The idea to perform data processing with light, without relying on any electronic components, has been around for quite some time. In fact, necessary components such as optical transistors are available. However, up to now they have not gained a lot of attention from computer companies.
This could change in the near future as packing densities of electronic devices as well as clock frequencies of electronic computers are about to reach their limits. Optical techniques promise a high bandwidth and low dissipation power, in particular, if only faint light pulses are needed to achieve the effect of switching. The ultimate limit is a gate-pulse that contains one photon only. A team of scientists has now managed to bring this almost utopian task into reality.
A team of scientists around Professor Gerhard Rempe, director of the Quantum Dynamics Division at the Max-Planck-Institute of Quantum Optics, succeeded in switching a medium — a cloud of about 200,000 ultracold atoms — from being transparent to being opaque for light pulses. This “single-photon-switch” could be the first step in the development of a quantum logic gate, an essential component in the field of quantum information processing.
The experiment starts with cooling a cloud of about 200,000 rubidium atoms down to a temperature of 0.43 micro-Kelvin (this is just above absolute zero, which corresponds to minus 273 degree Celsius). The atoms are held in an optical dipole trap created by the crosswise superposition of two laser beams. The cloud is irradiated by two light pulses separated by 0.15 micro-seconds. The pulses are extremely weak, they contain on average one or even less photons. The first pulse — the so-called gate-pulse — gets absorbed inside the cloud. To be precise, it is stored as an atomic excitation, as it brings one of the atoms into a highly excited Rydberg state. The mere presence of the Rydberg atom leads to a shift of the corresponding energy levels of the other atoms in the cloud. Hence, the wavelength of the second pulse — the target pulse -no longer meets the requirements for excitation and gets blocked. In other words, the cloud of atoms acts as a medium which, on capturing one single photon, switches from being transparent to opaque. The storage of the photon can be maintained as long as the Rydberg state survives, i.e. for about 60 micro-seconds.
The original article was written by Olivia Meyer-Streng.
February 19, 2014
Max Planck Institute of Quantum Optics
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