Electrostatic force spectroscopy of near-surface localized states

Electrostatic force microscopy at cryogenic temperatures is used to probe the electrostatic interaction of a conductive atomic force microscopy tip and electronic charges trapped in localized states in an insulating layer on a semiconductor. Measurement of the frequency shift of the cantilever as a function of tip-sample shows discrete peaks at certain voltages when the tip is located near trap centers. These discrete changes in frequency is attributed to one by one filling of individual electronic states when the quantized energies traverses the substrate conduction band fermi energy as tip-sample voltage is increased. Theoretical analysis of the experiment suggests that such measurement of the cantilever frequency shift as a function of bias voltage can be interpreted as an AC force measurement, from which spectroscopic information about the location, energy and tunneling times of localized states can be deduced. Experimental results from study of a sample with InAs quantum dots as trap centers is presented.Comment: 26 pages, 10 figure

Quantum correlation in degenerate optical parametric oscillators with mutual injections

We theoretically and numerically study the quantum dynamics of two degenerate optical parametric oscillators with mutual injections. The cavity mode in the optical coupling path between the two oscillator facets is explicitly considered. Stochastic equations for the oscillators and mutual injection path based on the positive $P$ representation are derived. The system of two gradually pumped oscillators with out-of-phase mutual injections is simulated, and its quantum state is investigated. When the incoherent loss of the oscillators other than the mutual injections is small, the squeezed quadratic amplitudes $\hat{p}$ in the oscillators are positively correlated near the oscillation threshold. It indicates finite quantum correlation, estimated via Gaussian quantum discord, and the entanglement between the intracavity subharmonic fields. When the loss in the injection path is low, each oscillator around the phase transition point forms macroscopic superposition even under a small pump noise. It suggests that the squeezed field stored in the low-loss injection path weakens the decoherence in the oscillators.Comment: 14 pages, 9 figures; v3: author added, minor updat