Archive for the ‘Weird ‘n’ spooky’ Category

Interference between photons that never meet

Wednesday, April 16th, 2008

Photon interference

The pantheon of impossible photon tricks grows ever larger. Today, a new addition from Andrew Shields and pals at Toshiba Research Europe in Cambridge, UK:

“We report an experiment in which two-photon interference occurs between degenerate single photons that never meet. The two photons travel in opposite directions through our fibre-optic interferometer and interference occurs when the photons reach two different, spatially separated, 2-by-2 couplers at the same time.”

Cool!

Ref: arxiv.org/abs/0709.0847 : Experimental Position-time Entanglement with Degenerate Single Photons

A survey of quantum programming languages

Wednesday, April 9th, 2008

It cannot be long before somebody breathes life into a useful quantum computer. And when that happens, an entirely new breed of keyboard monkey will be born: the quantum computer programmer.

This strange animal will have to work with the weird and wonderful tools of the quantum world, such as superposition of quantum bits, entanglement, destructive measurement and the no-cloning theorem.

Clearly no conventional programming language has operators and data structures that can handle these concepts but a growing number of physcists have been developing languages that can. Today, Donald Sofge at the Naval Research Laboratory in DC has kindly surveyed them and their history.

He divides them into three categories:

i) Imperative Programming Languages which use statements to change the global state of a program.  Classical examples include FORTRAN, C and Java. Quantum examples include QCL (quantum computation language) which was probably the first proper quantum programming language (it was developed by Bernhard Omer at the Technical University ofVienna about 10 years ago).

Another example is Q Language developed by Stefano Betelli and colleagues at Trento University in Italy.

ii) Functional Quantum Programming Languages by contrast,map inputs to outputs to perform mathematical transformations. This idea has influenced the development of conventional languages such as Lisp, ML and Haskell.

Quantum versions include QFC (quantum flow charts) proposed by Peter Selinger at Dalhousie University in Canada and QML developed by Thorsten Altenkirch at the University of Nottignham in the UK.

iii) Others

A number of people have developed languages aimed specifically at supporting cryptographic protocols. A good example is cQPL based on Selinger’s QP. Another language, CQP (communicating quantum
processes), relies on quantum process algebras to model systems that combine both quantum and classical elements.

Sofge’s paper makes a fascinating, if technical read. But if you’re a young programmer wondering what you’ll be working on in 30 years time, get your Landau and Lifshitz out of the attic and start working through it.

Ref: arxiv.org/abs/0804.1118: A Survey of Quantum Programming Languages

Qutrit breakthrough brings quantum computers closer

Friday, April 4th, 2008

Toffoli gate

The folks playing with quantum computers have been claiming for years that their gadgets will one day make today’s supercomputers look like quivering lumps of jelly. But so far, their computers have yet to match the calculating prowess of a 10-year old with ADHD.

The most exciting work so far has been on universal quantum logic gates, the building blocks of any computer. A number of groups have built and demonstrated these and one team even took their gates for the computing equivalent of a run round the block by factorising the number 15.

The trouble is that, to do anything useful with universal quantum gates, you need at least dozens and preferably hundreds of them, all joined together. And because of various errors and problems that creep in, that’s more or less impossible with today’s technology.

Which is why a breakthrough by an Australian group led by Andrew White at the University of Queensland is so exciting. They have built and tested quantum logic gates that are vastly more powerful than those that have gone before by exploiting the higher dimensions available in in quantum mechanics. For example, a qubit can be encoded in a photon’s polarisation. But a photon has other dimensions which can also be used to carry information, such as its arrival time, photon number or frequency. By exploiting these, a photon can easily be used as a much more powerful three level system called a qutrit.

This is how the Ozzie team have exploited the idea: during a computation, their gates convert qubits into qutrits, process the quantum information in this more powerful form and then convert it back into qubits. All using plain old vanilla optics.

That allows a dramatic reduction in the number of gates necessary to perform a specific task. Using only three of the higher dimension logic gates, the team has built and tested a Toffoli logic gate that could only have been constructed using 6 conventional logic gates. And they say that a computer made up of 50 conventional quantum logic gates could be built using only 9 of theirs.

That’s a significant reduction. What’s more, they reckon that these kinds of numbers are possible with today’s linear optics technology.

That means these guys are right now bent over an optical bench with screwdrivers and lens cloths at the ready, attempting to build the world’s most powerful quantum computer. We may see the results–a decent factorisation perhaps–within months.

