What the Fridge magnet?

“You UNDID your parachute?!” “Had to. It was on fire.”
1,000,000 lemmings can’t be wrong
A bad analogy is like a leaky screwdriver
Anybody else think it’s ironic that “minimalism” is such a long word?
bad poetry / oh noetry!
Current mood: geocidal
Delete all files? Hit any key to continue.
Don’t blow up the universe! That’s where I keep all my stuff!
four out of five doctors agree: SHUT UP
Free Tibet! (one per customer)
Hey, I’ve got an idea… and it doesn’t involve high explosives!
How much do ya gotta BOMB people to get ’em to quit HATING you?
I had no shoes and I thought I was unlucky until someone beat me to death with a spade
I just shot a dolphin. I did it on porpoise
I like cats, too. Let’s exchange recipes.
I’d rather be rich than stupid.
If everybody really is out to get you, what you have is JUSTIFIED paranoia.
If you find yourself struggling with loneliness, you’re not alone. And yet you are alone. So very alone.
If you spot a tornado, always remember to stay absolutely still. Its vision is based on movement.
I’m only acting retarded, what’s your excuse?
In Los Angeles, you can always find a party. In Soviet Russia, the Party can always find YOU
Is it my fault your planet orbits a BALL of FIRE?
Never underestimate the power of stupid people in large groups.
Now I’m radioactive! That can’t be good!
Now you can borrow enough money to get completely out of debt!
Okay, smartass, if this is football, where’s all the ice?
poetry is pompous / any fool can do it / pineapple apocalypse
Quoth the server, “404”
Remember: The old adage “Fight fire with fire” does not apply to non-metaphorical fires.
Seven Deadly Sins? I thought it was a to-do list!
Sixteen days? That’s almost two weeks!
The solution to asylum seekers is better signs to the asylum

Funny UNIX t-shirt phrases

$> man woman
$> Segmentation Fault (Core Dumped)

sudo rm -r /

chown -r us ./base

grep my arp, chmod

You are made of space-time

Lee Smolin is no magician. Yet he and his colleagues have pulled off one of the greatest tricks imaginable. Starting from nothing more than Einstein’s general theory of relativity, they have conjured up the universe. Everything from the fabric of space to the matter that makes up wands and rabbits emerges as if out of an empty hat.

It is an impressive feat. Not only does it tell us about the origins of space and matter, it might help us understand where the laws of the universe come from. Not surprisingly, Smolin, who is a theoretical physicist at the Perimeter Institute in Waterloo, Ontario, is very excited. “I’ve been jumping up and down about these ideas,” he says.

This promising approach to understanding the cosmos is based on a collection of theories called loop quantum gravity, an attempt to merge general relativity and quantum mechanics into a single consistent theory.

The origins of loop quantum gravity can be traced back to the 1980s, when Abhay Ashtekar, now at Pennsylvania State University in University Park, rewrote Einstein’s equations of general relativity in a quantum framework. Smolin and Carlo Rovelli of the University of the Mediterranean in Marseille, France, later developed Ashtekar’s ideas and discovered that in the new framework, space is not smooth and continuous but instead comprises indivisible chunks just 10^-35 metres in diameter. Loop quantum gravity then defines space-time as a network of abstract links that connect these volumes of space, rather like nodes linked on an airline route map.

From the start, physicists noticed that these links could wrap around one another to form braid-like structures. Curious as these braids were, however, no one understood their meaning. “We knew about braiding in 1987,” says Smolin, “but we didn’t know if it corresponded to anything physical.”

Enter Sundance Bilson-Thompson, a theoretical particle physicist at the University of Adelaide in South Australia. He knew little about quantum gravity when, in 2004, he began studying an old problem from particle physics. Bilson-Thompson was trying to understand the true nature of what physicists think of as the elementary particles – those with no known sub-components. He was perplexed by the plethora of these particles in the standard model, and began wondering just how elementary they really were. As a first step towards answering this question, he dusted off some models developed in the 1970s that postulated the existence of more fundamental entities called preons.

Just as the nuclei of different elements are built from protons and neutrons, these preon models suggest that electrons, quarks, neutrinos and the like are built from smaller, hypothetical particles that carry electric charge and interact with each other. The models eventually ran into trouble, however, because they predicted that preons would have vastly more energy than the particles they were supposed to be part of. This fatal flaw saw the models abandoned, although not entirely forgotten.

