martedì 14 luglio 2009

Forgotten news: Einstein's spooky action acts at 10,000 times the speed of light.

(download PDF file about Salart's experiment)

Satellite view of Geneva region where the experiment was performed.
A spooky effect that could in theory connect particles at the opposite ends of the universe has been measured and found to exert its unsettling influence more than 10,000 times faster than the speed of light.
The effect, once described by Albert Einstein as 'spooky action at a distance' underpins quantum teleportation, a Star Trek like ability, and the next generation of encryption methods and superfast quantum computers too.
Now, by making measurements in two Swiss villages separated by 11 miles, Daniel Salart, a doctoral student working with the team of Prof Nocolas Gisin at the University of Geneva, has run detailed measurements and concluded that if this spooky action really exists, it must act faster than light.The new work lays down a lower speed limit of 10,000 times the speed of light. Quantum weirdness still rules OK.This study in the journal Nature suggests that a physical signalling mechanism that connects the villages is deeply implausible, because of the well known limit of the speed of light.Yet the effect is real, none the less, and rests on the peculiar properties of the subatomic world.
These are described by quantum mechanics which is routinely called strange, bizarre or counter-intuitive because the mathematics of the theory make predictions that seem to run counter to our own experiences, a feature famously summed up by the late physicist John Bell.
Yet experiment after experiment back them up. And the Swiss "Bell experiment" by Salart supports this wacky worldview once again.
The origins of this new experiment published in the journal Nature rest, in part, on Bell's ideas and an intellectual dispute between Albert Einstein, who hated quantum theory's unsettling take on reality, and Niels Bohr, the Danish father of atomic physics.
In 1935, Einstein outlined one such perplexing feature in a thought experiment with his colleagues Boris Podolsky and Nathan Rosen.
They first noted that quantum theory applied not only to single atoms but also to molecules made of many atoms. So, for example, a molecule containing two atoms could be described by a single mathematical expression called a wave function.
Einstein realised that if you separated these atoms, even by a vast distance, they would still be described by the same wave function. In the jargon, they were "entangled", as if their fate was connected in some way.
This may not sound so special: after all, anyone with a cell phone can achieve something similar, talking to someone on the other side of the planet with ease. The difference is that even if entangled particles are separated by billions of light-years, the fate of one instantly affects the fate of all its partners.
Einstein famously dismissed even the theoretical possibility of entanglement as "spooky action-at-a-distance".
But the reality of entanglement was first demonstrated by French scientists in 1982, notably by Alain Aspect, using light emitted by atoms driven by lasers to create pairs of entangled photons.
In the experiment, each pair was split up and the two photons sent off in opposite directions towards devices that measured their properties.
According to standard physics, the devices should show a certain degree of similarity in the properties of the two entangled photons. The precise amount should, however, be limited by the finite speed of light: roughly speaking, the photons should not have enough time to "compare notes" with each other.
The French team found, however, that the entangled photons were far more similar than expected on the basis of communication at the speed of light. In fact, the results showed that the photons were somehow communicating instantaneously - as if they were not really separated at all.
Then Dr Charles Bennett of IBM and others theorised that entanglement can make a "quantum phoneline" that could "teleport" the details (quantum state) of one particle to another over an arbitrary distance without knowing its state. This opened up the possibility that a transporter could transmit atomic data - even people and also opened up new opportunities for computing.
Tests have all but ruled out a classical (that is a non-quantum) explanation for these correlations between entangled photons, by waves and particles moving between them, but the lingering possibility remains that a first event could influence a second one, if the means of influence act faster than the speed of light.
To look for this, Mr Salart entangled their photon pairs using a source in Geneva, then passed them through fibre-optical cables of exactly equal length to the villages of Jussy and Satigny, which lie respectively east and west of Lake Geneva.
Here, the photons' entanglement was checked by an identical pair of instruments to reveal consistent entanglement of their photons, and the effects of the Earth's rotation taken into account, so they conclude that any signal passing between the entangled photons is, if not instantaneous, travelling at least ten thousand times faster than light.
