Search

Niobium-based nanobridge superconducting quantum interference devices (SQUIDs) have shown very low noise performance and high-frequency operation. We describe how we are bringing together these two aspects of nanobridge SQUIDs with the aim of realizing single spin-flip detection using microwave inductive readout techniques, where the nanobridge SQUID is integrated into a superconducting coplanar waveguide resonator. Using electron beam lithography (EBL) is ideal for fabricating nanobridge junctions and has the advantage of being easily scalable compared to using a focused ion beam. In this article, we demonstrate that EBL is suitable for fabricating nanobridge junction SQUIDs, and they exhibit very comparable performance to previous focused ion beam milled SQUIDs. Our intrated devices show potential to be used for flux-tunable sensors.

Since the carrier density and resistivity of graphene are heavily dependent on the Fermi level, Josephson junctions with graphene as the weak link can have their I – V properties easily tuned by the gate voltage. Most of the previous work on superconductor–graphene–superconductor (SGS) junctions and superconducting quantum interference devices (SQUIDs) were based on mechanically exfoliated graphene, which is not compatible with large scale production. Here, we show that SGS junctions and dc SQUIDs can be easily fabricated from chemical vapor deposition (CVD) graphene and exhibit good electronic properties. The SGS junctions can work without any hysteresis in their electrical characteristics from 1.5 K down to a base temperature of 320 mK, and the critical current can be effectively tuned by the gate voltage by up to an order of magnitude. As a result, dc SQUIDs made up of these junctions can have their critical current tuned by both the magnetic field and the gate voltage.

Superconducting QUantum Interference Devices (SQUIDs) based on nanobridge junctions have shown increasing promise for single particle detection. This paper describes the development of the fabrication of improved and reproducible nanobridge junctions fabricated by focused ion beam (FIB) milling from niobium thin films. Although the very low noise properties of nanobridge SQUIDs are well known, the nature of the milling process is little understood at the level of local superconducting properties. In this paper, we report the results for nanobridge Josephson devices and SQUIDs, which we believe are the first to be made by Xenon (Xe) FIB milling. Temperature-dependent current–voltage behavior, microwave-induced Shapiro steps, and SQUID response to magnetic fields have been measured. We make preliminary comparisons with nominally identical devices milled from Nb thin films using either Xe or Ga ions.

We discuss steps toward the readout of the position of a nanoelectromagnetic system (NEMS) beam resonator using a Nb nanosuperconducting quantum interference device (nanoSQUID). We describe our fabrication procedure for coupling the nanoSQUID and a suspended Al-coated Si3N4 NEMS resonator together by a combination of focused ion beam lithography and nanomanipulation. We discuss typical electrical characteristics of the integrated devices, and independent postfabrication atomic force microscope nanoindentation measurements of the elastic properties of the integrated resonator to estimate its natural frequencies of vibration. We compare and discuss the response of a nanoSQUID with current-carrying and superconducting screening (noncurrent carrying) modes of operation of the resonator

NanoSQUID sensors (Superconducting Quantum Interference Devices with nanoscale dimensions) are regarded as a useful tool for sensitive magnetic measurements and have achieved a low noise level. The nanoSQUIDs comprising nanobridge junctions are fabricated by photolithography followed by focused ion beam patterning. While previously such self-shunted nanobridge junctions have shown hysteretic characteristics at 4.2 K, we propose an approach to increase the operational temperature in order to reduce the critical current of the device. We report the design of a readout system, comprising all necessary components to achieve a low noise readout, which is limited by the nanoSQUID noise. It consists of a variable temperature (5-10 K) liquid helium probe stick, a coil system and a SQUID preamplifier. The orthogonal coil system enables particle magnetization up to 100 mT in parallel to and a flux bias of up to 28 mT perpendicular to the nanoSQUID, to set the working point. The nanoSQUID achieves a low white noise level of 0.23 μΦ0/√Hz including a readout contribution of 0.10 μΦ0/√Hz.

The ability to reduce SQUID dimensions into the sub-micrometer or nanometer regime points the way towards novel applications, particularly in emerging fields such as quantum information processing, single-photon/particle detection, and experimental studies of nano-scale entities such as Bose-Einstein condensates. We report here on our ongoing work combining traditional thin-film and photolithographic fabrication processes with computer-aided-design software and focused ion beam milling to realize sub-micrometer superconducting structures. Their magnetic field sensitivity, noise behavior, spatial resolution, and prospects for magnetic spin detection are discussed.

