CCD camera A camera based on a charge coupled device, consisting of an array of pixels, each of which builds up an electrical charge as photons hit it.
collimator lens A lens that collects together rays of light to make them aligned.
conduction band The range of energies of an electron in an atom in a material that enables the electron to move freely through the material.
Cooper pairs A pair of fermions (usually electrons) that act as a single particle, bound together by interaction with vibrations in the material that they are passing through. They are responsible for low-temperature superconductivity.
diode laser A laser producing light by stimulated emission from a semiconductor, used in telecommunications, CD and DVD players, laser pointers and printers.
doping Adding an impurity to a semiconductor to change its electrical properties. This makes it much easier for an electron to reach the conduction band or to be accepted into the valence band.
integrated circuit A ‘chip’ – a thin sheet of semiconductor, usually silicon, with an electronic circuit printed on it.
Josephson junctions A pair of superconductors separated by a thin layer. If a voltage is applied across the junction it causes a high frequency oscillation, providing an extremely accurate measure of the voltage. See here.
metamaterial A material specially constructed with unusual electromagnetic properties. Many metamaterials have a negative refractive index, giving them the potential to make unusually powerful lenses or to conceal an object by bending light around it (a process known as ‘cloaking’).
Moore’s law An observation in 1965 by Intel founder Gordon Moore that the capacity of electronic devices roughly doubles annually. Modified to doubling each 18 months or two years, this has proved remarkably accurate, although this now appears to be slowing.
nanoparticles Small pieces of a material around 1–100 nanometres across. Objects at this scale have physical characteristics that are very different from those of larger particles.
Pauli Exclusion Principle The observation that two of the same kind of fermion (electrons, for instance) can’t be in the same quantum state at once. For instance, electrons in the same atom can’t have the same quantum numbers. See here.
photonic lattices A material forming a regular lattice that acts on light as semiconductors do on electrons. They are used to produce high-quality lenses and found in Nature causing the swirls of an opal and the iridescence of a peacock’s tail.
photonics Methods of controlling, switching and amplifying light. They are the optical equivalent of electronics.
refractive index A measure of the degree to which a substance bends light as it passes between that substance and another material. It is linked to the velocity of light in the material.
resonance The tendency of a system to vibrate more strongly at particular frequencies. It can be applied to an object like a bell, or to a cavity, like an organ pipe or laser cavity.
semiconductor laser see diode laser
SQUIDs Superconducting Quantum Interference Devices that use Josephson junctions to detect small changes in voltage produced by a shifting magnetic field. They are used to make sensitive magnetometers employed in applications from MRI scanners to unexploded bomb detectors.
stimulated emission The mechanism behind a laser. An atom is pushed into an excited state by a flash of light or electrical current. When hit by an incoming photon, it gives off a second photon with identical frequency. This contrasts with spontaneous emission, which does not involve an incoming photon.
superconductivity The ability of extremely cold materials to conduct electricity without resistance and to expel an electromagnetic field. See here.
valence band The band of electrons in an atom still bound to the atom and which is responsible for many of the atom’s chemical properties.
We use lasers on a daily basis: they scan our barcodes at the supermarket checkout and they are a key component of CD and DVD players. It may be surprising to learn that these everyday machines are quantum since they depend on the unique energy levels of atoms at their heart. An atom’s electrons can be ‘excited’ to different, precise values by absorbing energy from heat or light. But the atom cannot stay excited all the time, so eventually it releases this energy as a photon of light with a precise frequency and returns to its ‘ground’ energy state. What happens if an excited atom that is already excited encounters a photon? Instead of absorbing it and releasing it at a random time in a random direction as before, a kind of resonance effect stimulates it to emit a second photon. This photon has exactly the same frequency direction and is in coherence – perfect step – with the incident photon. In a laser a collection of atoms are brought into an excited state by pumping them with a high voltage, so the atoms in an excited state outnumber those in the ground state. Reflecting the emitted photons between mirrors in a cavity stimulates further emissions, generating a powerful laser beam.
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A laser is a light source that works via stimulated or cooperative emission from many atoms to produce a highly organized beam, with all the wavefronts matching up, monochromatic and strongly directional.
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The laser wielded by Austin Powers’ character Dr Evil probably uses carbon dioxide gas as a lasing material, because it emits in the infrared so would fry anything it focuses on. The vast majority of existing lasers are the far less powerful semiconductor or diode lasers that are used for electronic devices and in communications. They typically emit red light and can be built into larger arrays.
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1920–2005
American physicist who coined the acronym LASER – standing for Light Amplification by Stimulated Emission of Radiation
1927–2007
American physicist who may have invented the first successful optical laser (some credit Gould)
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Sophie Hebden
Lasers use reflecting cavities repeatedly to stimulate atoms in order to produce coherent photons.
