EPJB Highlight - Picosecond-range control over information processing

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Published on 12 January 2015

Optical manipulation is key to reaching the necessary speed to control the furtive underlying physical mechanism used in quantum information processing

Quantum computing will, one day, bring quicker information processing. One of the keys to such speed is being able to control the short-lived physical phenomenon holding quantum information, also known as quantum bits (qubits). A new study presents a novel optical manipulation technique to control one possible kind of qubit—represented, in this case, by polarised electron spins—exposed to an ultra-short pulsed laser in the picosecond-range. Jorge Budagosky and Alberto Castro from the University of Zaragoza, Spain, have tested this novel optics approach using a quantum dot—nanoscopic artificial structures with a small number of electrons—in a study published in EPJ B.

Time is of the essence in information processing. The time required to perform a logical operation in a quantum computer—which physically translates as rotation of the electron spin polarisation—needs to be much smaller than the duration of spins retaining their orientation. Thus, rapid control would allow a large number of logical operations before the spin changes its orientation.

The authors used optical manipulation relying on very high-frequency—terahertz—laser pulses to induce a 180⁰ rotation of the polarisation of the spin of a single electron confined in a semiconductor quantum dot. Thus making such operations dramatically faster than those triggered by magnetic or even electric fields, as previously done. The indirect coupling of the laser pulse with the electrons, facilitated by the spin-orbit interaction, is the underlying mechanism of this approach.

They then used a set of mathematical tools to define the most effective manipulation technique. That involves identifying the optimal pulse shape necessary to achieve a predefined specific spin orientation, which means finding a pulse whose profile or shape makes it possible to perform spin rotations from an initial orientation to the predefined final orientation and, thus, control information processing.

J. A. Budagosky and A. Castro (2015), Ultrafast single electron spin manipulation in 2D semiconductor quantum dots with optimally controlled time-dependent electric fields through spin-orbit coupling, European Physical Journal B, DOI: 10.1140/epjb/e2014-50700-5