Could it be that Australia is about to become the center of the quantum computing world?

Ref: arxiv.org/abs/0804.0272: Quantum Computing using Shortcuts through Higher Dimensions

A new class of photon gun

Monday, March 31st, 2008

Photon gun

Photons are easy to produce, at least en masse. But making them one at a time in a controlled fashion is much harder. Until recently the only trick physicists had for this was to reduce the brightness of a beam until it contained only one photon at a time, on average.

Of course, the “on average” clause allows for any manner of multiphoton sin: the photons may be produced in pairs or bunches or not at all. That’s a serious problem for techniques such as quantum cryptography because any extra photons are equivalent to a leak of information that an eavesdropper can use to crack the code.

Enter the photon gun brigade, a growing band of physicists attempting to build devices that can fire identical, single photons on demand. The single photon part of this problem has been cracked by a number of groups who have built guns that work by inducing atoms, ions, molecules or quantum dots to fluoresce in a cavity.

What has turned out to be more difficult, but just as important, is making each of these single photons identical. What tends to happen is that the cavites vibrate in various ways and this subtly distorts the wavelength of the photons they produce. The result is that each photon is very slightly different, which spoils their ability to interfere and become entangled with each other.

Here’s the breakthrough: Andrew Shields, at Toshiba’s research labs in Cambridge, UK, plus a few pals from a nearby university, say they can create the photons using a quantum dot that they zap with two precisely timed voltage pulses. The first pulse injects charge carriers into the diode while the second suddenly shifts the quantum dot’s emission characteristics so that it can emit light. When that happens, the dot emits a photon of a specific energy.

But the key is to use such short voltage pulses that the photon doesn’t have time to be influenced by its surroundings. And that means the quantum dot always emits photons of precisely the same wavelength.

So it looks as if Shields and co have a neat new trick up their sleeves, albeit one that has to be performed at 4K. But it’s also an entirely new mechanism for producing photons of this kind so improvements are gonna be forthcoming. How long before we see a commercial version?

Ref: arxiv.org/abs/0803.3700: Indistinguishable Photons from a Diode

Entanglement beats gravitational test

Friday, March 21st, 2008

Gravity and entanglement

When does a quantum measurement end?

Surprisingly, quantum physicists cannot agree. Some say the measurement ends when you register a result on a piece of classical equipment such as a photomultiplier. Others says the measurement ends when the information in the quantum system has irreversibly leaked into the environment. There are still more who believe in the manyworlds interpretation of quantum mechanics and say a quantum measurement never ends but exists ad infinitum in several parallel universes.

This may sound like an ineffectual academic scrap but it actually has hugely important consequences for the quantum property of entanglement.

Entanglement is the state in which two physically separated particles share the same quantum existence, so that a measurement on one instantaneously affects the other. Yep, that’s instantaneously. It’s what Einstein described as “spooky action at distance”.

For some years, physicists have been measuring this “spooky action at a distance” in tests known as Bell experiments.

These tests depend crucially on the measurement ending quickly. Because if it were to drag on, the particles might be able to communicate at light speed by some currently unknown mechanism.

But because nobody has actually determined when a measurement ends, all the experiments to date are potentially open to this loophole.

Perhaps there is no spooky action at a distance after all, just long quantum measurements during which the particles communicate at the speed of light in some quite ordinary way.

Now Nicolas Gisin and colleagues at the University of Geneva have closed this loophole using the ideas of the Oxford theorist Roger Penrose. A few years ago, he suggested that the end of a quantum measurement is realted to the gravitational energy of the mass distribution of the resulting quantum superposition. In other words, the measurement ends when a massive object receives a decent kick.

So Gisin and buddies set up a Bell experiment which involved sending entangled photons in each direction from the midpoint of an 18 km fibre. At the ends of the fibre were piezoelectric actuators attached to small but massive mirrors. When the photons hit, they triggered the actuators causing the mirrors to move and deflect a beam of light.

The experiment was carefully set up so that the mirrors were heavy enough to please Penrose and far enough apart that no light speed signal could travel between them in the time it took for a pair of entangled photons to “kick” them.

The result? Gisin’s team confirmed that “spooky action at a distance” still governs the behaviour of the entangled photons.

If you believe Penrose, this is the first experiment to ever prove “spooky action at a distance”. Impressive, huh?

More interesting, is the idea of gravity and quantum mechanics coming under the microscope in the same experiment for the first time.