Bilson-Thompson took a different tack. Instead of thinking of preons as particles that join together like Lego bricks, he concentrated on how they interact. After all, what we call a particle’s properties are really nothing more than shorthand for the way it interacts with everything around it. Perhaps, he thought, he could work out how preons interact, and from that work out what they are.

To do this, Bilson-Thompson abandoned the idea that preons are point-like particles and theorised that they in fact possess length and width, like ribbons that could somehow interact by wrapping around each other. He supposed that these ribbons could cross over and under each other to form a braid when three preons come together to make a particle. Individual ribbons can also twist clockwise or anticlockwise along their length. Each twist, he imagined, would endow the preon with a charge equivalent to one-third of the charge on an electron, and the sign of the charge depends on the direction of the twist.

The simplest braid possible in Bilson-Thompson’s model looks like a deformed pretzel and corresponds to an electron neutrino (see Graphic). Flip it over in a mirror and you have its antimatter counterpart, the electron anti-neutrino. Add three clockwise twists and you have something that behaves just like an electron; three anticlockwise twists and you have a positron. Bilson-Thompson’s model also produces photons and the W and Z bosons, the particles that carry the electromagnetic and weak forces. In fact, these braided ribbons seem to map out the entire zoo of particles in the standard model.

Bilson-Thompson published his work online last year (www.arxiv.org/abs/hep-ph/0503213). Despite its achievements, however, he still didn’t know what the preons were. Or what his braids were really made from. “I toyed with the idea of them being micro-wormholes, which wrapped round each other. Or some other extreme distortions in the structure of space-time,” he recalls.

It was at this point that Smolin stumbled across Bilson-Thompson’s paper. “When we saw this, we got very excited because we had been looking for anything that might explain braiding,” says Smolin. Were the two types of braids one and the same? Are particles nothing more than tangled plaits in space-time?

Smolin invited Bilson-Thompson to Waterloo to help him find out. He also enlisted the help of Fotini Markopoulou at the institute, who had long suspected that the braids in space might be the source of matter and energy. Yet she was also aware that this idea sits uneasily with loop quantum gravity. At every instant, quantum fluctuations rumple the network of space-time links, crinkling it into a jumble of humps and bumps. These structures are so ephemeral that they last for around 10^-44 seconds before morphing into a new configuration. “If the network changes everywhere all the time, how come anything survives?” asks Markopoulou. “Even at the quantum level, I know that a photon or an electron lives for much longer that 10^-44 seconds.”

Markopoulou had already found an answer in a radical variant of loop quantum gravity she had been developing together with David Kribs, an expert in quantum computing at the University of Guelph in Ontario. While traditional computers store information in bits that can take the values 0 or 1, quantum computers use “qubits” that, in principle at least, can be 0 and 1 at the same time, which is what makes quantum computing such a powerful idea. Individual qubits’ delicate duality is always at risk of being lost as a result of interactions with the outside world, but calculations have shown that collections of qubits are far more robust than one might expect, and that the data stored on them can survive all kinds of disturbance.

In Markopoulou and Kribs’s version of loop quantum gravity, they considered the universe as a giant quantum computer, where each quantum of space is replaced by a bit of quantum information. Their calculations showed that the qubits’ resilience would preserve the quantum braids in space-time, explaining how particles could be so long-lived amid the quantum turbulence.

Smolin, Markopoulou and Bilson-Thompson have now confirmed that the braiding of this quantum space-time can produce the lightest particles in the standard model – the electron, the “up” and “down” quarks, the electron neutrino and their antimatter partners (www.arxiv.org/abs/hep-th/0603022).

All from nothing at all

So far the new theory reproduces only a few of the features of the standard model, such as the charge of the particles and their “handedness”, a quantity that describes how a particle’s quantum-mechanical spin relates to its direction of travel in space. Even so, Smolin is thrilled with the progress. “After 20 years, it is wonderful to finally make some connection to particle physics that isn’t put in by hand,” he says.

The correspondence between braids and particles suggests that more properties may be waiting to be derived from the theory. The most substantial achievement, Smolin says, would be to calculate the masses of the elementary particles from first principles. It is a hugely ambitious goal: predicting the masses and other fundamental constants of nature was something string theorists set out to do more than 20 years ago – and have now all but given up on.

