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arXiv:2107.05929

Superconducting microwave magnetometer for absolute flux detection

Simon Günzler, Patrick Winkel, Rieger, Dennis, Kiril Borisov, Martin Spiecker, Alexey Vladimirovich Ustinov, Ioan M. Pop, Wolfgang Wernsdorfer

Published Jul 13, 2021• 1 citation
arXivHTMLPDF

Abstract

Superconducting quantum interference devices (SQUIDs) are among the most sensitive detectors for out-of-plane magnetic field components. However, due to their periodic response with short modulation period $M = 1 Φ_0$, determined by the magnetic flux quantum $Φ_0 \approx 2.068\times 10^{-15}\,\mathrm{Wb}$, it is difficult to infer the value of the magnetic flux unambiguously, especially in case the magnetic flux enclosed in the SQUID loop changes by many flux quanta. Here, we demonstrate that by introducing a second degree of freedom in the form of a second SQUID, we substantially enhance the modulation period $M$ of our device without sacrificing sensitivity. As a proof of concept, we implement our idea by embedding two asymmetric direct current SQUIDs into a common tank circuit. By measuring the reflection coefficient of the device, we extract the two lowest energy eigenfrequencies as a function of the external magnetic flux created by a superconducting field coil, from which we experimentally deduce a modulation period $M \geq 15 Φ_0$, as well as the magnetic offset-field $B_0 = 22\,\mathrm{nT}$ present in our experiment.

DOI
10.48550/arxiv.2107.05929

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Superconducting quantum interference devices (SQUIDs) are among the most sensitive detectors for out-of-plane magnetic field components. However, due to their periodic response with short modulation period M=1Φ0M = 1 Φ_0M=1Φ0​, determined by the magnetic flux quantum Φ0≈2.068×10−15 WbΦ_0 \approx 2.068\times 10^{-15}\,\mathrm{Wb}Φ0​≈2.068×10−15Wb, it is difficult to infer the value of the magnetic flux unambiguously, especially in case the magnetic flux enclosed in the SQUID loop changes by many flux quanta. Here, we demonstrate that by introducing a second degree of freedom in the form of a second SQUID, we substantially enhance the modulation period MMM of our device without sacrificing sensitivity. As a proof of concept, we implement our idea by embedding two asymmetric direct current SQUIDs into a common tank circuit. By measuring the reflection coefficient of the device, we extract the two lowest energy eigenfrequencies as a function of the external magnetic flux created by a superconducting field coil, from which we experimentally deduce a modulation period M≥15Φ0M \geq 15 Φ_0M≥15Φ0​, as well as the magnetic offset-field B0=22 nTB_0 = 22\,\mathrm{nT}B0​=22nT present in our experiment.
We have measured the characteristics of a novel SQUID magnetometer pumped at 9 GHz. The device comprises a niobium point-contact shunting a low impedance superconducting microwave transmission line which isolates the junction at high frequencies from a toroidal input transformer. The junction is matched to a 50 Ω coaxial transmission line and the SQUID is operated in a reflection mode. For low values of the weak link critical current we observe a modulation of the phase of the reflected microwave signal periodic in the applied flux. The modulation persists at high microwave power levels indicating that the junction response is maintained to at least 200 GHz. For large values of critical current, amplitude modulation is observed. We have obtained a differential sideband power of 230 pW/(\Phi_{0})^{2}which is close to the value predicted by a simple model. In flux-locked loop operation we have obtained an energy sensitivity referred to the input terminals of 7 × 10-31J/Hz, which is superior to that so far reported for any other SQUID.
Sensitive magnetometers that can operate in high magnetic fields are essential for detecting magnetic resonance signals originating from small ensembles of quantum spins. Such devices have potential applications in quantum technologies, in particular quantum computing. We present a novel experimental setup implementing a differential flux measurement using two Direct-Current Superconducting QUantum Interference Device (DC-SQUID) magnetometers. The differential measurement allows for cancellation of background flux signals while enhancing sample signal. The developed protocol uses pulsed readout, which minimizes on-chip heating since sub-Kelvin temperatures are needed to preserve quantum spin coherence. Results of a proof of concept experiment are shown as well.
We propose a novel device, based on a series of dc SQUIDs (direct current Superconducting QUantum Interference Devices), each one with different sensitive area, that can be used to perform absolute measurements of magnetic field. With this array, we overcome the limit of a single dc SQUID of being sensitive only to variations of the linked magnetic flux. The overall voltage-flux characteristic of our device, in fact, has a non-periodic shape that allows a clear identification of the point with zero magnetic field. We present a discussion of the working principle and of the main characteristics, which have been partially tested on a prototype.
It is theoretically explained that a response of a superconducting quantum interference device (SQUID) is periodically dependent on total magnetic flux coupling to the SQUID ring (Φ) and its period is a flux quantum (Φ(o)=h/2e, where h and e, respectively, express Planck's constant and elementary charge). For example, the voltage of an electromagnetically oscillated rf-SQUID or a current biased dc-SQUID is thought to be periodically dependent on Φ with a period of Φ(o). In this paper, we propose an accurate method to check the periodicity of a SQUID response by using a set of sensing coils covered with a superconducting sheath. As a demonstration, we measured periodicity of a commercially available thin-film type rf-SQUID response in magnetic flux ranging up to approximately 4300Φ(o). Its flux dependence was periodic below about 3400Φ(o).
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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.