Mostly Quantum

Cascaded downconversion. A laser beam creates photon pairs in a noninear crystal via the process of downconversion. One photon of the created pair then pumps a second crystal and is again downconverted. The result is a photon triplet.

Single-photon pair sources based on the nonlinear process of spontaneous parametric downconversion are still a relatively young development. They are however probably one of the most successful tools of modern experimental science in terms of the massive impact they had on the field of quantum information processing. They provided the first bright source of  entangled particles, which were then used in countless proof-of-principle experimental demonstrations which have shaped the field we’re working in. Examples include violations of Bell inequalities, the first quantum state teleportation, entanglement purification, multi-partite entanglement, quantum computing, both in the circuit and the cluster-state paradigm, the entire field of entangled-state quantum communication, and so on.

The hand-waiving explanation for the downconversion process is that a photon from a strong laser beam, which is focussed in the nonlinear crystal at the heart of the source, is “split” into a photon pair. However, the actual downconversion of a single photon has not been observed before. Until now. We have just demonstrated exactly this effect – the downconversion of a single photon, which was itself created as one photon of a downconverted pair. The experiment was carried out at IQC, in the group of Thomas Jennewein, and was published in Nature today.

We have a new paper, ‘Quantum entanglement distribution with 810 nm photons through telecom fibers‘, in Applied Physics Letters as of today (also available on the arXiv here). The experiment has been done in Thomas Jennwein’s group at IQC in Canada and demonstrates that entangled photons at 810 nm can be transmitted through 1550 nm, standard telecom fibers with reasonably high fidelity.

Most quantum information processing experiments are performed at wavelengths around 810 nm where silicon avalanche photodetectors are most efficient. The standard wavelength used in the telecom industry is however 1550 nm, which is where fiber loss is minimal. In our new experiment we show that, even though 1550 nm fibers are bi-modal for 810 nm, they can still be used for the transmission of entanglement at that shorter wavelength. As it turns out, the cross-talk between the two supported modes in the fiber is not critical and we can employ simple mode filtering, either temporal or spatial, to get a high-fidelity signal. In other words, for medium distance quantum information experiments, you might just as well use the much cheaper (and easily available) telecom fibers instead of expensive custom single-mode fibers for shorter wavelengths.

This idea goes back to efforts by Paul Townsend and then Gerald Buller and co-workers, who have pioneered high clock-rate quantum key distribution protocols in telecom fibers with weak coherent states at 850 nm (see this recent paper, for example).

The fact that telco fibers are bi-modal around 810 nm and that the cross-talk between these modes is low begs the question whether one could transmit spatially encoded qu-dits through standard telecom fibers, which would allow QKD with higher coding density. This is certainly a long stretch in practical terms. Interestingly, the group of Han Woerdman in Leiden has demonstrated just such an experiment in a hollow-core fiber, coincidentally pre-published on the arXiv the same day as our paper, so it’s definitely an idea worth thinking about.

QCMC is over and the physics entourage has moved on to the ICAP conference in Cairns or back home. Watching all those talks has prompted me to share my thoughts on chairing a conference session. Here they are:

1. Your Speakers

The job of a session chair is really not that hard. It therefore always puzzles me that some chairs don’t know who the speakers in their session are, how to pronounce their names and what they are talking about. So please – memorize the list of speakers and try to talk to them before the session starts. It’s a real bonus to know the title instead of awkwardly reading it off the first slide of the presentation. A particularly nice touch would be to actually introduce the speakers, i.e. where they are from, what they are working on, etc.

2. Question Time

When a presentation is over the chair usually prompts the audience for questions. If there aren’t any, you should try not to embarrass the speaker by repeatedly asking for questions, e.g.: “No questions? Really, no questions? Maybe you there, at the back? Ah, you were just scratching your nose. Well obviously, the talk was clear enough, haha.”.

Often, the chair will himself come up with a question to fill this dreaded silence from the audience. This is not bad as such, the chair is supposed to be an expert on the session topic after all. However, it should still be a relevant question, which may indeed have come up during the talk. Alternatively, many chairs invent token questions from the title and abstract of the respective talks but these can usually be spotted easily. Also, they can backfire pretty quickly, such as when the speaker doesn’t understand your question, asks you to clarify it and therefore exposes that you have no idea (as witnessed at QCMC). So – if you have a good question, ask it. Maybe even ask first, before referring to the audience. That might break the ice. If you don’t, just let it go, the next speaker will thank you for the extra 30 seconds.

Another unnecessary thing is to tell the audience that there is time for “one or two short questions”. A question may be short but the answer might not be. And please, don’t interrupt a discussion just because you’re one minute late and the coffee is waiting outside. It’s not the speaker’s fault that he came last in the session. There will always be more coffee and discussions is what a conferences are for.

3. The Equipment

Know how to use the presentation hardware. Usually, you will have tech support, but what if you don’t? It’s therefore a good idea to check with the speakers whether they have all tried out their talks, be it on the conference system or their own computer. Also, have a laser pointer ready if at all possible.

4. The Audience

Have you noticed that there is someone in the audience who will have a question, or rather a long and not very relevant comment, for every single speaker? It is within your rights to ignore him.

Finally, a request to the audience proper. Some speakers knowledge of the conference language does often not extend beyond giving their talk, which is actually quite a brave thing to do. If during the first question it becomes clear that they struggle to understand, please don’t go on torturing them.

These are my thoughts on the topic. A Google search has just unsurprisingly revealed to me that there are excellent guides for Session Chairs on the web. This one here for example.

In 5 days the QCMC 2010 is taking off here in Brisbane. We’re looking forward to one week of exciting Science and free coffee.

Th French magazine “Pour la Science” has just reprinted the 2008 Seed magazine article “The Reality Tests” on our Vienna Leggett inequality experiments.

The original experiment on a violation of the Leggett inequality by Simon Groeblacher and colleagues garnered a lot of attention. My modest contribution was to perform more conclusive measurements for an inequality with less assumptions.

The actual implications of these experiments are a little blown out of proportion by the Seed magazine article. They have however sparked a lot of additional interest in research of the foundations of quantum mechanics beyond Bell inequalities. Most proficient have been the folks in Switzerland and Singapore, who picked up on Simon’s paper with amazing speed and then almost beat us to the second experiment, which we ended up publishing back-to-back. It didn’t take them long to then work out an improved version of Leggett’s inequality which they published in Nature Physics. Lead author was Cyril Branciard, who is now a colleague of mine in Brisbane and has helped Pour la Science translate the Seed magazine article into French. Thus closes the circle.