Density domains of a photo-excited electron gas on liquid helium

The Coulombic effect on the stability range of the photo-excited electron gas on liquid helium is shown to favor formation of domains of different densities. Domains appear to eliminate or greatly reduce regions with negative conductivity. An analysis of the density domain structure allows explaining remarkable observations reported recently for the photo-excited electron gas.Comment: 5 pages, 5 figure

Multiphoton antiresonance

We show that nonlinear response of a quantum oscillator displays antiresonant dips and resonant peaks with varying frequency of the driving field. The effect is a consequence of special symmetry and is related to resonant multiphoton mixing of several pairs of oscillator states at a time. We discuss the possibility to observe the antiresonance and the associated multiphoton Rabi oscillations in Josephson junctions.Comment: 4 pages, 3 figures; corrected referenc

Multiphoton Antiresonance and Quantum Activation in Driven Systems

We show that nonlinear response of a quantum oscillator displays antiresonant dips and resonant peaks with varying frequency of the driving field. The effect is a consequence of special symmetry and is related to resonant multiphoton mixing of several pairs of oscillator states at a time. We also discuss escape from a metastable state of forced vibrations. Two important examples show that the probability of escape via diffusion over quasienergy is larger than via dynamical tunneling provided the relaxation rate exceeds both of them. Diffusion dominates even for zero temperature, so that escape occurs via quantum rather than thermal activation. The effects can be studied using Josephson junctions and Josephson-junction based systems.Comment: An invited talk at "Unsolved Problems of Noise", 200

Novel Radiation-induced Magnetoresistance Oscillations in a Nondegenerate 2DES on Liquid Helium

We report the observation of novel magnetoresistance oscillations induced by the resonant inter-subband absorption in nondegenerate 2D electrons bound to the surface of liquid helium. The oscillations are periodic in 1/B and originate from the scattering-mediated transitions of the excited electrons into the Landau states of the first subband. The structure of the oscillations is affected by the collision broadening of the Landau levels and by many-electron effects.Comment: 4 figure

Relaxation of a qubit measured by a driven Duffing oscillator

We investigate the relaxation of a superconducting qubit for the case when its detector, the Josephson bifurcation amplifier, remains latched in one of its two (meta)stable states of forced vibrations. The qubit relaxation rates are different in different states. They can display strong dependence on the qubit frequency and resonant enhancement, which is due to quasienergy resonances. Coupling to the driven oscillator changes the effective temperature of the qubit.Comment: To appear in Phys. Rev. A (2010

Qubit state detection using the quantum Duffing oscillator

We introduce a detection scheme for the state of a qubit, which is based on resonant few-photon transitions in a driven nonlinear resonator. The latter is parametrically coupled to the qubit and is used as its detector. Close to the fundamental resonator frequency, the nonlinear resonator shows sharp resonant few-photon transitions. Depending on the qubit state, these few-photon resonances are shifted to different driving frequencies. We show that this detection scheme offers the advantage of small back action, a large discrimination power with an enhanced read-out fidelity, and a sufficiently large measurement efficiency. A realization of this scheme in the form of a persistent current qubit inductively coupled to a driven SQUID detector in its nonlinear regime is discussed.Comment: 10 pages, 6 figures. To appear in Phys. Rev.

Spectrum of an oscillator with jumping frequency and the interference of partial susceptibilities

We study an underdamped oscillator with shot-noise frequency fluctuations. The oscillator spectrum is determined by the interference of the susceptibilities for different eigenfrequencies. Depending on the parameters, it has a fine structure or displays a single asymmetric peak. For nano-mechanical resonators with a fluctuating number of attached molecules, the spectrum is found in a simple analytical form. The results bear on various types of systems where the reciprocal correlation time of frequency fluctuations can be comparable to the typical frequency jumps