A. Gruber

Single spin states in a defect center resolved by optical spectroscopy

Individual paramagnetic defect centers in diamond nanocrystals have been investigated by low-temperature high-resolution optical spectroscopy. Narrow fluorescence excitation spectral lines have been found, indicating transitions between individual spin sublevels. Spectral diffusion is explained by cross relaxation among spin sublevels and by the presence of excited electrons in the conduction band of diamond. The relaxation times are in the millisecond range. The system may be useful for quantum information processing with individual electron spins.

Magnetic resonance on single nuclei

Abstract The magnetic resonance signal of individual hydrogen nuclei has been detected. The experiments have been performed on a single pentacene molecule with the aid of optically detected electron nuclear double resonance. Two nuclear magnetic resonance lines of 30 kHz width have been observed as a 3% change in the fluorescence intensity of a single molecule. The results can be reproduced by a spin Hamilton operator describing the interaction of a single electron spin with two hydrogen nuclei.

Magnetic resonance on single molecules in an external magnetic field. The Zeeman effect of a single electron spin and determination of the orientation of individual molecules

Abstract The interaction of a single electron spin with an external magnetic field is presented. The broadening and shift in the electron spin resonance lineshape is discussed on the basis of classical ensemble theories. Due to a novel frequency stabilization scheme we were able to perform time consuming investigations at different orientations of the external magnetic field with respect to the molecule. Two different orientations of the molecules in the sample have been found which coincide in their relative orientation with two sites of matrix molecules. The orientational disorder of the molecules belonging to one of these sites is below 8°.

Correlation spectroscopy of individual molecules immobilized on surfaces under ambient conditions

Abstract Individual dye molecules, immobilized on surfaces have been observed under ambient conditions with confocal microscopy. The temporal evolution of the fluorescence intensity of these molecules is studied via the autocorrelation function of the fluorescence intensity. Nonexponentially decaying correlation functions with time constants ranging from 50 ms to 5 s have been observed for different molecules. A statistical analysis on 40 different molecules yields typical correlation times around 100 ms and 6 s.

Projection noise in the optically detected magnetic resonance signal of a single electron spin.

The spin quantum noise owing to a single molecular triplet electron spin jumping from one eigenstate to another has been detected. As a result of the superposition of two spin eigenstates in the triplet level of the molecule by a microwave pi/2-pulse an increase in the noise of the fluorescence intensity of a single molecule is detected. This increase in noise is attributed to the decay of coherence between the two spin eigenstates.

Detection of a single electron spin

The past decade has been characterised by the unpredicted blossoming of ultra high spectrally and spatially resolving microscopic and spectroscopic techniques. Well known examples of these are scanning tunneling microscopy and high resolution laser spectroscopy. The ultimate goal in resolution and sensitivity is the detection of single atoms or molecules. This has been achieved by both techniques. Atomic resolution in various scanning probe techniques is rather common at present /1/. The possibility to isolate and detect single atoms in laser spectroscopy has been demonstrated approximately 10 years ago /2/. Recently it has also been shown that it is possible to do optical spectroscopy on single molecules in liquids /3/ as well as in solids at low temperature /4,5/. In this communication we want to report results on electron spin resonance (ESR) on single molecules and hence on single electron spins in solid matrices at low temperature.