So the effect is real but, if one wanted to explain it by a transmission mechanism with waves and particles, therein lies madness. Dr Terence Rudolph of Imperial College, London, remarks that "any theory that tries to explain quantum entanglement... will need to be very spooky - spookier, perhaps, than quantum mechanics itself".
Note that Einstein's ban on faster-than-light communication remains intact: while the photons compare notes instantaneously, the contents of those notes are beyond our control, and so can't be used to transmit any useful messages.
In an attempt to rule out any kind of communication between entangled particles, physicists from the University of Geneva have sent two entangled photons traveling to different towns located 18 km apart – the longest distance for this type of quantum measurement. The distance enabled the physicists to completely finish performing their quantum measurements at each detector before any information could have time to travel between the two towns.
In an attempt to rule out any kind of communication between entangled particles, physicists from the University of Geneva have sent two entangled photons traveling to different towns located 18 km apart – the longest distance for this type of quantum measurement. The distance enabled the physicists to completely finish performing their quantum measurements at each detector before any information could have time to travel between the two towns. Many other experiments have observed quantum nonlocality – the “spooky interaction at a distance” that occurs between two entangled particles – and also known as a violation of Bell inequalities. But, as physicists Daniel Salart, et al., explain in a recent issue of Physical Review Letters, these Bell tests might not have gone far enough. If quantum measurements aren’t finished until after a mass has moved (as the team assumes here), then the Bell violations in previous tests might merely have been due to some type of classical communication between particles unknown to today’s physics.
In their experiment, the physicists sent pairs of entangled photons from Geneva through optical fibers leading to interferometers in two other Swiss towns: Satigny and Jussy, located 8.2 and 10.7 km away, respectively. The distance between the interferometers in Satigny and Jussy was 18 km.
With this large distance between the interferometers, the physicists could perform a more complete quantum measurement than has previously been done. Somewhat surprisingly, physicists have never decided exactly when a quantum measurement is finished (when the “collapse” occurs, if there is any).
Different interpretations of quantum mechanics lead to different answers. The most common view is that a quantum measurement is finished as soon as the photons are absorbed by detectors. Previous experiments have been set up to allow enough distance between particle detectors to prohibit communication under this view. But there are also other views of when the measurement is finished, including “when the result is secured in a classical system,” “when the information is in the environment,” or even that it is never over – a view that leads to the many worlds interpretation.
The Swiss team followed a view proposed independently by Penrose and Diosi, which assumes a connection between quantum measurements and gravity, and requires a macroscopic mass to be moved. In this view, the measurement takes more time than it does for a photon to be absorbed by a detector. The significance of the Swiss test is that it is the first “space-like separated” Bell test under the Penrose-Diosi assumption.“There is quite a large community of physicists that speculates on possible connections between quantum gravity and the measurement problem,” coauthor Hugo Zbinden told “The advantage of the Penrose-Diosi model is that it is testable using today's technology.”
In the physicists’ experiment, the detection of each photon by a single-photon detector triggers a voltage to a piezoelectric actuator. The actuator expands, which in turn causes a tiny gold-surfaced mirror to move. By measuring the mirror displacement, the researchers could confirm by the gravity-quantum connection that the quantum measurement had been successfully finished.All of the steps – from photon detection to mirror movement – take about 7.1 microseconds, which is significantly less than the 60 microseconds it would take a photon to cover the 18 km between interferometers. So measurements made simultaneously at each of the interferometers could not be been influenced by anything traveling at – or even a few times more than – the speed of light.
“The significance of our experiment lies entirely in achieving space-like separation, even under the assumption that a quantum measurement is only finished after a macroscopic mass has moved, as in the Penrose-Diosi model,” Zbinden explained.
Altogether, the experiment serves to fill a loophole by ruling out any kind of communication between two entangled particles separated by a distance, provided the collapse happens only after a mass has moved. By spatially separating the entangled photons, the test once again confirms the nonlocal nature of quantum correlations.
More information: Salart, D.; Baas, A.; van Houwelingen, J. A. W.; Gisin, N.; and Zbinden, H. “Spacelike Separation in a Bell Test Assuming Gravitationally Induced Collapses.” Physical Review Letters 100, 220404 (2008).

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