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.