As with string theory, devising experiments to test for the new theory will also be difficult. This is a problem that plagues loop quantum gravity in all its guises, because no conceivable experiment can probe space down to 10^-35 metres.

Ironically, the best arena in which to look for experimental proof might be the largest scales in the universe, not the smallest. “The closest anyone is getting to making predictions is in the area of cosmology,” says John Baez, a mathematician and expert on quantum gravity at the University of California, Irvine. Markopoulou is now trying to think of ways of testing the braid model using the fossil radiation left over from the big bang, the so-called cosmic microwave background that permeates the universe. Physicists believe that the patterns we see today in that radiation may have originated from quantum fluctuations during the earliest moments of the big bang, when all of the matter in the universe was crammed into a space small enough for quantum effects to be significant.

Meanwhile, Markopoulou’s vision of the universe as a giant quantum computer might be more than a useful analogy: it might be true, according to some theorists. If so, there is one startling consequence: space itself might not exist. By replacing loop quantum gravity’s chunks of space with qubits, what used to be a frame of reference – space itself – becomes just a web of information. If the notion of space ceases to have meaning at the smallest scale, Markopoulou says, some of the consequences of that could have been magnified by the expansion that followed the big bang. “My guess is that the non-existence of space has effects that are measurable, if you can only see it right.” Because it’s pretty hard to wrap your mind around what it means for there to be no space, she adds.

Hard indeed, but worth the effort. If this version of loop quantum gravity can reproduce all of the features of the standard model of particle physics and be borne out in experimental tests, we could be onto the best idea since Einstein. “It’s a beautiful idea. It’s a brave, strange idea,” says Rovelli. “And it might just work.”

Of course, most physicists are reserving judgement. Joe Polchinski, a string theorist at Stanford University in California, believes that Smolin and his colleagues still have a lot of work to do to show that their braids capture all of the details of the full standard model. “This is in a very preliminary stage. One has to play with it and see where it goes,” Polchinski says.

“Atoms and people may be down to the way space-time tangles up on itself”

If the new loop quantum gravity does go the distance, though, it could give us a new sense of our place in the universe. If electrons and quarks – and thus atoms and people – are a consequence of the way space-time tangles up on itself, we could be nothing more than a bundle of stubborn dreadlocks in space. Tangled up as we are, we could at least take comfort in knowing at last that we truly are at one with the universe.

Time Travel 2008

As you may have heard, this will be the year. The Large Hadron Collider – the most powerful atom-smasher ever built – will be switched on, and particle physics will hit pay-dirt. Yet if pair of Russian mathematicians is right, any advances in this area could be overshadowed by a truly extraordinary event. According to Irina Aref’eva and Igor Volovich, the LHC might just turn out to be the world’s first time machine.

It is a highly speculative claim, that’s for sure. But if Aref’eva and Volovich are correct, the LHC’s debut at CERN, the European particle physics centre near Geneva in Switzerland, could provide a landmark in history. That’s because travelling into the past is only possible – if it is possible at all – as far back as the creation of the first time machine, and that means 2008 could become Year Zero: a must-see for the discerning time traveler.

Aref’eva and Volovich are sensible and well respected mathematicians, based at the Steklov Mathematical Institute in Moscow, so they are not actually suggesting that visitors from the future are imminent. What they are saying is that since causality – the idea that effect must follow cause – is one of the most fundamental principles of physics, the notion that it might be tested at the LHC is worth pushing as far as possible.

Their work has yet to be recognised by a peer-reviewed journal, but that hasn’t stopped some other physicists from taking a keen interest.

For decades, physicists have strived to come up with plausible mechanisms for time travel. Our best description of how space and time behave comes from Einstein’s general theory of relativity, so researchers have been looking for some flaw in it – or some as yet unappreciated aspect – in the hope that this might do the trick. The time machine blueprints flowing from such endeavours have never got off the drawing board, but with the LHC we might have finally done it, albeit accidentally.

When the LHC is running at full throttle, it will imbue each of the particles travelling around its 27-kilometre circumference with around 7 teraelectronvolts (TeV) of energy. That may not be much in everyday terms: 1 TeV barely matches the kinetic energy of a flying mosquito. However, when concentrated into a subatomic particle – a trillionth the size of a mosquito – it can do extraordinary things to the fabric of the universe.