The peculiarities of quantum theory, such as uncertainty, superpositions and superconductivity, are usually found only in specialized, low-temperature conditions. However, the consequences of the discrete quantization of energy states are seen all the time in, say, the bonds between atoms and the colours of objects. One of the most significant technological applications of this quantization occurs in the transistor, the electronic device made from a semiconductor that is at the heart of all digital computing and IT. A semiconducting material contains electrons in a ‘band’ of quantum energy states, rather like a reservoir entirely filled with water, separated by an energy gap from another band empty of electrons. If electrons can gain enough energy to reach the empty band, they can move around and carry an electrical current. Only a few electrons can pick up enough energy from ambient heat to do this under a transistor’s normal operating conditions, which means that the flow of current can be finely controlled by doping – adding electrons to or removing them from the reservoir by dispersing other kinds of atom into the material – and by applying electric fields. In this way the current passing through a transistor can be controlled and directed electrically, so that it can act as a switch or an amplifier in digital electronics.
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Transistors, which make digital electronics and computers possible, exploit the quantization of electron energy states in semiconducting materials.
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Early commercial transistors, made from germanium, cost several dollars each in 1960 and measured about 12 mm across. Miniaturization of transistors made from silicon has now reached the point at which about 2 billion can be housed on a single silicon microprocessor chip, at a cost of about 0.0001 cents apiece. This decline in cost is one aspect of Moore’s law, more usually expressed in terms of the number of transistors on an integrated-circuit chip – which doubles about every 18 months.
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WALTER BRATTAIN, JOHN BARDEEN, WILLIAM SHOCKLEY
1902–87, 1908–91 & 1910–89
American physicists and members of the team that invented the transistor at Bell Telephone Laboratories in 1947, for which they shared the 1956 Nobel Prize in Physics
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Philip Ball
The use of transistors has transformed electronics, from individual components to integrated circuits.
Electron microscopes function in a similar way to optical microscopes. In optical microscopes a collimator lens focuses light on a glass slide containing, for example, bacteria. The light is then collected by an objective lens that enlarges the image and focuses it on an eyepiece or CCD camera. In principle, an electron microscope functions in a similar way. But instead of glass lenses, magnets deflect the electrons. A hot cathode produces the electrons, which are then accelerated by an electric field, just like in a cathode ray tube. A magnetic collimator focuses the electrons on the sample, and the electrons that pass through the sample are focused by another set of magnetic lenses on a fluorescent screen, where the electrons form an image that we can see. The resolution of an optical microscope is limited by the wavelength of light, and its magnification is limited to 2,000; anything smaller than a wavelength of light, such as viruses, remains invisible. However, electrons behave as particles, but also as waves. Their wavelength is much smaller than that of light, and therefore electron microscopes can magnify up to 10 million times and ‘see’ much smaller objects, such as viruses – even atoms.
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Because electrons have a much shorter wavelength than photons, their amplification is much higher than that of their optical counterparts.
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The electron microscope demonstrates the dual nature of electrons. When electrons pass through magnetic lenses, their path becomes bent and they behave like particles. Passed through a sample, electrons are diffracted and bend around obstacles, behaving as a wave: they bend to a far lesser extent, resulting in much sharper images. This explains the electron microscope’s significance to science: biologists, for instance, can view cell components that were invisible with optical microscopes; nanotechnology would be impossible without it.
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MAX KNOLL & ERNST RUSKA
1897–1969 & 1906–88
German electrical engineer and physicist who pioneered the apparatus that help construct the electron microscope in 1931
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Alexander Hellemans
The magnetic field of an electron microscope focuses electrons as a lens does visible light.
In the 1970s, researchers including American physician Raymond Damadian, American chemist Paul Lauterbur and British physicist Peter Mansfield developed Magnetic Resonance Imaging (MRI). It provides a non-surgical way of seeing soft tissues inside our bodies that can help diagnose diseases and injuries ranging from cancer to torn ligaments. Inside the scanner, a patient is surrounded by a magnetic field generally 30,000–60,000 times greater than the Earth’s magnetic field. This is produced by the scanner’s powerful magnet – usually a superconducting electromagnet. Our bodies are 65% water, and every hydrogen atom in this water contains a proton that spins like a top, making each proton behave like a small magnet. The scanner’s large magnetic field causes the protons to spin in a particular way. Radio waves directed into the patient’s body then alter how the protons spin, and so change their magnetization. Switching these radio waves off again lets the protons ‘relax’ back to how they were spinning previously, and the signals they emit as they relax are electronically recorded. The time it takes a proton to relax depends on the type of tissue surrounding it. So computer software turns this information and the other detected signals into images revealing the different tissues.
