Tuesday, March 24, 2009

Quantum resource or classical control? The environment decides...

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.

Monday, January 26, 2009

A syllogistic argument

For some unknown reason I was reading up on syllogistic argument the other day and came up with this:
  • All classical trajectories are deterministic (i.e. not probabilistic).
  • (if there exists a correspondence limit for quantum mechanics then) Some quantum trajectories are classical (in terms of expectation values of observables).
  • Hence, some quantum trajectories are deterministic (¿ in terms of expectation values of observables ?).
not sure that this is even vaguely surprising yet I get the feeling there is a mistake in the logic. If so - I would like to know where - hence posting it here for constructive criticism.

Monday, January 12, 2009

Reconstruction of non-classical cavity field states with snapshots of their decoherence

Last year at the ICTP's Workshop on Quantum Phenomena and Information: From Atomic to Mesoscopic Systems Serge Haroche presented: Reconstructing the Wigner function of photonic Schrödinger cats in a cavity: a movie of decoherence.

While this was one of the best talks of the conference I was mildly disappointed that there were no actual movies of the decoherence process. The situation has now changed with the publication of the paper below in Nature - movies of the reconstructed Wigner function are available for download on the links given below. They really are well worth a look.

Nature 455, 510-514 (25 September 2008) 

Reconstruction of non-classical cavity field states with snapshots of their decoherence

Samuel Deléglise, Igor Dotsenko, Clément Sayrin, Julien Bernu, Michel Brune, Jean-Michel Raimond & Serge Haroche

The state of a microscopic system encodes its complete quantum description, from which the probabilities of all measurement outcomes are inferred. Being a statistical concept, the state cannot be obtained from a single system realization, but can instead be reconstructed1 from an ensemble of copies through measurements on different realizations2, 3, 4. Reconstructing the state of a set of trapped particles shielded from their environment is an important step in the investigation of the quantum–classical boundary5. Although trapped-atom state reconstructions6, 7, 8 have been achieved, it is challenging to perform similar experiments with trapped photons because cavities that can store light for very long times are required. Here we report the complete reconstruction and pictorial representation of a variety of radiation states trapped in a cavity in which several photons survive long enough to be repeatedly measured. Atoms crossing the cavity one by one are used to extract information about the field. We obtain images of coherent states9, Fock states with a definite photon number and 'Schrödinger cat' states (superpositions of coherent states with different phases10). These states are equivalently represented by their density matrices or Wigner functions11. Quasi-classical coherent states have a Gaussian-shaped Wigner function, whereas the Wigner functions of Fock and Schrödinger cat states show oscillations and negativities revealing quantum interferences. Cavity damping induces decoherence that quickly washes out such oscillations5. We observe this process and follow the evolution of decoherence by reconstructing snapshots of Schrödinger cat states at successive times. Our reconstruction procedure is a useful tool for further decoherence and quantum feedback studies of fields trapped in one or two cavities.