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Scientists stop light for a minute, breaking records

Professor Thomas Halfmann and his team of scientists at the University of Darmstadt, Germany, have managed to stop a pulse of light inside a crystal for an entire minute. © Katrin Binner/ TU Darmstadt
Professor Thomas Halfmann and his team of scientists at the University of Darmstadt, Germany, have managed to stop a pulse of light inside a crystal for an entire minute. © Katrin Binner/ TU Darmstadt

Scientists at the University of Darmstadt, in Germany, have trapped a pulse of light inside a crystal for a minute, and used it to store an image, raising the possibility of light-based computers that could work faster than today’s electronic processors and transistors.

The results could have practical significance in future computer systems that operate using light, and could pave the way for quantum computing and communications.

‘We are reaching the principal limits of conventional electronic data processing,’ said Professor Thomas Halfmann, who coordinates the EU-funded project Marie Curie Initial Training Network - Coherent Information Processing in Rare-Earth Ion Doped Solids (CIPRIS).

Light usually travels at a speed of just under 300 million metres per second, making it the fastest thing in the universe, and computer scientists and physicists believe that computers in the future need to be optical to achieve faster processing speeds.

Currently, optical technology is mainly confined to communication networks, where light carries information through optical fibres. However, at the ends of the fibres the light signals have to be converted to and from the electrical signals that computers use to process information.

In the future, it is hoped that optical quantum computers will be able to process information using light, and the first step towards this is being able to store optical data in quantum systems.

‘We need media to store light and this is what is called an optical or quantum memory,’ said Prof. Halfmann. ‘What we have done is demonstrate an optical memory in a solid-state quantum system that can store light for one minute.’

They did it by using a control laser to manipulate the speed of the light in the crystal, which contained a low concentration of ions – electrically charged atoms – of the element praseodymium. When the light source then came into contact with the crystal, it rapidly decelerated. The scientists then switched off the laser beam and the light came to a complete halt.

‘In simple terms you transfer energy from one oscillator, the light field, into the other oscillator, the atom, and there it stays and then you can retrieve it afterwards.’

Professor Thomas Halfmann, the coordinator of MC ITN - CIPRIS

Technically, the light wasn’t stopped, but it was converted into the atomic medium, Prof. Halfmann explained. ‘What happens is we convert the light pulse into something called an atomic oscillator.

‘In simple terms, you transfer energy from one oscillator, the light field, into the other oscillator, the atom, and there it stays and then you can retrieve it afterwards.’

Storing an image

The researchers imprinted an image consisting of three stripes onto the light pulse, demonstrating that they could store the image inside the crystal for a minute and then retrieve it, smashing the previous image storage record, which was less than ten microseconds.

The fact they stored an image is significant for developments in computing because, while a single light pulse contains one ‘bit’ of data, an image contains many.

‘These stripes are just one simple image you can store, but essentially you can store any image,’ Prof. Halfmann said.

Professor Halfmann in the laboratory. © Katrin Binner/ TU DarmstadtProfessor Halfmann in the laboratory. ©Katrin Binner/ TU Darmstadt

Usually light storage times are very short because ‘perturbing environments’ disrupt the oscillation. However, the team managed to achieve their record-breaking storage time by protecting the oscillator with magnetic fields and high-frequency pulses.

Using complex algorithms, they were able to optimise the laser beams, magnetic field and high-frequency pulses so that the oscillation lasted almost as long as theoretically possible in the crystal.

Prof. Halfmann likened the process of trapping the light to a person running through a crowd at a funfair with a briefcase full of papers. ‘When you run, you collide with people and if you collide often enough you lose your suitcase, your information, your papers.

‘So, what we do is we shield the information with these magnetic fields and we protect it somehow. It is like running through the funfair with bodyguards, big tough guys, around you. They protect you on your way through the crowd and nothing happens to you and your suitcase with your papers.’

The scientists have almost reached the theoretical storage limit of the crystal they used in this research, which is 100 seconds. But, they already have a different type of crystal set up in their laboratory and have started working with it.

Although there is some debate over the exact timeframe, the new crystal is theoretically capable of storing a light pulse for between a few hours to a week, and Prof. Halfmann believes that in two to three years they will again be very close to the limit of that crystal.

Quantum computers could be much more powerful

Quantum computers could revolutionise science by offering a new way of solving complex problems beyond the scope of standard transistor-based computers.

While normal computers are limited to ‘bits’, short for binary digits ( 0 or 1), quantum computers could be much more powerful because they could store information in qubits, short for quantum bits, using photons or atoms.

That’s because an atom can simultaneously have different energy states, or a photon of light may have multiple polarisations.

It means that a quantum computer would be able to solve many types of data encryption that are used today.

However, functioning quantum computers are still five or 10 years in the future, many researchers say, and it could take a couple more decades to reach the stage where quantum computers harness enough qubits to perform significant mathematical tasks.

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