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

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

Gravitational Waves – behind the scenes

I had the honour to interview David Reitze, one of the leaders in gravitational wave physics, when he visited Canberra earlier this year.

Bit surprised he wasn’t one of the Nobel laureates… but anyway, ahead of the exciting announcement tomorrow, (what have they found this time?!),  here’s the interview.

 

And for those who just want the highlights:

Continue Reading Gravitational Waves – behind the scenes

End of content

No more pages to load