According to general relativity, everything in the universe is played out on a stage that has three dimensions of space and one of time. The strange thing about this space-time is that it gets warped by the mass and energy of the universe’s contents. This is what lies at the root of gravitational attraction. The mass of the Earth, for instance, distorts the surrounding space, causing everything in its vicinity to feel a pull towards it.

It’s harder to visualise the distortion of time, but it does happen to a tiny extent in the presence of any matter or energy. What’s more, a large enough concentration of mass or energy can distort time so much that it loops back on itself like a rubber sheet rolled up to make a cylinder. These loops are known to physicists as “closed timelike curves” and they ought, at least in theory, to allow us to revisit some past moment in time.

The first person to show how a closed timelike curve could form was the Austrian mathematician Kurt Gödel. In 1949, he demonstrated that if the universe were spinning, relativity should allow this spin to create conditions in which time looped back on itself. If you could get yourself onto this loop, you would keep revisiting the same moment until you got off.

The idea that relativity allowed time travel bothered Einstein when Gödel showed him the results of his calculations, but it wasn’t really a problem: to the best of our knowledge, our universe is not spinning, so time travel couldn’t happen this way. Neither did the world end in 1976 when Frank Tipler of Tulane University in New Orleans, Louisiana, showed how an extremely massive and infinitely long, fast rotating cylinder would create a similar opportunity to travel through time: it is, after all, not a machine that is going to get built any time soon.

Things got more interesting in 1988, when Kip Thorne and colleagues at the California Institute of Technology in Pasadena showed that wormholes, or tunnels through space-time, would allow time travel (Physical Review Letters, vol 61, p 1446). In this case a wormhole would close a loop in time.

Traveling through it is a bit like taking a tunnel under a hill: you could get to the other side by going over the hill, but the tunnel gets you there faster. If you choose your wormhole carefully – or take an existing one and move its entrances around – you could even emerge from the wormhole before you went in at the other end.

Space-time shock

This is where the LHC comes in. It could, Aref’eva and Volovich believe, create wormholes and so allow some form of time travel. Each particle travelling through the LHC creates a kind of shock wave in space-time, a gravitational ripple that distorts the space and time around it. When two such waves are heading towards each other, the outcome could be spectacular. Under certain conditions, the colliding gravitational waves will rip a hole in space and time.

What those conditions are depends on the precise nature of space-time – something we don’t yet know enough about. While Einstein’s relativity theory provides a description of space-time’s properties on a large scale, this is only an approximation. Finding out just how much energy it might take to rip holes in the fabric require an understanding of quantum gravity – a microscopic description of spacetime that is still beyond our reach.

Nevertheless, it is conceivable that the LHC could achieve the conditions needed for ripping a hole in space-time. The conventional view among physicists is that quantum gravity does not become important until you deal with phenomena that occur at energies of around 1016 TeV. However, a team led by Nima Arkani-Hamed from the University of California, Berkeley, has shown that quantum gravity could kick in at energies as low as 1 TeV (Physical Review Letters, vol 84, p 586).

Aref’eva and Volovich’s speculation about strange space-time effects began with the realization that the LHC might be powerful enough to make mini black holes. Two protons colliding with a combined energy of 14 TeV might create black holes 10-18 meters in diameter. That idea is intriguing enough, but it is only one possibility. Last year, Aref’eva and her colleagues were again playing about with Einstein’s equations, looking for ways in which closed timelike curves might arise (see Diagram). It was then that they came across the possibility that the LHC might create a time machine (www.arxiv.org/abs/0710.2696). “We realized that closed timelike curves and wormholes could also be a result of collisions of particles,” Aref’eva says.

The possibilities this raises are being taken seriously by some physicists. “This is an interesting paper,” says J. Richard Gott of Princeton University in New Jersey, who suggested as long ago as 1991 that accelerating particles could be a route to time travel. In a paper published at the time in Physical Review Letters, he suggested that if super-energetic particles were aimed so that they missed each other by a small amount, they would warp the space-time around them enough that the interaction of their two warped space-times could form a closed timelike curve.

In Gott’s calculations, however, the final outcome wasn’t clear: the deformed space-times might well form a black hole instead of a time machine. “The twisting of space and time required to make a time machine are similar to that required to make a black hole,” Gott says. Now Aref’eva and Volovich have calculated that wormholes and mini black holes have an equal chance of being created by the LHC, and that a wormhole might even appear as frequently as every couple of seconds.

