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Nanowires approach the quantum realm

Silicon is appealing for nanotechnology applications because the techniques to make silicon-based devices are so well advanced. Different methods have been developed for the growth of silicon nanowires such as laser ablation, catalyst-free methods and solution techniques. Such methods produce nanowires with different orientations and diameters that are covered by an oxide sheath 1 to 3 nm thick. The smallest silicon nanowire made so far has a diameter of between 3 to 5 nm. However, theory predicts that significant quantum size effects only come into play at diameters of less than 3 nm.

Lee and co-workers fabricated silicon nanowires using an oxide-assisted growth method that produces wires with diameters in the range of a few to tens of nanometres. The resulting wires consist of a single crystalline silicon core and an oxide sheath, which is about one third of the diameter. The researchers removed this oxide layer and terminated the surface with hydrogen to produce an oxidation resistant wire.

The team then used scanning tunnelling spectroscopy to determine the electronic band gaps of the nanowires. They found that the band gap increases as the diameter decreases – from 1.1 eV for 7 nm diameter wires to 3.5 eV for 1.3 nm wires. This is in agreement with previous theoretical predictions and provides experimental evidence for the quantum size effect on the electronic density of states in silicon nanowires.

The researchers now hope to use these nanowires for use in light-emitting diodes and lasers. “We also want to apply our method to a host of other scientifically and technologically important semiconducting nanowires, such as zinc oxide, zinc sulphide, gallium nitride and germanium”, Lee told PhysicsWeb.

Hubble photographs the Boomerang Nebula

Raghvendra Sahai from the Jet Propulsion Laboratory and Lars-Ake Nyman from the European Southern Observatory and the Onsala Space Observatory, Sweden first studied the Boomerang Nebula in 1995 using the radio telescope at La Silla, in Chile. They showed that it was the only object cooler than the cosmic microwave background radiation.

Sahai and Nyman believe that the Boomerang is colder than most other expanding nebulae because it is losing its mass about 100 times faster than other similar dying stars. “As the object is losing so much mass, the gas around it – especially carbon monoxide – self-shields itself and the photons from the microwave background do not penetrate deep into the outflow,” says Nyman.

Hubble has shown in its high-resolution images that the “Bow-Tie Nebula” might be a more apt description for the Boomerang, which was originally named by Australian astronomers in 1980.

The images show faint arcs and filaments embedded within the diffuse gas of the smooth “bow-tie” lobes in the nebula. This shape is very different from other observed planetary nebulae, which have lobes that resemble bubbles blown in the gas. Researchers are not certain how these lobes are created but they believe that the overall shape of the Boomerang was created by a 500 000 kilometre-per-hour wind blowing ultra-cold gas away from the dying star.

Qubits are on solid ground

Photons, atoms or trapped ions can be used as qubits but it should be easier to build working devices using solid-state qubits. Quantum computing works on two basic quantum mechanical principles. The first is the superposition of states, which is a one-particle phenomenon. The second is entanglement, which involves two or more particles.

The spin of a particle can point in two opposite directions, “up” and “down”, but the particle can also exist in a superposition of these states. This superposition also holds true for two-particle states including entangled states. When two particles are entangled they behave as one, regardless of how far apart they are.

For quantum computing to work, however, these entangled states must be made to interact in a controlled manner.

Tsai and co-workers used micron-sized “boxes” of superconducting material that were connected to a Josephson junction – a type of superconducting “reservoir” – via a capacitor. A Cooper pair of electrons can tunnel from the junction onto the box. The box is the qubit, which can exist in two states: one state has an excess of Cooper pairs while the other has no excess Cooper pairs. The qubits are made to interact using the capacitor, which leads to a mixing of two-particle states and thus entanglement of the qubit pair.

Although the team has not yet measured a specific entangled state, they have shown that the qubit pair is strongly entangled. “This result shows that it is indeed possible to construct a quantum logic gate using such a solid-state device,” Tsai told PhysicsWeb. “A quantum computer could be made using such gates as the basic units.”

