Decoherence and 1/f noise in Josephson qubits

We propose and study a model of dephasing due to an environment of bistable fluctuators. We apply our analysis to the decoherence of Josephson qubits, induced by background charges present in the substrate, which are also responsible for the 1/f noise. The discrete nature of the environment leads to a number of new features which are mostly pronounced for slowly moving charges. Far away from the degeneracy this model for the dephasing is solved exactly.     

1/f Flux noise in Josephson phase qubits

We present a new method to measure 1/f noise in Josephson quantum bits (qubits) that yields low-frequency spectra below 1 Hz. A comparison of the noise taken at positive and negative bias of a phase qubit shows the dominant noise source to be flux noise and not junction critical-current noise, with a magnitude similar to that measured previously in other systems. Theoretical calculations show that the level of flux noise is not compatible with the standard model of noise from two-level state defects in the surface oxides of the films.     

金属薄膜电迁移1/f噪声与1/f2噪声统一模型

应用晶粒边界自由体积的概念建立了能够统一描述金属薄膜1f噪声与1f2噪声的模型.该模型表明,结构完整的多晶金属薄膜产生的电噪声为1f噪声,当金属薄膜受到电迁移损伤而形成空洞时就会引入1f2噪声的成分.在电迁移应力实验中,观察到金属薄膜1fγ噪声在空洞成核前γ约为10,一旦发生空洞成核,即突增至16以上,这一规律与本模型的预测相符合 关键词： 金属薄膜 1fγ噪声 电迁移 自由体积     

Probing noise in flux qubits via macroscopic resonant tunneling

Macroscopic resonant tunneling between the two lowest lying states of a bistable rf SQUID is used to characterize noise in a flux qubit. Measurements of the incoherent decay rate as a function of flux bias revealed a Gaussian-shaped profile that is not peaked at the resonance point but is shifted to a bias at which the initial well is higher than the target well. The rms amplitude of the noise, which is proportional to the dephasing rate 1/tauphi, was observed to be weakly dependent on temperature below 70 mK. Analysis of these results indicates that the dominant source of low energy flux noise in this device is a quantum mechanical environment in thermal equilibrium.     

1/f noise in varistors

An experimental investigation was made of low-frequency fluctuations of the open-circuit voltage of mass-produced Varistors made of silicon carbide. It was concluded that the main charge transport mechanism in varistors may be the Frenkel ionization of impurities.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 11–15, November, 1980.     

Decoherence of qubits: results beyond Markovian approximation

In this paper the master equation method is used to calculate the relaxation and decoherence times of a qubit. The results are beyond Markovian approximation, where the noise spectrum is assumed to be wide-band, so that they are valid for not only the wide- but also the narrow-band noises, which may be the main decoherence source in solid-state qubits. Moreover, for some special cases, analytical results can be achieved, which are consistent with those derived by others.     

A theory of 1/f noise

Letu() be an absolutely integrable function and define the random process where thet _i are Poisson arrivals and thes _i, are identically distributed nonnegative random variables. Under routine independence assumptions, one may then calculate a formula for the spectrum ofn(t), S _n(), in terms of the probability density ofs, p_s(). If any probability density p_s() having the property p_s() I for small is substituted into this formula, the calculated S_n() is such that S_n() 1 for small . However, this is not a spectrum of a well-defined random process; here, it is termed alimit spectrum. If a probability density having the property p_s() for small , where > 0, is substituted into the formula instead, a spectrum is calculated which is indeed the spectrum of a well-defined random process. Also, if the latter p_s is suitably close to the former p_s, then the spectrum in the second case approximates, to an arbitrary, degree of accuracy, the limit spectrum. It is shown how one may thereby have 1/f noise with low-frequency turnover, and also strict 1/f ^1– noise (the latter spectrum being integrable for > 0). Suitable examples are given. Actually, u() may be itself a random process, and the theory is developed on this basis.     

Decoherence of quantum information of qubits by stochastic dephasing

Decoherence of purity, distinguishability and entanglement of qubit states is investigated under the influence of stochastic dephasing which obeys the Gauss–Markov process and the two-state jump Markov process. The quantum teleportation and quantum dense coding under the influence of stochastic dephasing are also discussed.     

The so-called 1/Δ f noise

A sinusoidal ac current of frequeencyf _c generates an excess noise in many solidstate conductors. The power spectrum of this noise is calculated starting from the autocorrelation function of the stochastic process. Besides a 1/Δf term the spectrum includes another contribution varying with 1/(f _c+f). In the low frequency range there is white noise.     

Quantum chaos and 1/f noise

It is shown that the energy spectrum fluctuations of quantum systems can be formally considered as a discrete time series. The power spectrum behavior of such a signal for different systems suggests the following conjecture: The energy spectra of chaotic quantum systems are characterized by 1/f noise.     

Decoherence due to elastic Rayleigh scattering

We present theoretical and experimental studies of the decoherence of hyperfine ground-state superpositions due to elastic Rayleigh scattering of light off resonant with higher lying excited states. We demonstrate that under appropriate conditions, elastic Rayleigh scattering can be the dominant source of decoherence, contrary to previous discussions in the literature. We show that the elastic-scattering decoherence rate of a two-level system is given by the square of the difference between the elastic-scattering amplitudes for the two levels, and that for certain detunings of the light, the amplitudes can interfere constructively even when the elastic-scattering rates from the two levels are equal. We confirm this prediction through calculations and measurements of the total decoherence rate for a superposition of the valence electron spin levels in the ground state of 9Be+ in a 4.5?T magnetic field.     

On the interpretation of 1/f noise

We propose a model of 1/f noise based on a random walk in a random potential. Numerical support for the model is given, and physical applicability discussed.     

From 1/f noise to multifractal cascades in heartbeat dynamics

We explore the degree to which concepts developed in statistical physics can be usefully applied to physiological signals. We illustrate the problems related to physiologic signal analysis with representative examples of human heartbeat dynamics under healthy and pathologic conditions. We first review recent progress based on two analysis methods, power spectrum and detrended fluctuation analysis, used to quantify long-range power-law correlations in noisy heartbeat fluctuations. The finding of power-law correlations indicates presence of scale-invariant, fractal structures in the human heartbeat. These fractal structures are represented by self-affine cascades of beat-to-beat fluctuations revealed by wavelet decomposition at different time scales. We then describe very recent work that quantifies multifractal features in these cascades, and the discovery that the multifractal structure of healthy dynamics is lost with congestive heart failure. The analytic tools we discuss may be used on a wide range of physiologic signals. (c) 2001 American Institute of Physics.     

Modeling 1/f noise using moving averages

A model of 1/f noise is considered, based on moving averages of ordern. The coefficientsα _k defining the model are calculated numerically using Seidel iteration which turns out to converge rapidly. The convergence is independent ofn which seems to be caused by the fact that the nonlinear problem solved is defined by a self-similar matrix. The coefficientsα _k appear to approach, with indefinitely growingn, valuesα _k =1/√k and thus the model has a kind of Fourier invariance. Physical interpretation of the invariance is suggested as well as of coefficientsα _k describing long-range correlations. Fractal sets of dimensiond=2.5 are proposed to play certain role in explaining the latter. This work was partly supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic (VEGA) under the Grant No. 1/3143/96 and by the Slovak Literary Fund.