Linear Hyperfine Tuning of Donor Spins in Silicon Using Hydrostatic Strain.

We experimentally study the coupling of group V donor spins in silicon to mechanical strain, and measure strain-induced frequency shifts that are linear in strain, in contrast to the quadratic dependence predicted by the valley repopulation model (VRM), and therefore orders of magnitude greater than that predicted by the VRM for small strains |ϵ|<10^{-5}. Through both tight-binding and first principles calculations we find that these shifts arise from a linear tuning of the donor hyperfine interaction term by the hydrostatic component of strain and achieve semiquantitative agreement with the experimental values. Our results provide a framework for making quantitative predictions of donor spins in silicon nanostructures, such as those being used to develop silicon-based quantum processors and memories. The strong spin-strain coupling we measure (up to 150 GHz per strain, for Bi donors in Si) offers a method for donor spin tuning-shifting Bi donor electron spins by over a linewidth with a hydrostatic strain of order 10^{-6}-as well as opportunities for coupling to mechanical resonators.

Controlling spin relaxation with a cavity.

Spontaneous emission of radiation is one of the fundamental mechanisms by which an excited quantum system returns to equilibrium. For spins, however, spontaneous emission is generally negligible compared to other non-radiative relaxation processes because of the weak coupling between the magnetic dipole and the electromagnetic field. In 1946, Purcell realized that the rate of spontaneous emission can be greatly enhanced by placing the quantum system in a resonant cavity. This effect has since been used extensively to control the lifetime of atoms and semiconducting heterostructures coupled to microwave or optical cavities, and is essential for the realization of high-efficiency single-photon sources. Here we report the application of this idea to spins in solids. By coupling donor spins in silicon to a superconducting microwave cavity with a high quality factor and a small mode volume, we reach the regime in which spontaneous emission constitutes the dominant mechanism of spin relaxation. The relaxation rate is increased by three orders of magnitude as the spins are tuned to the cavity resonance, demonstrating that energy relaxation can be controlled on demand. Our results provide a general way to initialize spin systems into their ground state and therefore have applications in magnetic resonance and quantum information processing. They also demonstrate that the coupling between the magnetic dipole of a spin and the electromagnetic field can be enhanced up to the point at which quantum fluctuations have a marked effect on the spin dynamics; as such, they represent an important step towards the coherent magnetic coupling of individual spins to microwave photons.

Reaching the quantum limit of sensitivity in electron spin resonance.

The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology and materials science, from in vivo imaging to distance measurements in spin-labelled proteins. ESR relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession. However, such signals can be very small, prohibiting the application of ESR at the nanoscale (for example, at the single-cell level or on individual nanoparticles). Here, using a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly four orders of magnitude. We demonstrate the detection of 1,700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise ratio, reduced to 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼ 0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new versatile toolbox for ESR at the nanoscale.

Nanoscale broadband transmission lines for spin qubit control.

The intense interest in spin-based quantum information processing has caused an increasing overlap between the two traditionally distinct disciplines of magnetic resonance and nanotechnology. In this work we discuss rigorous design guidelines to integrate microwave circuits with charge-sensitive nanostructures, and describe how to simulate such structures accurately and efficiently. We present a new design for an on-chip, broadband, nanoscale microwave line that optimizes the magnetic field used to drive a spin-based quantum bit (or qubit) while minimizing the disturbance to a nearby charge sensor. This new structure was successfully employed in a single-spin qubit experiment, and shows that the simulations accurately predict the magnetic field values even at frequencies as high as 30 GHz.

Chiral separation of oxprenolol by affinity electrokinetic chromatography-partial filling technique using human serum albumin as chiral selector.

The intrinsic characteristics of capillary electrophoresis have made this technique a powerful tool in the chiral separation field. The present paper deals with the enantiomeric separation of oxprenolol enantiomers by affinity electrokinetic chromatography-partial filling technique using human serum albumin (HSA) as chiral selector. Several experimental conditions and variables affecting the separation such as pH, HSA concentration and plug length, background electrolyte concentration, temperature and voltage were studied. Baseline separation of oxprenolol enantiomers was obtained in less than 8 min under the following selected conditions: electrophoretic buffer composed of 50 mM Tris-(hydroximethyl)-aminomethane (Tris) at pH 8.5; 190 microM HSA solution applied at 50 mbar for 225 s as chiral selector; oxprenolol samples contained 190 microM HSA solution injected hydrodynamically at 30 mbar for 2s and the electrophoretic runs performed at 30 degrees C applying 15 kV voltage. The proposed methodology was applied for the analysis of two pharmaceutical preparations. Resolution, accuracy, reproducibility, speed and cost of the proposed method make it suitable for quality control of the enantiomeric composition of oxprenolol in drugs. The results show that a different affinity between oxprenolol enantiomers and HSA exists and can contribute to the pharmacokinetic differentiation of these enantiomers.

Evaluation of the pH effect of formulations on the skin permeability of drugs by biopartitioning micellar chromatography.

