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New phase for quantum computers

One of the biggest challenges in quantum computation is to built “fault tolerant” logic gates. In the mid-1990s it was shown that this requires the time evolution of a quantum sub-system to be controlled by the state of a second sub-system. The evolution of the first system can manifest itself in several ways, including a phase shift. In the Oxford experiment this phase shift can be geometric.

The geometric phase was predicted by Michael Berry in 1984. Basically, if the Hamiltonian describing a quantum system is changed extremely slowly, and eventually returns to its initial value, then the system acquires a quantum phase that depends on the area enclosed by the changing Hamiltonian. This is in addition to the well-known dynamical phase associated with time evolution. The geometric phase has since been detected in many different experimental systems.

Jones and co-workers performed their experiment at room temperature in a solution containing chloroform (CHCl3) labelled with carbon-13. The quantum systems were the spins on the hydrogen and carbon-13 nuclei, and the size of the geometric phase shift applied to the hydrogen nucleus depended on the spin state of the carbon-13 nucleus. However, the team stresses that the geometric approach to NMR quantum computation has “no particular advantage” over more conventional methods.

Atom opticians think small

Mara Prentiss and colleagues at Harvard University in the US mounted two short wires on a sapphire substrate (Phys. Rev. Lett. 84 1124). When electric currents flow in opposite directions through the wires, a magnetic field is generated. An external magnetic field can then be used to cancel this field in the region between the wires. This zero-field region attracts cold, neutral atoms and channels them along the waveguide. The scheme also works when the external field is replaced by another pair of wires.

Eric Cornell’s group at the University of Colorado in Boulder has developed a similar scheme that uses longer wires (Phys. Rev. Lett. 83 5194). While the short-wire technique is ideal for constructing miniature beam splitters, longer wires can be curved. It should be possible to exploit this in the development of atomic interferometers.

“There are many advantages to using a substrate,” says Nynke Dekker of the Harvard team. “The substrate can be used as a heat sink for the wires, which allows large currents to be passed through them.” Another advantage is that standard techniques from the semiconductor industry could be used to fabricate atom circuits.

Meanwhile, a group led by Ed Hinds of Sussex University in the UK has made a waveguide for atoms by embedding four wires in a silica fibre with a hollow core (Phys. Rev. Lett. 84 1371). Current flowing through the wires creates a magnetic field that guides cold atoms back up the fibre. The coupling efficiency – the number of atoms guided into the fibre – is much higher than the substrate alternatives. “It’s a tricky experiment,” says Matt Key of Sussex, “but it is also a significant advance towards making an atom fibre, the cold-atom analogue of the optical fibre.”

Black holes ‘mimic’ the Sun

The team claim to have evidence for: an thin outer layer that is hot; a warm middle layer that is also optically thick; and a “cold” inner layer. The similarity of these layers to that of the corona, chromosphere and photosphere of the Sun suggest that similar physical processes may be at work in both types of object.

In a binary black hole system, the strong gravitational pull of the black hole rips gas and other material from its companion star. This material forms a spiralling accretion disk around the black hole. As the material in the disk approaches the inner region of the black hole, its gravitational energy is released as X-rays and gamma-rays.

Measurements from three satellites — ASCA, the Compton Gamma-ray Observatory and the Rossi X-ray Timing Explorer — indicate that there are three separate components, all with different temperatures, associated with the emissions. Computer modelling by the US-China team suggests that the similarity with the Sun is due to the viscosity of the accretion disc.

Climate discrepancy solved

Previously is was thought that the result might have been due to some inadequacy in the data for the troposphere. However, Dian Gaffen from the US National Oceanic and Atmospheric Administration and colleagues have analyzed the data carefully and confirmed that although the temperature changes with height, the current average temperature measurements for the troposphere are correct (Science 287 1242).

Moreover, in related work, Benjamin Santer from the Lawrence Livermore National Laboratory and colleagues have shown that part of the discrepancy could come from uneven coverage by satellite and ground-based measuring stations, and from the vast plumes of material that are pumped into the atmosphere by volcanic activity (Science 297 1247).

“Understanding the difference between surface and tropospheric temperature trends is crucial for modelling climate and planning future climate monitoring,” says David Parker from the UK Meteorological Office in Bracknell.

New light on organic LEDs

Light emission from organic materials occurs through two separate processes: fluorescence – the process by which today’s organic semiconductors produce light – and phosphorescence. In fluorescence, a material absorbs charge carriers such as electrons and electron holes. The opposite charges meet up and combine to form an excited state known as an exciton. The material then returns to the ground state by emitting a photon.

Phosphorescence works on a similar principle, but gives a longer-lived emission by taking advantage of electron spin. If two electrons in the excited state have the same spin – called a ‘triplet’ state – then the material takes longer to move to a ground ‘singlet’ state (where electrons have opposite spin). Nevertheless, both processes are relatively inefficient and to make matters worse, many phosphorescent compounds do not work at room temperatures.

Forrest and his colleagues realised that by harnessing both techniques, they could improve the efficiency of organic light-emitting diodes. The group placed alternating layers of fluorescent and phosphorescent compounds in their device. Energy pumped into the system excites singlet and triplet states into the phosphorescent compound. These in turn excite ‘singlet’ states of the fluorescent material, which then emit light. The increased light efficiency reduces heat production in the device and thus extends its lifetime.

