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NS 8 jun 2002

Pulp fiction? Genetically modified forests aim to clean up the paper industry

THE first paper has been made from genet- ically modified trees despite an attack on a stand of GM poplars by environmentalac- tivists. The trees yield slightly Tore paper with the use of less chemicals.

To make paper, one of the two main com- ponents of wood, lignin, must be removed. This is usually done by heating with alkalis and bleaching. To make the process more efficient, an international team funded by the European Union modified poplars so their lignin would break down more easily. Stands of the trees were planted in France and Britain in 1995. In 1999, those in Britain had to be harvested several months early aftey they were damaged by activists. - The team has now published the results of its field trials. The best variety required 6 per cent less alkali to process, while pulp yield was up by 3 per cent, says team mem- ber Claire Halpin of the University of Dundee. The trees grew normally, although their roots decomposed more easily.

Their qualities should be attractive to industry. But conventional breeding over the past two or three decades has probably produced even greater gains, points out Gra- ham Bernard of Britain's Paper Federation. And how much further the GM strains can be improved isn't clear. More drastic modi- fications have hindered growth.

Activists worry that large stands of altered trees could harm forest ecosystems. But the team found no significant differences in soil chemistry or bacteria under the GM trees.

Not everyone is convinced that the trees are as tough as normal, though. David Andow, an entomologist at the University of Minnesota, points out that the rate of insect attack was so low for all trees in the study that it's hard to tell how the GM strains would tolerate serious infestations.

The experimental strains won't ever be grown commercially. But the team does now plan to produce other GM strains of poplar and pine for paper production.

Environmentalists question the need for such trees, saying chemical pulping should be replaced with more eco-friendly options. But these methods are more expensive and produce lower-quality paper. Kurt Kleiner More at: Nature Biotechnology (vol 20, p 607)

QUANTUM SUPERBRAINS

The prize is a machine powerful enough to take on life, the Universe and everything. Justin Mullins commentates on the race to build a quantum computer

IN THE quantum steeplechase, the runners are competing for a prize beyond riches: a computer so powerful it can simulate the most complex and ipysterious aspects of the Uni- verse without breaking sweat, leaving today's suvercomt)uters looking like ageing nags.

Many of the runners have already attracted widespread interest. But could this be a race in which the fences are so menacing, the dis- tances so vast and the water jump so expan- sive that nobody makes it to the finish? It wouldn't be the first time. Forty years ago, researchers claimed they could harness nuclear fusion to generate cheap, clean power. Today, such power stations are as far off as ever. Pushing technology beyond the limits of known science is always risky. No one can tell what unimaginable complexities will ar,ise or what laws of physics will emerge to thwart you. Quantum physicists will have several tricky fences to jump before they can build their devices, and their dreams may rapidly change into nightmares. What makes a quantum computer such a glittering prize? The key is in the nature of the information it would exploit. Think of ordinary information and you probably imag- ine the Os and ls of binary code, but that changes when you work with quantum parti- cles. Information becomes a strange, ethereal substance, because quantum bits or quoits can be both a 0 and a 1 at the same time. And when put to work inside a quantum computer, they can do extraordinary things.

Take the spin of an electron. Think of it as being like the spin of a basketball with its axis pointing either up or down. Let's say spin "up" corresponds to a 0 and spin "down" to a 1. But the electron can also be placed in a dual existence known as a superposition of states in which its spin is both up and down, a 0 and a 1 at the same time. Perform a calculation with this ghostly electron and you compute an answer using both the 0 and the 1. That's two calculations for the price of one. While a single qubit can be in a super- position of two states, a pair of quoits can be in a superposition of four states. The states can be written 00, 0 1, 10 and 11 (meaning: both spins down; first one down and second one up; and so on). So with two qubits, the system can be in one, some or all of these four states at once. That way you can do four calculations at once. And then it gets really interesting. By the same logic, three quoits can be in a superpo- sition of eight states, four in 16, five in 32 and so on. This exponential increase means that with only a few hundred quoits it is possible to represent simultaneously more numbers than there are atoms in the Universe. With this power, quantum computers would make today's supercomputers look like pocket calculators. They could crack the most fiendish codes, answer problems once deemed unsolvable and run simulations so authentic they can't be distinguished from reality. And that's just the start, says Anton Zei- linger, a qantum physicist at the University of Vienna. "Just look at the way ordinary com- puters were first used and how they are used today. Nobody could have imagined it." That's why physicists the world over are racing to build a quantum computer, backed by ner- vous governments, power-hungry military organisations and profit-hungry companies. But creating a quantum computer is a daunting task. The quantum states used to store information are fragile things-you only have to look at a superposition for it to immediately collapse into a single state. This means that just reading out the result of a computation is tricky, and quantum pro- grams have to be designed to make it as easy as possible (see "Quantum software", p 27).