Detectors with sufficient sensitivity to resolve the energy of single photons and particles combined with rapid response times are becoming increasingly important. We describe work in progress on a recently-proposed type of sensor, which we have named the Inductive Superconducting Transition Edge Detector (ISTED). We have theoretically estimated the energy sensitivity of such a sensor, in the case where the heat absorber is incorporated within the sensitive area of a nano-scale SQUID which acts as an inductive readout device. Recent results on the experimental characterization of prototype devices in the temperature range 5-8 K are presented, and ongoing work which is intended to achieve smaller device dimensions together with lower operating temperatures is described.

Double Josephson junction HTS resistive SQUIDs have been fabricated using an on-chip shunt Au resistor across a gap in an YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// loop, providing a low shunt resistance of R/sub s/ /spl sim/26 /spl mu//spl Omega/ at T/spl les/77 K. The dependence of the heterodyne oscillation linewidth of the R-SQUID on the thermal noise across the shunt resistor is under investigation with a view to the development of an absolute noise thermometer. Signal powers /spl sim/nW have been measured and the device has been shown to be tunable from 5 to 50 MHz. The measured oscillation linewidth (/spl sim/30 kHz at 17 K) is believed to be the narrowest reported for such an HTS device, however non-thermal broadening of the linewidth is also observed, as has been reported for single junctions at GHz frequencies. We describe measurements on such devices over a range of temperatures and bias conditions.

NanoSQUIDs made from Nb thin films have been produced with nanometre loop sizes down to 200 nm, using weak-link junctions with dimensions less than 60 nm. These composite (W/Nb) single layer thin film devices, patterned by FIB milling, show extremely good low-noise performance $\sim\!\! 170\ {\rm n}\Phi_{0} $ at temperatures between 5 and 8.5 K and can operate in rather high magnetic fields (at least up to 1 T) . The devices produced so far have a limited operating temperature range, typically only 1–2 K. We have the goal of achieving operation at 4.2 K, to be compatible with the best SQUID series array (SSA) preamplifier available. Using the SSA to readout the nanoSQUIDs provides us with a means of investigating the intrinsic noise of the former. In this paper we report improved white noise levels of these nanoSQUIDs, enabling potential detection of a single electronic spin flip in a 1-Hz bandwidth. At low frequencies the noise performance is already limited by SSA preamplifier noise.

Micro-sized SQUID sensors have been designed and fabricated in conventional Nb/AlOx/Nb junction technology. Two types are presented here: a microSQUID gradiometer with two small loops, each with a diameter of 3 μm , and an integrated susceptometer having 30 μm or 60 μm circular loops and on-chip field coils. Both sensor types have been read out by a SQUID array, acting as a low-noise preamplifier. The microSQUIDs were operated in a temperature range of 4.2 K to 7 K, whereas the susceptometers are rather intended for operation below 4.2 K. Important SQUID parameters such as flux noise, maximum operation field, and nonlinearity were measured at different temperatures. The best white flux noise level achieved with a microSQUID was 135 nΦ0/Hz1/2 at 4.2 K. Micro-sized magnetic beads were investigated with microSQUIDs, whereas susceptometers were used to measure the susceptibility of thin wires up to 100 kHz.

Recent developments in cryogenic nano-electromechanical (NEMS) resonators have shown that they can address some fundamental physics, for example in achieving the ground state of a mechanical resonator and entanglement with an electromagnetic resonator. In contrast, little work has been reported on using what is arguably the most sensitive measuring device, a SQUID, to directly interact with, and thus interrogate, a NEMS resonator. We report here our initial experimental results aimed towards forming an optimized coupled micro/nano-mechanical resonator and a focused ion beam patterned Nb SQUID, possessing exceptionally low noise (~200nΦ0/Hz1/2 above 1 kHz), and operating above 4.2 K. We describe our first results from a paddle-shaped mechanical resonator with a diameter of 15 μm coupled to a Nb SQUID loop. Finally, we describe the construction of our first true nanoscale-coupled, double-clamped cantilever and nanoSQUID (rectangular loop area 100 nm × 900 nm).

We report on the low-frequency noise performance of niobium dc superconducting quantum interference devices (SQUIDs), which contain nanobridges fabricated by focused ion beam lithography as the active Josephson elements. The devices have feature sizes down to 70 nm. We have measured devices of different loop sizes in two readout configurations: nano-scale loop sizes in small signal mode using a series SQUID array as a low-temperature pre-amplifier, and larger micron-scale loop sizes in a conventional flux-locked loop. We investigate the different contributions to the low frequency noise and report on electrical measurements made in applied magnetic fields of up to 0.5 T (in-plane) and 0.1 T (perpendicular to the plane) at operating temperatures around 7 K. We compare the measurements with the existing theories of noise in a dc SQUID.