A major challenge along the path to realising a quantum computer and the associated large-scale quantum hardware is the initialisation and reading out of the minuscule quantum bits. The spin rotation of both individual electrons and atomic nuclei functions as a quantum bit: left spin is a '0', and right spin is a '1'. Atomic nuclei are highly stable quantum bits as they scarcely interact with their surroundings. However, this property also makes it difficult to read out the state of atoms. A team from Delft University of Technology under the leadership of FOM scientist Dr.ir. Ronald Hanson has now solved this problem by using a captured electron as an intermediate station in the measurement.
The researchers can now determine the state (spin direction) of the electron quickly and accurately. They do this by sending laser pulses of an accurately preset wavelength at the electron. The electron absorbs the light and transmits it again if it is in the '0' state but does nothing if it is in the '1' state.
To read out the spin direction of the atomic nuclei as well, the researchers first perform a quantum operation. The electron then gets entangled with the atomic nuclei, and the information about the atom is transferred to the electron. The researchers subsequently read out the state of the electron and from this derive the original state of the atomic nuclei.
This measurement technique is special because the measurement does not alter the state of the atomic nuclei. It is therefore an ideal way of preparing the atomic nuclei for further quantum calculations and so for use in a quantum computer. The researchers demonstrated their technique on a mini-quantum computer comprising four quantum bits on a diamond chip.
The discovery that the fundamental natural laws of quantum mechanics make it possible to perform ultrafast calculations and transmit information completely securely, has elicited a worldwide race to construct the necessary 'quantum hardware'. For the past few years diamond has been the favourite material as quantum states are scarcely disrupted in this. This new read-out technique for quantum bits in diamond provides the researchers with many possibilities. For example, they want to test interesting phenomena such as teleportation and multiparticle entanglement in the laboratory. The read-out also makes it possible to implement the correction of elementary quantum errors, an essential aspect of a large-scale quantum computer.
The research was supported by a Marie Curie Intra European Fellowship of the Seventh European Community Framework Programme, FOM, the European Commission (SOLID) and the Research Corporation for Science Advancement (RCSA).