1. Pure and applied quantum physics
2. Putting general relativity to the test
3. Good year for planets
4. Supersolid helium
5. Ultracold Fermi gases
6. Physicists target viruses
7. Electrons in a spin
8. Liquids go against the flow
9. Smallest atomic clock
10. Particles and prizes

1. Pure and applied quantum physics

2004 was the year in which quantum cryptography was used in a commercial transaction for the first time. On 21 April the Mayor of Vienna transferred money from the City Hall to Bank Austria Creditanstalt over a fibre-optic cable using a quantum "key" made of single photons to ensure that the transfer was completely secure. At present classical keys -- essentially random numbers that must be kept secret by the two parties involved -- are used to keep such transactions secure.

The quantum key was generated at the bank by using a crystal to convert photons from a laser into "entangled" pairs of photons: one photon in every pair remained at the bank while the other was sent to the City Hall. By measuring the polarizations of the photons, the two parties were able to generate identical strings of ones and zeros that could be used as the key.

In addition to relying on quantum mechanics rather than computer algorithms to generate the keys in the first place, quantum cryptography has the added advantage that eavesdroppers can be easily detected. In December several of the groups involved in the Vienna experiment shared the European Union's Descartes prize for their work on quantum cryptography.

Elsewhere quantum physicists notched up a series of experimental breakthroughs in fundamental research with entangled states containing up to five photons being produced, atoms being teleported for the first time, and the boundary between the quantum and classical worlds being explored with carbon-70 molecules.

Cryptography system goes underground
Quantum cryptography wins Descartes prize
Entanglement beats the diffraction limit
Entanglement breaks new record
Teleportation breaks new ground
Looking at decoherence

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2. Putting general relativity to the test

In January, radio astronomers in Australia and in the UK reported that they had discovered the first double pulsar system. Pulsars are extremely dense, rapidly rotating neutron stars that are a million times more massive than the Earth, but measure just tens of kilometres across. The discovery will allow physicists to perform the most stringent tests of Einstein's general theory of relativity to date.

April saw the launch of Gravity Probe B, which has been designed to test two predictions of general relativity: the Lense-Thirring effect and the geodetic effect. The former is caused by massive bodies such as stars and planets "dragging" space-time with them as they rotate, while the latter is due to the distortion of space-time caused by the body. The first results from the mission are expected in 2006.

In October, however, physicists in Italy and the US reported that they had used existing satellite data to measure the Lense-Thirring effect with an accuracy of 10% and that their results agreed with the predictions of general relativity.

First double pulsar comes into view
Gravity Probe B takes off
Relativity passes latest test

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3. Good year for planets

2004 was a busy year on Mars with two rovers, Opportunity and Spirit, landing on the red planet and Mars Express going into orbit around our near neighbour. All three missions found evidence for ancient water on Mars -- a feat that was selected as the breakthrough of the year by Science magazine.

Images from Opportunity revealed numerous layered outcrops, similar to sedimentary rocks on Earth, suggesting that liquid water had once flowed through the rocks, while Spirit found that water may have altered volcanic rocks at Gusev crater. Meanwhile the OMEGA instrument on board Mars Express obtained the first direct evidence for frozen water on the surface of Mars by analysing the spectrum of sunlight reflected from the planet.

Elsewhere astronomers discovered Sedna, the most distant object ever seen observed in the solar system -- a feat that was selected as the top story of the year by Astronomy magazine. And still further away, three separate teams discovered three planets that are between 10 and 20 times as big as the Earth. These "super-Earths" are the smallest of the 140 or so extrasolar planets detected so far.

Mars Express finds water - official
Rovers display spirit of discovery
Tales from the transneptunian sea
A small step for extrasolar planets

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4. Supersolid helium

Physicists at Pennsylvania State University in the US created a new "supersolid" phase of matter by cooling helium-4 to ultracold temperatures. Supersolid helium-4 behaves like a superfluid -- a liquid that flows without resistance -- but has all the characteristics of a crystalline solid. Liquid helium-4 displays superfluidity when it is cooled to below about 2 Kelvin. The Penn State experiments, which were carried out at a range of pressures between 26 and 66 bars, show that solid helium-4 becomes a superfluid at temperatures below 230 millikelvin. Superfluid behaviour has now been observed in all three phases of matter - gas, liquid and solid.

Supersolid is seen in the lab
Evidence for supersolid is firmed up

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5. Ultracold Fermi gases

In July physicists at Innsbruck in Austria reported the strongest evidence to date for superfluidity in an ultracold gas of fermionic atoms when they observed the "pairing gap" in an ultracold Fermi gas for the first time. The observation of a similar gap in low-temperature superconductors in the late 1950s was a major milestone in the quest to understand these materials.