The superconducting flux qubit developed by the Delft-NEC team, on the other hand, comprises three Josephson junctions in a loop. The two quantum states in this system are macroscopic currents consisting of billions of Cooper pairs travelling around the loop in opposite directions. The qubit can undergo hundreds of oscillations between these two states, and can be read with a superconducting quantum interference device.

Superconductor transistor breaks records

At present single-electron transistors (SETs) and superconducting quantum interference devices (SQUIDS) are the most promising candidates for read-out devices in solid-state quantum computers. SETs work well with large impedances and SQUIDS with small impedances. However, both run into problems at intermediate impedances, of about 1 megaohm.

The Helsinki team has built a “Bloch oscillating transistor” consisting of three junctions. The first is a Josephson junction in which two superconducting layers are separated by a thin insulating layer. The second is a “normal” tunnel junction and the third, a large resistance. The Josephson junction measures less than 100 nanometres across.

The researchers inject a base current made up of single electrons into one side of the Josephson junction and find that a “supercurrent” of Cooper pairs emerges on the other side. Cooper pairs form when the electrons in a superconducting material overcome their mutual repulsion as a result of their interactions with vibrations of the crystal lattice.

The device works by setting up Bloch oscillations in the Josephson junction. Normally, Bloch oscillations only occur in the ground state, E₀. However, electrons can also tunnel from E₀ to E₁ in a process known as Zener tunnelling, and the device will only work if the electrons can be made to “relax” back down to the ground state.

The researchers achieve this by injecting a current of “quasiparticles” into the normal junction, which then allows relaxation between the two energy levels. The team observed a current gain of 30 and a power amplification of 5 in their device.

The resistance in the circuit simply acts as “island” that suppresses unwanted fluctuations in the system.

Teleportation moves on

In quantum teleportation the sender, normally called Alice, instantaneously transfers the quantum state of a particle to a receiver, called Bob. In most experiments so far Alice has teleported the quantum state of a photon – defined in terms of its polarization – to Bob. The photon itself is not transferred: rather Bob’s photon acquires exactly the same polarization as Alice’s. The uncertainty principle means that Alice cannot know the exact state of her photon, but another feature of quantum mechanics called “entanglement” means that this is not an obstacle to teleporting the state to Bob.

Quantum entanglement essentially allows two particles to behave as one, regardless of how far apart they are. Photons can be entangled so that if one is vertically polarized, for instance, then the other photon in the pair is always horizontally polarized.

In a standard teleportation experiment a laser is directed at a crystal with nonlinear optical properties. Occasionally the photon will be “down-converted” into two lower energy photons, and sometimes these photons will have their polarizations entangled. In a teleportation experiment the beam is reflected back through the crystal to sometimes produce a second pair of entangled photons. By convention the photons in the first pair are labelled 2 and 3 (for mode 2 and mode 3), and those in the second pair are 1 and 4. Photons 1 and 2 are directed to Alice, photon 3 is sent to Bob and photon 4 is used as a trigger. A variety of mirrors, beamsplitters and polarizers are used to direct the photons to four detectors labelled D1, D2, D3 and T (for trigger).

The aim of the experiment is to transfer, or teleport, the polarization of photon 1 to photon 3. This is normally done by making a joint measurement on photons 1 and 2 which changes the polarization of the latter in such a way that photon 3 – which is entangled with it – always acquires the same polarization as the first photon. In other words the quantum state of photon 1 – which was unknown to Alice – has been teleported to photon 3.

The experiment was set up such that detectors D1, D2 and T all register photons at the same time when teleportation takes place. However, under certain circumstances – the emission of two photons each into modes 1 and 4 – it is possible for the same three detectors to register events even though there is no photon in mode 3 to teleport to. These spurious events mean that the D3 detector must also register – and destroy – a photon to be sure that teleportation has taken place.

By using a filter to reduce the intensity of the photons that are going to be teleported the researchers were able to significantly reduce the number of spurious detection events. The Vienna team could be 97% certain that the state had been teleported to photon 3 without actually having to detect it. Such a high accuracy means that the teleported photons could be used in “quantum repeaters” for long distance communication. The team now hopes to combine these results with a technique known as “entanglement purification” to further develop quantum communication over long distances.

