The microchips that the researchers are working on developing are about the size of a one-cent piece.

© WWU/P. Leßmann

A technology that works like a brain? In times of artificial intelligence, this doesn't seem so far away - for example, when a cell phone recognizes faces or languages. However, computers still quickly reach their limits when it comes to more complex applications, which is partly due to the fact that their computing units and data storage are traditionally separate from each other. As a result, all data has to be sent back and forth. In this respect, the human brain is many steps ahead of even the most modern computers, because it processes and stores information in the same place: at the synapses, connections between nerve cells, of which there are around 100 trillion in the brain. An international team of researchers from the Universities of Münster, Oxford and Exeter has now succeeded in developing hardware that could pave the way towards brain-like computers: The nanoscientists produced a chip on which a network of artificial neurons extends that works with light and can imitate the behavior of nerve cells in the brain.

The researchers were able to show that such an optical neurosynaptic network is able to "learn" information and calculate and recognize patterns based on it - just like a brain can. As the system works exclusively with light and not with electrons as is traditionally the case, it can process data many times faster. "This integrated photonic system is an experimental milestone. The approach could later be used in many areas to evaluate patterns in large amounts of data, for example in medical diagnostics," says study leader Prof. Dr. Wolfram Pernice, physicist at the University of Münster (WWU).

Advantages of optical systems

Most existing approaches for so-called neuromorphic networks are currently based on electrons, whereas optical systems that use photons, i.e. particles of light, are still in their infancy. The principle that the German and British scientists are now presenting works like this: Optical waveguides that can transmit light are placed on the microchips. The researchers equip the optical waveguides with so-called phase change materials. These are already used today in storage media such as rewritable DVDs. They are characterized by the fact that they drastically change their properties depending on which phase state they are in. The materials alternate between a crystalline state, in which their atoms arrange themselves in a regular manner, and an amorphous state, in which their atoms organize themselves in an irregular manner. The phase change can be triggered by light by heating the material with a laser beam. "The fact that the material reacts so strongly and drastically changes its properties makes it well suited to mimicking synapses and the transmission of excitation between two neurons," says first author Johannes Feldmann, who carried out a large part of the experiments as part of his doctoral thesis at the WWU.

In their current study, the researchers succeeded for the first time in combining many nanostructured phase change materials to form a neurosynaptic network. The nanoscientists developed a chip with four artificial neurons and a total of 60 synapses. The structure of the chip, built up in different layers, was based on the so-called wavelength-division multiplexing technique - a process in which light is transmitted on different channels within an optical nanocircuit.

Artificial network learns from examples

To test the extent to which the system is able to recognize patterns, the researchers "fed" it with information in the form of light pulses and applied two different machine learning algorithms. Here, an artificial system "learns" from examples and can generalize them in the end. In the two algorithms used - both supervised and unsupervised learning - the artificial network was ultimately able to recognize a searched pattern, including four consecutive letters, based on given light patterns.

"With our system, we have taken an important step in the direction of computer hardware that behaves similarly to neurons and synapses in the brain and is capable of processing real tasks," says Wolfram Pernice. "By working with photons instead of electrons, we can make optimal use of the known potential of optical technologies - not only to transmit data as before, but also to store and process it in one place," emphasizes co-author Prof. Dr. Harish Bhaskaran from Oxford University.

Conceivable use in cancer research

In principle, such hardware could be used to automatically identify cancer cells, for example. However, further steps are necessary before such applications can be realized. For example, the researchers need to increase the number of artificial neurons and synapses and increase the depth of the neuronal networks. This can be done, for example, with optical chips produced using silicon technology. "This step is to be taken in the EU joint project 'Fun-COMP'," says Prof. Dr. David Wright from Exeter University, co-author and head of the Fun-COMP project.