Andrey L. Pankratov

Suppression of timing errors in short overdamped josephson junctions.

The influence of fluctuations and periodical driving on temporal characteristics of short overdamped Josephson junctions is analyzed. We obtain the standard deviation of the switching time in the presence of a dichotomous driving force for arbitrary noise intensity and in the frequency range of practical interest. For sinusoidal driving the resonant activation effect has been observed. The mean switching time and its standard deviation have a minimum as a function of driving frequency. As a consequence the optimization of the system for fast operation will simultaneously lead to the minimization of timing errors.

Optimization of the phase-locked flux-flow oscillator for the submm integrated receiver

The Superconducting Integrated Receiver (SIR) comprises in one chip a planar antenna integrated with an SIS mixer, a superconducting Flux Flow Oscillator (FFO) acting as Local Oscillator (LO) and a second SIS harmonic mixer (HM) for FFO phase locking. Free-running FFO linewidth well below 10 MHz is required to ensure phase-locked operation of an SIR. Comprehensive experimental study of the Nb-AlOx-Nb FFO linewidth and other main parameters has been carried out in order to achieve this goal. Essential dependence of the FFO linewidth on its width and idle region dimension has been found. It makes possible an optimization of the FFO design and selection of the best FFO parameters for practical operation of the SIR.

Form and width of the spectral line of a Josephson flux-flow oscillator

The behavior of a Josephson flux-flow oscillator in the presence of both bias current and magnetic field fluctuations has been studied. To derive the equation for slow phase dynamics in the limit of small noise intensity the Poincare method has been used. Both the form of spectral line and the linewidth of the flux-flow oscillator have been derived analytically on the basis of known frequency modulation theory technique, known limiting cases are considered, limits of their applicability are discussed and appearance of excess noise is explained. Good coincidence of theoretical description with experimental results has been demonstrated.

Spectral properties of phase-locked flux flow oscillator

In the present paper, the results of theoretical modeling of flux flow oscillator (FFO) included into phase-locked loop (PLL) system and its comparison with experiment are outlined. To describe theoretically the dynamics of phase-locked FFO, we have considered two models: first order PLL system and the PLL system with integral-proportional filter. While the first order PLL system may be treated analytically even in the frame of nonlinear PLL model and gives satisfactory agreement with experimental results, it does not describe all peculiarities of spectral density, which may be described well by a more sophisticated model with the integral-proportional filter. The quantitative prediction of spectral ratio improvement due to increase of PLL regulation bandwidth is given.

Line width of Josephson flux flow oscillators

A combination of wide-band electronic tunability and moderate free-running line width makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated submillimeter-wave SIS receivers. The possibility of FFO phase locking at all frequencies of interest has to be proven before one initiates real FFO applications. To achieve this goal a comprehensive set of line width measurements of the FFO operating in different regimes has been performed. FFOs with tapered shape have been successfully implemented in order to avoid the superfine resonant structure with voltage spacing of about 20 nV and extremely low differential resistance, recently observed in the IVC of the standard rectangular geometry. The obtained results have been compared with existing theories and FFO models in order to understand and possibly eliminate excess noise in the FFO. The intrinsic line width increases considerably at voltages above the boundary voltage because of the abrupt increase of the internal damping due to Josephson self-coupling. The influence of FFO parameters, in particular the differential resistances associated both with the bias current and with the applied magnetic field on the radiation line width, has been studied. Possible means of decreasing the free-running FFO line width will be discussed.

Underdamped Josephson junction as a switching current detector

We demonstrate the narrow switching distribution of an underdamped Josephson junction from the zero to the finite voltage state at millikelvin temperatures. We argue that such junctions can be used as ultrasensitive detectors of the single photons in the GHz range, operating close to the quantum limit: a given initial (zero voltage) state can be driven by an incoming signal to the finite voltage state. The width of the switching distribution at a nominal temperature of about T = 10 mK was 4.5 nA, which corresponds to an effective noise temperature of the device below 60 mK.

Lifetime of the superconductive state in short and long Josephson junctions

AbstractWe study the transient statistical properties of short and long Josephson junctions under the influence of thermal and correlated fluctuations. In particular, we investigate the lifetime of the superconductive metastable state finding the presence of noise induced phenomena. For short Josephson junctions we investigate the lifetime as a function both of the frequency of the current driving signal and the noise intensity and we find how these noise-induced effects are modified by the presence of a correlated noise source. For long Josephson junctions we integrate numerically the sine-Gordon equation calculating the lifetime as a function of the length of the junction both for inhomogeneous and homogeneous bias current distributions. We obtain a non-monotonic behavior of the lifetime as a function of the frequency of the current driving signal and the correlation time of the noise. Moreover we find two maxima in the non-monotonic behaviour of the mean escape time as a function of the correlated noise intensity.

Drastic Suppression of Noise-Induced Errors in Underdamped Long Josephson Junctions

The thermal jitter of transmission of magnetic flux quanta in long Josephson junctions is studied. While for large-to-critical damping and small values of bias current the physically obvious dependence of the jitter versus length σ ∼ √ L is confirmed, for small damping starting from the experimentally relevant α = 0.03 and below strong deviation from σ ∼ √ L is observed, up to nearly complete independence of the jitter versus length, which is exciting from fundamental point of view, but also intriguing from the point of view of possible applications.

Minimization of timing errors in reproduction of single flux quantum pulses

We present the analysis of the mean switching time and its standard deviation of an overdamped Josephson junction, driven by a direct current and a single flux quantum (SFQ) pulse. The performed analysis allows us to find the optimal value of the bias current of the clock generator, responsible for the shape of SFQ pulse, which minimizes noise-induced switching errors.

On certain time characteristics of dynamical systems driven by noise

Abstract The moments of the transition time of non-linear dynamical systems driven by noise are introduced. It is demonstrated that the well-known recurrence formula for the moments of the first passage time (FTP) of the absorbing boundary by a Brownian particle can also be used to obtain the exact values of the moments of the transition time in non-linear dynamical systems with noise, described by a symmetric potential profile.