It won’t be the last. There are plenty of other mysteries about gravity that quantum mechanics can probe. It’s about time physicists bit the bullet and started testing them.

Ref: arxiv.org/abs/0803.2425: Space-like Separation in a Bell Test assuming Gravitationally Induced Collapses

Future brightens for quantum imaging

Tuesday, March 18th, 2008

Quantum illumination

This is the idea behind quantum imaging: create an entangled pair of photons and send one towards the object you want to image and hang on to the other.

But then what? For some time, physcists have been whisperin’ about the extraordinary potential of this technique. Some imagine that it might be possible to create images of objects that cannot otherwise be seen, objects inside black boxes, for example, or black holes.

The thinking is that the photon you hold in your hand can somehow tell you something about the object it’s entangled cousin has hit. So you can create an image of an object without ever seeing its reflection.

But pin physicists down about what they mean and they start a-mumblin’ and a-dribblin’ incoherently. Despite rumours from places like Boston University where various bods are testing the idea in the bowels of the physics department, nobody has ever provided experimental evidence that quantum illumination is anything but a hatful of hot air.

So if ever a field needed an injection of common sense, this is it. Step forward quantum theorist and all round bright spark Seth Lloyd from MIT. He’s taken the thinkin’ and given it a thorough shakin’ by the scruff of its neck.

Lloyd doesn’t give any credence to the ideas of reflection-free imaging but he’s found something almost as good. Lloyd has calculated that illuminating an object with entangled photons can reduce increase the signal to noise ratio of the reflected signal by a factor of 2^e, where e is the number of bits of entanglement. That’s an exponential improvement.

What’s more, the improvement occurs even if the entanglement is completely destroyed during the process of reflection. So quantum illumination could help image anything that is currently hard to distinguish because of noise.

That’s impressive but Lloyd’s ideas raise quite a few questions, such as how to perform the required entanglement measurement on the returning photon. That’s for the experimentalists to sort out although there’s no easy and obvious answer.

So although a clever piece of work, it could be a while before we’re posing for quantum snapshots from Kodak.

Ref: arxiv.org/abs/0803.2022: Quantum Illumination

Single photons bounced off orbiting satellite

Monday, March 17th, 2008

Quantum satellite

Quantum physicists have been sending qubits through the atmosphere encoded in individual photons for years now. The work is the foundation of a new type of quantum communication that is perfectly secure from eavesdropping.

But there are challenges in setting up a global system of quantum communication. Not least is the problem of decoherence, in which noise destroys the quantum nature of the information as it travels though the atmosphere. This has limited the distance record for this kind of transmission to 144km (although longer distances are possible through optical fibres).

The obvious way around this is to send the signals through space via a satellite. When sent straight up, the photons need only travel through 8 kilometres of atmosphere and so are much less likely to decohere.
On Friday, Anton Zeilinger’s group in Vienna announced that they had taken the first step in this direction by bouncing single photons off an orbiting satellite soome 1400km above the Earth.

The team used a 1.5 metre telescope called the Matera Laser Ranging Observatory in Italy to bounce single photons off the Ajisai geodetic satellite, an orbiting disco ball that is used for laser ranging measurements.

Quantum communication with entangled photons can only be done by sending and detecting them one at a time so the experiment is a crucial step in making space-based quantum communication possible.

However, the team also tried bouncing photons off several other disco balls such as Lageos II, without success.

But give them their due. The experiment proves that it is possible to use existing laser ranging equipment to send and receive single photons from orbiting satellites.

“Our findings strongly underline the feasibility of Space-to-Earth quantum communication with available technology,” says the team.

Of course, this isn’t a demonstration of quantum communication itself in space. That will require an orbiting source of entangled photons.

So all they need now is somebody to build and launch a satellite that can produce and transmit entangled photons. Any takers?

Ref: arxiv.org/abs/0803.1871: Experimental Verification of the Feasibility of a Quantum
Channel between Space and Earth

Holographic quantum computing

Wednesday, March 5th, 2008

Holographic quantum computing

After a decade or so in the lab, holographic data storage is about to burst into the hardware market big time.

Its USP is that holographic data is stored globally rather than at specific sites in the storage medium.

It is written using a pair of lasers to create an interference pattern that is recorded in the storage medium. It can then viewed by illuminating that area with a laser to recreate the pattern. Crucially, you can add and view more data by changing the angle at which you address the medium and this gives huge storage potential.

