table) or very large (from Voyager 1 back to Earth), but the principles are the same. Quantum communication changes the nature of the data—it's the state of a quantum system—but that information still needs to be carried across space.

In most modern research, that process involves entanglement: linking the quantum state of the transmitter and receiver. Unfortunately, these quantum states are fragile. While a bit stored in computer memory is relatively stable, a quantum state can be altered by random interactions with its environment. But new model for quantum communication has been proposed that would not require either memory or entangling the quantum states of the transmitter and receiver.

W. J. Munro, A. M. Stephens, S. J. Devitt, K. A. Harrison, and Kae Nemoto have designed a system where quantum bits (qubits) were transferred by individual photons, but interpreted using a special algorithm designed to contain a lot of redundancy and avoid data loss. Since the states of the transmitter and receiver were not entangled (or copied), they don't need to remain coherent, obviating the need for quantum memory. The actual data transfer could take place over fiber optic cables, and the receiver could itself be used as a transmitter, forming a repeater for larger networks.

While this is a conceptual model for the moment, if implemented, it could help resolve some of the problems in quantum communications, or at least provide a workable stopgap solution until entanglement-based networks come of age.

The data in a quantum system is encoded as its quantum state—all the relevant physical parameters, including photon polarization. (An atom's quantum state is typically very complicated, involving the nuclear properties and configuration of every electron. As a result, quantum computing and communication based on atoms often relies on isolating a handful of those properties, such as the spin of one electron.) Quantum communication involves sending this state information between points.

A great deal of the research on quantum communication and computing involves entanglement: the linking of the quantum states of two systems separated in space. Measuring the state of one of those systems reveals the state of the second system instantaneously, no matter how widely they are separated, which is why entanglement is so useful for quantum communication. The problem then becomes that quantum systems can "decohere" due to environmental influences, changing their state and losing the information.

(Since no data is exchanged through entanglement, it's still necessary to use additional communications channels to tell the receiver what to do with their measurement. Additionally, the act of preparing entangled systems usually involves exchanging a photon, which while very fast is far from instantaneous. So, quantum networks are still limited by the speed of light.)

Thus, in these networks, quantum memory is essential for entanglement-based communication—storing the quantum state for a sufficient period. They're limited by the speed of establishing the entangled transmitter-receiver pair, and by how long the states remain coherent. The proposed model bypassed that problem entirely by not requiring the transmitter's quantum state maintain coherence longer than necessary to interact with a messenger photon.

In the researchers' proposal, both the transmitter and receiver contained a simple matter-based quantum system, such as a single electron spin. The state of the transmitter system was prepared in a particular way (the data), which was then encoded in the state of a messenger photon, in the form of a redundant code. The photon can then be sent down fiber optic cable to the receiver, where interaction with a similar system extracts the state of the original. At no point are the two matter-based systems required to be in the same state (as in quantum teleportation) or entangled, so their relative configuration is irrelevant to communication. Similarly, even though the transmitting system was prepared to be in a specific state, it didn't need to maintain that state once it interacts with the photon.

A lot of the paper was devoted to a discussion of the redundancy algorithm, which I will happily abstain from explaining. However, its purpose was to make it possible for communication even with photon losses, which are inevitable in fiber optic communication.

Without the need for entanglement or quantum memory, and with the redundancy algorithm, this model could potentially provide a much simpler method for quantum communication. The reduced number of steps also would result in faster speed, not for the communication itself, but for preparation and analysis of the quantum systems involved. All of this would have big benefits for quantum computers, which will need to manage communications between different components (like the memory and processor), and have data shuffled into them, possibly from remote locations.

A practical implementation of this project hopefully will be forthcoming soon.

Nature Photonics, 2012. DOI: 10.1038/nphoton.2012.243 (About DOIs).