Dermal absorption of chemicals is an area of increasing interest for the pharmaceutical and cosmetic industries, as well as in dermal exposure and risk assessment processes. Biopartitioning micellar chromatography (BMC) is a mode of reversed phase micellar chromatography that has proved to be useful in the description and prediction of several pharmacological properties of xenobiotics including oral drug absorption, ocular and skin drug permeability. The present paper deals with the application of biopartitionig micellar chromatography to evaluate the pH effect on the skin permeability of twelve non-steroidal anti-inflammatory drugs and lidocaine. For this purpose the BMC retention of the whole set of compounds at several pHs between 3.5 and 8 was obtained. Using the BMC retention-permeability model previously reported, the permeability of the compounds at different pH values was estimated. The predicted permeability values at different pH values for ketoprofen, lidocaine, salicylic acid and ibuprofen agree with those experimental reported in literature for these compounds using excised human and rat skin.

Chiral separation of bupivacaine enantiomers by capillary electrophoresis partial-filling technique with human serum albumin as chiral selector.

Capillary electrophoresis (CE) is a powerful technique for enantiomer separations due to its intrinsic high separation efficiencies, speed of analysis, low reagent consumption and small sample requirements. However, some chiral selectors present strong background UV absorption providing high detection limits. The present paper deals with the application of the partial-filling technique to the separation of bupivacaine enantiomers by capillary electrophoresis using human serum albumin (HSA) as chiral selector. In this procedure the cationic surfactant cetyltrimethylammonium bromide (CTAB) was used as a dinamic capillary coating in order to reduce the electro-osmotic flow and detect both bupivacaine enantiomers out of the chiral selector plug. Several experimental conditions such as CTAB concentration, pH, HSA concentration and plug length, background electrolyte concentration, temperature and voltage were studied. Under the selected conditions it is possible to detect the separated enantiomers out of the HSA plug in less than 4 min using 50 mM Tris pH 8 as background electrolyte with 50 microM CTAB, at 30 degrees C and using a separation voltage of 25 kV. The proposed methodology was then validated for analytical purposes and applied to the analysis of pharmaceutical preparations commercially available. The results obtained with the proposed methodology were in good agreement with those declared by the manufacturers. The simplicity, sample throughput, accuracy, reproducibility and low cost of the proposed method make it suitable for the control of the enantiomeric composition of bupivacaine in pharmaceuticals.

Biopartitioning micellar chromatography to predict skin permeability.

Dermal absorption of chemicals is an area of increasing interest to the pharmaceutical and cosmetic industries, as well as in dermal exposure and risk assessment processes. In this paper the capability of biopartitioning micellar chromatography (BMC) as an in vitro technique to describe compound percutaneous absorption is evaluated. A multivariate study (principal component analysis, partial least squares) is performed in order to evaluate the importance of some physicochemical variables on the skin permeability constant values. From these results, a quantitative retention-activity relationship model for predicting the skin permeability constants that uses the BMC retention data and melting point as descriptor variables was obtained from a heterogeneous set of 43 compounds. The main advantage of the proposed methodology is that it allows the obtention of permeability constants of ionic compound and it can be very useful to predict the effect of pH of vehicle on the skin permeability of xenobiotics.

Biopartitioning micellar separation methods: modelling drug absorption.

The search for new pharmacologically active compounds in drug discovery programmes often neglects biopharmaceutical properties as drug absorption. As a result, poor biopharmaceutical characteristics constitute a major reason for the low success rate for candidates in clinical development. Since the cost of drug development is many times larger than the cost of drug discovery, predictive methodologies aiding the selection of bioavailable drug candidates are of profound significance. This paper has been focussed on recent developments and applications of chromatographic systems, particularly those systems based on amphiphilic structures, in the frame of alternative approaches for estimating the transport properties of new drugs. The aim of this review is to take a critical look at the separations methods proposed for describing and predicting drug passive permeability across gastrointestinal tract and the skin.

QRAR models for central nervous system drugs using biopartitioning micellar chromatography.

The capability of biopartitioning Micellar Chromatography, BMC, to describe and estimate pharmacokinetic and pharmacodynamic parameters of central nervous system drugs is reviewed in this article. BMC is a mode of micellar liquid chromatography, MLC, that uses micellar mobile phases of Brij35 (polyoxyethilene(23) lauryl ether) prepared in physiological conditions (pH, ionic strength). The retention of a drug in this system depends on its hydrophobic, electronic and steric properties, which also determine its biological activity. The results of BMC studies suggest that this in vitro approach is an attractive useful tool to be implemented into the lead optimization step of drug development scheme.

Retention-property relationships of anticonvulsant drugs by biopartitioning micellar chromatography.

Epilepsy may be considered as a group of disorders with only one thing in common: the fact that recurrent anomalous electrochemical phenomena appear in the central nervous system. Different classes of drugs are included under the generic term of anticonvulsant drugs. All of them work by decreasing discharge propagation in different ways. Biopartitioning micellar chromatography (BMC) is a mode of reversed-phase liquid chromatography, which can be used as an in vitro system to model the biopartitioning process of drugs when there are no active processes. In this paper, relationships between the BMC retention data of anticonvulsant drugs, their pharmacokinetics (oral absorption, protein binding, volume of distribution, clearance, and renal elimination) and their therapeutic parameters (therapeutic, toxic and comatose-fatal concentration, and LD50) are studied and the predictive ability of models is evaluated.