We have just had the following article published on line:
M.J. Everitt, W.J. Munro and T.P. Spiller "Quantum-classical crossover of a field mode" Phys Rev A 79(3) 032328 (2009). [published version] [eprint]
Below is a brief summary that is supposed to be accessible to a general audience.
In this work we show that we can take a photonic device and use it either as a quantum resources or a classical control field simply by changing its environment. Quantum communication technologies are a reality today, and the first steps are now being taken towards other new technologies that sense, process and store information using quantum resources. These new technologies get their power by leveraging properties, such as "spooky action at a distance", only seen in quantum systems. So we can operate them, these new technologies must have a conventional - classical - IT interface. Furthermore, the quantum resources need to be controlled with classical sources, such as electromagnetic fields. However, we know that everything is actually made of quantum parts! So this begs a question: Under what circumstances are fields quantum - and thus part of the technology resources - and under what conditions are they classical - and thus part of the control interface? A "standard" answer to this question is size: A field with one photon (one quantum of light) is clearly quantum, and a large coherent field containing many photons is classical. In our work we demonstrate that the actual answer is rather more subtle than this. Indeed, it is possible to take a field with fifty or more photons in it, and allow it to be highly quantum (part of the resources), or force it to be classical (a control field) by changing its environment. So size is a factor, but it's not the only thing that matters. In the end how a system behaves is also determined by what it interacts with.