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The detailed images of our internal organs and tissues revealed by MRI scanners have revolutionized the treatment of many diseases and injuries.
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A variation on MRI known as functional MRI, or fMRI, shows which areas of the brain are activated when we move a part of our body or respond to a picture. fMRI is sometimes used to help plan for brain surgery, so surgeons can avoid damaging essential areas when removing cancerous tumours or diseased regions. fMRI is also revealing more about Alzheimer’s disease, and Multiple Sclerosis, and can show how medicinal drugs affect the brain.
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HEIKE KAMERLINGH ONNES
1853–1926
Dutch discoverer of superconductivity in 1911
PAUL LAUTERBUR & PETER MANSFIELD
1929–2007 & 1933–
American and British scientists who shared the 2003 Nobel Prize in Physiology or Medicine for their work leading to the development of MRI
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Sharon Ann Holgate
MRI scanners build a series of cross-sectional images from the electromagnetic radiation given off when radio waves and strong magnetic fields change the spins of protons.
In 1962 Brian Josephson predicted that Cooper pairs – pairs of linked electrons that travel with no resistance through superconductors – should also tunnel through a non-superconducting or an insulating barrier between two superconductors. Electrons can jump over a tiny gap or insulating layer between two conductors, an effect known as quantum tunnelling. However, unlike electrons tunnelling from one conductor to another conductor at room temperature, Cooper pairs do not require an electric field to coax them through the barrier. All the Cooper pairs in a superconductor share the same wave function, and the difference in the phase of the wave function on each side of the insulating barrier causes the Cooper pairs to tunnel spontaneously through the barrier. However, if a voltage is applied over the junction, the Josephson current is replaced by an oscillating current of a very high frequency. The frequency depends only on the applied voltage. Since frequencies can be measured with higher precision than voltages, Josephson junctions are used as very precise voltmeters. If a Josephson junction is part of a closed loop, the voltage over the junction changes even with extremely weak magnetic fields. Called superconducting quantum interference devices (SQUIDs), they are able to measure the magnetic fields created by the human brain.
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The unexpected appearance of a superconducting current when two superconductors are separated by a tiny gap is promising a host of applications.
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Because Josephson junctions can function
as very fast logic gates, researchers are investigating their application in ultrafast computers. An interesting quantum property of Josephson junctions is that in a superconducting loop they can cause the superposition of two Josephson currents in opposite directions simultaneously. Researchers are currently investigating how tiny superconducting loops can be connected together to store quantum data or to form a quantum computer.
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LEO ESAKI & IVAR GIAEVER
1925– & 1929–
Japanese and Norwegian physicists who, along with Brian Josephson, shared the Nobel Prize in Physics in 1973 for their work in electron tunnelling
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Alexander Hellemans
A superconducting quantum interference device (SQUID) can detect small variations in magnetic fields, including those produced by the brain.
Like many great physicists, Brian Josephson showed an intuitive grasp of the subject from an early age. As an undergraduate at Cambridge, he constantly impressed his teachers with the depth of his understanding, and by the time he obtained his first degree – at the unusually young age of 20 – he had already published his first research papers. Choosing to remain at Cambridge, he pursued a PhD in the area of physics that intrigued him the most: a phenomenon called superconductivity that occurs at extremely low temperatures. He was still working on his PhD when he wrote the paper for which he is most famous, ‘Possible New Effects in Superconductive Tunnelling’. It was known that quantum theory allows particles to ‘tunnel’ through otherwise impenetrable barriers, but Josephson showed how this could produce a hitherto unknown effect – since dubbed the Josephson effect – in the context of superconductors. This was one of the first examples of a quantum effect that can be exploited at macroscopic scales, and it led to the now well-established technology of the Josephson junction.
Josephson was still only 33 when he shared the 1973 Nobel Prize in Physics with two other researchers who had worked on quantum tunnelling. Although he was the youngest of the three, he received half the prize while the other two received a quarter each. It was only in the following year, 1974, that Josephson was finally made a full professor at Cambridge – a post he held until his retirement in 2007.
By the late 1970s Josephson was becoming disillusioned with mainstream physics, feeling that it ignored large areas of experience which, given the chance, might be illuminated by quantum theory, or a yet-to-be-discovered extension of it. He developed an interest in Eastern philosophy, meditation and higher states of consciousness, and in 1988 – still under the auspices of the physics department at Cambridge – he set up his long-running Mind-Matter Unification Project. This was concerned with such subjects as language, music and cognition – which, although respectable topics in other academic departments, were not normally the province of a theoretical physicist. In more recent years Josephson has written on distinctly non-academic subjects, including telepathy and homeopathy, which has inevitably brought him into conflict with his more conservative peers. Josephson himself remains open-minded about what he calls ‘heretical science’. On his website he adopts the motto of the Royal Society, nullius in verba, which he paraphrases as ‘take nobody’s word for it’.
