Nuclear spin squeezing via electric quadrupole interaction

Control over nuclear spin fluctuations is essential for processes that rely on preserving the quantum state of an embedded system. For this purpose, squeezing is a viable alternative, so far that has not been properly exploited for the nuclear spins. Of particular relevance in solids is the electric quadrupole interaction (QI), which operates on nuclei having spin higher than 1/2. In its general form, QI involves an electric field gradient (EFG) biaxiality term. Here, we show that as this EFG biaxiality increases, it enables continuous tuning of single-particle squeezing from the one-axis twisting to the two-axis countertwisting limits. A detailed analysis of QI squeezing is provided, exhibiting the intricate consequences of EFG biaxiality. The initial states over the Bloch sphere are mapped out to identify those favorable for fast initial squeezing, or for prolonged squeezings. Furthermore, the evolution of squeezing in the presence of a phase-damping channel and an external magnetic field are investigated. We observe that dephasing drives toward an anti-squeezed terminal state, the degree of which increases with the spin angular momentum. Finally, QI squeezing in the limiting case of a two-dimensional EFG with a perpendicular magnetic field is discussed, which is of importance for two-dimensional materials, and the associated beat patterns in squeezing are revealed.Comment: Published version in contents, 10 pages, 9 figure

Quadrupolar spectra of nuclear spins in strained InGaAs quantum dots

Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling, strain affects the magnetic environment as seen by the nuclear spins. To guide prospective single-QD nuclear magnetic resonance (NMR) as well as dynamic nuclear spin polarization experiments, an atomistic insight to the strain and quadrupolar field distributions is presented. A number of implications of the structural and compositional profile of the QD have been identified. A high aspect ratio of the QD geometry enhances the quadrupolar interaction. The inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. Regarding the NMR spectra, both Faraday and Voigt geometries are investigated, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally-forbidden transitions. Moreover, it is shown that from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. By means of the element-resolved NMR intensities it is found that In nuclei has a factor of five dominance over those of As. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. At these values the nuclear spin depolarization rates of the respective nuclei get maximized due to the noncollinear secular hyperfine interaction with a resident electron in the QD.Comment: Published version, 13 pages, 9 figure

Stark effect, polarizability and electroabsorption in silicon nanocrystals

Demonstrating the quantum-confined Stark effect (QCSE) in silicon nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other materials. Here, the recent experimental data from ion-implanted Si NCs is unambiguously explained within the context of QCSE using an atomistic pseudopotential theory. This further reveals that the majority of the Stark shift comes from the valence states which undergo a level crossing that leads to a nonmonotonic radiative recombination behavior with respect to the applied field. The polarizability of embedded Si NCs including the excitonic effects is extracted over a diameter range of 2.5--6.5 nm, which displays a cubic scaling, $\alpha=c D^3$, with $c=2.436\times 10^{-11}$ C/(Vm), where $D$ is the NC diameter. Finally, based on intraband electroabsorption analysis, it is predicted that p-doped Si NCs will show substantial voltage tunability, whereas n-doped samples should be almost insensitive. Given the fact that bulk silicon lacks the linear electro-optic effect as being a centrosymmetric crystal, this may offer a viable alternative for electrical modulation using p-doped Si NCs.Comment: Published version, 7 pages, 7 figure

Nuclear magnetic resonance inverse spectra of InGaAs quantum dots: Atomistic level structural information

A wealth of atomistic information is contained within a self-assembled quantum dot (QD), associated with its chemical composition and the growth history. In the presence of quadrupolar nuclei, as in InGaAs QDs, much of this is inherited to nuclear spins via the coupling between the strain within the polar lattice and the electric quadrupole moments of the nuclei. Here, we present a computational study of the recently introduced inverse spectra nuclear magnetic resonance technique to assess its suitability for extracting such structural information. We observe marked spectral differences between the compound InAs and alloy InGaAs QDs. These are linked to the local biaxial and shear strains, and the local bonding configurations. The cation-alloying plays a crucial role especially for the arsenic nuclei. The isotopic line profiles also largely differ among nuclear species: While the central transition of the gallium isotopes have a narrow linewidth, those of arsenic and indium are much broader and oppositely skewed with respect to each other. The statistical distributions of electric field gradient (EFG) parameters of the nuclei within the QD are analyzed. The consequences of various EFG axial orientation characteristics are discussed. Finally, the possibility of suppressing the first-order quadrupolar shifts is demonstrated by simply tilting the sample with respect to the static magnetic field.Comment: Published version, 17 pages, 18 figure

Wavelet-based Ramsey magnetometry enhancement of a single NV center in diamond

Nitrogen-vacancy (NV) centers in diamond constitute a solid-state nanosensing paradigm. Specifically for high-precision magnetometry, the so-called Ramsey interferometry is the prevalent choice where the sensing signal is extracted from time-resolved spin-state-dependent photoluminescence (PL) data. Its sensitivity is ultimately limited by the photon shot noise (PSN), which cannot be sufficiently removed by averaging or frequency filtering. Here, we propose Ramsey DC magnetometry of a single NV center enhanced by a wavelet-denoising scheme specifically tailored to suppress PSN. It simply operates as a post-processing applied on a collected PL time series. Our implementation is based on template margin thresholding which we computationally benchmark, and demonstrate its signal-to-noise-ratio improvement over the standard quantum limit by up to an order of magnitude for the case of limited-time-budget measurements.Comment: 10 pages, 3 figures, comments are welcom