Popular Science Feature - The God Effect, Updated

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Feature - The God Effect, Updated by Brian Clegg

In my book The God Effect (published in 2006 by St Martin's Press) I describe quantum entanglement, the remarkable physical phenomenon that seems to allow to particles to be separated to opposite sides of the universe, yet a change to one is instantly reflected in the other. The strange capabilities of entanglement make it ideal for generating unbreakable encryption, making quantum computers work, even powering quantum teleportation. As it is a field where new breakthroughs are happening all the time, this feature will be regularly updated with the latest news from the entanglement front.

November 15, 2007 - A new application for entanglement
Many very sensitive measuring devices (for instance used in imaging, remote sensing, astronomy and spectroscopy) are based on the interference between quantum particles, but have problems with disruption of the incredibly sensitive instruments by the particles themselves. Now a group of Australian researchers led by B. L. Higgins have made it possible to take measurements to the absolute limit thanks to using an entanglement-based approach originally devised for quantum computers.

November 8, 2007 - Keeping the tangle
One of the problems with constructing a quantum computer is that the entangled state can be much more delicate in this situation than the long-range entanglement trials used for testing quantum encryption. Sabrina Maniscalco of the University of Turku in Finland and her team have found that by scanning the space around an entangled pair of particles, they can delay the collapse of the entanglement with its accompanying release of a photon. This relies on the "quantum Zeno effect" which shows that taking measurements on a quantum particle will slow down its decay. It's not possible to take measurements of the qubit, as this would alter its state, but by taking measurements in the space around it, the decay is delayed.

April 19, 2007 - Real or local?
All along, since Einstein's original EPR paper, the problem with entanglement is that either quantum mechanics is wrong, or the idea of local reality - that particles either have all the information they need to establish, for instance, their polarization or momentum, or they can be linked at a distance. When entanglement was proved to exist, it meant there was something wrong with the idea of local reality. Now one small further step has been taken. In experiments by Anton Zeilinger's team, written up in the journal Nature, it has been shown that some types of non-locality aren't viable. We aren't all the way to understanding the lack of real locality, but this experiment whittles down the possibilities as to what can be going on.

March, 2007 - Entangled Afar
Relentless entanglement researcher Anton Zeilinger has recently broken the record for sending entangled photons from one place to another. The ultraviolet photons travelled from one of the Canary Islands (La Palma) to another (Tenerife), where they were picked up by a telescope after travelling 144 kilometers. The Canaries is the location of the telescope Zeilinger hopes to use to communicate entangled photons with a satellite.

September 28, 2006 - Can entanglement rupture causality?
As explained in The God Effect, the instant connection at any distance of entanglement can't be used to send a message, as it is purely random, but researcher John Cramer has proposed a way of using entanglement that could reverse the timing of cause and effect. It relies on the way that light can be measured as a particle or as a wave when passing through two slits - but not both. Streams of entangled photons are sent off in two directions, One goes immediately through a pair of slits, and an instrument registers whether the outcome was a measurement as waves or particles. The second stream goes through a delay line, arriving at slits a little later. In this case a moveable detector forces the measurement to be waves or particles. Because of the spooky link of entanglement the theory is that the light passing through the first detector will be forced to be in the appropriate form, even though the measurement was taken before the action that decided what the outcome should be. Bear in mind that this experiment hasn't been carried out yet, and some doubt it's practicality - but it is being taken seriously by the science community.

May 26, 2006 - Entanglement as gravity wave detector
Ye Yeo and colleagues at the National University of Singapore have suggested that naturally entangled particles could be used to detective gravity waves, the difficult to detect gravitational aftershock of extreme cosmic events like supernovas. While other scientists have argued that the chances of detecting a disruption of entanglement caused by gravity waves are very slight, they have been impressed by the linking of two such dramatic concepts.

March 6, 2006 - almost cloning
As readers of The God Effect will know, it's impossible to clone a quantum particle - you can't make an exact copy with all the quantum states identical, as any attempt to measure things changes the original particle. However, teams at the University of Tokyo in Japan and the University of York in the UK have come up with a technique called telecloning that allows a particle's quantum state to be almost duplicated in a number of different locations. In the experiment, three laser beams were entangled. A measurement was then made on one beam, which enabled the experimenters to modify the other two beams so they became near-clones of the state of the original beam.

December 8, 2005 - Quiet progress
Just to emphasize how much entanglement remains in the science news even when there's no great breakthrough to report, the December 8, 2005 edition of the prestigious journal contained no less than four entanglement-related pieces. As well as an article by leading entanglement researcher Anton Zeilinger, there was a news item and two separate papers on advances in distributing entanglement across a structure, and storing single photons in atomic memories to later release them. These small steps on the way to quantum computing and encryption may not always be reported, but are laying the foundation for use of entanglement in real world applications.

December 2, 2005 - Seriously entangled
Most entanglement experiments work on a particular property - spin for example - but Paul Kwiat and his team at the University of Illinois have gone further, managing to produce pairs of photons that are entangled every way imaginable. Using special non linear crystals, the teams managed to get the photons entangled in all 36 of their different quantum states. Such "hyper-entanglement" would be required for a true matter transmitter to duplicate all the states of the original particles.

November 30, 2005 - Secure, but only when I want it to be
A potential problem with quantum encryption is that it is just too secure. But a team working at the Hefei National Laboratory for Physical Sciences in China have found a way to provide quantum encryption that is secure to the outside world, but needs a third party to help. The scheme demonstrated by Yu-Ao Chen and his colleagues imagines that a boss wants to keep control over whether or not his employees can exchange secure information (they don't explain why the boss would want this - any suggestions?) By using a complex four photon entanglement, the team have set up a mechanism where the two colleagues can only establish a secret key for decoding encrypted data if the boss cooperates, even though the boss can't access the data.

June 23, 2005 - Quantum computers and the goldfish effect
In The God Effect, I point out that most of the technologies proposed to use as qubits, the fundamental units of quantum computers, can't keep in a usable state for very long. Now it looks like this could be a universal limit. Jasper van Wezel, Jeroen van den Brink and Jan Zaanen of Leiden University (Netherlands) claim to have proved that there is a universal rate at which quantum information decays. The rate gets shorter as the qubit gets smaller - for some of the current technologies, the maximum time it could "remember" its state is around a second. This doesn't mean disaster for quantum computing, however, merely an even stronger requirement for the use of entanglement to teleport the state from qubit to qubit like a hot potato jiggled from hand to hand.

June 17, 2005 - Is entanglement really instantaneous?
In principle, entanglement's spooky connection operates instantaneously. This is difficult to demonstrate definitively, but in experiments using fibre optic links between two villages 10 km apart, Nicolas Gisin and his team have shown that the effect takes place at a minimum of between 10,000 and 10,000,000 times the speed of light (depending on what the speed is measured relative to). Gisin's most recent experiments beneath the waters of lake Geneva have never failed to support entanglement's action. Gisin commented "Entanglement is a new explanation for how things happen in the universe. It's on the same level as cause and effect: there's cause, effect and entanglement."

Despite all this experimental verification, some scientists still believe that there could be loopholes which would deny the reality of entanglement's instantaneous connection - but this doesn't stop entanglement's applications growing by the year. Even if there are still dissenters on exactly what is happening - and some commentators have suggested it may never be possible to confirm definitively - the fact remains that entanglement does makes the seemingly impossible happen on a daily basis.

Brian Clegg's book The God Effect will be available in 2006.

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