How to make a pure downconversion photon

It’s been almost a year since I last added some news here but that doesn’t mean nothing has happened in the lab. We’ve made some very good progress on one of our major goals, the creation of spectrally pure single photons via parametric downconversion.

This progress is summarised in one theory paper published earlier last year and a more recent preprint describing our experimental results. The short summary is that we found not one but two new algorithms for creating downconversion photons with joint Gaussian spectra — and both outperform all previous results in particular for short nonlinear crystals. Although often overlooked, nonlinearity tailoring is the third and equally crucial requirement of producing spectrally pure photons via what is inaccurately summarised as group-velocity matching, the other two being the actual group-velocity matching and the matching of the downconversion and pump bandwidth.

The new domain engineering algorithm was put to the test experimentally and the results look pretty good. Without any spectral filtering, we measure a lower bound on spectral purity close to 91% which is pretty close to the ideal target purity of just above 95% (which could be higher but isn’t due to known and fixable issues with the pump laser). For comparison, a standard crystal with a rectangular nonlinear profile restricts the purity to under 81%. This means we can ditch the frequency filters from our experiments and ultimately reach much higher multi-photon brightness — the key to at least in principle scalable photonic quantum technology.

Quantum circuit discovery for quantum-enhanced logistics

Not a result (yet), but an intro to a new research direction for our lab. Financed and supported by the Oxford Quantum Hub NQOIT, we’re collaborating with Prof. David Corne from Heriot-Watt’s School of Mathematical & Computer Sciences and local logistics services provider Route Monkey to develop quantum enhanced-software via circuit discovery. HW press release below:

Professor David Corne from the School of Mathematical and Computing Sciences, and Dr Alessandro Fedrizzi, from the School of Engineering and Physical Sciences, have been working with Livingston-based company Route Monkey to create innovative algorithms for the company’s transport and travel systems.

The team has now launched a project with NQIT to develop, test and commercialise quantum algorithms.

Route Monkey’s optimisation solutions eliminate unnecessary mileage and improve vehicle utilisation, typically helping to reduce fleet costs by up to 20 per cent and substantially cutting carbon emissions. The algorithms are capable of making millions of calculations in a relatively short space of time, vastly improving on manual transport planning.

Dr Alessandro Fedrizzi from Heriot-Watt University, said, “Quantum computers use the fundamental laws of nature to solve certain tasks faster than classical computers.

“In this collaboration with Route Monkey, we’re developing quantum-enhanced software for real-world applications.”

Colin Ferguson, chief executive of Route Monkey said, “With Heriot-Watt University and the NQIT Hub, we can address the increasingly complex challenges of moving people and goods around our cities, while simultaneously cutting wasted miles and reducing emissions from road transport.”

Founded in 2009, Route Monkey initially focused on developing complex algorithms that provide route optimisation and scheduling software solutions for fleet and transport managers. Route Monkey has expanded its algorithm portfolio to support low carbon vehicles and is now the UK’s leading provider of optimisation solutions for both ultra-low emission vehicles and the energy management of their charging stations.

Phase I of the project is more or less completed thanks due to the fabulous Vaclav Potosek, who unfortunately (for us) had to move on and is now in Prague. But we hope to continue and indeed expand this fruitful collaboration with NQIT’s help.

Testing `spooky’ causation

Last year saw the completion of a decade long quest to perform a successful Bell experiment without the three major and a few minor experimental loopholes. With local causality laid to rest, we can now start to unravel which of the many assumptions made by John Bell are incompatible with quantum mechanics. In a new experiment now published in Science Advances, we did just that., we did just that by allowing for `spooky’ causation between measurement outcomes. From the EQUS press release:

Research by an international team from Australia, the United Kingdom and Germany has shown that the gap between quantum phenomena and classical intuition is even bigger than previously thought.

In 2015, the universe was officially shown to be weird when a series of experiments demonstrated that entangled quantum particles remain instantly connected, no matter how far apart they are, through what Einstein famously dismissed as “spooky action at a distance”.

While it was shown that entanglement does not follow the classical rules of cause and effect, researchers continue to puzzle over how it really works.

