French revolution in quantum photonics

7 March 2016

Three sources of single photons. . Image credit: Niccolo Somaschi

Photonics, the technology that powers the global internet, uses bright lasers to function.

Quantum photonics will allow currently impossible capabilities in not only communication, but in sensing, metrology, and even computation, however it requires single particles of light—photons.

The problem is that to date there have been only approximate single-photon sources: these are physically large and hard to multiplex—making 6 independent photons at a time currently occurs at a lower rate than gravitational-wave detection!

A French-Australian team has cleared this decades-long roadblock away, developing a single photon source that is a million times smaller in volume, and 20 times brighter, than existing sources.

The new sources use quantum-dots—artificial atoms made of 10,000 or so semiconductor atoms—sandwiched between two microscopic mirrors that are housed inside a pillar a tenth the thickness of the finest human hair.

“The source is so bright because we use the mirrors to engineer the quantum vacuum, making the quantum-dot very likely to emit light in one direction, unlike natural light sources that emit light in all directions”, said Professor Pascale Senellart, lead of the team at the Centre National de la Recherche Scientifique, Paris.

“We have developed a technique that uses electric fields to ensure that the dot emits light at exactly the right colour for the mirrors it lies between”, she continues.

The new sources are a remarkable improvement over the current state-of-the-art sources, which use millimetre scale crystals, require expensive detectors to flag photon production, and mostly produce no light at all.

“The last twenty years have seen many proposals for exciting future technologies, all of which assumed single photon sources”, notes Juan Loredo, PhD student in the team at the University of Queensland, Brisbane, “We think the new devices will have the same effect in quantum photonics as moving from room-sized mainframes to personal computers did in computing”.

“These sources are the first scalable single-photon technology: literally, the future is looking bright”.

The paper can be found at http://dx.doi.org/10.1038/nphoton.2016.23

Caption: Three sources of single photons. The semiconductor quantum dot—about 10,000 atoms, represented by a red dot at the centre of the cavity—is inserted in the centre of the cavity, which consists of a 3 µm pillar connected to a circular frame by guides that are 1.3 µm wide. By applying electrical voltage to the cavity, the wavelength of the emitted photons can be tuned and the charge noise totally eliminated. Image credit: Niccolo Somaschi – Laboratoire de photonique et de nanostructures (CNRS)

Media:
Mr Juan Loredo (+61 4 2140 8439, juan.loredo1@gmail.com) and Professor Andrew White (+61 4 6625 6329, andrew.white@uq.edu.au), School of Mathematics and Physics, University of Queensland, Australia.
Professor Pascale Senellart, Laboratoire de Photonique et de Nanostructures, Centre National de la Recherche Scientifique, France (+33 6 28 35 55 26, Pascale.Senellart@lpn.cnrs.fr).
Ms Tara Roberson, Communications officer for ARC Centre for Engineered Quantum Systems, Australia (+61 4 0451 6635, t.roberson@uq.edu.au).

Official press release.

Quantum reality check

9 February 2015

Schrödinger’s cat highlights a long-standing dilemma in quantum mechanics: is the cat really alive and dead, or is the weirdness just in our head?

Researchers at The University of Queensland have now made major progress in answering this question.

Using four-dimensional states of photons, and subjecting them to very precise measurements, they ruled out the popular view that describing the cat as dead and alive is just due to a lack of knowledge about its real state.

As with all objects in quantum physics, the cat is described by the quantum wavefunction.

Dr Alessandro Fedrizzi, from the UQ School of Mathematics and Physics (SMP), said that although the quantum wavefunction is our central tool for describing physical systems in quantum mechanics, it is still unclear what it actually is.

“Does it only represent our limited knowledge about the real state of a system, or is it in direct correspondence with this reality?” he said.

“And is there any objective reality at all?”

This debate has remained purely theoretical for decades, until three teams of quantum theorists — including co-authors UQ’s Dr Cyril Branciard and Dr Eric Cavalcanti from The University of Sydney — recently proposed experimental tests to answer this question.

Lead author and UQ PhD student Mr Martin Ringbauer said that the new approach tests whether the competing interpretations of the wavefunction can explain why we cannot tell quantum states apart with certainty, which is a central feature of quantum mechanics.

“Our results suggest that, if there is objective reality, the wavefunction corresponds to this reality,” Mr Ringbauer said.

