Teleportation fidelity the big winner in the quantum lottery

Sophie Zhao

by Phil Dooley

Running your quantum system as a lottery turns out to be a way to improve the transmission of data via quantum teleportation.

Researchers at the Research School of Physics used a probabilistic twist to develop a new transmission protocol that set a new record in data transmission: 92 percent fidelity, which more than halves the loss in previous experiments.

The new protocol will enable encrypted data, for example in finance or military settings, to be sent with higher accuracy.

“Our protocol improves the capability of the quantum teleporter to protect fragile quantum states during long-distance transmission, making the system resilient to noise and loss,” said lead researcher Dr Sophie Jie Zhao, from the Department of Quantum Sciences and the CQC2T ARC Centre of Excellence, who is the lead author in the team’s publication in Nature Communications.

Quantum teleportation is already being used in encrypted networks. It allows information to be shared instantly between linked, or entangled, quantum objects. 

However, the entanglement between the objects can easily be destroyed by interactions with external entities. This at once makes quantum teleportation extremely secure – as any tampering instantly destroys the data transfer – but also very prone to degradation through noise due to environmental interactions.

With entanglement degradation limiting their existing teleportation’s fidelity and distance, the team set their mind to improving the teleportation efficacy by leveraging the paradoxes of the Heisenberg Uncertainty Principle.

In these experiments, the ends of the teleportation link are two photons from the same source, which creates entanglement in their properties. These photons are sent to two separate locations, untouched, which leaves their properties unknown, and able to appear in any possible state.

The signaller then gets the information to be teleported to interact with one of the photons, and measures the photon’s properties – in this case amplitude and phase – making the photon choose a state. This causes the other photon (the receiver) to instantly choose its state as well. Because the two photons are linked, information about the signaller’s experiment can be deduced by the receiver.

This deduction relies on the sender separately conveying to the receiver the result of the experiment. This does not reveal the teleported information, as it is the result of the mashup between that information and the original photon. However, this result acts as a key that allows the receiver to work backwards from the result at their end and disentangle the teleported information.

It is crucial that the sender can’t know what the teleported information is – that would constitute a measurement and collapse the quantum information, said University of Queensland researcher and CQC2T member Professor Tim Ralph.

“The information needs to be hidden in uncertainty so the sender doesn’t know exactly what they are sending. The more they know about the signal, the more they destroy it,” he said.

Quantum uncertainty resulting from the mixing of possible states can be cancelled out with the key, however uncertainty resulting from noise from entanglement degradation is harder to cancel out.

To filter this noise the team leveraged the fact that the mixed states have a Gaussian distribution. They realised that a lottery, a protocol in which a subset of the measurements was selected randomly in a way that actually narrowed the Gaussian distribution, while other measurements were randomly discarded, could help filter out noise.

“Adding an element of chance to our protocol has the effect of distilling the quantum information,” Dr Zhao said.

“The post-selection effectively biases the Gaussian distribution in favour of high-amplitude outcomes than outcomes close to the origin of phase space, hence acting as an amplifier. Since this amplification is noiseless and takes over from part of the amplification applied by the receiver in standard teleportation protocols, the teleported states suffer less from the noise added due to imperfect entanglement.”

An interesting quirk of the system is that the balance between the probabilistic factor and the noise reduction can be tuned. By simply reducing the probability of measurements being selected in the protocol the teleportation fidelity can be increased.

To achieve their record 92 percent fidelity the team used a success rate of less than one in a hundred thousand, sampling the system for around two hours.

In the new protocol, the success of the teleportation relies on the stability of the laser system, instead of being limited by environmental noise, Dr Zhao said.

“You can always get better fidelity if you are willing to sacrifice your success rate. But then you need a longer sampling time.

“If the system were stable enough to allow us to sample for say, 20 hours, then I believe we could go above 95 percent,” she said.

Originally published on ANU Physics website

Continue Reading Teleportation fidelity the big winner in the quantum lottery

Schrodinger’s Cat – the video is now live!