Associate Professor Alessandro Fedrizzi from Heriot Watt University said that one popular explanation is that entangled objects could affect each other instantaneously, which ignores the universal speed-of-light limit.

He said, “In our experiment, we showed that this model cannot explain the experimental observations”.

We all intuitively understand and apply the concepts of cause and effect every day in our lives. Martin Ringbauer from the ARC Centre of Excellence for Engineered Quantum Systems said, “Picture yourself in a room where someone is flicking a light switch. Intuition and experience lets you establish a simple causal model: the switch causes the lights to turn on and off. In this case, correlation implies causation.”

“If we could entangle two lights, you would see them turn on and off at random, regardless of how far apart they are, with no obvious switch and in perfect lockstep. Einstein’s preferred explanation of this mysterious effect was that there must be a hidden light switch which acts as a common cause for our entangled lights.”

“In our experiment, our team set up individual photons to act like entangled lights and subjected them to two tests. In the first test, we essentially flicked the light switches ourselves to test a causal hypothesis. In the second, we tested a theory from one of our collaborators Rafael Chaves, which proposed that nonlocal causality cannot explain quantum entanglement.”

These results, published in the journal Science Advances, bring us a step closer to understanding the nature of this “spooky action at a distance”. Besides the fundamental importance, they also have potential applications for cyber-security. They can for example be used to increase the level of trust we have into quantum encryption devices.


Lead author Martin Ringbauer at work in Andrew White's UQ QTech lab. Photo taken by co-author Christina Giarmatzi. Our work was based on theory by Rafael Chaves in Freiburg, and local theory support was provided by Fabio Costa.
Lead author Martin Ringbauer at work in Andrew White’s UQ QTech lab. Photo taken by co-author Christina Giarmatzi. Our work was based on theory by Rafael Chaves in Freiburg, and local theory support was provided by Fabio Costa.

For the experts, what we really mean by `spooky’ causation, or `nonlocal causality’ as we labeled it in the title, is outcome dependence. Outcome dependence refers to the possibility that the outcomes of two measurements on entangled particles could have a classical cause-and-effect relation. We found that they don’t, no matter at which speed this cause-and-effect relation could be established.

So what does this mean, is this just another test of something-or-other that doesn’t actually enhance our understanding of quantum foundations? On the contrary. Outcome dependence was in fact a contender for explaining entanglement in a number of circles, in particular for people interested in the philosophy of science. And before you scoff at that as a hardened physicist, philosophers interested in quantum foundations have thought longer and harder about concepts such as causation, correlations, the arrow of time, and others than many of our own colleagues in the field.

So what are the caveats? On the one hand, we had to apply fair sampling — the requirements to beat the detection loophole in the otherwise device-independent 3-setting inequality we tested are in fact more stringent than for Bell (or rather: Eberhard) inequalities. Space-time separation is however not a problem, since our hypothetical causation could happen above light-speed anyway and still be ruled out. Furthermore we didn’t test for parameter (in)dependence.

What this shows though is that causal modelling and its application to quantum foundations has given us a fresh viewpoint, including that it allows for novel tests. It will be very exciting to see where the program of quantum causal models, which are now adapted to reconcile quantum mechanics with a causal world view will lead.

UPDATE: Physicsworld puts a nice spin on our results here.

DerStandard reports in German.

And here’s me playing a dubious expert on BBC radio, alongside science communicator Chad Orzel (starting at the 19:00 minute mark).

Equate Scotland

This summer we’ve had the pleasure of hosting Ella Wyllie in our lab, via the Equate Scotland initiative. From their newsletter:

EPS Equate Scotland students

The school of Engineering and Physical Sciences is delighted to welcome three Equate Scotland summer students to the Heriot-Watt campus. The students are Ella Wyllie, an undergraduate at University of Strathclyde, and Emily Badsvik and Rachel Sansom, both physics undergraduates at Heriot-Watt University. All three students won prestigious awards to work as research assistants in the field of quantum optics.

Rachel, Emily, and Ella (from the left) in one of our HW research labs.

Ella, who joins the Mostly Quantum research team, said that she was very happy to be awarded the position. She said, “It’s great to be part of such an active team. I’ve been thrown straight into exciting research – it’s great to have this opportunity.” All three students are looking for careers in science, so their placements are excellent opportunities to strengthen their CVs.