In other words, Schrödinger’s cat really is in a state of being both alive and dead.

As measurements improve further, physicists will be left with two possible interpretations of the wavefunction: either the wavefunction is completely real, or nothing is.

The authors of the study, published in Nature Physics, are Mr Martin Ringbauer, Mr Benjamin Duffus, Dr Cyril Branciard, Dr Eric Cavalcanti, Professor Andrew White and Dr Alessandro Fedrizzi.

The study can be found at http://dx.doi.org/10.1038/nphys3233.

The work was supported by the Australian Research Council Centres of Excellence for Engineered Quantum Systems and Quantum Computation and Communication Technology, as well as the Templeton World Charity Foundation.

Watch Dr Alessandro Fedrizzi present a short video on the research here.

Media: Mr Martin Ringbauer (+61 7 3365 2444, m.ringbauer@uq.edu.au); Dr Alessandro Fedrizzi (+61 7 5336 7031, fedrizzi@physics.uq.edu.au); Professor Andrew White (+61 4 6625 6329, agx.white@gmail.com); Faculty of Science Engagement Officer Aarti Kapoor (+61 0449 863 208, a.kapoor@uq.edu.au)

Official press release from the University of Queensland.

Doctor Who meets Professor Heisenberg

19 June 2014

Lead author and PhD student Martin Ringbauer, from UQ’s School of Mathematics and Physics, said the study used photons – single particles of light – to simulate quantum particles travelling through time and study their behaviour, possibly revealing bizarre aspects of modern physics.

“The question of time travel features at the interface between two of our most successful yet incompatible physical theories – Einstein’s general relativity and quantum mechanics,” Mr Ringbauer said.

“Einstein’s theory describes the world at the very large scale of stars and galaxies, while quantum mechanics is an excellent description of the world at the very small scale of atoms and molecules.”

Einstein’s theory suggests the possibility of travelling backwards in time by following a space-time path that returns to the starting point in space, but at an earlier time-a closed timelike curve.

This possibility has puzzled physicists and philosophers alike since it was discovered by Kurt Gödel in 1949, as it seems to cause paradoxes in the classical world, such as the grandparents paradox, where a time traveller could prevent their grandparents from meeting, thus preventing the time traveller’s birth.

This would make it impossible for the time traveller to have set out in the first place.

UQ Physics Professor Tim Ralph said it was predicted in 1991 that time travel in the quantum world could avoid such paradoxes.

“The properties of quantum particles are ‘fuzzy’ or uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations,” he said.

Professor Ralph said there was no evidence that nature behaved in ways other than standard quantum mechanics predicted,but this had not been tested in regimes where extreme effects of general relativity played a role, such as near a black hole.

“Our study provides insights into where and how nature might behave differently from what our theories predict.”

Examples of the intriguing possibilities in the presence of closed timelike curves include the violation of Heisenberg’s uncertainty principle, cracking of quantum cryptography and perfect cloning of quantum states.

Published in Nature Communications, the paper “Experimental Simulation of Closed Timelike Curves” includes Dr Matthew Broome, Dr Casey Myers, Professor Andrew White and Professor Timothy Ralph, all from The University of Queensland. http://www.nature.com/ncomms/2014/140619/ncomms5145/full/ncomms5145.html.

The work was supported by the Australian Research Council Centre of Excellence for Engineered Quantum Systems and Centre of Excellence for Quantum Computation and Communication Technology.

Media: Mr Martin Ringbauer (+61 7 3365 2444 or 0478 919 844 or m.ringbauer@uq.edu.au), Professor Tim Ralph (+61 7 3346 9693 or ralph@physics.uq.edu.au) and Professor Andrew White (+61 4 6625 6329 or agx.white@gmail.com).

Official press release from the University of Queensland.

UQ team challenges the limits of famous quantum principle

5 February 2014

A team of physicists is challenging the very limits of Heisenberg’s famous uncertainty principle by measuring quantum particles with unprecedented accuracy.

Physicists from The University of Queensland have performed joint measurements on single light particles with accuracy never seen before, and developed methods that could help improve the most sensitive quantum sensors.

Martin Ringbauer, PhD student at UQ’s School of Mathematics and Physics and lead author of the experimental study, said the findings help answer long-standing open questions in quantum mechanics.