Interactions with the environment also disturb any quantum state-a problem known as decoherence. So the qubits have to be kept isolated. On the other hand, they have to be somehow connected, because the computer must be able to link up the quoits to perform logical operations. There are two operations from which all others can be derived. The first flips a single qubit: if the qubit is in a superposition of a little bit of 0 and a lot of 1, it will be flipped to a lot of 0 and a little bit of 1. For example, that could involve passing an electron through a magnetic field that flips its spin from up to down. The second is a controlled- NOT or CNOT gate, which flips one qubit depending on the state of the other. For this to happen, one qubit must somehow feel the presence of the other and influence it. And a quantum computer requires the mani- pulation of many qubits in this way, each protected from decoherence and able to interact with all the others. It isn't easy.

And yet in the mid-1990s, scientists found one way to do it. Using a technique originally developed for medical imaging called nuclear magnetic resonance or NMR, information can be stored deep inside molecules in the spins of the atomic nuclei. The advantage of nuclear spins is that they are almost entirely cut off from the environment-they barely interact with their surroundings. Yet because spinning nuclei act as tiny magnets, they can be controlled using a magnetic field or the elec- tromagnetic fields in radio waves. In a strong magnetic field, the spins wobble at slightly different frequencies depending on their chemical environment. By zapping the molecule with radio waves tuned to these resonant frequencies, you can manipulate each nucleus individually.

Calculate with chloroform

One molecule that can act as a 2-qubit quan- tum computer is chloroform (CHC'3)' It has one hydrogen and one carbon nucleus. A quantum calculation involves applying a magnetic field and then zapping the molecule with a series of 'radio pulses that encode a sequence of 1 and 2-qubit [email protected] tions. Flipping one qubit i relatively straight- forward-you just hit it with. its resonant radio frequency. The 2-qubit operation is more complicated. The two nuclear magnets don't interact directly, but are coupled together by the electrons around them, which act a bit like a spring. The strength of this coupling depends on the two nuclear magnetic states, and that can be exploited to create a CNOT gate. If the hydrogen spin is up, for example, a certain radio pulse will jangle the connecting spring so as to flip the carbon spin. But if the hydrogen spin is down, the connecting spring becomes sensi- tive to different frequencies, so the same radio pulse won't do anything to the carbon spin.

With a bigger molecule, there are more nuclei in different chemical contexts, so you can have more quoits. Indeed, several groups have built working NMR computers that can manipulate up to seven qubits. A IO-qubit machine is even rumoured to exist. Last year, a team at IBM factored the number iS using a 7-qubit NMR device. But here so-called liquid NMR runs out of steam. "The whole show stops at 15 qubits," says Leo Kouwen- hoven, a quantum physicist at the Delft Uni- versity of Technology in the Netherlands.

That's because to read out the result of a calculation, you have to measure the magnetic field produced by the reoriented nuclear spins, and because it's such a weak signal you have to use huge numbers of molecules. Worse, the chloroform molecules start out with random spin states, distributed almost equally, which confuses the output si4nal. The signal fades rapidly with the number of quoits, which means you need still more molecules. NMR computers built so far already need around 102,3 moleCUles floating in a tub of solvent. Scientists do not expect to be able to handle more than a dozen or so qubits before the signal fades away.

A really useful quantum computer would need maybe 100 qubits. With only 10 or 12, liquid NMR is forever limited to solving school sums. In the quantum computing steeplechase, NMR is out of the race. Clearly, some technology is needed that allows you to -read,out the state of a single quantum system, so you don't need a whole bucketful. Ions seem promising, because you can use light to read out the spin state of a single ionised atom. What's more, scientists have already developed ways to handle indi- vidual ions trapped in magnetic fields. Since an ion's nucleus can be used to store a qubit, it wasn't long before they realised that by coupling two or more ions together they could create a simple quantum logic gate.