Many of you have heard me play this song live, and I’ve been planning to make a video for it.  The question was, how? Enter Wasabi the WonderCat, who offered to star in the video, some gentle impulse from Dr Kip Stewart, and I had inspiration.

Finally after 10 months of hard work teaching myself to animate, here it is!

The song was born as I wondered about how Schrodinger’s cat felt about being in a box for more than 80 years. You see, he was first put there to prove a point.

The originators of the Copenhagen Interpretation, Niels Bohr and Werner
Heisenberg proposed that reality as we knew it didn’t exist, things were
blurred across multiple states, until a measurement was made.

On the other hand, Einstein and Schrodinger found this preposterous, and
to illustrate came up with the idea of the cat in the box. 

While the fame of Schrodinger’s Cat’s has spread, it didn’t really settle the debate.

I want to know, what does the cat think, being in the box for nearly a century? Surely, it’s the dogs’ turn now!

Continue Reading Schrodinger’s Cat – the video is now live!

Adelaide Fringe shows!

Excited to be in Adelaide in 2019 for my first Fringe!  Doing seven shows in six days, all at the Rob Roy Hotel, 106 Halifax St South Adelaide.

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Tuesday 12th March: Physics in the Pub  (booked out)

 

 

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Wednesday 13 March – Solo Show

The Most Amazing Planet in the Universe – an Astronomer’s Ode to Earth

 

 

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Thursday 14th March – Sunday 17th March

The Poet’s Guide to Science – a hilarious play of modern dilemmas, featuring working scientists

Continue Reading Adelaide Fringe shows!

Quantum pancake reveals clues to better electronics

First published on Cosmos Magazine site, 25/9/18

An experiment with a cloud of ultracold atoms squashed into a quantum pancake has revealed never-before seen quantum effects that could lead to more efficient electronics, including high temperature superconductors.

A team at Swinburne University in Australia observed a quantum anomaly in lithium-6 gas cooled to a few billionths of a degree above absolute zero and squashed between two laser beams.

“We’re seeing quantum mechanics that’s visible on a macroscopic scale – a large collection of tens of thousands of atoms all behaving quantum mechanically,” says project leader Chris Vale, a researcher at Swinburne’s Centre of Excellence in Future Low-Energy Electronics Technologies.

The team’s work is published in the journal Physical Review Letters, concurrently with that of a team from Heidelberg in Germany which reported similar results.

The Australian team used laser beams focused into a flat plane to create a pancake of lithium-6 gas with a radius of 200 microns and the thickness of a single atom – around 500 nanometers. Then the researchers compressed the gas slightly with a magnetic field, to set it vibrating. Shining another laser from below, they measured the vibration frequency of the gas cloud by watching the shadow of the cloud on a camera.

The frequency of this radial vibration, known as a breathing mode, gave the telltale sign of a quantum anomaly: it vibrated 2.5% faster than the classical model predicted.

Symmetry in the classical model dictates that gas properties such as pressure and density should scale in a straightforward way as the size of the gas cloud oscillates. But the full quantum analysis predicts a higher frequency: the classical theory breaks down because of strong interactions between the gas particles.

To further test the model, the scientists jammed more atoms between the laser beams, and turned the pancake from a two-dimensional crepe into a fatter flapjack. Its three-dimensional nature conformed to the classical model.

Vale says the interactions between the gas particles in the quantum crepe was mid-way between two well-known states of matter: Bose-Einstein condensates, in which atoms in a gas interact strongly and exhibit uniform quantum behaviour, and low temperature superconductivity, in which electrons in a material form weakly-bound pairs known as Cooper pairs that can carry electricity through a material with no energy loss.

“There’s a continuous crossover between these two limits, what happens in the middle is not well understood,” he says

“There’s a lot of interesting physics there. For example, this is where we find the highest superfluid transition temperatures, in the intermediate zone where the binding energy of the fermion pairs is similar to the natural energy scale for the system, the Fermi energy.”

