Archive for the ‘Weird ‘n’ spooky’ Category

Graphene quantum computers could be built with today’s technology

Thursday, August 14th, 2008


Is there anything graphene cannot do?

The great graphene gold rush continues today with the news that graphene nanoribbon could be the key ingredient of the next generation of quantum computers.

The trick with quantum computing is to use qubit-carrying particles that are easy to manipulate so that their quibits can be written and read, that interact with each other so that the qubits can be processed in logic gates but are robust in the sense that thay do not easily interact with the environment so that data isn’t needlessly lost.

Photons are the current darlings of the quantum computing crowd because they do not interact easily with the environment and can be relatively easily manipulated themselves (although getting photons to interact with each other is hard).

But electron spins are also a good prospect because they can be easily controlled and interact readily with each other. Their downside is that it is hard to insulate them from stray magnetic and electric fields in the environment, so storing them is hard.

Now it looks as if graphene nanoribbon may come to the rescue. Guo-Ping Guo and pals from the University of Science and Technology of China in Hefei say that z-shaped graphene ribbons can easily store electrons in the corners of their Zs, where they can be read and written to. And by placing two Zs close to each other on a graphene strip, the electrons can also be made to interact with each other.

Materials scientists have recently worked out how to make Z-shaped graphene reliably in the lab so all the ingredients are in place for a test device to be knocked up shortly.

As Guo-Ping Guo and buddies put it: “Due to recent achievement in production of graphene nanoribbon, this proposal may be implementable within the present techniques.”

Ref: Quantum computation with graphene nanoribbon

Spooky action at a distance gets spookier

Friday, August 8th, 2008


Take a pair of entangled photons and perform a measurement on one of them. According to the strange laws of quantum mechanics, this measurement immediately influences the state of the second photon, no matter how far apart they are. Einstein bridled at the possibility that an instantaneous influence could take place. He called it spooky action at a distance and the term stuck.

Today spooky action at a distance just got spookier. Stephen Harris at Stanford University posts details of an experiment in which he zaps one photon in an entangled pair in a way that ensures that this photon is modulated.

But his extraordinary result is that after zapping the first photon, he can then zap the second photon in way that negates (or enhances) the modulation  of the first.

That’s not just spooky, it’s double spooky.  It opens up the possibility of a few neat new tricks such as pulse shaping photons at a distance. And it raises some interesting questions about the nature of entanglement and how it may come to be exploited in future.

Ref: Nonlocal Modulation of Entangled Photons

Schroedinger-like PageRank wave equation could revolutionise web rankings

Thursday, August 7th, 2008


The PageRank algorithm that first set Google on a path to glory measures the importance of a page in the world wide web.  It’s fair to say that an entire field of study has grown up around the analysis of its behaviour.

That field looks set for a shake up following the publication today of an entirely new formulation of the problem of ranking web pages. Nicola Perra at the University of Cagliari in Italy and colleagues have discovered that when they re-arrange the terms in the PageRank equation the result is a Schroedinger-like wave equation.

So what, I hear you say, that’s just a gimmick. Perhaps, but the significance is that it immediately allows the entire mathematical machinery of quantum mechanics to be brought to bear on the problem–that’s 80 years of toil and sweat.

Perra and pals point out some of the obvious advantages and disadvantages of the new formulation.

First, every webpage has a quantum-like potential. The topology of this potential gives the spatial distribution of PageRank throughout the web. What’s more, this distribution can be calculated in a straightforward way which does not require iteration as the conventional PageRank algorithm does.

So the PageRank can be calculated much more quickly for relatively small webs and the team has done a simple analysis of the PageRanking of the .eu domain in this way. However, Perra admits that the iterative method would probably be quicker when working with the tens of billions of pages that make up the entire web.

But the great promise of this Schroedinger-like approach is something else entirely. What the wave equation allows is a study of the dynamic behaviour of PageRanking, how the rankings change and under what conditions.

One of the key tools for this is called perturbation theory. It’s no understatement to say that perturbation theory revolutionised our understanding of the universe when it was applied to quantum theory in the 1920s and 1930s.

The promise is that it could do the same to our understanding of the web and if so, this field is in for an interesting few years ahead.

Ref: Schroedinger-like PageRank equation and localization in the WWW

Creating random numbers the quantum way

Wednesday, August 6th, 2008


The stream of high quality papers continues from the lab of Andrew Shields at Toshiba Research in Cambridge, UK. Today, his team unveils a new type of quantum random number generator and a fine looking machine it appears to be.

Here’s the idea. Create a stream of single photons are emitted at random intervals that depend entirely on quantum processes–an attenuated continuous wave laser should do the trick. Fire them at a gated photon detector which accurately records their (entirely random) arrival time. The arrival time within a gated time period is then a random number ready for use in quantum cryptography or whatever app you happen to need it for.

The team uses the souped up photodiode that we saw a couple of weeks back to make the photon detections at rates of 4MB/s. And Shields says 100MB/s is possible–that’s two orders of magnitude faster than existing quantum random number generators.

