Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Applied Physics Letters

Optimization of dc SQUID linear amplifiers and the quantum noise limit

Claudia D. Tesche

Published Sep 1, 1982• 35 citations
Publisher

Abstract

A model for inductively coupled linear amplifier and magnetometer circuits employing superconducting quantum interference devices (dc SQUID’s), which is accurate to all values in the coupling strength, is described. The model explicitly includes the interaction between the input circuit and the dc SQUID. The optimization of the untuned linear amplifier is discussed in detail. The uncertainty principle constraint on the noise temperature of a linear amplifier is shown to place a constraint on the minimum achievable values of the noise figures of an isolated dc SQUID in the quantum noise limit.

DOI
10.1063/1.93542

Related papers

Analysis of strong inductive coupling on SQUID systems

Claudia D. Tesche

A technique for the analysis of complex superconducting circuits containing one or more current biased inductively coupled Josephson junction interferometers is described. The equations of motion for the system are shown to separate into a set of non-linear junction equations relating the voltage drops across the junctions to the currents passing through the junctions, and a set of linear equations expressing the quantization of the fluxoid around the superconducting loops and the corresponding linear circuit equations for the normal loops. In many cases, the linear equations can be integrated, resulting in an effective elimination of the normal circuit loops from the problem. The circuit is thus reduced to that of an equivalent set of coupled SQUIDs whose device parameters are a function of the original circuit parameters and the inductive coupling strengths. The dc SQUID tuned linear amplifier is analyzed to display the method. As a result of this analysis, the optimal noise temperature of a tuned SQUID amplifier is derived as a function of the current and voltage noise spectral densities, Svand Sj, and forward transfer function, Vφ. The results obtained demonstrate that the behavior of the SQUID cannot be isolated from that of the rest of the circuit in the manner assumed in previous analyses. The existence of a quantum noise limit on the noise temperature of any phase preserving linear amplifier is used to infer that the noise figures must satisfy the constraint[(S_{v}/2LV_{\phi}^{2})(LS_{j}/2)-S_{vj}^{2}]^{1/2}>\hbarThe behavior of several simple circuits containing a pair of inductively coupled current biased interferometers is discussed to display the extension of the method to circuits containing more than one interferometer.

IEEE Transactions on Magnetics•May 1, 1983•12 citations

Optimization of a Low-Tc DC SQUID Amplifier With Tightly Coupled Input Coils

J. Pleikies, O. Usenko, G. Frossati +1 more

We optimized the design and operation of a low-Tc direct current superconducting quantum interference device (dc SQUID) with an integrated coupling coil of 1.5 muH inductance taking into account typical effects observed for similar devices. Numerical simulations were performed on a model including the capacitance of the Josephson junctions, thermal noise of the integrated shunt- and damping- resistors as well as a complex frequency dependent impedance of the SQUID loop originating from the integrated coils. The experimentally and numerically determined characteristics and sensitivity are in good agreement. A minimum additional coupled energy resolution of 700 (h/2p) and 250 (h/2p) was measured at a temperature of 4.2 K and 1.5 K, respectively.

IEEE Transactions on Applied Superconductivity•Jun 1, 2009•6 citations

A coupled DC SQUID with low 1/f noise

Heikki Seppä, M. Kiviranta, Alexandre Satrapinski +3 more

A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.>

IEEE Transactions on Applied Superconductivity•Mar 1, 1993•25 citations

Quantum noise theory for the dc SQUID

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

The noise temperature of a dc superconducting quantum interference device (SQUID) coupled to a tuned input circuit is computed using the complete quantum expression for the equilibrium noise in the shunt resistance of each junction. At T = 0, where the noise reduces to zero-point fluctuations, the noise temperature for an optimized system is hn/kB ln2, where n is the signal frequency. The computation is extended to nonzero temperatures, and it is shown that a SQUID operated at 1K can approach the quantum limit.

Applied Physics Letters•Mar 1, 1981•93 citations

Optimization of input impedance and mechanism of noise suppression in a DC SQUID RF amplifier

Michael A. Tarasov, Z. G. Ivanov

The optimal input impedance and noise of a DC SQUID RF amplifier at frequencies of the order of 1 GHz with a resonant input matching circuit have been studied analytically, numerically, and experimentally. A model for noise temperature and power gain has been developed for the practical resonant input tank circuit. A new effect of the output noise increasing or decreasing with changing the sign of voltage-to-flux transfer coefficient has been observed experimentally and explained analytically. The different values of noise temperature for the opposite dV/d/spl Phi/ values have been interpreted using a model with partially correlated current and voltage noise sources. The equivalent layout for optimal input matching of a SQUID amplifier comprising series and parallel resonant circuits has been presented. Using such a matching circuit and SIS junction as a signal source the SQUID amplifier noise temperature about 1 K has been measured at 1.1 GHz.

