The noise in the resonant soliton mode of long and narrow Josephson tunnel junctions (Josephson transmission lines or JTLs) have been measured in the frequency range from 0.1 Hz to 25 kHz by means of a DC SQUID. The measured white noise was found, to within a factor of two, to be equal to the Nyquist voltage noise in a resistance equal to the dynamic resistance R/sub D/ of the current-voltage characteristic of the bias point. In contrast, measurements of the linewidth of the microwave radiation from the same JTL showed that the spectral density of the underlying noise voltage scaled as R/sub D//sup 2//R/sub S/ where R/sub S/ is the static resistance. The origin of the different behavior is not known.The noise in the resonant soliton mode of long and narrow Josephson tunnel junctions (Josephson transmission lines or JTLs) have been measured in the frequency range from 0.1 Hz to 25 kHz by means of a DC SQUID. The measured white noise was found, to within a factor of two, to be equal to the Nyquist voltage noise in a resistance equal to the dynamic resistance R/sub D/ of the current-voltage characteristic of the bias point. In contrast, measurements of the linewidth of the microwave radiation from the same JTL showed that the spectral density of the underlying noise voltage scaled as R/sub D//sup 2//R/sub S/ where R/sub S/ is the static resistance. The origin of the different behavior is not known.

Sep 1, 19900 citations

Noise performance of superconductive magnetometers based on long Josephson tunnel junctions

C. Granata, Antonio Vettoliere, R. Monaco

The low-current fluctuations at cryogenic temperatures together with the low dynamical resistance in the resonant states of Josephson tunnel junctions allow for the realization of superconducting oscillators up to the THz range with ultra-low spectral linewidth. By virtue of the Josephson frequency-voltage relationship, we show that the same properties can be exploited for the practical realization of magnetic flux-to-voltage transducers based on the flux–flow in long Josephson tunnel junctions whose intrinsic low-frequency voltage fluctuations at amount to few , that is, too small to be measured by any present semiconductor electronics. Nevertheless, by using a double transformer SQUID amplifier we demonstrate that the (amplitude) voltage spectral density, , of an all-niobium sensor does not exceed the level of and is not affected by excess noise at least down to few hertz. Such ultra-low white noise, corresponding to a magnetic field noise , together with a highly linear and broadband voltage responsivity over a wide magnetic flux range, makes the flux–flow magnetometers potentially competitive with SQUID-based devices.The low-current fluctuations at cryogenic temperatures together with the low dynamical resistance in the resonant states of Josephson tunnel junctions allow for the realization of superconducting oscillators up to the THz range with ultra-low spectral linewidth. By virtue of the Josephson frequency-voltage relationship, we show that the same properties can be exploited for the practical realization of magnetic flux-to-voltage transducers based on the flux–flow in long Josephson tunnel junctions whose intrinsic low-frequency voltage fluctuations at amount to few , that is, too small to be measured by any present semiconductor electronics. Nevertheless, by using a double transformer SQUID amplifier we demonstrate that the (amplitude) voltage spectral density, , of an all-niobium sensor does not exceed the level of and is not affected by excess noise at least down to few hertz. Such ultra-low white noise, corresponding to a magnetic field noise , together with a highly linear and broadband voltage responsivity over a wide magnetic flux range, makes the flux–flow magnetometers potentially competitive with SQUID-based devices.