Now Karl Tordrup and colleagues at the University of Aarhus in Denmark have used the idea as inspiration for the design of a quantum computer. Their machine consists of an array of molecules that can each store a qubit. But instead of addressing them individually, Tordrup imagines storing quantum data in them as a group, by zapping them with the right kind of laser-created interference pattern. This is essentially quantum data storage, the holographic way.

What makes the idea interesting is that the group reckons that information can be processed by transferrring it to a nearby superconducting box in which the required operations can be performed. The processed data is then sent back again.

The big advantage of this idea is that, while stored in holographic form, the quantum data is incredibly robust. While any single molecule errors affect all qubits, they do so only very weakly. It also means that the molecules need only be addressed as a group, not as individuals which does away with a significant challenge that other designs of computer face

But there are problems too. The qubits will have to be protected from decoherence while in the superconducting box and travelling to and from it. And although the molecules do not need to be addressed individually, they do need to be held almost perfectly still. Those are toughies.

All they need to do now is build the thing.

Ref: arxiv.org/abs/0802.4406: Holographic Quantum Computing

When humans become entangled

Monday, March 3rd, 2008

human-entanglement

Something curious is happening at Nicolas Gisin’s lab at the University of Geneva. Gisin is a world expert in entanglement, the ghostly quantum phenomenon in which two or more particles become so deeply linked that they share the same existence, even when far apart.

Entanglement is now a routine resource in many labs: it can be generated, studied and even passed from one particle to another. It is usually measured using two detectors–Alice and Bob in the lingo of quantum physicists–which analyse pairs of incoming photons to see whether there is any spooky-action-at-a-distance, as a Einstein called it. In these so-called “Bell experiments”, spooky action rules.

Given the amazing properties of entangled photons, it was never going to be long before curious postdocs pointed these photons on themselves, in the manner of Nobel Prize winning Barry Marshall who famously swallowed H Pylori bacteria to see if it gave him ulcers, or more fittingly like Jeff Goldblum in The Fly.

What would happen if two humans–let’s call them Alf and Bess–replaced the lifeless Alice and Bob?

I guess most physicists would say that the process of observation in the eye is macroscopic, it involves large numbers of photons, and so any quantum effects would be drowned out.

Not so, reckons Gisin. It has long been known that the eye is sensitive enough to detect a mere handful of photons. He and a couple of pals, Nicolas Brunner and Cyril Branciard, have calculated that, were the eye a lifeless detector, it could be used to carry out the kind of Bell experiments described above.

Thus entanglement could in principle be seen,” conclude the group.

That’s a loaded statement if ever there was one. It implies that two humans could become entangled, if only for a brief moment.

Unfortunately, there is no room in the paper to discuss what “mantanglement” would be like. How long would Alf and Bess be mantangled? For as long as entangled photons bombard their retinas or longer? What would Alf and Bess feel?

I wonder if Gisin, Brunner and Branciard already know the answer to these questions, and whether we’ll be hearing some more interesting news from the Gisin lab in the months to come.

Ref: arxiv.org/abs/0802.0472: Can one see entanglement?

The quantum graphity question

Wednesday, February 20th, 2008

Emergent gravity

Physicists have been searching for a quantum theory of gravity some time. Most believe that the new theory will require some kind of modification to general relativity or quantum theory.

One of the ideas in vogue at the moment is that general relativity is actually an emergent phenomenon from some deeper physics.

Now Tomasz Konopka from Utrecht University in the Netherlands and others are developing an idea called quantum graphity that could prove it.

The thinking is that on a fundamental level the universe is like a dynamic graph with vertices and nodes. At high energies, this graph is highly connected and symmetric. But at low energies, it condenses into a system that has properties such as a geometry, thermodynamics and locality (the property that distant objects cannot influence each other).

That sounds suspiciously like the universe we live in, which is manna to theoretical physicists.

Could this so-called “quantum graphity” be the deeper physics from which general relativity emerges?

Who knows but a few physicists think it’s worth exploring further. The idea raises various interesting questions. What is the nature of the temperature at which the condensation occurs? Is it a real physical temperature or something else? How exactly do the familiar properties of geometry and gravity behave in this model?

And, most important of all, what evidence might we look for to confirm the idea that a condensation took place earlier in the history of the cosmos? If these guys can make some kind of testable prediction, then quantum graphity will alreayd be one giant step ahead of every other attempt to create a quantum theory of gravity.

Ref: arxiv.org/abs/0801.0861: Quantum Graphity: a Model of Eemergent Locality