By studying a two-dimensional system, Vale hopes to spark developments of new materials for the electronics industry, for example topological insulators or high-temperature superconductors.

In the current highest-temperature superconductors, ceramics based on copper oxide, the superconducting current is carried in two-dimensional layers within the material – although exactly how the electrons pair up is not fully understood.

The power of the experiments at Swinburne is their simplicity, says Vale – the microscopic properties of the lithium atoms and their interactions are precisely known, which is not always possible in more complex materials such as solids.

“In a sense, our cold atoms are acting as a quantum simulator, where we can test models of many-body physics with precisely known inputs that can be difficult to pin down in other materials,” he says.

Continue Reading Quantum pancake reveals clues to better electronics

Wave of the Century

Originally published in ANU Reporter:

The discovery of gravitational waves is the culmination of a search by a generation of ANU physicists, reports DR PHIL DOOLEY, BSc (Hons) ’90, PhD ’99.

An excited hush fell over the briefing room at Parliament House as Professor David McClelland stepped up to the microphone.

“I’m pretty sure you all know by now but I want to say it. We’ve done it,” he said as his voice quavered.

Spontaneous applause broke out, as McClelland allowed himself a smile. Camera flashes popped and TV cameras zoomed in.

“We detected a wave that was generated 1.3 billion years ago when two black holes crashed into each another… the most violent event ever witnessed.”

The announcement was sweet reward for McClelland, an ANU laser physicist who has spent his career working towards this moment.

Albert Einstein predicted the existence of gravitational waves but thought they were too small for humans to ever detect.

To prove Einstein wrong and right in a single stroke is rare treat for a scientist.

“This is a moment that will be remembered for a thousand years,” McClelland said.

Gravitational waves are vibrations of space and time themselves, one of the most outlandish predictions of Einstein’s 1916 General Theory of Relativity. Yet, they appeared exactly as predicted and join the long list of successes of Einstein’s theory over the last century.

The first success of Relativity came three years after Einstein’s publication, when a solar eclipse allowed astronomers to pick out the tiny deflection of distant starlight by the sun’s gravity.

25 years of chasing Einstein’s theory

In 1990, at the ANU Physics Department, McClelland and Emeritus Professor Hans Bachor AM gathered people from across the world to assess the challenges of that dream.

The microscopic effect was exactly what Einstein’s revolutionary concept of space and time that was warped by huge masses predicted. The charismatic genius became a superstar overnight.

Einstein’s vision of stretchy space was not just static, it also predicted large masses moving or colliding would make waves through space, effectively a spacequake.

To make a gravitational wave of any significance needs a cataclysmic event happening to a huge mass, a star exploding or black holes colliding.

But such events are rare and unlikely to happen close to Earth, so scientists need to extend their search beyond our galaxy.

It would be 75 years before physicists would dare dream of looking for gravitational waves.

In 1990, at the ANU Physics Department, McClelland and Emeritus Professor Hans Bachor AM gathered people from across the world to assess the challenges of that dream.

One-by-one, a who’s who of gravitational wave specialists laid out the seemingly impossible tasks.

The basic concept was clear: a stable laser beam bouncing between mirrors in a vacuum.

When a gravitational wave came past and warped the space between the mirrors, it would momentarily disturb the stability of the laser.

Surpassing scientific challenges

Gravitational waves are vibrations of space and time themselves, one part of Einstein’s 1916 General Theory of Relativity.
Gravitational waves are vibrations of space and time themselves, one part of Einstein’s 1916 General Theory of Relativity.

But the practicalities were not so simple.

“Everything needed to be roughly a factor of 1000 more powerful or sensitive than we had at the time,” Bachor recalls.

Laser technology was weak. Nobody knew how to build a good enough mirror, it had to be massive and perfectly smooth, and with losses less than one part in a billion.

The experiment would need to be enormous, preferably 10 or more kilometres long.

