Nanowires are candidates for enabling the exchange of quantum information between light and matter. The rapid control of a single electron spin by solely electrical means brings this possibility closer. See Letter p.1084
The quest to develop ways to store and manipulate quantum information in condensed-matter systems is establishing a tool kit for controlling the nanoworld — one that promises far-reaching technological innovation. One example is the idea of encoding data, both classical1 and quantum2, in the spin orientation of a single electron (its intrinsic magnetic moment). During the past five years, this vision has largely been realized3,4,5,6,7, and researchers are now turning to other goals, such as high-speed control of the spin orientation and the suppression of 'decoherence' processes that lead to a loss of quantum information. Innovative methods in quantum control8,9 and new material systems are leading the way in tackling this next generation of challenges.
On page 1084 of this issue, Kouwenhoven and co-workers10 report an experiment that exploits the unique material properties of an indium arsenide (InAs) semiconductor nanowire to rapidly control the quantum state of a single electron spin using only electric fields. Beyond just flipping the spin orientation of a single electron, the authors tailor the precise timing of electric-field pulses to extend the spin coherence time (during which the information encoded in the quantum state of the spin is preserved).
Controlling electron and nuclear spins is central to magnetic resonance technologies such as magnetic resonance imaging. These technologies use radio- or microwave-frequency magnetic fields to manipulate some 1023 spins in macroscopic volumes. On the nanometre scale, the application of spatially selective, oscillating magnetic fields is a formidable challenge, which makes controlling single spins difficult. Although proof-of-principle experiments have shown that nanometre-scale magnetic control is possible11, the time it takes to rotate the orientation of the electron spin magnetically is long and does not allow for many rotations within a spin coherence time. This limitation inhibits the use of this technique for quantum information processing.
Kouwenhoven and colleagues' experiment10 addresses this shortcoming by moving from magnetic to all-electric fields to achieve rapid control over the spin. Although an interaction between an electron's spin and an applied electric field is forbidden, if it is strong enough a quantum interaction known as spin–orbit coupling provides a means of controlling spins using oscillating electric fields, and is at the heart of the new field of 'spintronics'.
Special relativity requires that an electron moving through an electric field experiences an effective magnetic field that couples its spatial motion (orbit) to its spin. In the simplest picture, spin–orbit coupling is possible because, from the viewpoint of the electron, it is the electric field that is moving, and time-varying electric fields generate a magnetic field that splits the electron's spin states in energy. The detailed picture of spin–orbit coupling has played a key part in the formulation of quantum mechanics.
For semiconductors in a magnetic field, the spin–orbit interaction can be much stronger than in an atom, owing to the high electron velocities and strong electric-field gradients produced by nuclei in the semiconductor crystal lattice12. As is the case in Kouwenhoven and colleagues' experiment, careful choice of material system and device geometry can lead to spin–orbit coupling that is so strong that the electron's spatial state and its spin cannot be considered separately: they collectively form a quantum state that preserves the long-lived spin component while allowing for manipulation through electric fields13,14.
The signature of spin–orbit control has previously been identified in gallium arsenide (GaAs) semiconductor quantum devices15, but the strong coupling in the InAs nanowire devices allows both faster control and the potential for the exchange of quantum information between optical and solid-state electronic systems. Indeed, optoelectronic devices16,17, such as semiconductor LEDs (light-emitting diodes), have recently been demonstrated in nanowire architectures that are similar to the authors' InAs nanowire, and the possibility of transferring the quantum state of a single spin to a single photon now seems viable. The creation of such hybrid quantum systems is pivotal because they allow the unique advantages of different quantum platforms to be combined to open up new quantum technologies. The iPhone provides the perfect example of how the tight integration of optical, mechanical and electrical devices can have a significant technological impact. In quantum mechanics, this kind of integration is not easy, owing, in part, to the nature of quantum measurement and the fragility of systems that manipulate quantum information.
For Kouwenhoven and colleagues' experiment10, an important but perhaps unexpected result is that the spin coherence lifetime, measured by a technique known as the Hahn echo pulse sequence, is significantly shorter than in GaAs. The authors' hunch is that this may result from the larger nuclear spin moment of indium compared with gallium or arsenic, which couples uncontrollably to the electron spin. To what extent this short time presents a fundamental problem requires further research, but will undoubtedly drive fresh innovation in the science and engineering of quantum systems.
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Comptes Rendus Chimie (2012)