“The uncertainty principle is one of the central features of quantum mechanics, which has been misunderstood for the longest time,” Mr Ringbauer said.

This “Heisenberg principle” states it is impossible to jointly measure two incompatible quantities, for example speed and location, of a quantum particle with perfect accuracy.

“This experimental work settles a decade-long debate — ‘Heisenberg-like’ relations do not hold for joint measurements,” he said.

“Now that we have a complete theory, as well as experimental evidence, it is probably time to update the textbooks.”

Almost a century ago, renowned quantum theorist Werner Heisenberg found fundamental limits on how well a quantum system can be prepared and measured, known as Heisenberg's uncertainty principle.

However, only the limit that pertains to the preparation of quantum systems has been quantified; the other two, relating to measurements, have long been a matter of debate, lacking a formal treatment.

These limits are: That it is impossible to jointly measure incompatible quantities, for instance, location and speed of a quantum object, with perfect accuracy; and that a measurement of one of these quantities necessarily disturbs the other.

Last year, UQ’s Cyril Branciard proposed a new set of “uncertainty relations”, for the joint measurement of incompatible quantities, which describe the minimal disturbance that will occur for a given measurement accuracy.

“Branciard’s relations quantify how accurately we can measure,” Mr Ringbauer said.

“Testing these relations, we are now able to show in the lab that we can actually reach this ultimate limit of accuracy,” he said.

The study was published in January in the journal Physical Review Letters. A related work by Kaneda et al. in the same journal, has found similar results.

Other authors of the paper include Mr Devon Biggerstaff, Dr Matthew Broome, Dr Alessandro Fedrizzi, Dr Cyril Branciard, and Professor Andrew White.

The research was supported by the Australian Research Centres of Excellence for Engineered Quantum Systems, and Quantum Computing and Communication Technology.

Media: Mr Martin Ringbauer (+61 7 3365 2444, m.ringbauer@uq.edu.au) or Prof. Andrew White (+61 4 6625 6329, agx.white@gmail.com) or Engagement Officer Aarti Kapoor +61 0449 863 208, a.kapoor@uq.edu.au).

Official press release from the University of Queensland.

Advancing scientific insights into quantum systems

11 October 2012

A paper published last year by the Centre of Engineered Quantum Systems (EQuS) researchers has been selected for the New Journal of Physics (NJP): Highlights of 2011.

The paper entitled, Two photon quantum walks in an elliptical direct-wire waveguide array, looks at the evolution of two-photon states in an elliptic array of waveguides.

The paper was deemed by the NJP to be seen as advancing scientific insight within the Physics community, and worthy of note in their latest publication.

Paper co-author Matthew Broome from the Centre said this work highlighted the feasibility of emulation of coherent quantum phenomena in three-dimensional waveguide structures.

“Using integrated optics provides an ideal test-bed for the emulation of quantum systems via continuous-time quantum walks,” Mr Broome said.

“We characterise the photonic chip via coherent light tomography and use the results to predict distinct differences between two, two photon inputs. We then compare these with the experimental observations.”

Other EQuS Researchers include J Owens, Devon Biggerstaff, M Goggin, A Fedrizzi, Trond Linjordet, Jason Twamley and Andrew White are named authors on the paper which appears in the thirteenth volume of the publication.

This work support the EQuS research into Synthetic Quantum Systems and Simulation that aims to harness quantum mechanical phenomena to enhance the functionality and power of information and communication technologies.

Photons are indispensable for quantum communication, and work, such as the research being conducted at EQuS, are leading the approach to quantum information processing and simulation.

The realisation of future technologies in these areas will require miniaturization and integration of high performance components, including single photon sources and detectors, and photonic quantum circuits for manipulating and distributing photons.

EQuS is an Australia Research Centre of Excellence that seeks to initiate the Quantum Era in the 21st century by engineering designer quantum systems.

Through focused and visionary research EQuS will deliver new scientific insights and fundamentally new technical capabilities across a range of disciplines.

Impacts of this work will improve the lives of Australians and people all over the world by producing breakthroughs in physics, engineering, chemistry, biology and medicine.

New Journal of Physics has an impact factor of 3.849 reaching over 1.5 million full-text downloads in 2011.

The paper can be downloaded here.

For more information about Research at EQuS visit equs.org or contact Lynelle Ross (lynelle.ross@uq.edu.au) or Matthew Broome (m.a.broome@physics.uq.edu.au).