None of this means we’re going to be time travelling by Christmas, however. There are still plenty of obstacles to opening a time portal. Not least of them is the fact that these are mini wormholes, so only subatomic particles are small enough to travel through them. Probably the best we can hope for is that this might provide a signature of the wormholes’ existence, Volovich says. If some of the energy from collisions in the LHC goes missing, it could be because the collisions created particles that have travelled into a wormhole.

The second obstacle is also to do with wormhole size. The mouth of a wormhole is like the mouth of a rubber balloon, in that it has a tendency to pull itself closed. The only way to avoid this is to prop the wormholes open with some strange kind of matter that exerts a push rather than a pull.

Is there any such stuff available? At this point, Aref’eva and Volovich extend their speculation into the mysteries of the “dark energy” that seems to be accelerating the expansion of the universe. Dark energy could, they say, be just what is needed to keep the entrance to a wormhole open, but to find out if that is even possible we need to know the answer to another crucial question: as space-time expands, does the density of dark energy increase, decrease or stay constant?

When physicists look at the way expanding space-time behaves, most interpret the observations as suggesting that the energy contained in every cubic centimeter of space-time stays constant: it is “persistent”, not, as one might expect, “diluted” by the expansion of the universe. There are, however, a minority of physicists who are putting their money on a third possibility – that as space-time expands, every cubic centimeter gains ever more energy. If dark energy did have this “phantom” nature, spacetime would contain an inherent push that could keep the mouths of LHC wormholes open – and perhaps even grow them big enough for people to pass through. “The observational evidence still allows for phantom energy,” says Robert Caldwell, a physicist at Dartmouth College in Hanover, New Hampshire.

Wormhole fingerprint

Unfortunately, we just don’t know yet which of the three possibilities is right. Francisco Lobo of the University of Lisbon in Portugal is among the minority who favor the existence of phantom energy – the kind Aref’eva and Volovich say would prop wormholes open. However, just as we’re getting ready to go back to the future, Lobo throws his own spanner in the works. “Even if one could, in principle, detect a wormhole signature it does not guarantee the presence of a time machine,” he says. We might see the fingerprint of a wormhole at the LHC, just as we might see the indicator of a black hole being formed, but that’s not enough to create a useful loop in time, Lobo reckons.

A wormhole is a loop protruding from “normal” space-time, like a handle protruding from a teacup. If you want to turn the wormhole into a time machine; you have to make sure the two ends of the handle meet the cup at just the right points in time. “One would have to create a time-shift between the wormhole mouths,” Lobo says.

Various schemes have been proposed to create such a time-shift, but all of them are exotic to say the least. Anchoring one end of a wormhole to a neutron star might do the job, for instance. The intense gravitational field of the star slows time, so the wormhole mouth near the star would develop a time difference with respect to the other mouth. It is conceivable that a time traveler could then jump in, emerge at some point in her past, then travel through normal space to the other end of the wormhole and hang around waiting to watch herself jumping in. It’s not the kind of operation we are going to be capable of in the foreseeable future, as Lobo points out.

Yet who knows? Perhaps future civilizations might work out how to stabilize and grow a wormhole, then manipulate the two mouths in order to create a time tunnel. If a combination of fast-moving particles and phantom energy does create a wormhole in Geneva this year, such an advanced civilization could find it in their history books, pinpoint the moment, and take advantage of their technology to pay us a visit.

This possibility forces us to confront the many paradoxes that time travel raises. The classic example is the time traveler who goes back to kill his grandfather before his own father is conceived – thus ensuring he is never born. Scenarios such as this prompted Stephen Hawking to suggest in 1992 that the laws of physics actually conspire against time travel. His “chronology protection conjecture” says that creating loops in time that would allow time travel has a kind of negative feedback, giving rise to physical phenomena that act to block the loops – as if there were a causality enforcement agency.

Aref’eva doesn’t fear these time cops, though. “In general relativity, one cannot just assert that chronology should be preserved without careful analysis,” she says. There are many solutions of Einstein’s equations that permit such paradoxes to arise, she points out; it is arrogant to declare that these situations can’t be manifest in reality just because we can’t see how they will play out. Perhaps, she says, the paradoxes will answer questions about free will or allow us to sift through the interpretations of quantum theory. Maybe you would find yourself unable or unwilling to kill your grandfather, or end up in a parallel universe where killing your grandfather would make no difference in the universe from whence you came. Until we build a time machine, we just can’t know.

