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Low-frequency noise in superconductor tunnel junctions

Alexander N. Vystavkin, Vladimir N. Gubankov, V. E. Zhuravlev, V. P. Koshelets, Yu. V. Obukhov, Тарасов

Published Dec 1, 1983• 1 citation
OpenAlex

Abstract

The purpose of the present work is to measure the low-frequency (30 Hz- 30 kHz) noise in superconductor insulator-superconductor (SIS) junctions in order to estimate the noise temperature at the output, identify the noise sources and the mechanisms by which the noise is transformed, and compare the findings with the analogous results for Josephson junctions. (AIP)

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Quantum mechanics can strongly influence the noise properties of mesoscopic devices. To probe this effect we have measured the current fluctuations at high-frequency (5-90G Hz) using a superconductor-insulator-superconductor tunnel junction as an on-chip spectrum analyzer. By coupling this frequency-resolved noise detector to a quantum device we can measure the high-frequency, non-symmetrized noise as demonstrated for a Josephson junction. The same scheme has been used to detect the current fluctuations arising from coherent charge oscillations in a two-level system, a superconducting charge qubit. A narrow band peak was observed in the spectral noise density at the frequency of the coherent charge oscillations.

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We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.

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Cooled low noise amplifier (LNA) is used as the first or second component of receiver system in radio astronomy. It is developed all over the world to improve its noise. Noise temperature is one of the most important parameters of LNA because it governs the observation time. It is usually measured by inputting two reference signal which have different power each other and measuring the responses. For accuracy, the power of these reference signals is required to be close to the noise of the LNA. In the conventional method output of a Avalanche diode is attenuated to 1/100 by a cold attenuator and used as a reference signal. In this method the error caused by the attenuator and the cable between the Avalanche diode and LNA is too large (a few 10 %) and another accurate noise measurement method is required. In this research we propose a new method using Superconductor-Insulator-Superconductor junction (SIS) as a reference signal. When biased, SIS generates small shot noise that is proportional to the bias voltage so it can be used as a reference signal that make an accurate noise measurement possible (a few % error). First, SIS and SIS housing is designed and fabricated for the shot noise to transmit without any loss and reflection. Then, noise temperature of the LNA is measured by using the conventional method and SIS shot noise method and compared. As a result, there was difference and it was shown that the capacitance of SIS is one of the candidate cause.

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It has been shown both theoretically [1] and experimentally [2,3] that the very sharp nonlinearity in the conductance of superconducting tunnel junctions can be used to implement millimeter wavelength mixers having high gain and very low noise, leading to the possibility of receivers that achieve the quantum noise limit. The junctions involved contain a thin insulating barrier between two superconductors, and are thus known as superconductor-insulator-superconductor (SIS) junctions. They are physically identical to Josephson junctions, but in this application the conductance is via the tunneling of quasiparticles (unpaired electrons), and so the Josephson effect is not involved.

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Noise parameters of SIS mixers

Larry R. D’Addario

It has been shown that low-noise receivers can be constructed at millimeter wavelengths by using mixers containing superconducting tunnel junctions as the nonlinear elements. This is possible because of both the low intrinsic noise of these devices and their potential for high conversion gain. Here, the quantum theory of mixing is used to derive the full-noise parameters and small-signal parameters of sinusoidally pumped SIS (superconductor-insulator-superconductor) junctions. These are then put into a form that allows the theory of two-port linear networks to be brought to bear, allowing calculation of such useful parameters as minimum noise temperature, optimum source impedance, available (or exchangeable) gain at minimum noise, and stability factor. These quantities are properties of the pumped junction that do not depend on the source or load impedance, but do depend on the terminations at the image and harmonic sideband frequencies. The harmonic sidebands are taken to be shorted, and the image termination dependence is studied. Numerical results are presented for both ideal (BSC theory) and practical (measured current-voltage characteristic) junctions. The noise parameters of the cascade connection of an SIS mixer and a (noisy) IF amplifier are considered, leading to a specification of the optimum coupling network between the two. It is noted that the SIS mixer is usually not unconditionally stable, but that oscillation can be avoided by careful design of the IF coupling network.< >

