Critical Current Noise Research Articles

Decoherence in Josephson-junction qubits due to critical-current fluctuations

We compute the decoherence caused by $1/f$ fluctuations at low frequency $f$ in the critical current $I_0$ of Josephson junctions incorporated into flux, phase, charge and hybrid flux-charge superconducting quantum bits (qubits). The dephasing time $\tau_{\phi}$ scales as $I_0/ \Omega \Lambda S_{I_0}^{1/2}(1$ Hz$)$, where $\Omega / 2\pi$ is the energy level splitting frequency, $S_{I_0}(1$ Hz$)$ is the spectral density of the critical current noise at 1 Hz, and $\Lambda \equiv |I_0 d \Omega / \Omega d I_0|$ is a parameter computed for given parameters for each type of qubit that specifies the sensitivity of the level splitting to critical current fluctuations. Computer simulations show that the envelope of the coherent oscillations of any qubit after time $t$ scales as $\exp (-t^2/ 2 \tau_{\phi}^2)$ when the dephasing due to critical current noise dominates the dephasing from all sources of dissipation. We compile published results for fluctuations in the critical current of Josephson tunnel junctions fabricated with different technologies and a wide range in $I_0$ and $A$, and show that their values of $S_{I_0}(1$ Hz$)$ scale to within a factor of three of $[ 144 (I_0/\mu{\rm A})^2/ (A/ \mu{\rm m}^2)]($pA$)^2/$Hz at 4.2 K. We empirically extrapolate $S_{I_0}^{1/2}(1$ Hz$)$ to lower temperatures using a scaling $T($K$)/4.2$. Using this result, we find that the predicted values of $\tau_{\phi}$ at 100 mK range from 0.8 to 12 $\mu$s, and are usually substantially longer than values measured experimentally at lower temperatures.

A device with two orthogonal low-T/sub c/ SQUID gradiometers - A comparison of series and parallel gradiometric SQUID's

We have developed a planar gradiometric device with two orthogonal SQUID gradiometers integrated on a substrate. In developing this new device, both series- and parallel-type gradiometric SQUID's (without pickup coils) suitable for the same pickup coils were fabricated. The parasitic inductance, parasitic areas, and cross talks of each SQUID were evaluated to compare their performances. Next, to evaluate the effects of changing field on the SQUID's, the critical current reduction and noise increase under an applied magnetic field were measured. These results show that the parallel SQUID is superior to the series SQUID; thus the parallel SQUID was chosen for use in the new single-chip gradiometric device.

Temperature dependence of the current-phase relation for YBa2Cu3O7−x step-edge Josephson junctions

The current-phase relation (CPR) of YBa2Cu3O7−x step-edge Josephson junctions was investigated experimentally at various temperatures. The investigated junctions were incorporated into a washer-shaped superconducting ring with inductance L∼300 pH. The ring was inductively coupled to a tank circuit with a resonance frequency between 9 MHz and 40 MHz. The CPR was obtained from the measurement of the impedance of the phase-biased junction. At high temperatures and low critical current the experimentally observed CPR is sinusoidal. The same results were obtained at low temperatures when the rms flux noise in the ring is negligible. In the case of larger critical current and finite noise, deviations of the apparent CPR from sinusoidal dependence were found. The deviations can be described by thermal fluctuations of the magnetic flux in the ring.

Vortex dynamics below the flux-lattice melting transition in YBa2Cu3O7- delta.

We report measurements of the critical current, differential resistance, and voltage noise below the vortex-lattice melting transition in clean, untwinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals. The differential resistance at dc current bias much greater than ${\mathit{J}}_{\mathit{c}}$ shows a sharp feature at the melting transition accompanied by a rapid drop in the voltage noise, demonstrating that the melting transition is insensitive to the static pinning potential. In the solid phase, the measurements all show a marked change from the low-field, pinned lattice to the high-field vortex-glass contiguous with the tricritical point in this sample.

Noise in (double) relaxation oscillation SQUIDs

We have modelled the effect of two intrinsic noise sources on the flux noise spectral density of (Double) Relaxation Oscillation SQUIDs ((D)ROSs) based on hysteretic Josephson tunnel junctions. An important noise source is the spread in the critical current of the SQUID due to thermal fluctuations. Critical current noise mainly determines the noise on the average output voltage of DROSs with high flux to voltage transfer. A second noise source is the spread in the relaxation frequency due to the random interaction between the Josephson oscillations and the relaxation oscillations during switching to the zero-voltage state. This effect can dominate the voltage noise of a ROS.

Critical current and magnetic flux noise of YBaCuO thin films in the presence of flux creep

The pinning potential U of YBaCuO films in weak magnetic fields is studied experimentally. It is shown that the pinning potential increases with the increase in the critical current density J/sub c/; values of U(T=0)=25-120 meV are obtained for the films with J/sub c/(T=0)=5.3*10/sup 5/-1.8*10/sup 7/ A/cm/sup 2/. The temperature dependence of the pinning potential U(T) is also obtained. The experimental results can be explained qualitatively by the pinning model of isolated vortices. The properties of the 1/f flux noise of the film are also discussed with the experimental results of the pinning potential. It is shown that the dependences of the flux noise on the temperature and the critical current density can be explained by the properties of the pinning potential.

Fluctuations of critical current and phase slippage in Josephson junctions.

The effect of critical current noise on the phase-slip- rate in an overdamped Josephson junction is calculated. A significant increase of the Kramers escape rate is predicted for submicron metal-insulator-metal junctions. The temperature dependence of the rate shows deviations from the Arrhenius law at low temperatures if the junction conductance fluctuations are produced by quantum ionic tunneling processes in the junction barrier. Implications for magnetization decay in high-${T}_{c}$ superconductors are discussed.

Phase-slip mechanism in ultrasmall Josephson junctions with fluctuating coupling

The rate of the phase-slip in an overdamped Josephson junction in the presence of both the thermal quasiparticle noise and the critical current fluctuations is calculated. Dramatic increase of the standard Kramers rate is predicted for large intensity of the critical current noise. For a temperature-independent noise intensity, produced by quantum ionic tunneling processes in the junction barrier, significant departures from the Arrhenius law take place in the T-dependence of the rates below a characteristics temperature which may exceed the temperature for the crossover from the thermal activation to the macroscopic quantum tunneling.