Spin Phenomena

Scope of research

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.

Experimental laboratories


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.

Optically detected nuclear magnetic resonance (ODNMR) laboratory. Andreas Waeber is studying nuclear spin coherence and fluctuations in strained (self-assembled) quantum dots.

Optically detected nuclear magnetic resonance (ODNMR) laboratory. Andreas Waeber is studying nuclear spin coherence and fluctuations in strained (self-assembled) quantum dots.

Optical spectroscopy laboratory. Dr Ata Ulhaq is working on microwave control of electron spins in self-assembled quantum dots.

Contact details

For any inquiries please contact Dr. Evgeny Chekhovich

Research highlights

Nuclear spin thermometry of quantum dots
Previous measurements of the state of a quantum dot nuclear spin bath were limited to mean-field approaches, which lack accuracy due to the spatial inhomogeneity of the nuclear polarization and uncertainty in hyperfine constants. We have developed an alternative NMR-based approach where the state of the nuclear spin ensemble is probed directly by measuring its spin temperature. Our studies lead to observation of record-high nuclear spin polarization in a quantum dot (~80%), revealed the non-equilibrium nature of the optically-cooled nuclei, and allowed the hyperfine constants of GaAs to be measured for the first time.

An NMR spectrum with a well-resolved triplet (top left) originating from the non-equdistant spin levels (bottom left) of the spin-3/2 nuclei in a weakly strained GaAs/AlGaAs quantum dot. NMR signals obtained by selectively saturating two out of three NMR transitions show nonlinear dependence on the initial polarization (right): model fitting of the data allows reconstruction of the nuclear spin level populations (horizontal bars in bottom left schematic) and a direct measurement of the nuclear spin temperature. For details see [E. A. Chekhovich et al, Nature Materials doi:10.1038/nmat4959 (2017)].

Probing nuclear spin bath fluctuations in strained quantum dots
One of the key challenges in spectroscopy is the inhomogeneous broadening that masks the homogeneous spectral lineshape. In strained self-assembled quantum dots inhomogeneous broadening of the nuclear spin ensemble is so large, that the standard spin-echo techniques using high power radio-frequency (rf) pulses no longer work. We have developed new NMR techniques that rely on low-power non-coherent rf pulses whose spectral profiles resemble a comb. This frequency comb NMR method not only gives measure of the hidden homogeneous NMR lineshapes, but also reveals the few-second-long correlation times of the subtle nuclear spin fluctuation dynamics, which can not be detected with standard NMR methods.

Left: Homogeneous lineshapes (red lines) of individual nuclei merge into an inhomogeneously broadened NMR spectrum of a strained quantum dot (green line). The entire nuclear spin ensemble of a dot is manipulated by a rf pulse with a comb profile (black line). Middle: By analyzing the nuclear spin dynamics under frequency-comb rf with different comb spacing it is possible to deduce the homogeneous linewidth. Right: When an additional frequency comb is used to heat the 71Ga nuclei the homogeneous linewidth of 75As increasing, analysis of these results reveal the correlation times of the spin bath fluctuation dynamics. For details see [A. M. Waeber et al, Nature Physics 12, 688 (2016)].

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.

Rabi

Rabi precession of the 75As nuclear spins in magnetic field Bz=8 T. Quadrupolar moment of the nucleus (arising from its non-spherical shape) interacts with the gradients of electric field E, resulting in increased nuclear spin coherence times. For details see [E. Chekhovich et al, Nature Communications 6, 6348 (2015)].

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.

Left: Photoluminescence spectra of an InGaAs/GaAs quantum dot: both “bright” and “dark” excitons are observed. Right: p-symmetry and d-symmetry orbitals comprising the valence band states as revealed by our isotope-selective measurements of the hole hyperfine constants. For details see [E. Chekhovich et al, Nature Physics 9, 74 (2013)].

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.

Left: Transmission electron microscope (TEM) image of an InGaAs/GaAs quantum dot. Right: “Inverse” NMR spectrum of an individual InGaAs/GaAs quantum dot. Contributions from arsenic, indium and gallium nuclei are clearly resolved. For details see [E. Chekhovich et al, Nature Nanotechnology 7, 646 (2012)].

Recent publications

2013

2012

2011

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