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 Welsh scientists find order in chaos
There is apparently some semblance of order in the weird world of chaos after all.
In a groundbreaking experiment, electronic engineers at the University of Wales, Bangor have proven that with two systems in a chaotic environment, one of them is able to anticipate a signal from the other before it is even sent.
This extraordinary saga began twelve months
ago when a German physicist, Dr Henning Voss at the University of Freiburg, controversially suggested that it was theoretically possible for a state of what he called anticipating synchronization to exist when two duplicate systems exist within a chaotic or apparently random environment. In simple terms he was putting forward the notion that under such conditions it was possible for a duplicate system to anticipate changes in the original system. It would achieve this by receiving a signal a fraction of a second before it was actually transmitted.
However, since that time this proposition has remained no more than an intriguing theory. That is until now. Bringing it firmly into the practical world of reality, the team at Bangor has just announced a successful experimental demonstration of it by using light signals transmitted by chaotic semiconductor lasers.
In an article in this months Physical Review of Letters, the leading international journal of physics, Professor Alan Shore describes how he and his colleagues achieved this remarkable feat. He is an electronic engineer working in the field of laser cryptographyusing lasers in a chaotic state for the secure transmission of data.

In traditional optical transmission, lasers are used to send data via optic fibre cables in non-chaotic form. In this mode the message travels at the speed of light. Work in transmitting data in chaotic form is leading towards greater security of transmission. The data is wrapped in a chaotic signal, which is created by using a simple mirror to reflect light back into the laser. The data can only be deciphered at the receiver by using an identical chaotic signal to unravel the signal. This is possible when the transmitter is used to drive the receiver in such a way that the chaotic dynamics of transmitter and receiver become identical. This is termed chaos synchronization.
In the first experimental demonstration of anticipating synchronization, the laser transmitted a continuous fluctuating signal in chaotic form. The changes in the pattern of the signal were recorded at the transmitter and receiver. Uncannily, the receiver recorded changes in the signal nanoseconds before the transmitter recorded the transmission of those changesthereby anticipating the synchronization of transmitter and receiver. The anticipation time was found to be equal to the time of flight from the transmitter to the receiver.
At present, we cannot fully explain our observations, says Professor Shore. We had expected that the anticipation time would depend upon the time taken for the light to travel between the laser and the external mirror which is used to drive the laser into chaos. We are currently developing a theoretical explanation of our observations.
Synchronization of chaotic external cavity lasers and message transmission and extraction has been accomplished and has been demonstrated by a number of groups including our own, said Professor Shore.
In order to carry forward this concept into practical use it is now necessary to confront basic performance issues and, in particular, to determine the rates at which information can be transmitted. Laboratory experiments have already confirmed the GHz message transmission capabilities of semiconductor lasers.
One of the difficulties faced in developing the use of synchronized lasers for secure transmission is that there is a finite time of flight between the transmitter and the receiver.
As the distance between transmitter and receiver is increased it could be considered that a basic limitation on system operation can be established due to the time it takes to synchronize the receiver and the transmitter. Indeed previous laboratory experimental demonstrations of laser chaos synchronization have shown that the receiver lags the transmitter.
What we have reproduced, however, is the state which was theorized by Voss of the University of Freiburg, in which when working with chaotic electronic oscillators, the receiver leads or anticipates the transmitter.
The work undertaken in our laboratory has provided the first experimental confirmation of the appearance of anticipated synchronization in chaotic external cavity lasers and indeed in any physical system. A particularly important feature of the experimental results is the demonstration that the anticipation time is precisely the time of flight between the

The significance of this observation for chaotic communications is immediately apparent: operating in a regime of anticipating chaos would ensure that the time of flight is not a fundamental constraint on system performance. We are actively exploring a number of other issues relevant to chaotic optical communications which follow from our observations.
The essential feature of the chaotic laser system we have studied is the finite time delay associated with the round-trip time of light in the laser external cavity. We point out that finite time delays are ubiquitous.
We would thus expect that any of those analogous systems which exhibit chaos should also be liable to anticipating synchronization. We thus hope that our work will act as a stimulus to explore the opportunities for observing anticipating synchronization in physical, chemical, biological and socio-economic systems.

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PVI 6/10/2001

Source: Bangor University


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