Damping out vibrations from the environment was another huge challenge. The smallest earth tremor, or a passing truck, threatened to obliterate the tiny ripples from distant spacequakes.

But they didn’t know the size and shape of the ripples they were searching for, says Professor Susan Scott, from the Research School of Physics and Engineering (RSPE).

“We really had no idea of the enormity of the task which lay ahead of us. We were searching for a needle in a cosmic haystack, extremely faint signals in very noisy data.

“We had early meetings with submarine intelligence experts but we quickly realised we were facing a much more daunting task than theirs,” she said.

But, as in 1916, the scientists were not discouraged.

“The amazing thing was that the mood at the end of the conference was ‘Let’s go for it,’ ” Bachor says.

“We were eternal optimists. These people were pioneers who had seen the progress over the previous 20 years. We trusted that the technology would evolve.”

The start of a technological revolution

Over the next 20 years technological revolution after technological revolution achieved the impossible.

But political realities posed a big obstacle to progress.

The Russians quickly realised they could not afford to build an experiment.

The UK could not find the land and teamed up with Germany to build a smaller experiment, GEO 600, only 600 metres long. Japan set up small experiments in disused mines.

Australia had the land but lost out on funding against stiff competition from more developed astronomy projects, such as the Square Kilometre Array, which could guarantee data.

The gravitational astronomy world’s hopes lay with the two four-kilometre long experiments at the Laser Interferometer Gravitational-Wave Observatory (LIGO) at either end of the US, and the French-Italian Virgo project.

The nations apportioned the tasks, and set to work.

Over the next 20 years technological revolution after technological revolution achieved the impossible.

Joining the hunt as an honours student in 1996, Daniel Shaddock, was dismayed by the challenge.

“When I first heard about the sensitivity needed for gravitational waves, I thought that’s crazy, we’re never going to see them,” said Shaddock, now a professor in the RSPE.

“But you start working through the challenges and before you know it, you’re in a position where you have the measurement technology that you need.”

Also drawn to the quest was Dutch student Bram Slagmolen, who travelled around the globe in 1999 to work with McClelland.

“We’ve done things I wouldn’t have dreamed of back then,” says Slagmolen, now a research fellow at ANU and the leader of the Australian Consortium for Interferometric Gravitational Astronomy.

“Although the final experiment would be on a large scale and hideously complex, we ran little experiments that gave us understanding and allowed us to go the next step.”

In Italy, the Virgo experiment was dogged by technical hitches, but by mid-2015, LIGO scientists – including many Australians led by McClelland – believed their twin US experiments could start, and flicked on the many systems required to isolate Einstein’s elusive echoes of intergalactic impacts.

The Universe delivers an extraordinary result

ANU Vice-Chancellor Brian Schmidt AC and Australian Chief Scientist Dr Alan Finkel AO at the gravitational waves discovery media event. Photo by Stuart Hay.
ANU Vice-Chancellor Brian Schmidt AC and Australian Chief Scientist Dr Alan Finkel AO at the gravitational waves discovery media event. Photo by Stuart Hay.

Then the Universe delivered something truly extraordinary.

The team had estimated they might have to spend a couple of years sifting through their data to find matched sets of tiny wobbles from outer space.

But while they were still doing final tests with simulated data, still days away from the first official science run, the detector twanged with a signal so clear that the team could see it above the background without any filtering.

“The signal matched perfectly to the predicted pattern of two black holes colliding, but it was so strong we thought it must be a blind injection of a simulated signal, ” Scott says.

“We were aware very quickly that this was not the case and so this signal that had thundered into our detectors was probably real, and from surprisingly large black holes.”

A long time ago in a galaxy far, far away, there had been an invisible collision that had unleashed more energy than any other event humans had witnessed.

The peak power had been 50 times that of total output of every star in the visible universe.

There was no accompanying flash of an explosion.

After a trip of 1.3 billion years across the cosmic pond, the ripples had died down so much that they moved LIGO’s mirrors by about one thousandth of the diameter of a proton.