Conductivity measurements made on a single strand of DNA

The Delft group fabricated the electrodes by making a slit in a silicon nitride film with standard lithography. A series of platinum layers were then sputtered across the slit until the gap was reduced to 4 nanometers. The electrodes were then immersed in a droplet of dilute DNA solution. A voltage applied between the electrodes generated an intense electric field, attracting a single molecule strand between the electrodes. Once the molecule was in place, the group could investigate how electrons are transferred in DNA.

"The results show that the charge carriers are being mediated by the molecular bands of DNA," says Dekker, "but more research is needed to explore DNA’s electrical properties under a large variety of conditions."

Last year Hans-Werner Fink and Christian Schönenberger from the University of Basel, Switzerland made the first measurements of the conductivity of a ‘rope’ of DNA molecules. They suggested that the molecules were ohmic conductors with fairly high conductivity.

Life, longevity, and a $6000 bet

The disagreement stems from a paper Gott published in Nature several years ago and recently discussed in the New Yorker magazine. In this paper Gott proposed a formula that suggests that by knowing how long an object, person, species or event has existed for, you can make a generalised prediction for how long it will survive in the future. “This Copernican principle is one of the most successful principles in science,” Gott told PhysicsWeb. According to Gott, the Copernican principle can be used to show that there is a 95% chance that the future duration of the human species, which has existed for 200,000 years, is between 1/39 and 39 times 200 000 years. “In other words,” says Gott, “the human species will probably die out sometime between 5100 and 7.8 million years from now.”

But Caves believes that by making such a prediction, Gott has thrown out rational, scientific inquiry and replaced it with a single, universal statistical rule. Caves argues that the exceedingly long time-scales used in the predictions make the theory worthless. “Anyone could have written down such a long time-scale and achieve the same ‘success’,” he says.

To challenge it, Caves surveyed all his colleagues and students in the physics department and asked them how many of them had pet dogs. He found 24 dog owners who provided Caves with details about the name, age, and breed of the dogs. Gott’s rule, he says, predicts that there is a 50% chance that each dog will survive to twice its present age. Caves then picked 6 of the oldest dogs (over 10 years old) and offered to bet Gott $1000 for each dog at odds of 2:1 in Gott’s favour, that they would not reach twice their age. “You can tell whether someone really believes his probabilistic predictions by challenging him to bet,” says Caves. “The fact that Gott won’t accept my bet means that he doesn’t believe his own predictions in this situation. Thus, he has already conceded the main point of my paper: his rule is not the universal rule that he pretends it to be.”

However, Gott argues that the bet is biased, and that his theory does work if a dog is randomly picked from the sample. “I think by the time all 24 dogs are dead my 95% Copernican formula will have worked in approximately 95% of the cases,” he says. “I think it will do well.”

Caves remains unconvinced. “I would be happy if he acknowledged that his rule is restricted to special circumstances and quit spewing out predictions for any phenomenon that is presented to him,” he says.

PhysicsWeb exclusive: Gott’s response

Molecules made in a Bose-Einstein Condensate

Molecules are poor candidates for the standard techniques used to create Bose-Einstein condensates because of their complicated internal vibrational and rotational structures. But by creating the molecules from atoms that are already inside a condensate, the Texas researchers have been able to put all the molecules into the same quantum state. The molecules, which have extremely low velocities, were at a temperature of just of 100 nanoKelvin. This allowed the researchers to perform a series of high-precision molecular spectroscopic measurements on the molecules. Indeed, their measurements of the molecular binding energy were 10 000 times more precise than any other previous experiment. However, the research has not yet led to a molecular Bose-Einstein condensate: their studies were on mixtures of atoms and molecules called a ‘two-species’ BEC.

CERN claims quark-gluon first

In the CERN experiments a 33 TeV lead ion beam was crashed into targets made of lead and gold inside the seven different experimental detectors. The collisions created energy densities twenty times that of ordinary nuclear matter in order to break down the forces that normally confine quarks within protons and neutrons. The collisions produced fewer J/psi particles and more strange particles than would otherwise be expected.

The real aim of such experiments, however, is to produce a “quark-gluon plasma”, in which the quarks and gluons exist in thermal equilibrium within the soup. The CERN researchers cannot say for sure that they have produced such a plasma since they have not managed to create a soup that exists for a long enough period of time. This requires a larger soup and therefore higher energies.

Higher energy experiments are expected to start this year at the Relatvistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory near New York. A press release issued by Brookhaven to coincide with the CERN announcement pointed out that the European lab had not scooped their experiment, but said that “their results do generate great confidence that the quark-gluon plasma may be directly observed for the first time when RHIC achieves full operational status.” RHIC should produce one of the key signatures needed to prove the existence of a quark-gluon plasma – the spectrum of thermal photons given off in the ion collisions.

Although Brookhaven regard this signature as vital, John Kinson of Birmingham University, and one of the researchers on the WA97 experiment, sees it as detail. “It is 99% certain that a quark-gluon plasma has been produced at CERN,” says Kinson. “RHIC will dot a few i’s and cross a few t’s.” Kinson adds that the heavy ion experiments to be carried out at CERN’s Large Hadron Collider, due to come online in 2005, will produce far higher energies than RHIC and believes that this “will be the real step forward.”

X-ray telescope burns-up

Meanwhile, ESA this week released the first two images from XMM-Newton. One shows the Magellanic Cloud, with hot gas released from supernova explosions and new stars being created in a interstellar nursery. The second image shows the different effects of galactic collisions. “In one collision a black hole shows through a dense cloud of material as an intense blue glow. In another a series of massive stellar explosions, triggered by the collision, emit copious X-rays, seen here as a red halo,” says Martin Turner of the Leicester University.

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