Andrew May
4 January 1940
Born in Cardiff, Wales
1960
Gains Bachelor’s degree in Natural Sciences from the University of Cambridge
1962
His paper on what would become known as ‘the Josephson Effect’ is published in the journal Physics Letters
1964
Gains his PhD from Cambridge
1964
The SQUID (superconducting quantum interference device), a highly sensitive magnetometer using Josephson junctions, is invented
1965
Begins a short stint as a research assistant professor at the University of Illinois
1967
Returns to Cambridge as Assistant Director of Research
1972
Becomes Reader (senior lecturer) in Physics at Cambridge
Shares the Nobel Prize in Physics with Leo Esaki and Ivar Giaever
1974
Becomes Professor of Physics at Cambridge
1983
Addresses US Congressional Committee on the subject of ‘higher states of consciousness’
1988
Sets up the Mind–Matter Unification Project at Cambridge
2007
Retires from his Cambridge professorship, but continues in active research
Among the factors limiting the downsizing of structures on electronic chips are quantum effects that interfere with the functioning of the circuitry. For example, when conductors come too close, electrons tunnel from one conductor to the other. However, researchers are trying to turn these quantum effects to their advantage. Quantum dots are tiny nanoparticles made of semiconducting materials including silicon, cadmium selenide, cadmium sulphide and indium arsenide. They are designed to be so small that quantum effects become apparent. Typically the size of 10–50 atoms (2–10 nanometres), they start behaving like atoms themselves. The electrons in the conduction bands start populating discrete quantum levels imposed by the Pauli Exclusion Principle. Therefore quantum dots are sometimes called ‘artificial atoms’. Because nanodots consist of semiconducting materials, there is a gap between the conduction band and the highest band. Photons can excite electrons in the valence band and bump them into the conduction band. These electrons can then jump back to the valence band while emitting a photon. Now the energy difference between the conduction band and the valence band can be tuned by changing the size of the nanoparticles, where the energy difference is the highest for the smallest particles.
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Quantum dots are nanoparticles, often attached to a substrate or active surface, which, because of their small size acquire quantum properties that are of interest to technology.
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The ability of quantum dots to eject two electrons simultaneously when hit by a single photon, means they offer the possibility of enhancing the efficiency of solar cells. Because they can be tuned to emit any colour of the spectrum, researchers are investigating how to use them in displays and in light-emitting diodes.
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WOLFGANG PAULI
1900–58
Austrian theoretical physicist who introduced the exclusion principle that took his name
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Alexander Hellemans
Quantum dots could be used to produce visual displays with an unequalled level of fine detail.
All optical devices work at the quantum level, and the interaction between photons of light and the atoms of objects, mirrors and lenses is explained by QED (quantum electrodynamics). However, recently there are many more direct applications of quantum theory in optics, sometimes known as photonics. Among the most dramatic possibilities here are quantum lenses – materials that manipulate photons in ways different from traditional lenses. Take, for instance, metamaterials. These substances have complex structures – for example, layers of lattices or patterns of tiny holes in a metallic sheet – that produce strange effects like a negative refractive index, bending light the opposite way to conventional lenses or prisms. Their structures give metamaterials the ability to focus on far smaller objects than a conventional lens, producing so-called super-lenses. Another example of a quantum optical structure is a photonic lattice, which acts on light in a way similar to a semiconductor acting on electrons. The iridescence of some butterfly wings is produced by natural photonic lattices. Photonic lattices could be employed in future optical computers, and crystals of these lattices are already used in special paint systems, reflection-reducing coatings on lenses and in high transmission photonic fibre optics.
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All optics involve quantum phenomena, but special quantum optical materials like metamaterials and photonic lattices manipulate photons in a way that conventional optics cannot duplicate.
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Because of their negative refractive index metamaterials can bend light around an object, making it disappear. This has already been achieved on a small scale, but is limited, as the materials used absorb too much light to provide total invisibility. However, there are alternative mechanisms that either optically amplify the restricted output of the metamaterial, or use a photonic crystal to control the way the light is diffracted, so we may have Harry Potter-style cloaking in the not-too-distant future.
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1929–
Russian physicist and the first to consider negative refractive index and metamaterials
1943–
British theoretical physicist who worked on theories of metamaterials for perfect lenses and invisibility cloaks
1965–
German physicist working on practical invisibility cloaks
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Brian Clegg