IEEE Transactions on Microwave Theory and Techniques•Jul 1, 1988•19 citations
The purpose of the present work is to measure the low-frequency (30 Hz- 30 kHz) noise in superconductor insulator-superconductor (SIS) junctions in order to estimate the noise temperature at the output, identify the noise sources and the mechanisms by which the noise is transformed, and compare the findings with the analogous results for Josephson junctions. (AIP)
Quantum mechanics can strongly influence the noise properties of mesoscopic devices. To probe this effect we have measured the current fluctuations at high-frequency (5-90G Hz) using a superconductor-insulator-superconductor tunnel junction as an on-chip spectrum analyzer. By coupling this frequency-resolved noise detector to a quantum device we can measure the high-frequency, non-symmetrized noise as demonstrated for a Josephson junction. The same scheme has been used to detect the current fluctuations arising from coherent charge oscillations in a two-level system, a superconducting charge qubit. A narrow band peak was observed in the spectral noise density at the frequency of the coherent charge oscillations.
We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.
Cooled low noise amplifier (LNA) is used as the first or second component of receiver system in radio astronomy. It is developed all over the world to improve its noise. Noise temperature is one of the most important parameters of LNA because it governs the observation time. It is usually measured by inputting two reference signal which have different power each other and measuring the responses. For accuracy, the power of these reference signals is required to be close to the noise of the LNA. In the conventional method output of a Avalanche diode is attenuated to 1/100 by a cold attenuator and used as a reference signal. In this method the error caused by the attenuator and the cable between the Avalanche diode and LNA is too large (a few 10 %) and another accurate noise measurement method is required. In this research we propose a new method using Superconductor-Insulator-Superconductor junction (SIS) as a reference signal. When biased, SIS generates small shot noise that is proportional to the bias voltage so it can be used as a reference signal that make an accurate noise measurement possible (a few % error). First, SIS and SIS housing is designed and fabricated for the shot noise to transmit without any loss and reflection. Then, noise temperature of the LNA is measured by using the conventional method and SIS shot noise method and compared. As a result, there was difference and it was shown that the capacitance of SIS is one of the candidate cause.
It has been shown both theoretically [1] and experimentally [2,3] that the very sharp nonlinearity in the conductance of superconducting tunnel junctions can be used to implement millimeter wavelength mixers having high gain and very low noise, leading to the possibility of receivers that achieve the quantum noise limit. The junctions involved contain a thin insulating barrier between two superconductors, and are thus known as superconductor-insulator-superconductor (SIS) junctions. They are physically identical to Josephson junctions, but in this application the conductance is via the tunneling of quasiparticles (unpaired electrons), and so the Josephson effect is not involved.
It has been shown that low-noise receivers can be constructed at millimeter wavelengths by using mixers containing superconducting tunnel junctions as the nonlinear elements. This is possible because of both the low intrinsic noise of these devices and their potential for high conversion gain. Here, the quantum theory of mixing is used to derive the full-noise parameters and small-signal parameters of sinusoidally pumped SIS (superconductor-insulator-superconductor) junctions. These are then put into a form that allows the theory of two-port linear networks to be brought to bear, allowing calculation of such useful parameters as minimum noise temperature, optimum source impedance, available (or exchangeable) gain at minimum noise, and stability factor. These quantities are properties of the pumped junction that do not depend on the source or load impedance, but do depend on the terminations at the image and harmonic sideband frequencies. The harmonic sidebands are taken to be shorted, and the image termination dependence is studied. Numerical results are presented for both ideal (BSC theory) and practical (measured current-voltage characteristic) junctions. The noise parameters of the cascade connection of an SIS mixer and a (noisy) IF amplifier are considered, leading to a specification of the optimum coupling network between the two. It is noted that the SIS mixer is usually not unconditionally stable, but that oscillation can be avoided by careful design of the IF coupling network.< >