For now our best hope of finding out about the limits of temporal law enforcement is to let the physicists and engineers carry on with their preparations at the LHC. Sure, there are unresolved issues about the scale at which quantum gravity kicks in; we are still arguing over whether the universe contains phantom energy; and we don’t even know if we have the likelihood of black holes and wormholes pinned down accurately. Nevertheless, the slim possibility remains that we will see visitors from the future in the next year.

Wouldn’t it be better to be prepared than not? Perhaps now is the time to increase the staffing levels at Geneva’s tourist information centre. And if you are a grandfather, you might want to check the small print on your life insurance.

Atomic Logic

URRAY HOLLAND dreams of the day when we can switch on a very different kind of computer. Forget pressing a button and waiting for your hard drive to whirr into action. If Holland’s idea works out, your computer’s circuits will materialise in front of you, made from little more than beams of light and a puff of gas. Your new machine would be more versatile, too. Flick a switch and the shimmering electronics will reconfigure a component from, say, data storage into an extra processor. In fact, you would be able to transform its parts into any electronic circuitry you like.

Welcome to the strange, shape-shifting world of atomtronics, where light beams are used to generate and control a current that is not a flow of electrons but a flow of atoms. Holland and his team at the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado, reckon they can use a current of atoms to build anything from batteries to amplifiers and even transistors, which could eventually become the building blocks of “atomtronic” computers. Admittedly, you might have a long wait. “Atomtronics won’t happen tomorrow,” says Holland. “As an idea though, it is really taking off.”

How so? For more than a decade, researchers have been using light to trap atoms, creating artificial crystals that are much like the real thing but on a larger scale. The atoms in these crystals mirror the behaviour of electrons in solid matter, and the prospect of being able to model real materials this way is proving irresistible. Not only are these collections of atoms large enough to see, physicists can control the interactions between atoms, switching a material from insulator to metal, say. At last they can explore phenomena that are difficult or impossible to study in real materials.

The idea of using light to trap atoms was suggested more than 30 years ago by Soviet physicist Vladilen Letokhov, and was developed in the US by Arthur Ashkin at Bell Telephone Labs in New Jersey. They proposed creating an interference pattern in a light beam ? a pattern of bright and dark stripes ? and using it to ensnare atoms. Place an atom in a laser beam, and the beam’s intense electric field induces a charge imbalance on the atom which, in turn, interacts with the light’s electric field. Their calculations showed that the effect would be to move the atom into the dark region in the interference pattern and hold it there.

Things get really interesting when you add more light beams. Create a criss-cross grid of laser beams, with a dark patch of interference where each pair of beams cross, and you have a chessboard array known as an optical lattice. Atoms sprinkled into the lattice fall into the dark “wells”, where they remain trapped like marbles in an egg box.

That’s the theory, anyway. In practice, atoms at room temperature whizz around at hundreds of metres per second and have far too much kinetic energy to be trapped by the minute forces generated in an optical lattice, so to make the idea work they have to be cooled. In 1992, Philippe Verkerk and his colleagues at the ?cole Normale Sup?rieure in Paris, France, used a combination of laser and magnetic fields to remove energy from a cloud of caesium atoms, slowing them down to less than walking pace ? the equivalent to cooling them to within a few millionths of a degree above absolute zero. At these temperatures, Verkerk’s caesium atoms fell into the optical lattice’s wells and stayed there, resembling beads on a string.

Measurements confirmed the material’s similarities to a real crystal ? though the caesium atoms in the traps were some 10,000 times further apart than those in the real thing. Today, research groups all over the world are studying ultracold atoms held in optical lattices. “Of course, you don’t capture all the complexity,” says physicist Immanuel Bloch at the University of Mainz in Germany. “We are trying to understand the interactions that matter at the simplest level.”

It might seem strange that optical lattices have so much in common with real materials. The reason lies in quantum mechanics. The properties of any real crystal depend on its energy landscape: the potential energy maxima are hills, while the minima are the valleys where atoms nestle. The ability of electrons to flow through a crystal, by moving between atoms in adjoining valleys, determines whether a crystal is a metal, a semiconductor or an insulator.

Classical physics struggles to explain how electrons do this. In the 1960s, British theorist John Hubbard developed a quantum mechanical model to describe this motion. He suggested that since electrons are quantum particles, they can “tunnel” through the hills, from one atom to its neighbour. Hubbard showed that the probability of this happening depends on the depth of the energy valley, the distance between neighbouring valleys and interactions between electrons on each atom.