Superconductor Science and TechnologyAug 12, 20148 citations

Measurements of quantum noise in resistively shunted Josephson junctions

R. H. Koch, Dale J. Van Harlingen, John L. Clarke

Measurements have been made of the low-frequency spectral density of the voltage noise in current-biased resistively shunted Josephson tunnel junctions under conditions in which the noise mixed-down from frequencies near the Josephson frequency (${\ensuremath{\nu}}_{J}$) to the measurement frequency ($\ensuremath{\ll}{\ensuremath{\nu}}_{J}$) is in the regime $h{\ensuremath{\nu}}_{J}>{k}_{B}T$. In this limit, quantum corrections to the mixed-down noise are important. The spectral densities measured on junctions with current-voltage characteristics close to the Stewart-McCumber model were in excellent agreement with the predicted values, with no fitted parameters. The mixed-down noise for a wide range of bias voltages was used to infer the spectral density of the current noise in the shunt resistor at frequency $\ensuremath{\nu}$. With no fitted parameters, this spectral density at frequencies up to 500 GHz was in excellent agreement with the prediction $(\frac{2h\ensuremath{\nu}}{R})coth(\frac{h\ensuremath{\nu}}{2{k}_{B}T})$. The presence of the zero-point term, $\frac{2h\ensuremath{\nu}}{R}$, at frequencies $h\ensuremath{\nu}>{k}_{B}T$ was clearly demonstrated. The current-voltage characteristics of a junction with ${\ensuremath{\beta}}_{L}\ensuremath{\equiv}\frac{2\ensuremath{\pi}{L}_{s}{I}_{0}}{{\ensuremath{\Phi}}_{0}}\ensuremath{\sim}1$ and ${\ensuremath{\beta}}_{C}\ensuremath{\equiv}\frac{2\ensuremath{\pi}{I}_{0}{R}^{2}C}{{\ensuremath{\Phi}}_{0}}\ensuremath{\ll}1$, where ${I}_{0}$ is the critical current, $C$ is the junction capacitance, and ${L}_{s}$ is the shunt inductance, showed structure at voltages where the Josephson frequency was near a subharmonic of the ${L}_{s}C$ resonant frequency. The additional nonlinearity of the $I\ensuremath{-}V$ characteristic caused mixing down of noise near higher harmonics of the Josephson frequency, thereby greatly enhancing the voltage noise. The measured noise was in good agreement with that predicted by computer simulations.Measurements have been made of the low-frequency spectral density of the voltage noise in current-biased resistively shunted Josephson tunnel junctions under conditions in which the noise mixed-down from frequencies near the Josephson frequency (${\ensuremath{\nu}}_{J}$) to the measurement frequency ($\ensuremath{\ll}{\ensuremath{\nu}}_{J}$) is in the regime $h{\ensuremath{\nu}}_{J}>{k}_{B}T$. In this limit, quantum corrections to the mixed-down noise are important. The spectral densities measured on junctions with current-voltage characteristics close to the Stewart-McCumber model were in excellent agreement with the predicted values, with no fitted parameters. The mixed-down noise for a wide range of bias voltages was used to infer the spectral density of the current noise in the shunt resistor at frequency $\ensuremath{\nu}$. With no fitted parameters, this spectral density at frequencies up to 500 GHz was in excellent agreement with the prediction $(\frac{2h\ensuremath{\nu}}{R})coth(\frac{h\ensuremath{\nu}}{2{k}_{B}T})$. The presence of the zero-point term, $\frac{2h\ensuremath{\nu}}{R}$, at frequencies $h\ensuremath{\nu}>{k}_{B}T$ was clearly demonstrated. The current-voltage characteristics of a junction with ${\ensuremath{\beta}}_{L}\ensuremath{\equiv}\frac{2\ensuremath{\pi}{L}_{s}{I}_{0}}{{\ensuremath{\Phi}}_{0}}\ensuremath{\sim}1$ and ${\ensuremath{\beta}}_{C}\ensuremath{\equiv}\frac{2\ensuremath{\pi}{I}_{0}{R}^{2}C}{{\ensuremath{\Phi}}_{0}}\ensuremath{\ll}1$, where ${I}_{0}$ is the critical current, $C$ is the junction capacitance, and ${L}_{s}$ is the shunt inductance, showed structure at voltages where the Josephson frequency was near a subharmonic of the ${L}_{s}C$ resonant frequency. The additional nonlinearity of the $I\ensuremath{-}V$ characteristic caused mixing down of noise near higher harmonics of the Josephson frequency, thereby greatly enhancing the voltage noise. The measured noise was in good agreement with that predicted by computer simulations.

Physical review. B, Condensed matterJul 1, 1982117 citations

Relaxation Oscillations in Inductively Shunted Josephson Tunnel Junctions

Nils P. Calander, T. Claeson, S. Rudner

Relaxation oscillations at subharmonics of the Josephson frequency may occur in a resistively shunted superconducting tunnel junction, where the shunt loop has a large inductance. We have studied the variation in the relaxation period with the shunt inductance and resistance, the Josephson critical current, and the bias current. Steps in the current-voltage curve were noted at bias points where the subharmonic number changed. Radiation powers of the order of a nW were coupled out of small, single, tunnel junctions at 10 GHz. Linewidths scaled with the frequency square as compared with those of the Josephson mode oscillations. Wide signals centered around harmonics of the relaxation frequency were due to amplified noise. Injection locking to a fraction of the frequency of a relatively strong signal occurred, while weaker signals were amplified. Gains up to 15 dB were achieved within a fairly large bandwidth, but the noise temperature was high. The importance of parametrically up-converted noise from low idler frequencies is emphasized.Relaxation oscillations at subharmonics of the Josephson frequency may occur in a resistively shunted superconducting tunnel junction, where the shunt loop has a large inductance. We have studied the variation in the relaxation period with the shunt inductance and resistance, the Josephson critical current, and the bias current. Steps in the current-voltage curve were noted at bias points where the subharmonic number changed. Radiation powers of the order of a nW were coupled out of small, single, tunnel junctions at 10 GHz. Linewidths scaled with the frequency square as compared with those of the Josephson mode oscillations. Wide signals centered around harmonics of the relaxation frequency were due to amplified noise. Injection locking to a fraction of the frequency of a relatively strong signal occurred, while weaker signals were amplified. Gains up to 15 dB were achieved within a fairly large bandwidth, but the noise temperature was high. The importance of parametrically up-converted noise from low idler frequencies is emphasized.

