We study decoherence in a qubit with the distance between the two levels affected by random flips of bistable fluctuators. For the case of a single fluctuator we evaluate explicitly an exact expression for the phase-memory decay in the echo experiment with a resonant ac excitation. The echo signal as a function of time shows a sequence of plateaus. The position and the height of the plateaus can be used to extract the fluctuator switching rate gamma and its coupling strength v. At small times the logarithm of the echo signal is proportional to t3. The plateaus disappear when the decoherence is induced by many fluctuators. In this case the echo signal depends on the distribution of the fluctuators parameters. According to our analysis, the results significantly deviate from those obtained in the Gaussian model as soon as v greater than or approximately equal gamma.

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Recent experiments by F. Yoshihara et al. [Phys. Rev. Lett. 97, 167001 (2006)] and by K. Kakuyanagi et al. (cond-mat/0609564) provided information on decoherence of the echo signal in Josephson-junction flux qubits at various bias conditions. These results were interpreted assuming a Gaussian model for the decoherence due to 1/f noise. Here we revisit this problem on the basis of the exactly solvable spin-fluctuator model reproducing detailed properties of the 1/f noise interacting with a qubit. We consider the time dependence of the echo signal and conclude that the results based on the Gaussian assumption need essential reconsideration.

A theoretical interpretation of the recent experiments of Astafiev et. al. on the T1-relaxation rate in Josephson Charge Qubits is proposed. The experimentally observed reproducible nonmonotonic dependence of T1 on the splitting EJ of the qubit levels suggests further specification of the previously proposed models of the background charge noise. From our point of view the most promising is the "Andreev fluctuator" model of the noise. In this model the fluctuator is a Cooper pair that tunnels from a superconductor and occupies a pair of localized electronic states. Within this model one can naturally explain both the average linear T1(EJ ) dependence and the irregular fluctuations. PACS numbers:Proposals to implement qubits using superconducting nanocircuits have undergone an amazing development during the last years [1,2,3,4,5]. In Josephson Charge Qubit (JCQ) information is encoded in the charge states of a Cooper pair box. The JCQ is manipulated by tuning gate voltage and magnetic flux. Both time resolved coherent oscillations in single and coupled JCQ have been recently observed [2,6]. Although decoherence is a severe limitation to the performances of these devices the dominant source of noise is yet to be identified. A significant step towards a characterization of the environment in a JCQ has been recently made by Astafiev et.al.[7] The experimental set-up consists of a Cooper pair box connected to a reservoir through a tunnel junction of SQUID geometry with Josephson energy E J pierced by an external magnetic field. Provided that E c ≫ E J ≫ T (where E c , E J and T are correspondingly charging energy, Josephson energy and temperature, k B = = 1). only two charge states |0 and |1 are relevant and the Hamiltonian of the box reads:where, C g is the gate capacitance, V g is the gate voltage and e denotes the electron charge. In the rotated basis {|+ , |− } the Hamiltonian (1) reads:where E = δE 2 c + E 2 J and θ = arctan(E J /δE c ). One can distinguish the off degeneracy working points (θ ≈ 0 and δE c ≫ E J ) and the degeneracy one (θ = π/2 and δE c = 0). Astafiev et.al.[7] measured the energy relaxation rate Γ 1 of the JCQ in a wide range of parameters. Two main features have been observed: (*) Linear increase of Γ 1 with E J at large E J , and (**) Small nonmonotonous fluctuations in the Γ 1 (E J )-function on this linear background.We do not believe that the existing experimental information is sufficient to identify a unique interpretation. However, it substantially reduces the range of possibilities. In this Letter we show that some models which have been used to study dephasing in JCQ can not explain these features. We propose a model where all of them appear naturally.Many different mechanisms can be responsible for decoherence in JCQ. We will consider three models, all based on the idea that the oxide layer close to some metallic reservoir, like one of the leads or gates or Cooper pair box itself, is disordered and thus hosts trapping centers, i.e. localized states for the electrons.

We study the dissipative dynamics of a qubit that is afflicted by classical random telegraph noise and it is subject to dynamical decoupling. We derive exact formulas for the qubit dynamics at arbitrary working points in the limit of infinitely strong control pulses (bang-bang control) and we investigate in great detail the efficiency of the dynamical decoupling techniques both for Gaussian and non-Gaussian (slow) noise at qubit pure dephasing and at optimal point. We demonstrate that control sequences can be successfully implemented as diagnostic tools to infer spectral proprieties of a few fluctuators interacting with the qubit. The analysis is extended in order to include the effect of noise in the pulses and we give upper bounds on the noise levels that can be tolerated in the pulses while still achieving efficient dynamical decoupling performance.

The decoherence of a qubit due to a classical non-Gaussian noise with correlation time longer than the decoherence time is discussed for arbitrary working points of the qubit. A method is developed that allows an exact formula for the phase memory functional in the presence of independent random telegraph noise sources to be derived.

It is known that silicon is an indirect band gap material, reducing its efficiency in photovoltaic applications. Using surface plasmons in metallic nanoparticles embedded in a solar cell has recently been proposed as a way to increase the efficiency of thin-film silicon solar cells. The dipole mode that dominates the plasmons in small particles produces an electric field having Fourier components with all wave numbers. In this work, we show that such a field creates electron-hole-pairs without phonon assistance, and discuss the importance of this effect compared to radiation from the particle and losses due to heating.

Evaporation of aqueous droplets of carbon nanotubes (CNTs) coated with a physisorbed layer of humic acid (HA) on a partially hydrophilic substrate induces the formation of a film of CNTs. Here, we investigate the role that the global geometry of the substrate surfaces has on the structure of the CNT film. On a flat mica or silica surface, the evaporation of a convex droplet of the CNT dispersion induces the well-known "coffee ring", while evaporation of a concave droplet (capillary meniscus) of the CNT dispersion in a wedge of two planar mica sheets or between two crossed-cylinder sheets induces a large area (>mm(2)) of textured or patterned films characterized by different short- and long-range orientational and positional ordering of the CNTs. The resulting patterns appear to be determined by two competing or cooperative sedimentation mechanisms: (1) capillary forces between CNTs giving micrometer-sized filaments parallel to the boundary line of the evaporating droplet and (2) fingering instability at the boundary line of the evaporating droplet and subsequent pinning of CNTs on the surface giving micrometer-sized filaments of CNTs perpendicular to this boundary line. The interplay between substrate surface geometry and sedimentation mechanisms gives an extra control parameter for manipulating patterns of self-assembling nanoparticles at substrate surfaces.