In 1998, a team of theorists led by Dieter Jaksch, then at the University of Innsbruck in Austria, suggested that Hubbard’s model should also work for atoms in an optical lattice (Physical Review Letters

, vol 81, p 3108). Here, atoms would tunnel between valleys corresponding to the dark regions in the laser interference pattern, creating the atom equivalent of an electron current flowing through a metal-like conductor.

Jaksch and his colleagues even predicted that physicists would be able to create an optical lattice that behaved like a type of insulator known as a Mott insulator. The secret was the choice of atoms ? they must repel each other when they collide ? and to make sure the valleys are so deep that it is very difficult for the atoms to tunnel through. In theory, the atom flow should stop altogether.

TransformersIn 2002, Ted H?nsch’s group at the Max Planck Institute for Quantum Optics in Garching, Germany, demonstrated this in spectacular style. Electrically neutral atoms can repel or attract each other, so H?nsch needed a way of tuning the force between them. Magnetic fields are perfect for this. The team made a 3D crystal using ultracold rubidium atoms bathed in a magnetic field and the light from six orthogonal laser beams. Cranking up the intensity of the laser made the wells deeper and stopped the atoms tunnelling, exactly as Jaksch’s group had predicted. At the turn of a dial, the researchers had morphed a material made of atoms and light beams from a metal into an insulator ? a material with entirely different characteristics.

Back at JILA, this set Holland wondering just how versatile the morphing optical lattices could be. If you could turn a metal into an insulator, he asked himself, could you also make a semiconductor? Perhaps you could even turn these atom semiconductors into electrical components ? an idea he dubbed atomtronics. “When we started we didn’t know how far we could push the analogy between electronics and atomtronics,” Holland says. “It’s not at all obvious that you could dream up a diode junction.” For a start, there is the fact that atoms are so much heavier than electrons ? over 150,000 times more massive in the case of rubidium. Unlike electrons, atoms are electrically neutral and their quantum mechanical spin is different. Can atomtronics be any match for electronics?

Holland’s team looked to the Hubbard model for answers. These were not easily found. Theorists can write down equations describing one atom in a lattice. Add another atom, and you have an interaction between the two atoms as well as between the atoms and the lattice. “Add more atoms and you soon need a supercomputer,” says Holland. “The equations get very hard very quickly.” He started with the simplest electric circuit possible: a wire joining the two ends of a battery. Connecting a wire to a conventional battery starts a chemical reaction that produces electrons, which travel along the wire from the battery’s negative terminal to its positive terminal.

In Holland’s scheme, the atomtronic equivalent of the negative terminal is a dense cloud of ultracold atoms trapped by laser beams. The positive end of the battery uses identical beams, but without the atom cloud. The wire between them is an optical lattice containing plenty of empty wells . The atomtronic current starts flowing when repulsive interactions among the atoms in the cloud become too great for some atoms. Their response is to escape by tunnelling into the optical lattice “wire”. From there, they tunnel their way along the length of the wire towards the empty trap. How fast the atoms hop, Holland’s team inferred by comparison with the Hubbard model, depends on the depth of the wells in the optical lattice.

With a simple circuit in the bag, Holland’s team next turned to the atomtronic equivalent of a diode. In conventional electronics, diodes are the simplest semiconductor devices you can build. Add a few extra atoms to a silicon crystal and you turn it from an insulator into a semiconductor. Toss in phosphorus or arsenic, whose atoms carry one more electron than silicon and you have an “n-type” semiconductor that conducts electrons. Add atoms with one less electron, such as boron or gallium, and you create a p-type semiconductor whose crystal structure is missing electrons. Known as holes, these absent electrons in effect move through the crystal and also conduct electricity.

By marrying n-type and p-type semiconductors together, you make a p-n diode junction that allows current to flow in one direction only. To get any measurable current, you need to connect the negative terminal of the battery to the n-type semiconductor and the positive terminal to the p-type. This arrangement causes electrons to flow from the n-type silicon across the p-n junction. Swap the battery around and the current stops.

