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Journal of Applied Physics

Direct observation of the current oscillation in a dc SQUID

Kiyotaka Nakajima, Akira Fujimaki, Yasuji Sawada

Published Nov 15, 1986• 3 citations
Publisher

Abstract

The current waveforms of the self-resetting oscillation in a dc superconducting quantum interference device (SQUID) are observed by means of a Josephson sampling system. The frequency of the oscillation is around 60 GHz. The time period of the self-resetting oscillation is experimentally measured as a function of the bias current to the SQUID. Theoretical and numerical analyses are presented for the self-resetting oscillation in the dc SQUID.

DOI
10.1063/1.337545

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The current waveforms of the self-resetting oscillation in a dc superconducting quantum interference device (SQUID) are observed by means of a Josephson sampling system. The frequency of the oscillation is around 60 GHz. The time period of the self-resetting oscillation is experimentally measured as a function of the bias current to the SQUID. Theoretical and numerical analyses are presented for the self-resetting oscillation in the dc SQUID.
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We describe a new dc-superconducting quantum interference device (SQUID) circuit of the FM type, which has a large coefficient for converting an applied magnetic flux into a frequency of MHz band or a lower band. The circuit consists of a SQUID amplifier composed of two stages, formed by a superconducting weak link and two dc-SQUID elements with nonhysteretic characteristics. In the first stage, the weak link and dc-SQUID elements are connected in parallel and operated by a common bias voltage. Since the respective elements oscillate with high frequencies by the ac Josephson effect, the two frequencies interact with each other, so that, the first stage generates a beat signal of a lower frequency. In the second stage, the other dc-SQUID element with a intrinsic flux gain enhances the beat signal generated during the first stage. A theoretical analysis of the circuit with computer simulation shows that the beat frequency is a periodical function of the magnetic flux, and the SQUID amplifier with two stages can improve the flux sensitivity.
The output voltage of a dc superconducting quantum interference device (SQUID) containing a resistive section Rs oscillates at a frequency νq =Vs/ φ0 (φ0=flux quantum) when a current controlled voltage Vs is maintained across Rs. These oscillations have been observed from ∼10−1 to 1011 Hz, using Rs values ranging from ∼10−10 to ∼10−3 Ω in low inductance (∼10−10 H) point contact SQUID loops. At high frequencies, they manifest themselves in the dc current-voltage characteristics by inducing extra current steps.
We have developed a radio frequency bias current scheme with reversal frequency up to 50 MHz for dc superconducting quantum interference devices (SQUIDs). The SQUID and the readout electronics, were connected via a quarter wavelength (/spl lambda//4) transmission line. This transmission line not only transported the SQUID signal, but also served as the resonant matching circuit. For a dc SQUID of inductance 115 pH (loop size 75/spl times/75 /spl mu/m/sup 2/), we measured a flux noise of 33 /spl mu//spl phi//sub 0///spl radic/Hz at 77 K in the flux locked loop configuration. The corner frequency of 1/f noise was less than 0.5 Hz.
A current biased dc superconducting quantum interference device behaves as an anharmonic quantum oscillator controlled by a bias current and an applied magnetic flux. We consider here its two level limit consisting of the two lower energy states \ensuremath{\mid}0⟩ and \ensuremath{\mid}1⟩. We have measured energy relaxation times and microwave absorption for different bias currents and fluxes in the low microwave power limit. Decoherence times are extracted. The low frequency flux and current noise have been measured independently by analyzing the probability of current switching from the superconducting to the finite voltage state, as a function of applied flux. The high frequency part of the current noise is derived from the electromagnetic environment of the circuit. The decoherence of this quantum circuit can be fully accounted for by these current and flux noise sources.