Official press release from the University of Queensland.

At the solstice: shining light on quantum computers

21 December 2012

An Australian-American team has shone light — literally — onto the question of whether quantum computers are actually more powerful than conventional counterparts.

“Famously, quantum computers promise a more efficient means of computation, for example using a technique known as `fast factoring’ to efficiently crack encryption codes that form the basis of today’s internet security,” said the study’s lead author, Dr Matthew Broome, of the University of Queensland.

Surprisingly it’s still not known whether quantum computers are the only way to do this efficiently, or whether conventional computers can solve the problem almost as quickly.

In a paper in Science this week, scientists from The University of Queensland and the Massachusetts Institute of Technology (MIT) described the first experimental steps towards answering this question, building a so-called `BosonSampling’ device.

The device implemented a form of quantum computation where a handful of single photons were sent through a photonic network.

The team then sampled how often the photons exited the network.

“Although this sounds simple, for large devices and many photons, it becomes extremely difficult to predict the outcomes using a conventional computer, whereas our measurements remain straightforward to do,” said Dr Broome.

Testing this device — proposed in late 2010 by co-author Associate-Professor Scott Aaronson, and his colleague Dr Alex Arkhipov, both from MIT — will provide strong evidence that quantum computers do indeed have an exponential advantage over conventional computers.

The experimental team leader at UQ Professor Andrew White said: “Scott and Alex’s proposal was a 94-page mathematical tour-de-force.”

“We genuinely didn’t know if it would implement nicely in the lab, where we have to worry about real-world effects like lossy circuits, and imperfect single photon sources and detectors.”

Confirming that the BosonSampling device behaves as expected paves the way for larger and larger instances of this experiment.

The prediction is that with just tens of photons it can outperform any of today’s supercomputers.

“I am excited to see that the first proof-of-principle demonstrations of BosonSampling have been shown — even if only with 3 photons, rather than the 30 or so required to outperform a classical computer,” said Associate-Professor Aaronson.

“I did not expect this to happen so quickly.”

Other researchers in this study are UQ’s Dr Alessandro Fedrizzi, PhD student Saleh Rahimi-Keshari, and Professor Tim Ralph, and MIT’s PhD student Justin Dove.

Financial support was provided by the Australian Research Council Centres of Excellence for Engineered Quantum Systems (EQuS) and Quantum Computing and Communication Technology (CQC2T), and the United States Government.

Related experimental work was published in the same issue of Science by J. Spring et al.

Media: Dr Matthew Broome (+61 4 0644 3479, m.a.broome@googlemail.com) or Prof. Andrew White (+61 4 6625 6329, agx.white@gmail.com) or School of Mathematics and Physics Communications & Marketing Officer Aarti Kapoor (+61 7 3346 9935, a.kapoor@uq.edu.au).

Official press release from the University of Queensland.

Australian-US collaboration leaps ahead in catching spooky light

11 January 2012

Experiments with entangled photons have led the way in the burgeoning fields of quantum information, communication and computation in the last decade.

Their biggest drawback has always been low photon-detection efficiencies, which has limited their potential applications.

Now, a joint experiment by Australian and US labs has fixed this problem, doubling the previous record in entangled photon detection ratio to 62 per cent, and closing the detection “loophole” in the strange phenomenon of quantum steering.

The experiment was conducted by researchers at The University of Queensland, Griffith University, the ARC Centre for Engineered Quantum Systems and the ARC Centre for Quantum Computation and Communication Technology in Australia; and the National Institute of Standards and Technology in Boulder, USA.

Austrian physicist Erwin Schrödinger first introduced the term steering in 1935 to highlight the ability of certain quantum particles to influence—or steer—each other no matter how far they are apart.

This striking effect is the result of quantum entanglement—a phenomenon that connects two particles in such a way that changes to one of the particles are instantly reflected in the other—something that Einstein famously described as “spooky action-at-a-distance”.

Steering allows two parties to verify if they have received quantum particles that share this quantum entanglement—even if one of the parties cannot be trusted.

However, if there are any loopholes—which occur due to problems with the experimental design or set-up—the parties will not be able to say that they have conclusively observed quantum steering.

“We overcame the detection loophole—where not all the photons can be detected—by combining a highly-efficient entangled photon source with state-of-the-art photon detectors,” said Dr Marcelo de Almeida of The University of Queensland.

