AN INTERNATIONAL team of researchers have found a way to use an external magnetic field to stop the loss of information from quantum bits.
Qubits are the basic logical elements of quantum information processing. While there are a number of possibilities for implementing qubits, a team of physicists headed by Alexander Bechtold and Professor Jonathan Finley at the Walter Schottky Institute of the Technical University of Munich and the Cluster of Excellence Nanosystems Initiative Munich (NIM) have presented a system comprising a single electron trapped in a semiconductor nanostructure.
The physicists evaporated indium gallium arsenide onto a gallium arsenide substrate to form their nanostructure. As a result of the different lattice spacing of the two semiconductor materials strain is produced at the interface between the crystal grids. The system thus forms nanometer-scale quantum dots.
When the quantum dots are cooled down to liquid helium temperatures and optically excited, a single electron can be trapped in each of the quantum dots.
The information is stored in the spin states of the electrons, while laser pulses read and alter the states optically from the outside. 1 and 0 are represented by spin up and spin down, with additional states of quantum mechanical up and down superpositions.
The advantage is the use of common semiconductor materials means they are compatible with standard manufacturing processes, and also harmonise perfectly with existing computer technology.
However, the researchers found that the strain in the semiconductor material also leads to a new mechanism that results in the loss of quantum information. The strain creates tiny electric fields in the semiconductor that influence the nuclear spin orientation of the atomic nuclei.
These uncontrolled fluctuations in the nuclear spin modify the spin of the electrons and thus causes loss of the stored information.
The researchers found a way to prevent this loss, by applying a magnetic field of around 1.5 tesla. This magnetic field, which is around the strength of the field created by a strong permanent magnet, stabilises the nuclear spins, resulting in the encoded information remaining intact.