The idea is to trap extremely cold ions with electromagnetic fields that hold them tightly in two dimensions but only weakly in the third (see Graphic, p 28). The ions repel each other and arrange themselves in a line with equal spacing, like beads on an elastic string.

One scheme uses beryllium ions, each forming a two-qubit logic gate. One qubit is stored in the energy level of an electron orbiting the ion, and a second in the vibra- tion of the ion within the trap. Both of these states can be in quantum superpositions: the electron can have both high and low energy, and the ion can be both vibrating and still.

A laser would be used to couple these two quoits and perform the CNOT operation. Only if the electron is in a particular energy state will a photon in the laser beam have the right energy to resonate with the ion, and so change its vibration.

With more than one ion, the vibration gets passed along the string, acting like a data bus to transfer information. David Wineland, a physicist at the National Institute of Stan- dards and Technology in Boulder, Colorado, has already demonstrated that a string of four ions can be linked in this way, and has performed the equivalent of a 2-qubit logic operation with them. But Wineland a nd his competitors around the world are rapidly approaching the hurdle of decoherence. Some quantum states are more fragile than others. The nuclear spins of NMR are quan- tum fortresses compared with ion vibrational states. Because ions are charged, any stray electric field can set up unwanted vibrations and turn the quantum information into quantum gobbledegook.

The problem of decoherence was once thought so [email protected] that quantum computers wouldn't work. But in 1995, Peter Shor at AT&T's Bell Laboratories in New Jersey and Andrew Steane at the University of Oxford came up with a plan for taming it. If you pass the quantum information from particle to particle before decoherence has a chance to dissipate it, it can stay intact indefinitely. But crucially, for this to work the quoits must stay coherent long enough to be passed around. Wineland thinks that should be possible for nuclear spin states, but for vibrational states it's not clear.

And there's another problem. Without the ability to join logic gates together, an ion trap machine would be no more powerful than a supermarket cash register. So Wineland and his team have been looking for ways to make the link. Adding ions to the string isn't an option because the number of modes of vibration rapidly becomes too great to handle. Instead, his hopes are pinned on physically moving ions between logic gates. But so far it hasn't worked. Wineland says the ions become overheated during their short journey and this destroys the information they carry. "We don't know what's causing the heating. Nobody has run into this before," he says.

One alternative to ions is neutral atoms, which cannot be influenced by stray electric fields. The qubit would be stored in the energy states of electrons orbiting their nuclei. An electron can be boosted to a higher energy level when a photon of the right wave- length strikes it, and a photon is generated when the electron falls back to its ground state. So an atom in an excited state could represent a 1, for example, and an atom in its ground state could be a 0. They could then share information via photons held in a mirrored cavity. But the disadvantage is that the atomic states exploited for transmitting and receiving the photons must be highly unstable, as otherwise they would never release their photons-and again this makes them vulnerable to decoherence. If that weren't bad enough, the cavities will never be perfect: some light, and hence some infor- mation, will always leak out. Neutral atoms don't seem like a front-runner.

Silicon dreams

Many researchers think that the techniques for manipulating individual ions and atoms are so clumsy that they can never be scaled up into a large quantum computer operating with hundreds of qubits. For that, they say, you need something solid-the quantum equivalent of a computer chip. David DiVin- cenzo, a physicist at IBM's T. J. Watson Research Center in Yorktown Heights, New Jersey, developed one of the earliest propos- als for a quantum computer of this type in 1998, and could be [email protected] as a solid state evangelist. 'We know what we have to do. It's just a matter of doing it, ".Ihe says. DiVincenzo's idea, which he developed with Daniel Loss at the University of Basle in Switzerland, is to use the spin of single elec- trons as quoits, and store them on the surface of silicon chips within structures known as quantum dots. Electrodes under each quan- tum dot can then flip electrons' spin or force two electrons on nearby dots to overlap. That makes them interact, allowing 2-qubit opera- tions. The answer is measured with a magnetic device that distinguishes spin up from spin down. If it works, many millions of dots could be fitted on a single sliver of silicon.