The new results, which agree with theoretical calculations by a second group in Finland, could help us understand more about high-temperature superconductivity and systems as diverse and exotic as neutron stars, atomic nuclei and quark-gluon plasmas. The gap was observed in experiments on "fermionic condensates", a new state of matter that was reported for the first time by a group in the US in January.

Fermi gas goes superfluid
Fermionic condensate makes its debut
Fermi gas approaches superfluid regime

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6. Physicists target viruses

Most physicists are used to detecting inanimate objects such as photons and electrons, but this year saw increased interest in the detection of living objects such as viruses. Different teams in the US modified a range of existing devices -- including field-effect transistors, atomic force microscopes and both nanoelectromechanical systems and magnetic sensors -- so that they could detect single viruses.

Meanwhile a team in France demonstrated the first purely electronic technique for detecting DNA by measuring the intrinsic charge of the molecule with an array of silicon transistors.

Nanodevices target viruses
Magnetic sensors tackle viruses
"ViriChip" shows up viruses
DNA detection made easy

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7. Electrons in a spin

The spin Hall effect, which causes "spin-up" and "spin-down" electrons to build up on opposite sides of a sample in the presence of an electric field, was observed for the first time this year. The ability to manoeuvre electron spins with an electric field rather than a magnetic field could prove useful for making "spintronic" devices that manipulate spin rather than charge.

Meanwhile physicists imaged the spin of an individual electron for the first time by combining magnetic resonance imaging with atomic force microscopy. This breakthrough was selected as the top physics story of the year by Physics News Update.

Finally, researchers in Switzerland and Sweden used a scanning tunnelling microscope to add and remove single electrons from individual gold atoms on a surface. The technique could ultimately lead to memory devices that store each bit on just one atom.

Hall effect takes a spin
Single spins come into view
STM turns atoms into ions

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8. Liquids go against the flow

Solids usually melt when they are heated but in September French physicists reported that a simple solution composed of two organic compounds becomes a solid when it is heated and a liquid again when cooled. The team says that hydrogen bonds are responsible for this novel behaviour.

Meanwhile physicists in Japan confirmed that liquids can exist in more than two different states at the same time -- contrary to what is commonly believed. The results suggest that liquid-liquid transitions can occur in any liquid.

Finally, another Japanese group found that strong magnetic fields can slightly increase the melting point of water. This result was unexpected because water is diamagnetic and therefore should not be affected by a magnetic field. Again, hydrogen bonds are thought to be responsible.

Law-breaking liquid defies the rules
Liquids double up
Magnetic effects seen in water

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9. Smallest atomic clock

For more than 50 years, atomic clocks have set the gold standard for time and frequency measurement but their applications have been limited by their complexity, size and expense. Now physicists in the US have built a "physics package" -- the components that lie at the heart of an atomic clock -- that is 100 times smaller than those in existing atomic clocks and several orders of magnitude more stable than conventional devices such as quartz crystal oscillators. The new clock opens the way for atomic-level timekeeping in portable, battery-operated systems such as global positioning receivers and wireless communications.

NIST unveils smallest atomic clock

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10. Particles and prizes

In 2004 particle physicists continued to report results that agreed with the Standard Model, such as the first observation of parity violation in collisions between electrons at Stanford, as well as results that required new physics beyond the model. The latter included the most precise measurement to date of the anomalous magnetic moment of the muon at Brookhaven and evidence for antineutrino oscillations from the KamLAND experiment in Japan.

However, the particle physics community was split over the existence of exotic particles that contain four quarks and an antiquark. First reported in 2003, roughly half of the experiments that have looked for "pentaquarks" to date have seen them, but the rest have not.

Elsewhere the particle physics community decided to design the international linear collider using cold or superconducting technology, although a decision on whether to go ahead and build the collider is not expected before 2010. Meanwhile three particle theorists -- David Gross, Hugh Politzer and Frank Wilczek -- shared the Nobel Prize in 2004 "for the discovery of asymptotic freedom in the theory of the strong interaction".

SLAC sees parity violation in electrons
Muons continue to defy Standard Model
Neutrino oscillations are here to stay
Charmed pentaquark appears at DESY
German lab wins linear collider contest
Strong-force theorists scoop Nobel Prize

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And finally: The physics of everything

As always, physicists did not restrict themselves to physics during 2004. Spiders, archaeology, Brazilian football and the EU constitution were just some of the topics to attract the attention of the physics community during the year (see below for a full list), while in August a bookmaker in the UK turned the tables and offered odds on various breakthroughs happening by 2010.

Reversal of fortune for Turin Shroud
Spiders get a grip
Physics and fame
Physics meets archaeometry in ancient Greece
The turbulent life of dolphins
Physicists tackle EU constitution
Physicists stop forgers in their tracks
Taking a chance on physics
The small world of Brazilian soccer
Fingerprint model makes an impression
Physicists tackle linguistics

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