These detectors—called transition edge sensors —were developed by Dr Sae Woo Nam and his team at the National Institute of Standards and Technology.

“The absorption of a single photon in such detectors causes a tiny change in the temperature which is sensed using superconducting effects,” Dr Almeida said.

“Closing the detection loophole requires efficiencies of above 50 per cent.

"The remarkably high efficiency of 62 per cent achieved in our experiment allows us to demonstrate conclusive steering.”

Dr Almeida’s UQ-based co-authors include PhD students Devin H. Smith, Geoff Gillett, Drs Alessandro Fedrizzi, Till J. Weinhold, and Cyril Branciard, and Professor Andrew G. White, all from the ARC Centre for Engineered Quantum Systems (EQuS) and the ARC Centre for Quantum Computation and Communication Technology (CQC2T), as well as Professor Howard M. Wiseman from Griffith University, also of CQC2T.

This record-breaking achievement, published in Nature Communications today, brings the researchers a step closer toward achieving even higher detection efficiency levels in the near future.

"If we can achieve 66 per cent, then we could perform secure quantum communication even if one party has untrustworthy equipment. Five years ago I would have thought that was impossible,” said Dr Almeida.

Media: Dr Marcelo de Almeida (07 334 67347, marcelo@physics.uq.edu.au) or School of Mathematics and Physics Communications & Marketing Officer Aarti Kapoor (07 3346 9935, a.kapoor@uq.edu.au)

Official press release from the University of Queensland.

Quantum engineers remove roadblock in developing next-generation technologies

Tuesday, March 8, 2011

From just 18 randomly selected white tiles (representing measurements) out of a potential 576, the researchers
		were able to estimate the behaviour of a quantum device (illustrated by the yellow section). Image credit: Alessandro Fedrizzi

An international team has removed a major obstacle to engineer quantum systems that will play a key role in the computers, communication networks, and even biomedical devices of the future.

With the process of miniaturisation advancing by the day, quantum effects will come to dominate our everyday lives.

At present it is extremely difficult to characterise quantum systems—the number of measurements required increases exponentially with the number of quantum parts. For example, an 8-qubit quantum computer would require over a billion measurements.

Caption: From just 18 randomly selected white tiles (representing measurements) out of a potential 576, the researchers were able to estimate the behaviour of a quantum device (illustrated by the yellow section). Image credit: Alessandro Fedrizzi

“Imagine that you're building a car but you can't test-drive it. This is the situation that quantum engineers are facing at the moment”, said UQ's Dr Alessandro Fedrizzi, co-author of the study that was recently published in Physical Review Letters.

“We have now found a way to test quantum devices efficiently, which will help transform them from small-scale laboratory experiments to real-world applications.”

The team also include UQ collaborators Dr Marcelo de Almeida, Professor Andrew White and PhD student Matthew Broome, as well as researchers from Princeton University, the Massachusetts Institute of Technology (MIT), and SC Solutions, Inc. The researchers adapted techniques from “compressive sensing”, a hugely successful mathematical data compression method and for the first time, have applied it to experimental quantum research.

“Audio signals have natural patterns which can be compressed to vastly smaller size without a significant quality loss: this means we now store in a single CD what used to take hundreds. In the same way, compressive sensing now allows us to drastically simplify the measurement of quantum systems“, said Dr Alireza Shabani, the study's main author from Princeton University.

“A common example for data compression is a Sudoku puzzle: only a few numbers will allow you to fill in the whole grid. Similarly, we can now estimate the behaviour of a quantum device from just a few key parameters“, said co-author Dr Robert Kosut from SC Solutions, Inc., who developed the algorithm with Dr Shabani, Dr Masoud Mohseni (MIT) and Professor Hershel Rabitz (Princeton University).

The researchers tested their compressive sensing algorithm on a photonic two-qubit quantum computer built at UQ, and demonstrated they could obtain high-fidelity estimates from as few as 18 measurements, compared to the 240 normally required.

The team expects its technique could be applied in a wide range of architectures including quantum-based computers, communication networks, metrology devices and even biotechnology.

More information: The paper, "Efficient Measurement of Quantum Dynamics via Compressive Sensing," by A. Shabani et al., was published in the March 2011 edition of Physical Review Letters. DOI:10.1103/PhysRevLett.106.100401.

