SKIFFS: Superconducting Kinetic Inductance Field-Frequency Sensors for sensitive magnetometry in moderate background magnetic fields

Abstract

We describe sensitive magnetometry using lumped-element resonators fabricated from a superconducting thin film of NbTiN. Taking advantage of the large kinetic inductance of the superconductor, we demonstrate a continuous resonance frequency shift of 27 MHz for a change in magnetic field of 1.8 µT within a perpendicular background field of 60 mT. By using phase-sensitive readout of microwaves transmitted through the sensors, we measure phase shifts in real time with a sensitivity of 1 degree/nT. We present measurements of the noise spectral density of the sensors, and find their field sensitivity is at least within one to two orders of magnitude of superconducting quantum interference devices operating with zero background field. Our superconducting kinetic inductance field-frequency sensors enable real-time magnetometry in the presence of moderate perpendicular background fields up to at least 0.2 T. Applications for our sensors include the stabilization of magnetic fields in long coherence electron spin resonance measurements and quantum computation.Disordered superconductors such as NbTiN, TiN and NbN have become ubiquitous in several fields of study due to their large kinetic inductance and resilience to large background magnetic fields 1,2 . Microwave kinetic inductance detectors 3-5 and superconducting nanowire single-photon detectors 6,7 fabricated from kinetic inductors are now routinely used in astronomy and imaging. Kinetic inductors can also be used in applications such as current-sensing 8 , magnetometry 9 , parametric amplification 10,11 , generation of frequency combs 12 , and superconducting qubits 13,14 .In this work, we take advantage of the kinetic inductance of a thin film of NbTiN to fabricate lumped-element resonators whose resonance frequencies are strongly dependent on the perpendicular magnetic field, changing by as much as 27 MHz for a field change of 1.8 µT.We demonstrate a method for real-time measurement of AC magnetic fields based on phase-sensitive readout of microwave transmission through the resonators, finding a detection sensitivity of 1 degree/nT. Our Superconducting Kinetic Inductance Field-Frequency Sensors (SKIFFS) are able to operate in perpendicular background magnetic fields at least as large as 0.2 T, and may find applications in quantum computation, where superconducting quantum interference devices (SQUIDs) based on Josephson junctions may not be applicable due to the large magnetic fields.Our SKIFFS are fabricated from a 7 nm NbTiN thin film (T C ∼ 9 K, R sheet = 252 Ω/ ) DC-sputtered reactively on a c-axis sapphire substrate using a NbTi alloy target and an Ar/N environment 15 . The device features are patterned using electron-beam lithography followed by reactive-ion etching with an SF 6 /Ar plasma. A scanning electron micrograph of a SKIFFS is shown in figure Fig. 1a. The sensor is a lumped-element microwave resonator fabricated from a 100 µm × 100 µm rectangular a) Electronic mail: asfaw@princeton. edu FIG. 1. (a) Scanning electron micrograph of a SKIFFS resonato...

Development of an Array of Kinetic Inductance Magnetometers (KIMs)

“…When the loop is incorporated into a resonator, the change in kinetic inductance can be seen as a shift in resonance, ω r = 1/ √ LC. [5] [12] [13] TL TL…”

Magnetic field sensing with the kinetic inductance of a high-Tc superconductor

“…Each device harnesses a certain type of a non-linearity of the kinetic inductance L k , such as that induced by temperature, electric current, or non-equilibrium quasiparticles. In sensor applications, radio-frequency (rf) techniques are often employed in observation of the variations of L k : a high sensitivity follows from the intrinsically low dissipation of the superconductors, manifesting itself as a high quality factor of resonator circuits, for example.The general advantages common to all L k sensors have motivated the development of kinetic inductance magnetometers (KIMs), devices that combine the L k current non-linearity with magnetic flux quantization 8,9 . In comparison to the most established category of sensitive magnetometers, i.e., superconducting quantum interference devices (SQUIDs), KIMs have certain benefits.…”

“…The general advantages common to all L k sensors have motivated the development of kinetic inductance magnetometers (KIMs), devices that combine the L k current non-linearity with magnetic flux quantization 8,9 . In comparison to the most established category of sensitive magnetometers, i.e., superconducting quantum interference devices (SQUIDs), KIMs have certain benefits.…”

“…Then, the inductance variation translates into a changing resonance frequency, a quantity which is probed by coupling the resonator weakly into a 50-Ω readout feedline. Two KIMs of this kind have recently been reported: the materials of choice have been NbN 8 and NbTiN 9 , both of which are low-T c superconductors whose disordered nature provides a magnetic penetration depth 15 λ exceeding several hundreds of nm.…”

“…In conclusion, we have demonstrated high-T c kinetic inductance magnetometers with a sensitivity of 4 pT/ √ Hz at 10 kHz and T = 77 K. They tolerate background fields of 9 − 28 µT, which is close to the Earth's field. We anticipate that changing the sensor geometry by implementing narrow constrictions 9 to reduce I c should allow for periodical resets, enabling operation at even higher fields. The constrictions should also help to increase the kinetic inductance fraction, a likely route towards a higher device responsivity.…”