Quantum is the preferred adjective in space operas to ask the audience to suspend disbelief on something technical that doesn’t make sense. If there is a scenario where you to have something, say “Communication”, but it would not make sense for communication to be possible in a given situation, you can just call it quantum communication.

Seeing “Quantum Communication” on the cover of last month’s IEEE Communications Magazine reminded me that the word actually means something.

Quantum communication exploits counterintuitive properties such as the wave / particle duality of light. To understand this, Dr. Imre Sandor, author of one of the papers in in last month’s IEEE Communication, suggests we start by envisioning a classical (non-quantum) communication system that communicates bits by sending horizontally or vertically polarized light, with one polarization representing a ‘0’ and the other a ‘1’. A horizontal and vertical detector could detect the polarization and recover the bits. We could even send intermediate angles to send more bits per symbol. The amplitude detected would be the cosine of the angle between the polarization of the light and the detector.

What if the light level is decreased to one photon (the smallest drop of light you can have) at a time? The photon either activates the horizontal or vertical detector, not both. If the photons arrive at a 45 degree angle to both detectors, the receiver will detect a random horizontal or vertical photon each time a photon is sent. The result will be like a coin toss but "more random". A coin’s eventual resting orientation may be too complicated to calculate while it’s spinning in the air, but the laws of motion describe its path and could predict its resting position if we had all the parameters. Which detector a photon sets off, however, is truly random and unpredictable.

Consider the previous example where the sender encodes ‘0’s and ‘1’s as horizontal and vertical photons that are well aligned to a receiver’s detectors. Imagine the channel is noisy and sometimes flips the horizontal and vertical components of photons, as in the diagram to the right. This bit-flip interference is as a random as the thought experiment above, “more random” than a coin toss.

If we could construct a device, however, that can distinguish between photons at a +45 degree angle and those at a -45 degree angle, we can undo the effects of the bit-flip interference altogether. A +45-degree photon [1 + j1] is unaffected by the bit-flip because its horizontal and vertical components are the same. The bit-flip will transpose the components of a -45 degree photon [1 - j1], making it a 135 degree photon [-1 + j1]. A vector at a 135 degree is the same as a negative magnitude vector at -45 degrees. If the detector can ignore the magnitude, and just look at the angle, it can correctly identify photons sent at +45 or -45, completely circumventing the bit-flip interference.

I cannot find information about the construction of such a device. One article refers to early trials doing this over fiber optic cable but unfortunately without a footnote. Most of the articles discuss using quantum effects for encryption. Quantum effects also out to be useful for channel access schemes, which is one of the biggest problems in M2M wireless. Something obscure like distributed channel access could end up being a killer app of quantum communication.

Conclusion

After reading articles and exchanging e-mails with one of the authors, quantum communication is only a little clearer to me than the space opera definition. Fifteen years ago I thought MIMO was useful only in that it was easy to do in MATLAB if you needed something to write a paper on. Now it cheaply implemented on many inexpensive Wi-Fi chipsets. I’m curious to see if quantum communication follows the same path and just how much practical benefit, in terms of link budget, channel utilization, and encryption robustness, comes from these techniques.

Further Reading (All are from the Aug 2013 edition of IEEE Communications Magazine and require IEEEXplore access.)

Guest Editorial: Quantum Communications - A quick summary of the applications of Quantum Communications

Quantum Communications: Explained for Communication Engineers - Takes you through several applications with simple analogies to things in the macroscopic world

Using Quantum Technologies to Improve Fiber Optic Communication Systems - Explains how quantum effects theoretically allow a system to exceed the Shannon Limit