Physica ScriptaJun 1, 198211 citations

Microwave noise in high-T c Josephson junctions

Erich N. Grossman, Leila R. Vale, David A. Rudman

We have measured the noise of YBa2Cu3O7−δ superconductor-normal-superconductor (SNS) junctions whose high normal-state resistances and characteristic frequencies make them suitable for THz frequency mixers. By directly measuring the 1 GHz power spectral density delivered to a low-noise 50 Ω radiometer system, the noise could be measured over a wide range of dc voltage and temperature, without complications due to 1/f noise, and without invoking any specific model. At a physical temperature of 4 K, the lowest noise junction had an available noise temperature of 31±2 K, corresponding to an effective noise temperature of the normal resistance of 9 K. The effective noise temperature of the normal resistance is approximately equal to the physical temperature at high temperatures, but approaches a limiting value at low temperatures, implying an excess current noise of unknown origin.We have measured the noise of YBa2Cu3O7−δ superconductor-normal-superconductor (SNS) junctions whose high normal-state resistances and characteristic frequencies make them suitable for THz frequency mixers. By directly measuring the 1 GHz power spectral density delivered to a low-noise 50 Ω radiometer system, the noise could be measured over a wide range of dc voltage and temperature, without complications due to 1/f noise, and without invoking any specific model. At a physical temperature of 4 K, the lowest noise junction had an available noise temperature of 31±2 K, corresponding to an effective noise temperature of the normal resistance of 9 K. The effective noise temperature of the normal resistance is approximately equal to the physical temperature at high temperatures, but approaches a limiting value at low temperatures, implying an excess current noise of unknown origin.

Applied Physics LettersMar 27, 19954 citations

Shot noise in YBCO bicrystal Josephson junctions

K. Y. Constantinian, G. A. Ovsyannikov, I. V. Borisenko +2 more

We measured spectral noise density in YBCO symmetric bicrystal Josephson junctions on sapphire substrates at bias voltages up to 100 mV and T=4.2 K. Normal state resistance of the Josephson junctions, R/sub N/=20-90 /spl Omega/ and I/sub C/R/sub N/ up to 2.2 mV have been observed in the experimental samples. Noise measurements were carried out within frequency bands of 1-2 GHz and 0.3-300 kHz. At bias voltages 10/spl les/V/spl les/60 mV a linear voltage dependence of noise power has been registered, while at V/spl les/5 mV a noticeable noise rise has been observed. The latter may explain the experimentally measured linewidth broadening of Josephson oscillations at mm and submm wave frequencies in high-Tc superconducting junctions. Experimental results are discussed in terms of bound states existing at surfaces of d-wave superconducting electrodes.We measured spectral noise density in YBCO symmetric bicrystal Josephson junctions on sapphire substrates at bias voltages up to 100 mV and T=4.2 K. Normal state resistance of the Josephson junctions, R/sub N/=20-90 /spl Omega/ and I/sub C/R/sub N/ up to 2.2 mV have been observed in the experimental samples. Noise measurements were carried out within frequency bands of 1-2 GHz and 0.3-300 kHz. At bias voltages 10/spl les/V/spl les/60 mV a linear voltage dependence of noise power has been registered, while at V/spl les/5 mV a noticeable noise rise has been observed. The latter may explain the experimentally measured linewidth broadening of Josephson oscillations at mm and submm wave frequencies in high-Tc superconducting junctions. Experimental results are discussed in terms of bound states existing at surfaces of d-wave superconducting electrodes.

IEEE Transactions on Applied SuperconductivityJun 1, 20035 citations

{I}_{0}

is the critical current,

C

is the junction capacitance, and

{L}_{s}

is the shunt inductance, showed structure at voltages where the Josephson frequency was near a subharmonic of the

{L}_{s}C

resonant frequency. The additional nonlinearity of the $I\ensuremath{-}V$ characteristic caused mixing down of noise near higher harmonics of the Josephson frequency, thereby greatly enhancing the voltage noise. The measured noise was in good agreement with that predicted by computer simulations.