Astronomers detect “missing” baryons

Observations suggest that baryons account for only 4% of the total mass-energy density of the universe, but stars, galaxies, intergalactic clouds and hot X-ray emitting plasma make up for only a third of the expected baryon density. Simulations of galaxy formation predict that a large number of these baryons may have escaped detection because they are very hot and have a low density.

One way of detecting this baryon matter directly is to look for characteristic ultraviolet absorption lines belonging to heavy elements in the spectra of sources such as quasars.

Nicastro and colleagues analyzed the absorption spectra of oxygen ions using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite and the Chandra and XMM-Newton X-ray observatories. The researchers measured the radial velocities of the oxygen clouds as determined by the Doppler shifts of their absorption lines. In this way they were able to locate the position and origin of these clouds in the galaxy.

The team found that the oxygen absorbers had high radial velocities, greater than 100 kilometres per second. This value is much higher than would be expected for clouds restricted to our galaxy alone. The researchers believe that the oxygen absorbers are spread throughout the entire “Local Group” of galaxies, which includes the Milky Way, Andromeda and about 30 other smaller galaxies and is an area about 5 million light years across.

The extended distribution of this hot gas agrees well with the very low gas densities deduced from x-ray absorption spectra of oxygen ions. The total mass of this ionized gas compares favourably with the mass of the Local Group of galaxies. The researchers estimate that the total mass of baryons in this region is about 10¹² solar masses, which is of the order of the mass needed to sustain a stable Local Group. This mass could account for up to 100% of the missing baryons in our Local Group.

Ancient radiation sheds new light on the universe

The photons in the cosmic microwave background were created in the Big Bang and then continually scattered by free electrons in the early universe. They were released when the universe had expanded and cooled enough to allow these electrons to combine with protons and form hydrogen atoms. The properties of the photons – which have been stretched to microwave wavelengths by the expansion of the universe — can therefore be used to study the universe at this “time of last scattering”.

A number of experiments have been carried out on the ground and in balloons to study the tiny temperature fluctuations in the microwave background caused by variations in the density of the early universe. WMAP, however, has been able to study these fluctuations in far more detail. And unlike previous experiments, it can record images of the microwave background across the whole sky.

“We’ve captured the infant universe in sharp focus, and from this portrait we can now describe the universe with unprecedented accuracy,” says WMAP Principal Investigator Charles L. Bennett of the Goddard Space Flight Center. “The data are solid, a real gold mine.”

The data indicate that the time of last scattering occurred about 380,000 years after the Big Bang and that the Universe is now 13.7 billion years old, give or take 1%. They also reveal that the earliest stars in the universe were created just 200 million years after the Big Bang.

In addition, the results support the idea that the universe underwent a period of rapid expansion when it was very young and that its geometry is flat. The data show that 4% of the universe is ordinary matter, 23% dark matter and 73% dark energy. The NASA researchers say that this dark energy is more likely to resemble Einstein’s “cosmological constant” than a negative-pressure energy field called “quintessence”, but they do not rule out quintessence.

WMAP is named in honour of David Wilkinson of Princeton University, a project team member who died in September last year. The satellite was launched in June 2001 and now orbits the Lagrange Point, a million miles from Earth. It will continue to observe the cosmic microwave background for another three years.

Dyes boost solar cells

In conventional solid-state solar cells, charge carriers – electrons and holes – are created by light absorption in a semiconductor. Subsequent transport and separation of the charge carriers, together with collection of the current, is carried out within the semiconductor itself. To accomplish all this at once, the semiconductor needs to be free of impurities, which increases manufacturing costs.

To overcome this problem, McFarland and Jing have developed a multi-layer device that separates the light-absorption and charge-carrier transport processes. Photons are collected using “photoreceptor” dye molecules placed on the surface of a thin gold film, which rests on a layer of semiconducting titanium dioxide. The photoexcited electrons from the dye molecules are first transferred to the gold layer and then to the conduction band in the titanium dioxide layer, thus producing a current.