Kouwenhoven has managed to set up a sin- gle qubit this way, and thinks he can produce a 3 or 4-qubit processor within four years. But he admits it is going to be hard. Decoherence is the hurdle again: like ions, electrons are susceptible to stray electrical fields.

It's possible that quantum states involving large numbers of particles would be more robust. A number of groups are toying with superconductivity, a quantum phenomenon in which current flows with zero resistance. In 1999, a team at the Delft University of Technology in the Netherlands designed a superconducting circuit in which the current flowed both clockwise and anticlockwise in a superposition of states. These circuits can easily be carved onto chips, which is why physicists say they could be joined together into a large-scale computer. Unfortunately, it's too soon for anyone to have attempted the 2-qubit operations needed even for small- scale computation, and the fundamental question of whether such a quantum system is more robust than a single quantum parti- cle has yet to be answered. The most advanced solid state approach is the NMR chip proposed by Bruce Kane at the University of Maryland (New Scientist, 13 January 2001, p 14). The idea is to bury an array of phosphorus atoms in silicon and place an electrode above each. The qubits are stored in the nuclear spin of each atom, and changing the voltage across the atom changes the frequency of the radio waves the nucleus responds to. And since the voltage over eat!h atom can be varied separately, each nucleus can be addressed individually. The phosphorus atoms are close enough for the electrons in their outer shells to interact in a way that can also be controlled by the elec- trodes. And two-qubit operations can be performed by transferring the qubit from the nucleus to the outer electrons, then allowing those of neighbouring atoms to interact.

At the Centre for Quantum Computer Technology at the University of New South Wales in Australia, Robert Clark heads a team that has taken on the challenge of building Kane's computer. They have had to work oui- how to place the phosphorus atoms in just the right spots and stop them migrating, and align the electrodes so that they sit precisely on top of the atoms. In March the Australian group announced that it had made a simple chip-an impressive achievement. Unfortu- nately, the group has so far been unable to read the quantum information stored in the chip and so cannot tell whether it works as expected or not (New Scientist, 30 March 2002, p 13). Another group at the University of Cambridge have suggested using sodium atoms instead of phosphorus, as they can be manoeuvred around inside the silicon with electric fields. But this still hasn't been turned into a quantum computer.

So neither electrons nor ions nor atoms look like reaching the finishing line any time soon. But there's still one quantum particle that physicists feel more comfortable playing with than most-the photon. And photons are extraordinarily robust: under the right circumstances they do not interact easily with other particles, and especially not with each other. With this built-in protection against decoherence, shouldn't they be the ideal quantum particles for quantum computation? Actually, no. The inability of photons to interact with each other means that under ordinary circumstances two-qubit operations are almost impossible. It's the old conundrum of needing qubits that are aloof, yet interactive.

Some have suggested ways to force the photons into partnership, for example using crystals that mediate an interaction between photons. But there is little progress to report. "It's a very difficult engineering challenge. Although nothing in principle prevents us, nobody has successfully demonstrated the technique," says Manny Knill of Los Alamos National Laboratory in New Mexico.

The race for the quantum computer is littered with disqualified runners. Yet there's cause for optimism. "Physicists believe they can build a quantum computer because the laws of physics allow it. There is no 'no-go' theorem that says we cannot do it," says Artur Ekert, a quantum physicist at the Uni- versity of Oxford. So in principle, problems such as decoherence can be beaten.

At least, that's how the laws of physics stand right now. But Ekert argues that even if the drive to build quantum computers uncovers some previously unknown law that puts a spanner in the works, then that will be important too. By this logic, physicists have an each-way bet. If somebody actually builds a quantum computer, they will pace proudly round the winner's enclosure displaying their victorious steed. And if somebody proves that nature will never allow useful quantum com- putation, the race will be quietly called off while physicists rejoice in their newfound understanding of the Universe.

But what of the third outcome, the fate that has dogged scientists working on nuclear fusion? What if the race proves so challeng- ing that the finishing line never looms into view? If so, the runners will continue charg- ing into a gloomy unknown, falling one by one until eventually there are none left. 1-1