Mostly Quantum

Yesterday, I watched the second part of the BBC’s “The race for absolute zero”, a 2007 documentary on the quest to reach zero temperature. I’m a big fan of the BBC and it’s documentaries and can thoroughly recommend “Absolute Zero”, which features eminent scientists such as Eric Cornell, Carl Wieman and Wolfgang Ketterle, who shared the 2001 physics Nobel prize for creating the first Bose-Einstein condensates (BEC). However, three quarters into the documentary, when BECs had finally been demonstrated in the lab, the following sentence came up:

“At last, quantum mechanics was more than just theoretical mumbo-jumbo”.

A surprising statement. Surely, quantum mechanics hadn’t been “mumbo-jumbo” at any time before, especially not as late as 1995!

I’ll put it down to sloppy writing, with the intention to be witty. A more suspicious mind could conceive that the writers were trying to (sub-) consciously appeal to a less scientific minded crowd. But is that really necessary in a scientific documentary, dedicated to explaining ultra-cold temperatures, by the most reputable broadcast company in the world?

It is easy for the scientific community to overlook these things. At least it was a documentary on science, right? Some of us were on TV, weren’t they? Did you see me, Mum, all those years were not in vain! But on the other hand it’s those little things which creep into the public’s minds and later, when it counts, make it near impossible to discuss important matters such as global warming. After all, the scientific “facts” behind that could just be more “theoretic mumbo-jumbo”.

We have just added a paper to the growing collection of experimental implementations of quantum walks. Quantum walks are the quantum analogue of random walks in the classical world. The theory of quantum walks has long been established, but only last year saw a number of decent experimental realizations, with neutral atoms, ions (here and here) and, of course, photons.

What makes our quantum walk special is that we can control the amount of decoherence in the walk. Decoherence, i.e. loss of coherence in the quantum system by interaction with the environment, is at the heart of many of the quantum protocols and biological processes that have partly spurred this renewed interest in quantum walks. We expect to use our new toy for a number of experiments on these phenomena in the near future.

EDIT: The paper has now been published in PRL.

I have a new paper, “Information complementarity in quantum physics” on the ArXiv preprint server. We studied the distribution of information in a whole zoo of pure and mixed two-photon quantum states and demonstrate experimentally that local information is complementary to information stored in correlations.

The paper contains the probably most extensive range of experimentally created two-photon quantum states so far and a number of insights about bipartite complementarity that were previously not widely known. Most of the experimental work was done by Bojan Skerlak, a Swiss student (and surf-tourist extraordinaire) from ETH who decided to do his master’s thesis here in Queensland. The theory was heavily influenced by Tomasz Paterek.

Our paper Discrete, tunable color entanglement has been published in Physical Review Letters.

Two photons can be entangled in many different degrees of freedom, such as their polarization (the all-time favorite in the community). Photon pairs created in spontaneous parametric downconversion are intrinsically entangled in energy (and thus in frequency) and momentum. The frequency entanglement however is continuous, i.e. the spectral components of each photon are entangled with each other. To verify this entanglement one can single out a pair of frequencies within the photon spectrum, which in earlier experiments was usually done after an actual measurement via interference filters or apertures.

In our experiment in contrast, only the two colors which show up in the final entangled state are present in the initial state. Hence the title ‘discrete’ color entanglement. The frequency bins in our experiment are well separated and we can even tune the frequency difference between them. In other words, each photon can be seen as a frequency qubit with a tunable energy gap. In addition, we were able to reconstruct a density matrix for the entangled states and calculate properties such as their tangle and purity.

Physics Today has highlighted our work here.

Sometimes, when chasing up a reference, I come across a journal websites which I haven’t used before. Once I find the desired article, I usually read the abstract and then proceed to downloading the paper in PDF format. One would think that this is the most common procedure for visitors to a journal website. Alas, this is not always easy. Take for example the following screenshot taken from the American Institute of Physic’s Journal of Mathematical Physics website:

Can you spot the PDF download link? Tricky, isn’t it? Just like looking for easter eggs. That’s maybe because the PDF button is only 20×20 pixel large. What makes it even worse is that there is no text to accompany it, which would allow a simple search for “pdf” in your browser. Interestingly, 12 out of the 14 (!) “full text options” for this article—among them for example “erratum alert”, or “blog this article”—do have an alternative text link. One can only assume that the publisher wants to maximize the time we spend on their site to maybe even click one of these other links out of frustration of not finding the PDF download.

Here’s my proposed fix to that:

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.

My name is Alessandro Fedrizzi, I’m a researcher at the Department of Physics and Centre for Quantum Computer Technology at the University of Queensland, Brisbane, Australia. This ‘blog’ is less a blog than a personal website. I will however — now and then, whenever I feel the urge — blog about things that interest me; things mostly — but not exclusively — quantum.