What’s more, the new device is much simpler than other quantum random generators. One popular approach is to send a photon through a beam splitter and see which way it goes. In principle, the outcome is perfectly random but in practice it ain’t because it’s almost impossible to make a beam splitter with perfect 50% probability split.

In practice, the data from these devices needs a certain amount of massaging which can be costly and time-consuming.

So Shields looks to be on a roll.  Exciting times in his lab.

Ref: A High Speed, Post-Processing Free, Quantum Random Number Generator

Quantum communication: when 0 + 0 is not equal to 0

Tuesday, August 5th, 2008


One of the lesser known cornerstones of modern physics is Claude Shannon’s mathematical theory of communication which he published in 1948 while juggling and unicycling his way around Bell Labs.

Shannon’s theory concerns how a message created at one point in space can be reproduced at another point in space. He calls the conduit for such a process a channel and the limits imposed by the universe on this process the channel capacity.

The capacity of a communications channel is hugely important idea. It tells you, among other things, the rate at which you can send information from one location to another, without loss. If you’ve ever made a phone call, watched television or surfed the internet you’ll have benefited from the work associated with this idea.

In recent years, our ideas about communication have been transformed by the possibility of using quantum particles to carry information. When that happens the strange rules of quantum mechanics govern what can and cannot be sent from one region of space to another. This kind of thinking has has spawned the entirely new fields of quantum communication and quantum computing.

But ask a physicist what the capacity is of a quantum information channel and she’ll stare at the floor and shuffle her feet. Despite years of trying, nobody has been able to update Shannon’s theory of communication with a quantum version.

Which is why a paper today on the arXiv is so exciting. Graeme Smith at the IBM Watson Research Center in Yorktown Heights NY (a lab that has carried the torch for this problem) and Jon Yard from Los Alamos National Labs have made what looks to be an important breakthrough by calculating that two zero-capacity quantum channels can have a nonzero capacity when used together.

That’s interesting because it indicates that physicists may have been barking up the wrong tree with this problem: perhaps the quantum capacity of a channel does not uniquely specify its ability for transmitting quantum information. And if not, what else is relevant?

That’s going to be a stepping stone to some interesting new thinking in the coming months and years. Betcha!

Ref: Quantum Communication With Zero-Capacity Channels

Can entanglement exist in biological systems?

Wednesday, July 9th, 2008


Can entanglement exist in biological systems? The usual argument against is that physicists have to work hard to produce entanglement in the carefully controlled conditions that exist in the lab. So it’s hardly likely that entanglement will ever be found in systems that are warm, wet and messy, like human bodies for instance.

But Sandu Popescu from the University of Bristol in the UK and Hans Briegel from the University of Innsbruk in Austria do a convincing job today of arguing otherwise. Their main point is that biological systems are thermodynamic open driven systems which are far from equilibrium. “In such systems error correction can occur which may maintain entanglement despite high levels of decoherence,” they say.

That’s a good point. But it is better made later in their paper when they point out that we already know entanglement can exist in open driven systems at room temperature.

[This] is absolutely clear, once one realizes that every quantum physics laboratory is such a system.

Nicely put! The paper is by no means a proof that biological systems exploit entanglement but it certainly challenges the naysayers to think more carefully about their case.

Ref: Entanglement and Intra-Molecular Cooling in Biological Systems? — A Quantum Thermodynamic Perspective

How to build a quantum internet

Monday, June 30th, 2008

Quantum internet

You could be forgiven for thinking that a quantum version of the internet is a couple of-afternoons-in-the-lab away from being plumbed into your living room. In reality, there are significant engineering challenges to overcome, says Jeff Kimble from the California Institute of Technology in Pasadena and one of the leading thinkers on the links between the quantum and information sciences.

If you want to know about some of the bigger hurdles that physicists face in wanting to build a quantum internet, you could do worse than look at his account of this field on the arXiv today.

Some of the challenges are particularly daunting such as the unambiguous creation and verification of entanglement.

But, ever the optimist,  he concludes:

“I have every confidence that extending entanglement across quantum networks will create wonderful scientific opportunities for the exploration of physical systems that have not heretofore existed in the natural world”.

Ref: The Quantum Internet

The popcorn experiment and spooky action-at-a-distance

Friday, June 27th, 2008

Macroscopic EPR experiment

In 1964, John Bell became fascinated by the EPR paradox, an idea that Einstein had dreamt up to highlight what he saw as a major flaw in quantum mechanics.

The paradox (called EPR after Einstein and his mates Boris Podolsky and Nathan Rosen) is a thought experiment involving two particles that share the same quantum state. The particles become separated. Then a measurement is made on one particle which immediately determines the state of the other, regardless of the distance between them. This, said Einstein, violates special relativity and is in an act of “spooky action-at-a-distance”.

For thirty years or so, physicists ignored this paradox, all that is, except Bell, a physicist at CERN, the European particle physics laboratory near Geneva.

Bell developed a set of inequalities that could be tested against experiment. If violated, Bell’s inequalities would prove that quantum mechanics and relativity really were at odds.

At first everbody ignored Bell’s ideas but in 1984, a French team succeeded in showing that quantum mechanics did violate the inequalities. Today Bell’s inequalities are routinely violated in quantum laboratories all over the world, leaving little doubt over the issue.

