Light marker describes quantum gas atom by atom

To allow the control of individual atoms, clever tricks are needed: in a quantum gas consisting of rubidium particles, physicists from the Max Planck Institute of Quantum Optics and the Ludwig Maximilian University in Munich have managed to accurately manipulate individual spins. Put simply: the spin represents the direction of an atom's rotation. Using a microscope especially developed for the purpose, the scientists working with Stefan Kuhr and Immanuel Bloch addressed individual particles in an atom system that is arranged in a wavelength lattice, much like an optical egg carton. The experiment provides a basis for processing information using atoms in an artificial light crystal - for example, as is planned for a quantum computer. Primarily, however, the work of the scientists paves the way for completely new ways of investigating quantum processes. For the first time, they were able to observe directly how individual solid particles - rubidium atoms - tunnelled through lattice potentials.

© S. Kuhr / I. Bloch, MPI of Quantum Optics

Sometimes the only way forward for physicists is to simulate a problem. To research the quantum world, measurements are often not accurate enough or simply not possible - and computations are not feasible. For instance, when looking for a detailed explanation of magnetism or high-temperature superconductivity, in which materials can conduct current without a loss at relatively high temperatures. However, only a precise understanding of such phenomena will allow scientists to develop materials that are suited for, say, resistance-free cables for everyday use. Deeper insight into high-temperature superconductivity or magnetism can be gained by simulating the corresponding materials in experiments using quantum systems that the scientists understand exactly and can control with great precision. The scientists at the Max Planck Institute of Quantum Optics in Garching and the Ludwig Maximilian University in Munich have recently provided new ways of doing this.

In a wavelength lattice, the physicists trapped up to 400 rubidium atoms, so that the atoms sat in the dents of the electromagnetic potential of the light beams like eggs in a carton. Once thus arranged, the spin of those individual atoms which the scientists had selected for manipulation, was flipped from one direction to the other.

To do this, the scientists first used an additional laser, which was tightly focused through a proprietary microscope, on the lattice site of the atom. The electromagnetic potential was superimposed on the optical egg carton, virtually denting it. In this way, the scientists introduced a controlled differential energy shift between two atomic spin states. The greatest shift occurred on the lattice site which the scientists aimed to address. Therefore, only the atom in this site reacted to the microwave pulse which the scientists applied to flip the spin.

Although the distance between the atoms is smaller than the wavelength of the addressing beam, the scientists were able to address individual lattice sites with this trick. "It is a prerequisite for describing individual atoms as quantum bits", explains Stefan Kuhr, who led the team at the Max Planck Institute of Quantum Optics. With its two spin directions, an atom could store the zero and one of a bit. How the bit can be read has only recently been discovered by the scientists from Munich and Garching. By applying quantum laws, physicists want to use such bits to construct quantum computers with much higher processing speeds than their conventional counterparts do.

Source: Max Planck Institute