IEEE Transactions on Applied Superconductivity•Jun 1, 1996•3 citations

Uncertainty principle limit to the energy sensitivity of SQUID’s and other linear amplifiers

Richard F. Voss

SQUID’s (Superconducting quantum interference devices) and other amplifiers are modeled by ideal noiseless linear amplifiers with current in and voltage en noise sources in their input circuits. For independent sources at frequency v the uncertainty principle requires that (e2ni2n)1/2 ≳2hv, where h is Planck’s constant. This limits the intrinsic energy sensitivity ε=Φ2n/2L of a SQUID of inductance L and effective flux noise power Φ2n to ε≳εmin=(e2ni2n)1/2 /2πν=h/π. Correlation of the noise sources or mismatch of the SQUID inductance is shown to degrade εmin.

Applied Physics Letters•Feb 1, 1981•17 citations
A model for inductively coupled linear amplifier and magnetometer circuits employing superconducting quantum interference devices (dc SQUID’s), which is accurate to all values in the coupling strength, is described. The model explicitly includes the interaction between the input circuit and the dc SQUID. The optimization of the untuned linear amplifier is discussed in detail. The uncertainty principle constraint on the noise temperature of a linear amplifier is shown to place a constraint on the minimum achievable values of the noise figures of an isolated dc SQUID in the quantum noise limit.
A technique for the analysis of complex superconducting circuits containing one or more current biased inductively coupled Josephson junction interferometers is described. The equations of motion for the system are shown to separate into a set of non-linear junction equations relating the voltage drops across the junctions to the currents passing through the junctions, and a set of linear equations expressing the quantization of the fluxoid around the superconducting loops and the corresponding linear circuit equations for the normal loops. In many cases, the linear equations can be integrated, resulting in an effective elimination of the normal circuit loops from the problem. The circuit is thus reduced to that of an equivalent set of coupled SQUIDs whose device parameters are a function of the original circuit parameters and the inductive coupling strengths. The dc SQUID tuned linear amplifier is analyzed to display the method. As a result of this analysis, the optimal noise temperature of a tuned SQUID amplifier is derived as a function of the current and voltage noise spectral densities, Svand Sj, and forward transfer function, Vφ. The results obtained demonstrate that the behavior of the SQUID cannot be isolated from that of the rest of the circuit in the manner assumed in previous analyses. The existence of a quantum noise limit on the noise temperature of any phase preserving linear amplifier is used to infer that the noise figures must satisfy the constraint[(S_{v}/2LV_{\phi}^{2})(LS_{j}/2)-S_{vj}^{2}]^{1/2}>\hbarThe behavior of several simple circuits containing a pair of inductively coupled current biased interferometers is discussed to display the extension of the method to circuits containing more than one interferometer.
We optimized the design and operation of a low-Tc direct current superconducting quantum interference device (dc SQUID) with an integrated coupling coil of 1.5 muH inductance taking into account typical effects observed for similar devices. Numerical simulations were performed on a model including the capacitance of the Josephson junctions, thermal noise of the integrated shunt- and damping- resistors as well as a complex frequency dependent impedance of the SQUID loop originating from the integrated coils. The experimentally and numerically determined characteristics and sensitivity are in good agreement. A minimum additional coupled energy resolution of 700 (h/2p) and 250 (h/2p) was measured at a temperature of 4.2 K and 1.5 K, respectively.
A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.>
The noise temperature of a dc superconducting quantum interference device (SQUID) coupled to a tuned input circuit is computed using the complete quantum expression for the equilibrium noise in the shunt resistance of each junction. At T = 0, where the noise reduces to zero-point fluctuations, the noise temperature for an optimized system is hn/kB ln2, where n is the signal frequency. The computation is extended to nonzero temperatures, and it is shown that a SQUID operated at 1K can approach the quantum limit.
The optimal input impedance and noise of a DC SQUID RF amplifier at frequencies of the order of 1 GHz with a resonant input matching circuit have been studied analytically, numerically, and experimentally. A model for noise temperature and power gain has been developed for the practical resonant input tank circuit. A new effect of the output noise increasing or decreasing with changing the sign of voltage-to-flux transfer coefficient has been observed experimentally and explained analytically. The different values of noise temperature for the opposite dV/d/spl Phi/ values have been interpreted using a model with partially correlated current and voltage noise sources. The equivalent layout for optimal input matching of a SQUID amplifier comprising series and parallel resonant circuits has been presented. Using such a matching circuit and SIS junction as a signal source the SQUID amplifier noise temperature about 1 K has been measured at 1.1 GHz.
SQUID’s (Superconducting quantum interference devices) and other amplifiers are modeled by ideal noiseless linear amplifiers with current in and voltage en noise sources in their input circuits. For independent sources at frequency v the uncertainty principle requires that (e2ni2n)1/2 ≳2hv, where h is Planck’s constant. This limits the intrinsic energy sensitivity ε=Φ2n/2L of a SQUID of inductance L and effective flux noise power Φ2n to ε≳εmin=(e2ni2n)1/2 /2πν=h/π. Correlation of the noise sources or mismatch of the SQUID inductance is shown to degrade εmin.