Monday, 23 November, 2015
Enroute to a quantum computer
Physicists at TU München detect mechanisms in semiconductor nanostructures which can cause stored quantum information to be lost and inhibit this by applying magnetic fields.
Quantum bits, or qubits in short, are the basic logical units of Quantum Information Processing (QIP), which might be the future of computer technology. A quantum computer composed of qubits could one day solve more complex problems at maximum speed, so the hope of researchers, as it processes problems on a quantum-mechanical basis. In principle, there are various possibilities of implementing qubits, each based on the use of isolated quantum systems. For example, photons can be used as well as confined ions or atoms, the state of which can be modified in a targeted manner with the help of a laser. The key questions with regard to their potential use as storage units are: (i) for how long can quantum information be stored in a system and (ii) which mechanisms lead to a loss of stored information. The physicist Alexander Bechtold in the group of Professor Jonathan Finley at the Walter Schottky Institute of Technische Universität (TU) München and Nanosystems Initiative Munich (NIM) has now presented a system consisting of one single electron which is trapped in a semiconductor nanostructure. The electron spin is used as the hardware that stores quantum information. The researchers were able to probe the various mechanisms that lead to the loss of quantum information for the first time and showed that stored information can nevertheless be maintained by applying external magnetic fields.
The physicists at TU München produced their nanostructure by growing semiconductor "artificial atoms" called quantum dots formed by evaporating the narrow bandgap semiconductor indium gallium arsenide onto a gallium arsenide substrate. As a consequence of the different lattice spacing of the two semiconductor materials strain is created in the crystal lattice resulting in the formation of nanoscale clusters of indium gallium arsenide - the quantum dots. By cooling the quantum dots to liquid helium temperatures and optically exciting them, it is possible to optically generate and trap a single electron in each quantum dot. The magnetic moment, or the spin state of each individual electron can then be used to encode quantum information. Using laser pulses, this quantum information can be read optically from the outside and also modified, making the system ideal hardware for building future quantum computers. Spin-up and spin-down correspond to the classical information states 0 and 1 but quantum-mechanical superpositions of spin-up and spin-down represent the quantum information and underpin the power of quantum technologies.
However, there is one problem explained Alexander Bechtold "We found out that the strain in the semiconductor material leads to a new and, until recently, unknown mechanism that results in the loss of quantum information". The strain creates tiny electric fields in the semiconductor that influence the magnetic moment, or nuclear spin orientation of each atomic nucleus contained in the quantum dot. "It's a kind of piezoelectric effect," said Bechtold, that results in uncontrolled fluctuations of the direction of the nuclear spins, and these can in turn modify the direction along which the spin of the electron is pointing. The direction of the electron spin, the quantum information, is lost over timescales of hundreds of nanoseconds. The team was able to provide exact evidence for even more loss mechanisms, e.g. that generally every electron spin is influenced by the spins of the surrounding ~100,000 atomic nuclei. "Both loss channels can be switched off if a magnetic field of approx. 1.5 tesla is applied," says Bechtold. This stabilizes the nuclear spins and the quantum information encoded in the electron remains protected, he explains.
"The system is extremely promising as hardware for solid-state quantum technologies," said Jonathan Finley head of the research group. "Semiconductor based approaches like quantum dots have the advantage that they can be integrated perfectly into semiconductor devices such as those used in optoelectronics and classical information technologies." They could even be equipped with electric contacts, he says, which means they can not only be controlled optically with the help of ultrafast lasers but, in addition, also controlled and switched using voltage pulses. "The ability to control the quantum state of an individual electron trapped in a semiconductor device is both exciting and certainly useful as hardware for quantum technologies."
“Three-stage decoherence dynamics of an electron spin qubit in an optically active quantum dot”, Alexander Bechtold, Dominik Rauch, Fuxiang Li, Tobias Simmet, Per-Lennart Ardelt, Armin Regler, Kai Müller, Nikolai A. Sinitsyn and Jonathan J. Finley, Nature Physics (2015)