Except for Joy Christian at the University of Oxford, who says that Bell’s inequalities ought to be violated on a macroscopic scale as well as the quantum level.

His assertion is based on an argument about the topology of space. In particular, he relies on a bizarre property of space that, like the EPR paradox,  physicists have tended to  ignore. It is this: turn an object through 360 degrees and it returns to its starting position, right? Actually, no. Not if you’re dealing with fermions such as protons and electrons which have a 720 degree symmetry. To get back to the start, you actually need to rotate them through two full turns.

Christian’s argument is that Bell’s inequalities take no account of this property, which he likens to taking an image apart pixel by pixel but without numbering them and then trying to put them back together again. He says this is the reason why Bell’s inequalities are violated, because they do not take account of the toplogy of space, not because of any spooky action-at-a-distance (although this doesn’t rule that out).

He suggests a somewhat tricky experiment that could be done on the macroscopic scale which would also violate Bell’s inequalities as strongly as on the microscopic scale. It involves measuring how balls pop apart when they’re heated,  like popcorn (this is not a joke, see the paper for full details).

So is Christian implying that there’s nothing strange about the quantum world that isn’t also strange about the macroscopic world? And that perhaps Einstein was onto something after all?

Obviously, we need to take a closer look at this “macroscopic world” everybody is talking about.

Ref: Can Bell’s Prescription for Physical Reality Be Considered Complete?
His says that a macroscopic test of Bell’s inequalities and today he explains why.

How to build a quantum eavesdropper

Friday, June 13th, 2008

Quantum eavesdropping

In the cat and mouse game of preparing and eavesdropping on secret messages, quantum encryption trumps all. At least, that’s what we’ve been told.

The truth is a little more complex. Quantum key distribution, the quantum technique by which a classical encryption key can be transferred, is perfectly secure in theory. In practice, there are a number of loopholes that can give an eavesdropper a grandstand view of the conversation.

Here’s one loophole. The security of quantum encryption schemes depends on our inability to make a copy of a quantum state. If that were possible, Eve could make a copy of the message and pass on the original without anybody being the wiser. But in the quantum world, copying anything destroys the original, so the sender and receiver can always tell if they’ve been overheard by examining the error rates in their message. If it rises above a certain limit, the line is not secure.

That would be pretty convincing were it not for our ability to make imperfect copies of quantum states without destroying the original. That’s a loophole that an eavesdropper can exploit to extract information from a quantum message without the sender or receiver knowing. It should work as long as Eve is careful to keep the error rate below the critical limit.

Today, Yuta Okubo from the University of Tskuba in Japan and a few mates outline the design of a quantum eavesdropper that works on just this principle. They’ve yet to build their device but the publication of its plans should raise the blood pressure in a few government agencies and more than one hi-tech start up that has been selling quantum encryption as a new generation of perfectly secure communication.

Ref: Proposal of an Eavesdropping Experiment for BB84 QKD Protocol with 1→3 Phase-covariant Quantum Cloner

Testing “spooky action-at-a-distance” on the International Space Station

Monday, June 9th, 2008

Entangled ISS

Entanglement is the strange and beautiful property of certain quantum particles to become so deeply linked that they share the same existence. According to quantum theory, that link should be maintained whatever the distance between the particles, whether the width of an atom or the diameter of the universe.

This led Einstein to claim that the instantaneous effects of entanglement would lead to “spooky action-at-a-distance” in violation of special relativity which prevents faster-than-light signals.

Nobody knows how the different predictions of relativity and quantum mechanics can be resolved. However, entanglement has been measured in numerous experiments over relatively short distances on Earth. The tests involve two entangled particles, photons say, being sent to distant experimenters who then perform measurements on them.

In every one of these tests, the results agree entirely with the predictions of quantum mechanics. And yet naysayers continue to unearth loopholes that allow them to claim that there is a way in which the results are fixed, perhaps because quantum mechanics works only only over the short distances that can be exploited on Earth or by the existence some kind of hidden variable that determines in advance how the particles will behave when they are separated.

There is one way to settle the matter for sure: send entangled photons to two orbiting astronauts on board different spacecraft with large relative velocities. That leaves no room for hidden variable theories or any other fix because the peculiarities of special relativity allow both astronauts to claim the measurement on their photon was performed before the other.

Today Anton Zeilinger from the University of Vienna in Austria, says he wants to try just such an experiment and has put together an impressive international team to design and promote idea. The team has submitted its proposal, called Space-QUEST, to the European Space Agency in the hope that one end of the experiment could hosted on the Columbus module, Europe’s orbiting laboratory attached to the International Space Station.

The other observer need only be on the ground since Zeilinger has already proven that single photons can be bounced off orbiting satellites and detected on the ground.

That should please mission planners for the International Space Station which has yet to host a single significant experiment in space. Zeilinger’s Space-QUEST experiment looks like a genuine attempt to push the envelope of physics. The quicker they get it into orbit, the better.

Ref: Space-QUEST: Experiments with Quantum Entanglement in Space