Media: Dr Alessandro Fedrizzi (07 3346 7715, a.fedrizzi@uq.edu.au) or Aarti Kapoor (07 3346 9935, a.kapoor@uq.edu.au)

Official press releases from the University of Queensland and Princeton University.

Quantum computers make light work of chemistry

Monday, January 11, 2010

Physicists have a problem.

They have an outstandingly successful theory of nature at the small scale—quantum mechanics—but have been unable to apply it exactly to situations more complicated than, say, 4 or 5 atoms—let alone a caffeine or cholesterol molecule.

Instead, they have developed a host of approximate methods to use quantum mechanics in fields such as biology, chemistry, and materials science, but this approach raises the concern that natural behaviours are being missed, and limits the development of new technologies.

Nearly thirty years ago Nobel Prize winning physicist Richard Feynman proposed a better solution: to use computers that are themselves quantum mechanical, a hypothetical device now known as a quantum computer.

This week an international team of scientists based in Australia and the US have done exactly that: building a small quantum computer and used it to calculate the precise energy of molecular hydrogen.

This groundbreaking approach to molecular simulations could have profound implications not just for chemistry, but also for a range of fields from cryptography to materials science.

The work, described this week in Nature Chemistry, comes from a partnership between a group of physicists—led by Professor Andrew White at the University of Queensland in Brisbane, Australia—and a group of chemists—led by Professor Alán Aspuru-Guzik at Harvard University, Cambridge, USA.

White's team assembled the physical computer and ran the experiments and Aspuru-Guzik's team coordinated experimental design and performed key calculations. "We were the software guys", says Aspuru-Guzik, "and they were the hardware guys".

"Our results agreed with those calculated using a traditional computer to within six parts to a million", says White, "which we were pretty happy with".

While modern supercomputers can perform approximate simulations, increasing the complexity of these systems results in exponential increase in computational time. Quantum computers promise highly precise calculations while using a fraction the resources of conventional computing.

This computational power derives from the way quantum computers manipulate information. In classical computers, information is encoded in bits, that have only two values: zero and one; quantum computers use quantum bits?qubits?that can have an infinite different number of values: zero, or one, or zero plus one, and so on.

Quantum computers also exploit the strange phenomena of entanglement, powerful correlations between qubits that Einstein once described as "spooky action at a distance".

When asked when quantum computers will leave the lab and appear on desktops, White smiles ?Later than I?d like but sooner than I think?, he replies.

"It's very early days for quantum technology", he continues, "most quantum computer demonstrations have been limited to a handful of qubits. A colleague of mine in Canada says that any demonstration with less than ten qubits is cute but useless—which makes me think of a baby with an abacus."

"However Alan and his team at Harvard have shown that when we can build circuits of just a few hundred qubits, this will surpass the combined computing power of all the traditional computers in the world, each of which uses many billions of bits."

"It took standard computing 50 years to get to this point, I'm sure we can do it in much less time than that!"

White's University of Queensland co-authors on the Nature Chemistry paper are Benjamin P. Lanyon, Geoffrey G. Gillet, Michael E. Goggin, Marcelo P. Almeida, Benjamin J. Powell, and Marco Barbieri. Financial support was provided by the Australian Research Council Federation Fellow and Centre of Excellence programs, and the US Army Research Office (ARO) and Intelligence Advanced Research Projects Initiative (IARPA).

For more information contact Professor Andrew White by phone, office: +61 7 3365 7902 or by email: agx.white@gmail.com. Background information at http:// quantum.info/news .

Official press releases from the University of Queensland and Harvard University.

Photo from http://en.wikipedia.org/wiki/Mantis_shrimp

The Secret World of Shrimps: Polarisation Vision at Its Best

Wednesday, May 14, 2008

A Swiss marine biologist and an Australian quantum physicist have found that a species of shrimp from the Great Barrier Reef, Australia, can see a world invisible to all other animals.

Dr Sonja Kleinlogel and Professor Andrew White have shown that mantis shrimp not only have the ability to see colours from the ultraviolet through to the infrared, but have optimal polarisation vision — a first for any animal and a capability that humanity has only achieved in the last decade using fast computer technology.

