The Nature Publishing Group (NPG) has just announced a new journal – Nature Communications.

This is apparently an attempt at providing a home for excellent research which is not broad enough to be interesting for the general readership of  Nature and at the same time not necessarily covered in a dedicated Nature research journal like Nature Physics or Nature Chemistry. Official examples include high-energy physics. The aims and scope of this newest addition to the NPG empire — whose official abbreviation will be Nat. Comms. — do however state that it will also feature contributions in areas which are already covered by these specialized journals. Confusing, isn’t it?

For me as an experimental physicist working in quantum optics this is quite exciting. The number of Nature journals which potentially cover our field of research has risen from 1 to 4 within just a few years: Nature, Nature Physics (released in October 2005), Nature Photonics (launched in January 2007) and now Nature Communications. The newest journal in this club could potentially attract quantum optics papers which do not necessarily contain exciting new experimental methods and thus ‘hard’ physics but are focused on conceptual questions.

An entirely different question is whether this explosion of top tier journals is a good thing for physics. Clearly, any journal with ‘Nature’ in its name will attract a lot of good papers. While I think it is a great idea to offer interdisciplinary fields a high-profile and high-impact place to publish their research I get the feeling that physics itself is spread out too far across many journals and that this could have an inflationary effect on the respective journals’ impact. Also, Nat. Comms. will for many groups be yet another rung on the ladder which starts with a rejection from Nature and descends into the nether regions of the publishing space. This will stretch the already lengthy period between initial submission of a paper and its publication even further.

Anyway, enough speculation. To see where Nature Communications will really be heading, we’ll have to wait for the first issue, which will be published in April 2010.

Our paper “Anti-symmetrisation reveals hidden entanglement” has been accepted to published in New Journal of Physics. This is one out of four papers which formed my PhD thesis and the last one to make it into a peer-review journal.

Two-photon interference pattern of frequency detuned photons superposed at a beamsplitter.

The paper offers a new take on the famous Hong-Ou-Mandel effect in quantum optics. Imagine two photons hitting a symmetric beamsplitter. They can either leave the beamsplitter together, in a single output mode, or apart, i.e. through separate output modes. In quantum physics, the probability amplitudes for these events can interfere. Two photons, indistinguishable in all respects (temporally, spatially, spectrally…), will bunch, i.e. they will always leave the beamsplitter together. Hong, Ou and Mandel demonstrated this phenomenon by changing the relative temporal delay of the photons and measuring them behind a beamsplitter in coincidence . For delays on the order of the photon coherence length, they observed a drop in the coincidence rate, with a minimum located at zero delay – the Hong-Ou-Mandel, or in short, HOM-dip (click here for an interactive demonstration). This effect is one of the pillars of experimental quantum optics with single photons and has been exploited in various flavours for a myriad of experiments.

One particularly interesting aspect of two-photon interference is that whenever the photons anti-bunch, which is the opposite of bunching, the photons must have been entangled to some extent. This is due to symmetry: In order to anti-bunch at a beamsplitter, the spatial part of the wavefunction of the photons must have been (partially) anti-symmetric. This in turn is a unique signature for entanglement, as pointed out in this comprehensive paper by K. Wang.

In our work, we argue that one can deliberately change the spatial symmetry of a quantum state to reveal entanglement on a beamsplitter. Experimentally, we achieve that by tuning the central frequency differences of two photons and measuring two-photon interference. The resulting patterns show frequent anti-bunching above the random level, a signature of the underlying frequency entanglement, which emanates from the creation process of the photons. The neat thing about the experiment is that the experimental method is very simple and does not require any frequency filtering unlike earlier quantum beating experiments. The neat thing about the concept is that it should be applicable to general quantum systems, beyond our simple demonstration with photons.