Transistor flipped by just a single photon

Blocked drain: one photon flips atomic transistor

Two independent teams of physicists in Germany have created the first high-gain optical transistors that can be switched using a single photon. Based on ultracold atomic gases, the devices make use of the "Rydberg blockade", whereby the creation of an atom in a highly excited state has a huge effect on the ability of the surrounding gas to transmit light. The research might lead to the development of all-optical logical circuits that could operate much faster than conventional electronics. The transistors could also find use in photon-based quantum-information systems of the future.

Communications and computing systems that use only light to transmit and process information have the potential to be faster and much more energy-efficient than those that use electronic signals. While optical-fibre communications is already widespread, the switching and processing of optically encoded data is usually done by converting light pulses to an electronic signal, which can then be easily processed. The electronic signal is then converted back to a light pulse.

Making photons interact

This time-consuming and energy-hungry process is necessary because photons do not readily interact with each other, which makes the design of all-optical components a major challenge that is currently being addressed by physicists and engineers. During the past few years, several research groups have made important breakthroughs in this area by showing that photons can be made to interact with each other in specially prepared samples of ultracold atomic gas.

Now, two independent teams led by Sebastian Hofferberth of the University of Stuttgart and Stephan Dürr of the Max Planck Institute of Quantum Optics near Munich have created devices in which a single "gate" photon can switch off a stream of as many as 20 photons. This gain of 20 is a huge improvement on previous attempts at optical switches, which either needed pulses of several gate photons to achieve gains greater than one or offered gains of much less than one for single-gate photons.

Both teams based their gates on gases of rubidium atoms that were cooled to temperatures below 1 mK. Normally, the gas is transparent to a beam of "source" photons, which can travel through the device and emerge via the "drain" – gate, source and drain being terms used to describe the control, input and output channels, respectively, of a conventional field-effect transistor.

Blocking the drain

When a gate photon is fired into the gas, it is absorbed by one atom, which puts that atom into a highly excited Rydberg state with one electron in an extremely large orbital. The large distance between this electron and the nucleus gives the atom a very large electric dipole moment, which shifts the energy levels of nearby atoms. This shift causes the gas to become opaque to light from the source, effectively switching the transistor off. The Rydberg state endures for about 1 μs, which is a surprisingly long time for an atomic system. This allowed Dürr and colleagues to use their transistor to switch off a stream of 20 source photons, while Hofferberth's team prevented 10 photons from reaching the drain of its device.

"This effect should make it possible – at least in principle – to cascade such transistors to solve complex computational tasks," says Dürr. He also points out that the experiments offer physicists a new and non-destructive way of studying the physics of Rydberg states. The ability to operate at the single-photon level also means that the transistors could find use in quantum-information applications such as secure quantum-communication systems or powerful quantum computers.

Another interesting aspect of the devices is that the gate photon is re-emitted by the gas when the Rydberg states decay – an effect that has been observed in other experiments. In principle, this means that the transistors could also be used as storage devices for quantum information.

Both experiments are described in separate papers in Physical Review Letters.

Scientists turn air into 'Optical Fibre'

Scientists say they have turned thin air into an 'optical fibre' that can transmit and amplify light signals without the need for any cables.

In a proof-of-principle experiment they created an "air waveguide" that could one day be used as an instantaneous optical fibre to any point on earth, or even into space.

The findings, reported in the journal Optica, have applications in long range laser communications, high-resolution topographic mapping, air pollution and climate change research, and could also be used by the military to make laser weapons.

"People have been thinking about making air waveguides for a while, but this is the first time it's been realised," says Professor Howard Milchberg of the University of Maryland, who led the research, which was funded by the US military and National Science Foundation.

Lasers lose intensity and focus with increasing distance as photons naturally spread apart and interact with atoms and molecules in the air.

Fibre optics solves this problem by beaming the light through glass cores with a high refractive index, which is good for transmitting light.

The core is surrounded by material with a lower refractive index that reflects light back in to the core, preventing the beam from losing focus or intensity.

Fibre optics, however, are limited in the amount of power they can carry and the need for a physical structure to support them.

Light and air

Milchberg and colleagues' made the equivalent of an optical fibre out of thin air by generating a laser with its light split into a ring of multiple beams forming a pipe.

They used very short and powerful pulses from the laser to heat the air molecules along the beam extremely quickly.

Such rapid heating produced sound waves that took about a microsecond to converge to the centre of the pipe, creating a high-density area surrounded by a low-density area left behind in the wake of the laser beams.

"A microsecond is a long time compared to how far light propagates, so the light is gone and a microsecond later those sound waves collide in the centre, enhancing the air density there," says Milchberg.

The lower density region of air surrounding the centre of the air waveguide had a lower refractive index, keeping the light focused.

"Any structure [even air] which has a higher density will have a higher index of refraction and thereby act like an optical fibre," says Milchberg.

Amplified signal

Once Milchberg and colleagues created their air waveguide, they used a second laser to spark the air at one end of the waveguide turning it into plasma.

An optical signal from the spark was transmitted along the air waveguide, over a distance of a metre to a detector at the other end.

The signal collected by the detector was strong enough to allow Milchberg and colleagues to analyse the chemical composition of the air that produced the spark.

The researchers found the signal was 50 per cent stronger than a signal obtained without an air waveguide.

The findings show the air waveguide can be used as a "remote collection optic," says Milchberg.

"This is an optical fibre cable that you can reel out at the speed of light and place next to [something] that you want to measure remotely, and have the signal come all the way back to where you are."

Australian expert Professor Ben Eggleton of the University of Sydney says this is potentially an important advance for the field of optics.

"It's sort of like you have an optical fibre that you can shine into the sky, connecting your laser to the top of the atmosphere," says Eggleton.

"You don't need big lenses and optics, it's already guided along this channel in the atmosphere."