Light marker describes quantum gas atom by atom
The spin of individual atoms in an optical crystal can be modified so that they can serve as quantum bits or explain unsolved quantum phenomena
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.
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.
However, Stefan Kuhr and his colleagues are mainly interested in the quantum marker, as it opens up new possibilities for exploring the uncharted regions of the quantum world. Using the quantum writer, they marked atoms that they then ‘kicked out’ of the lattice with the beam. The light pulses only removed the marked atoms, since they were set to their spin state. “This was proof that we had only modified the spin state of the designated atoms”, says Christof Weitenberg, who conducted part of the experiments. This way, the scientists were also able to create any given pattern using the rubidium atoms in the light crystal; a line, a star and even the quantum mechanical symbol of the wave function Ψ.
However that was not all: a line of rubidium atoms, positioned across the otherwise empty optical egg carton was used for a further quantum experiment dealing with atomic tunnelling through the optical lattice. In the experiment, the particles tunnelled the relatively low electromagnetic lattice potential from one lattice site to the next one – a behaviour that is rather unthinkable for eggs in a carton. In order to facilitate this process for the atoms, the scientists lowered the potentials of the lattice somewhat in one direction. Gradually, the particles would then hop along this axis from lattice site to lattice site. The atomic pattern thereby changed exactly as predicted by theory. “This was the first time that we observed this in an experiment”, says Stefan Kuhr, who has now taken up a professorship at the University of Strathclyde in Glasgow.
Finally, the tunnelling experiment was repeated with gently vibrating atoms. To achieve the vibrations, the scientists excited the particles by introducing a pointing offset of the addressing beam in relation to the light lattice. The rubidium atoms now tunnelled the lattice even faster. Thus, the scientists obtained the two different rates at which the excited atoms and the non-excited atoms changed lattice site. These were used to calculate that most of the non-excited atoms were in fact in their ground state.
“In so doing, we have found the evidence that our methods are suitable for quantum simulations or for a quantum computer”, says Immanuel Bloch from the Max Planck Institute of Quantum Optics. If the quantum marker pushed the atoms arbitrarily and uncontrolled by changing their spin state, it would be unfit for these types of applications. Only when physicists know their system exactly are they able to interpret the result correctly as soon as the computation or simulation begins.
The scientists are now seeking to explore the quantum world even further. Above all, they want to focus on the secrets of systems in which relatively large forces act between the particles – like the electrons of a superconductor or of a magnetic material. If they flip individual spins in such systems, or remove particles, they can observe how the whole system reacts to the disturbance. “It tells us a lot about which laws these systems abide by”, says Immanuel Bloch. Using atoms, the scientists imitate the behaviour of electrons that are more than 100,000 times lighter, much more mobile and electrically charged. It is something only true simulation pros are able to do.