The main advantage of the method is that an unusually large number of the photons absorbed by the dye layer – about 10% – generate electric current. In addition, the device is based on electrons only, which makes it less sensitive to impurities and imperfections.

The team now hopes to increase the internal quantum efficiency of their device. “This requires investigations of improved absorbers such as other dyes and quantum dots, improved coupling of the absorber to the metal film, and decreasing film thickness,” McFarland told PhysicsWeb.

Novel photomasks make 3-D microstructures

In conventional photolithography a flat, patterned “photomask” is placed on top of a silicon wafer that has been coated with a light sensitive layer known as a “resist”. Ultraviolet light is shone through the mask and exposes parts of the underlying material. Chemical etching then reveals the pattern created by the photomask. Light exposure is “all-or-none” and the process results in resist features of uniform height. The fabrication of three-dimensional structures thus requires several exposure steps, which is time-consuming and costly.

To overcome this “all-or-none” limitation, Folch and co-workers made a grey-scale photomask that allows differing amounts of light to pass through it. The photomask is made from a polymer that is transparent to ultraviolet light and contains light absorbing dyes. The concentrations of dye can be varied to allow more or less light through the system, which allows the researchers to accurately “sculpt” a three-dimensional resist surface. A variety of complex shapes with varying heights – such as “wedges” and “staircases” – can be formed in a few seconds.

“The advantage of the technique is that it allows for an arbitrary number of grey-scales,” Folch told PhysicsWeb. Previous approaches could only manage a few grey-scale levels.

Although the method has not been optimized, the team believes that the photomasks could be used in a wide variety of applications in addition to the fabrication of computer chips. These include microoptic elements, tissue-engineering scaffolds and other biotechnological applications. “We are interested in combining these devices with live cells,” Folch added. “The photomasks are particularly suited for cellular applications because the polymer from which they are made is biocompatible.”

Go, go, go – seminal moments in science

Which chemist, who inspired the fashion for breathing in the newly discovered nitrous oxide (laughing gas), almost killed himself while testing the effects of carbon monoxide? The answer can be found in Science Firsts, which describes some of the most important discoveries in science. Written by the freelance science journalist Robert Adler, the book picks out 35 landmark discoveries from Thales’ model of the universe in the 6th century BC to the birth of Dolly the cloned sheep in 1997.

Physics features prominently, with many of the usual suspects – Fermi, Hubble, Einstein, Rutherford, Marconi, Planck, Curie, Newton, Galileo and Copernicus getting separate chapters for their key findings. While it must have been hard for the author to limit himself to 35 seminal moments, Faraday’s discovery of electromagnetic induction is conspicuous by its absence. Faraday, after all, was the protegé of the gas-guzzling Sir Humphrey Davy and is said by some to have been Davy’s “greatest discovery”.

Another new book that covers similar ground is The Eureka! Moment by librarian Rupert Lee. It contains more scientific discoveries that Science Firsts – 100 in total – but limits itself to those that took place in the 20th century. So while the discovery of the neutron, the positron, the neutrino, the transistor and quarks (but not the laser) are featured, there is little on the work of Newton or the others on whose shoulders he stood. The book also surveys the state of science in the year 2000 and predicts what lies ahead only the discovery of the Higgs particle and grand unification, as far as physics is seemingly concerned.

So which is the better book? Science Firsts is stylishly written, with each chapter containing a strong storyline that draws out the personalities of the scientists involved. And with 35 chapters spread over 200 pages, each topic is given the space to breathe. There is also an excellent reference list containing a wealth of other popular-science titles. The Eureka! Moment, in contrast, is more formal in tone and crams 100 essays into just 250 pages, which barely gives each subject the justice it deserves. The book also lacks a bibliography, with the only references being to the journals in which the work in question was originally published. Science Firsts looks the better buy.

Buy the book
Science Firsts: From the Creation of Science to the Science of Creation Wiley
The Eureka! Moment: 100 Key Scientific Discoveries of the 20th Century amazon.co.uk/amazon.com

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