"The mantis shrimp is a delightfully weird beastie," said Professor White, of the University of Queensland. "They're multi-coloured, their order and genus names mean `mouth-feet' and `genital-fingers'; they can move each eye independently, they see the world in 11 or 12 primary colours as opposed to our humble three, and now we find that this species can see a world invisible to the rest of us."

Dr Kleinlogel, is based at the Max Planck Institute for Biophysics in Frankfurt, and collected the shrimp from the reef. She notes that "...scuba divers know them as 'thumb-splitters', they've got wickedly strong claws and are very aggressive!"

Most animals can tell how fast the electric field in a light wave is oscillating, which is perceived as colour. (Blue light oscillates faster than green, which is faster than red). The direction of the oscillation is known as polarisation: many animals, from budgerigars to ants have some form of polarisation vision. Since the 1950s animals have been shown to use linear polarisation vision for navigation, for finding food, for evading hunters, and for sex, or as Professor White says "...for the four eff's: feeding, fighting, fleeing and ... flirting".

Commonly polarisation vision is quite restricted: in its simplest form different directions of polarisation show up as lighter or darker patches — you can see this yourself by looking at clear blue sky with polarising sunglasses. But polarisation is more subtle than this: the electric field of the light can oscillate back and forth in a line or around and around in a circle, or anywhere in between.

Video by Sonja Kleinlogel. An O. scyllarus is trained to grab a left-hand circular polarised feeding cube and not the two right-hand circular polarised feeding cubes. He is rewarded with a prawn for the right choice.

The two scientists have shown that shrimp of the species Gonodactylus smithii have eyes that simultaneously measure four linear and two circular polarisations, enabling them to determine both the direction of the oscillation, as well as how polarised the light is.

"This is very useful because natural light can vary from strongly polarised, like the glare off snow or water, to unpolarised, like the sun," Professor White said.

"Any changes to the amount of polarisation instantly tells the animal that something is going on."

Colleagues at The University of Queensland have recently found a related species where the males reflect circular polarisation from their bodies, and hypothesized that circular polarisation vision is used for sexual signalling. Professor White smiles and says, "I think of that as the `prawnographic' hypothesis".

He continues, "It can't be the whole story in our case, though. We found the same structures in the eyes of both boy and girl mantis shrimps, and yet neither have circularly polarised markings on their bodies. Each eye measures the six polarisation components that are precisely required for optimal polarisation vision. In fact, the physics we used to understand what was going on is the same physics that we use in quantum computing for optimal storage of information."

"It is this unique talent — to measure linear and circular polarisation simultaneously — which presents a completely new concept of polarisation vision," Dr Kleinlogel continues. "There wouldn't be much point in only being able to see circular polarisation as it is extremely rare in nature. Even the polarized light reflected from some shrimp's bodies is only weakly circular polarised and often contains more linear polarisation."

"We doubt that circular polarisation is used exclusively as a secret shrimp sex signal! It makes more sense that mantis shrimp evolved both circular and linear polarisation receptors to work together so they can detect tiniest changes in any polarisation."

Prof. White notes, "Some of the animals they like to eat are transparent, and quite hard to see in sea-water - except they're packed full of polarising sugars - I suspect they light up like Christmas trees as far as these shrimp are concerned." "And of course", Dr Kleinlogel concludes, "they can still flirt with each other using fancy polarisation cues!

Press release here.

Experimental demonstration of Shor's algorithm with quantum entanglement

Wednesday, December 19, 2007

In the December 21 issue of the scientific journal Physical Review Letters a combined Australian-Canadian research team from Brisbane and Toronto have reported the first-ever unambiguous execution of a quantum calculation. By manipulating quantum mechanically entangled photons—the fundamental particles of light—the prime factors of the number 15 were calculated.

Although the answer to this problem could have been obtained much more quickly by querying a bright 8 year old, the result is significant because it was calculated using a quantum-mechanical program called Shor's algorithm (named after its discoverer, Prof. Peter Shor of the Massachusetts Institute of Technology). Previous theoretical work has shown that this program, when applied to larger numbers, could be used to crack cryptographic codes that are unbreakable using conventional computers. An essential ingredient of the power of quantum computers is entanglement: the apparently nonsensical correlations between particles that Einstein famously called "spooky action at a distance". The Australian-Canadian team showed that entanglement was present throughout their calculation.