The summer placement scheme is targeted at providing women with additional opportunities in STEM subjects. For more details of Equate Scotland and their initiatives, please visit

Environmentally enhanced coherent transport

We have a new paper on environmentally enhanced transport out in Nature Communications, “Enhancing coherent transport in a photonic network using controllable decoherence“.

Here’s what Ivan and Andrew have to say about it:

Quantum researchers find noise isn’t always bad

A team of researchers from the ARC Centre of Excellence for Engineered Quantum Systems and the Friedrich Schiller University Jena have shown how noise can help transfer energy faster and more efficiently.

Energy is transported by waves. Noise, or something that disturbs the wave, usually inhibits wave motion and can slow it down substantially.

Recent research has indicated that there are certain situations in which noise improves wave transport in a phenomenon called Environment-Assisted Quantum Transport (ENAQT).

The lead researcher Dr. Ivan Kassal, who is working at the forefront of research on quantum effects in photosynthesis, said that ENAQT was first proposed as an explanation for the energy transfer which occurs in plants and bacteria when they harvest light during photosynthesis.

“Plants and bacteria harvest light using large antenna complexes. This light, or energy, is transported to reaction centres where the first chemical steps take place.”

“The transport in the antenna is partially wave-like and very noisy,” said Dr Kassal.

“By demonstrating that noise can improve the efficiency of wave transport, we have taken steps towards understanding this process and applying our understanding to the creation of renewable-energy devices.”

Professor Andrew White, chief investigator in the ARC Centre of Excellence for Engineered Quantum Systems, said that this research is the first implementation of controlled quantum decoherence in integrated optics, which will allow novel quantum computation techniques that take advantage of noise.

Professor White said, “It also opens the possibility of applying ENAQT to engineered quantum systems, for instance using controlled noise to help waves to get from their source to their destination faster and more efficiently.”

This paper can be found in Nature Communications at

On DWave’s latest progress

Google’s joint announcement with DWave has hit the airwaves and just as with the three previous generations DWave reports that their quantum annealer achieves significant speedups over classical algorithms. The good news IMO is that the scientific manuscript that underpins the press release is upfront about the main caveat, which is that the speedup all but disappears when the DWave machine is compared to algorithms optimised for the specific problem that the DWave machine solves. All of this is explained in much detail by retired DWave chief critic Scott Aaronson in a blog post and a Q&A here.

I thought this was a good opportunity to show a picture of me holding 12 generations of DWave chips. I took this at a somewhat cringeworthy DWave sales event at the University of Queensland a while ago.

Twelve generations of DWave's quantum annealing chip.
Twelve generations of DWave’s quantum annealing chip.

The DWave sales team tried to sell the previous generation machine to our IT department (for a cool 10 million if I remember correctly), with the promise of real-world speedups in all kinds of fantastic applications. Funnily enough they never mentioned the small detail of diminishing returns once apples were compared to apples.

The UK Quantum Technology Showcase

An extraordinary event took place for the first time in the UK a couple of weeks ago: the UK National Quantum Technology Showcase 2015. According to the official announcement, this was “a unique opportunity to meet the four UK Quantum Technology Hubs – a consortium of 17 UK universities, and find out about the latest advances in imaging, sensing, metrology, secure communication and computing.”

In practice, around 300 people from business, industry and of course the university sector met in the halls of the Royal Society in London to have a first glimpse at more or less commercially ready quantum technology emerging from the involved quantum labs.

Most of this technology was, apart from the Toshiba quantum key Distribution setup and our very own Heriot-Watt / Edinburgh University ultrafast single-photon camera presented by my colleague Jonathan Leach’s team, still in a very early stage of development. However, it was still exciting to see the momentum generated by this unique push to move quantum technology into the mainstream. The turnout was nothing less than impressive, almost 300 people showed up as you can see in this slightly blurry photo of Miles Padgett giving his address.2015-11-11 11.16.23

One of the key quotes that I took away from the event (by Prof David Kelpy, if I remember correctly) was that quantum technology needs to be demystified. “Spooky” action needs to turn into a more mundane language to lower the entry hurdles for the hundreds of engineers and others that will need to be trained to bring quantum technology to the markets within the decade. I agree. More than a century after the dawn of quantum mechanics it is time that we start treating this field of physics like any other, especially when communicating to the public. Hopefully, not too much of the wonder and excitement that we experience in our every day work will disappear in this process.