The signal was imperceptible to normal humans but clear as a bell in the detector in Louisiana and then, seven milliseconds later, in the Washington State detector.

Just the beginning of gravitational wave physics

It’s like the moment when Galileo first turned a telescope towards the skies and started a new epoch of astronomy.

McClelland’s search is not over now their tool is working.

“This is just the beginning,” he says.

“It’s like the moment when Galileo first turned a telescope towards the skies and started a new epoch of astronomy. Here we shall begin a whole new and fundamentally different way of observing the Universe.”

Bram Slagmolen was jubilant.

“I was always convinced we would see them, it was just a question of when,” he says.

“We’ll definitely see more now, by the end of the year we should have multiple detections, in the bag, it will be fabulous for the whole community.”

Aside from observing the antics of invisible black holes, gravitational waves help scientists see further back in time than conventional astronomy, says McClelland’s colleague Professor Susan Scott.

“This is a big deal. One of our holy grails is to understand the beginning of the Universe.”

And so another search begins.

Continue Reading Wave of the Century

A new spin on data storage

Seems physicists are inventing particles faster’n I can write about em. Who ever knew you could call a twisted magnetic field a particle?

First published here: http://www.research.a-star.edu.sg/research/7691/a-new-spin-on-data-storage, 8 May 2017.

2skyrmions
Magnetic field patterns in two types of skyrmions (Wikimedia)
—

Study into spirals of magnetic spin showcases potential of layered materials for future data storage

Tiny spirals of magnetism called skyrmions could be used as ultrahigh density energy-efficient data carriers.

Jarvis Loh, Gan Chee Kwan and Khoo Khoong Hong from the A*STAR Institute of High Performance Computing have modeled these minute spin spirals in nanoscopic crystal layers. They found that alternating layers of manganese silicide (MnSi) and cobalt silicide (CoSi) forms a promising material architecture.

“Skyrmions are nanosized entities, only tens of nanometers, so they hold the promise of higher storage density than the current technology,” said Gan.

Storage based on skyrmions would represent binary data such as ‘1’s and ‘0’s as clockwise and anticlockwise spin spirals, respectively. Skyrmions can improve energy efficiency as they can be created and manipulated with currents significantly smaller than those required for conventional magnetic hard disk technology.

Skyrmions had been experimentally observed in manganese silicide, prompting the team to explore simulations of manganese silicide in its pristine form and in combination with similar materials.

The team selected cobalt silicide because cobalt sits close to manganese in the periodic table, and its similar lattice characteristics mean it should combine well with manganese silicide. Cobalt also has strong magnetic properties — it is ferromagnetic.

The team’s simulations showed that coupling cobalt silicide to manganese silicide enables the spin spirals in manganese silicide to be engineered. “What’s interesting is that we can now vary the size of skyrmions in an easy and elegant way,” Loh said.

In the skyrmion’s center the magnetic spin of the atoms is flipped 180 degrees relative to the spin on its outside edge; between the edge and the center the spins progressively tilt between the two extremes. Critical in the size of skyrmions is the ability of the material to support high relative tilt between neighboring atoms in the lattice, which enables the skyrmion to be packed into a smaller spiral.

The team found that adding cobalt silicide layers to the manganese silicide layers increased the possible relative tilt. However there is an upper limit — for cobalt silicide layers double the thickness of the manganese silicide, the material ceased to support skyrmions and transitioned to a more conventional ferromagnetic behavior.

One of the attractions of skyrmions as a data storage medium is their robustness, says Loh. “Unlike current magnetic storage, skyrmions are resistant to defects in the lattice. They are topologically protected.”

The team plans to apply their successful approach to other potential architectures, such as nanowires.

The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing.

Reference

  1. Loh, G. C., Khoo, K. H. & Gan, C. K. Helimagnetic order in bulk MnSi and CoSi/MnSi superlattices Journal of Magnetism and Magnetic Materials 421, 31–38 (2017). | Article
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