Last year, Holland’s team reported a design for an atomtronic diode
that they believe might be the easiest to make (Physical Review A
, vol 75, p 023615). One way to do this is to create an optical lattice as usual and then expose one half of it to laser light with a slightly different frequency. The effect is to raise the energy of this part of the lattice and the atoms it contains. This step in the energy landscape is what acts as the p-n junction in a diode. Holland’s calculations show that if you connect an atomtronic battery to the device, the atoms flow in one direction only. Ultracold atoms can tunnel easily through the high-energy half of the lattice. When they reach the junction, they simply drop down to the low-energy region and carry on tunnelling until they reach the other end of the battery. However, this is not true for atoms starting in the low-energy half of the lattice. When they reach the p-n junction, they do not have sufficient energy to make the leap. The atoms soon fill up the low-energy half of the lattice and turn it into a Mott insulator, blocking the flow of current.

Holland thinks there is no limit to what physicists could make using lasers and atoms. His group has designed an atomtronic transistor by joining two p-n diodes together, mirroring conventional transistors made from p-n-p semiconductor junctions. As the building block of all kinds of digital logic gates, an atomtronic transistor is the key to memory and even microprocessor chips. This raises the question: could you build an entire computer from light beams and atoms? “In principle, it should work,” Holland says.

Could they ever replace silicon chips? Probably not. Atoms are much heavier than electrons so atomtronic devices will always run more slowly than ones driven by electrons. But the research might still speed up your PC. Computer power has roughly doubled every two years as manufacturers pack twice the number of transistors into integrated circuits. This requires ever smaller transistors. Chip maker Intel has now launched a processor made from transistors just 45 nanometres wide and plans to reduce this to 16 nanometres by 2013. The laws of quantum mechanics could stymie progress beyond this, though. Intel has already found that electrons can tunnel through transistors’ insulating layers, threatening to create a short-circuit. While novel materials could help solve this, other quantum effects could still wreak havoc.

Holland believes atomtronics might help tackle these problems. “We need to understand these effects if we want our electronics to keep working,” he says. “Atoms offer exquisite control. You can use them to understand the physics and feed that back to the electronics.”

Quantum logicBeyond that, atomtronics might help the development of quantum computers. In theory, these computers would manipulate huge amounts of data stored in quantum bits, or qubits. The main difficulty is that you need to preserve the delicate superposition that qubits exist in. But in 1999 Gavin Brennan and colleagues at the University of New Mexico in Albuquerque suggested that neutral atoms in an optical lattice might offer an excellent route to stable qubits. Since neutral atoms in a lattice are protected from outside disturbances, atomtronics might offer a route to a more robust quantum computer that uses light to start a calculation and to read the answers.

So far no one has made an atomtronic diode or transistor, let alone an atomtronic quantum computer. In fact, no one has even created optical lattices with the shapes suggested by Holland. “All these experiments are very hard,” says Dana Anderson, an experimental physicist who works with Holland at JILA.

Anderson is taking a different approach to atomtronics, trapping the ultracold atoms with magnetic fields generated by wires on a chip rather than with laser beams. Design such a chip carefully, as Anderson and several groups around the world have, and you can cool a small cloud of ultracold atoms, trap it and then transport it around the circuit. He believes this could eventually lead to atomtronics circuits comprising several transistors packed together. “I think that it’s harder to do with optics, but I don’t want to underestimate the ability of my colleagues.”

He is right to respect their abilities. By playing around with laser beams, researchers have already made triangular lattices, as well as lattices that trap atoms spinning in certain directions, and lattices with wells of alternating depth. Bloch believes it’s only a matter of time before someone sculpts the optical lattices needed for a diode. This could even happen in the next year, he says.

Eventually, it could be possible to morph the atomtronic circuits from one type into another. Markus Greiner and his group at Harvard University are attempting to shape artificial crystals by shining laser light through a hologram onto a cloud of ultracold atoms. Similar techniques have been used by David Grier’s group at New York University to simultaneously manipulate hundreds of objects in the micrometre scale using a single laser beam. Grier’s holograms are computer-generated and created by shining laser light through an LCD screen, which splits the light into any number of beams and moves the objects such as cells at will. By adjusting the pattern on the screen, the researchers can create new holograms. One day, perhaps, this same idea will work with atomtronics.

To adapt the method to atoms that are thousands of times smaller is “very demanding”, says Bloch. If they pull it off, the technique could eventually mean you could watch an atomtronic diode transform into an atomtronic transistor and back again at the flick of a switch. Holland’s dream would finally be on the way to reality.