Nature’s exemplary summary paragraph

Every time we write a manuscript for a bold abstract journal the broad readership question is asked. Will they understand? Journals like Nature encourage you to think that way. To help their authors’ out in this endeavour, Nature supplies an exemplary summary paragraph, which is that bold, fully-referenced oddity that replaces the abstract in a Letter to Nature. This summary paragraph, we are told, should start with “One or two sentences providing a basic introduction to the field, comprehensible to a scientist in any discipline.” And this is their first sentence:

During cell division, mitotic spindles are assembled by microtubule-based motor proteins.

And here’s us, more than a century after the dawn of quantum theory, agonising over whether the broad readership will understand what quantum correlations are.

How to become a GOOD experimental physicist — part 0

Nobel laureate and super-determinism conspiracist Gerard ’t Hooft has an online guide on how to become a GOOD theoretical physicist. He lists required subjects and key concepts in each subject and links to mostly online resources for self-study of those concepts.

I’ve always considered it a shame that there isn’t anything similar for experimental physicists. Indeed, experimental physics is a bit of an afterthought in most university physics courses. A syllabus usually lists all kinds of theory skills that graduated physicists should have, but practical skills such as “knows how to use an oscilloscope” is usually not part of it. Sure, that’s too specific and instead there are those conceptual problem-solving related items that supposedly cover the practical aspects of being a physicist, but those are usually assumed to either happen on their own, or at most through the unstructured undergraduate labs that are sprinkled throughout a degree. Those labs usually represent a rather random collection of skills covered for historical reasons rather than a comprehensive framework, but more on that elsewhere.

So back to the topic, which is recommended resources on how to become a good experimentalist. I’ll start this out as a series of posts until I either get bored, or until I have enough material to collate it into a separate website. I will not presume to have anything of remotely the quality that G’tH offers, I simply haven’t put enough work into this. But hopefully it evolves and we will see what happens. Also, there is not yet any emphasis on free or online resources, I’ll write this off the cuff and refine it later.

G’tH starts out with languages, and similarly I will start Part 0 with a soft skill.

Project Management

Project management (PM) is crucial for any experimentalist and I swear that many a tragically failed PhD attempt could have been turned into a successful career had there been a modicum of project management knowledge. You don’t need full PM certification, even just the rudimentary basics will make a big difference. Set an achievable and measurable goal with a reasonable timeline, break it down into work packages, define milestones and deliverable for these work packages, and then define tasks and related activities to achieve those. And then learn how to time manage those tasks, how to assess the risks, and how to review and re-evaluate the work breakdown structure later when something goes wrong—as it inevitably will. On a higher up level, learn how projects form a program, and how a program can further a vision. For reading, any basic literature on the topic should do, so why not start with Project Management for Dummies.

I didn’t need to read that, luckily, because I attended professional development seminar on the topic in the first year of my PhD. Shortly after that I offered to impart my newfound wisdom on the rest of the group. I still remember quite well that Anton Zeilinger, who taught me a lot about having the right kind of vision, scoffed at the idea of having well defined, measurable goals. He insisted that that was for business people, that the mere idea would restrict us in our creativity to pursue whatever was the really important goal. I agree that we shouldn’t bow to some arbitrary goal (unless your stakeholders demand it, which is yet another thing PM helps you assess), but that’s for a group leader to keep in mind and a PhD student should instead still stick to goals that can be achieved.

And there are other aspects of project management that need adaptation in experimental physics. Rigid time planning for example is a challenge, due to the unpredictable nature of experimental work. However, that is part of proper risk management, and being aware of it helps decide when a project might need to be scrapped in order for deadlines to be achieved. But I digress, this really should be turned into a separate post on PM in the lab. TLDR: learn project management, it’s good for you!

Which concludes the warm-up to this series. The next part will deal with another bunch of not-yet-physics related subjects such as electronics and programming.