Shor's Algorithm
Think of a number—15, for example—what are its prime factors? Recalling from your school days that primes are numbers divisible only by themselves and 1, then the prime factors of 15 are 3 and 5. But as the number becomes bigger and bigger the problem becomes more and more difficult: what are the prime factors of 133, or 1633 or 2934331? (Answers: 133=7x19, 1633=23x71, and 2934331=911x3221). What is difficult for your brain is also difficult for conventional computers. This is not just a problem of interest to pure mathematicians: the computational difficulty of factoring very large numbers forms the basis of widely used internet encryption systems. An efficient solution to this problem will have very far-reaching implications for communications security—quantum computers will be able to crack these codes.

In any computer a problem must be broken down into manageable chunks: classical computers use two-level systems called bits (binary digits); quantum computers use two-level quantum-mechanical systems called qubits (quantum bits). A qubit is like a coin that can be heads (on), tails (off) or simultaneously heads AND tails (on and off) or any possible combination in-between! This is impossible with normal bits. One qubit is described by three pieces of information, two qubits by fifteen; three qubits by sixty-three, and so on: quantum memory sizes grow exponentially with the number of qubits. Performing an operation on just one of these qubits—for example swapping 1 and 0—simultaneously performs an operation on all possible configurations of the quantum memory. In effect, using the combination of an exponentially large memory and massive quantum parallelism, provided by entanglement, allows simultaneous storage of all possible outcomes of a mathematical procedure, with clever down-selection giving the correct result.

The experiment
In the Brisbane experiment, single photons were used as qubits, with up to four being manipulated at once. Using a complex configuration of optical elements—by which photons can be created, sent into multiple paths simultaneously, then recombined—a simplified version of the factoring algorithm was performed, equivalent to factoring the number 15. The initial proposal for optical approach to quantum computing was made by Dr. Emmanuel Knill (of the National Institute of Standards and Technology, Boulder, Colorado), Prof. Raymond Laflamme (director of the Institute for Quantum Computing at the University of Waterloo, Ontario, Canada) and Prof. Gerard Milburn (University of Queensland, Australia).

In addition to executing the quantum computer program, for the first time the true quantum mechanical nature of the device was confirmed at every step of the experiment. Pushing this envelope and identifying the best, most scalable architecture for a quantum computer is a very active area of research, with teams around the world working on a diverse range of technologies: photons, ions in silicon, atoms or ions in vacuum chambers, and superconducting electrical circuits to name just a few. The Australian-Canadian team are part of two international efforts to make a quantum computer: the Australian Centre for Quantum Computer Technology, led by Prof. Robert Clark at the University of New South Wales, and the US program for Optical Quantum Computing, led by Prof. Paul Kwiat at the University of Illinois.

Almost sixty years ago to the day, the team of Bardeen, Brattain, and Shockley revealed the first transistor to the world, an ungainly device consisting of a wire whisker touching a chunk of metal. Now millions of times smaller, transistors are found by the billions in applications undreamed of by the original inventors: from cell phones in the middle of Africa to iPods at the local bus stop. Functional large-scale quantum computers may be as many years away as the transistor is from the modern computer, and it is equally hard to know how they will change the world—but change our world they will.

Demonstration of an all-optical quantum controlled-NOT gate

Thursday, November 20, 2003

Background.
Quantum computers potentially offer tremendous computational power. One of the key elements in a possible quantum computer is the controlled- NOT, or CNOT, gate. In the November 20, 2003, issue of Nature, a team of researchers report an experiment performed in UQ's Quantum Technology Laboratory that unambiguously demonstrates and comprehensively characterises an optical quantum CNOT gate. The team consists of Drs Jeremy O'Brien and Geoffrey Pryde, Assoc.-Prof. Timothy Ralph, and Dr Andrew White, of the University of Queensland, and Dr David Branning, now of the Rose-Hulman Institute of Technology, Indiana.

This image represents the operation of the CNOT gate. The pair of spots at the bottom left are output modes of the photon source used to generate control and target qubits in the experiment. This image has been repeated to form the truth table: the left column represents the ideal input qubits (dim = 0, bright = 1); the right column represents the qubit outputs measured in the lab, where the intensities are scaled by the probabilities measured in our experiment. The entangled state is not shown, as it would look very strange indeed!

Paper and press release
Nature paper web site; Local copy of paper (PDF); UQ media release and contact details.

Media reports of this work