Electron and hole spins in solid-state nano-structures possess unique properties with favourably long spin coherence, and with high potential for fast coherent optical, magnetic and electrical control. The major challenge we undertake in our studies on this subject is the realisation of spin-qubits based on single spins with coherence in the ms range, and the development of optical and microwave control strategies enabling fast hole and electron spin operations in single quantum dots (QDs). In III-V semiconductor nano-structures the dominant mechanism of spin information leakage to the environment (decoherence) is magnetic interaction with slowly fluctuating nuclear spins of the lattice. In our experiments we have learned a great deal about nuclear spins, and have developed new nuclear magnetic resonance (NMR) techniques for control of tiny ensembles of nuclear spins in semiconductor QDs. These studies have opened new possibilities for non-invasive structural studies using nano-NMR, and in future will allow exploration of unusual collective behaviour of nuclear spins in nano-structures.
Our main experimental technique is photoluminescence spectroscopy of individual semiconductor quantum dots at low temperatures and high magnetic fields. In our laboratories we have three liquid-helium bath cryostats equipped with superconducting magnets (including one vector magnet system). To detect photoluminescence we use high-resolution single and double spectrometers coupled to state of the art low-noise, liquid-nitrogen-cooled CCD cameras (detection efficiency up to 90%). Top-range radiofrequency and microwave equipment is used for NMR and microwave-spin-control projects respectively.
Recent research highlights
Coherent NMR spectroscopy on self-assembled quantum dots
Using high-sensitivity optical detection we demonstrated for the first time coherent NMR spectroscopy on few thousand nuclear spins in individual self-assembled quantum dots. Large quadrupolar effects arising from the lattice mismatch, driving the quantum dot self-assembly, were found to increase significantly the nuclear spin coherence times T2. Such increase in T2 is a clear sign of the strain-induced suppression of the nuclear spin fluctuations, which opens the way for achieving long electron and hole spin coherence in self-assembled quantum dots.
Hole hyperfine interaction in quantum dots
Using photoluminescence spectroscopy of both “bright” and “dark” (optically forbidden) excitons we were able to measure the interaction of the valence band holes with nuclear spins. Furthermore, using NMR techniques we found a way to probe hole hyperfine interaction constants individually for each isotope. To our surprise we found that cations and anions have opposite signs of the hyperfine constants. This can only be explained if we take into account significant contribution of the d-symmetry atomic orbitals into the valence band states, which were previously thought to be constructed of p-orbitals only.
Structural analysis of strained quantum dots using NMR
Nuclear magnetic resonance (NMR) is a powerful analysis tool used in chemistry, biology and material science studies. However, application of NMR to strained nanostructures, such as self-assembled quantum dots, has been problematic due to huge strain-induced broadening of the NMR spectra. We have developed a special “inverse” NMR technique, which effectively solved this problem and allowed us for the first time to probe chemical composition and distribution of strain within a few-nanometer volume of an individual quantum dot.