We report here recent work at the U.K. National Physical Laboratory on developing nanoscale SQUIDs using Dayem bridge Josephson junctions. The advantages are simplicity of fabrication, exceptional low-noise performance, toward the quantum limit, and a range of novel applications. Focused ion beam patterned Nb SQUID, possessing exceptionally low noise (~200 nΦ0/Hz1/2above 1 kHz), and operating above 4.2 K can be applied to measurement of nanoscale magnetic objects or coupled to nanoelectromechanical resonators, as well as single particle detection of photons, protons, and ions. The limited operating temperature range may be extended by exposing the Dayem bridges to carefully controlled ion beam implantation, leading to nonreversible changes in junction transition temperature.
We report here recent work at the U.K. National Physical Laboratory on developing nanoscale SQUIDs using Dayem bridge Josephson junctions. The advantages are simplicity of fabrication, exceptional low-noise performance, toward the quantum limit, and a range of novel applications. Focused ion beam patterned Nb SQUID, possessing exceptionally low noise (~200 nΦ0/Hz1/2above 1 kHz), and operating above 4.2 K can be applied to measurement of nanoscale magnetic objects or coupled to nanoelectromechanical resonators, as well as single particle detection of photons, protons, and ions. The limited operating temperature range may be extended by exposing the Dayem bridges to carefully controlled ion beam implantation, leading to nonreversible changes in junction transition temperature.
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.
The requirements of quantum metrology and nanoscience are driving the need for single-particle detection capability across a wide variety of physics areas, including quantum information processing, single-photon detection, nanoelectromechanical systems, nanomagnetism and spintronics. The single particles to be detected include atoms, molecules, photons, spins and even phonons, in the future. Nanoscale superconducting quantum interference devices (nanoSQUIDs) represent a new manifestation of an old but exciting superconducting technology which addresses some of these requirements. In this paper we describe a straightforward approach to fabricating Nb microbridge weak links using combined optical lithography and focused ion beams which may be used to fabricate nanoSQUIDs. The devices show non-hysteretic current?voltage characteristics and demonstrate very low noise, even at operating temperatures above 4.2?K. To improve our understanding of the superconducting properties of the Josephson microbridge/nanobridge junctions which have proved very successful in realizing low noise nanoSQUIDs we have carried out a combination of investigations. These include cryogenic resistance versus temperature (R(T)) and current?voltage characteristic measurements, atomic force microscope scans, controlled gallium (Ga) ion milling and implantation, and ion beam track modelling of these Josephson devices and Nb thin films.
Science and industry demand ever more sensitive measurements on ever smaller systems, as exemplified by spintronics, nanoelectromechanical system, and spin-based quantum information processing, where single electronic spin detection poses a grand challenge. Superconducting quantum interference devices (SQUIDs) have yet to be effectively applied to nanoscale measurements. Here, we show that a simple bilayer deposition route, combining photolithography with focused ion beam patterning, produces high performance nanoscale SQUIDs. We present results of noise measurements on these nanoSQUIDs which correspond to a magnetic flux sensitivity of around 0.2μΦ0∕Hz1∕2. This represents one of the lowest noise values achieved for a SQUID device operating above 1K.
The increasing demands of quantum metrology and nanoscience are driving the requirements for single particle detection capability across a wide variety of physics applications, including QIP, single photon detection, NEMS, nanomagnetism and spintronics. NanoSQUIDs represent a new manifestation of an old but exciting superconducting technology which addresses some of these requirements. We will describe a straightforward approach to fabricating Nb microbridge weak-links which may be used in nanoscale SQUIDs. These devices have demonstrated very low noise, even at operating temperatures above 4.2 K. We will also describe the performance of prototype devices made using this technique for energy resolving single photon detection, nanoparticle spin detection and NEMS resonator readout.
Dayem Bridge Based Nano-SQUID for High Sensitive Nanoscale Applications A recent progress in nanotechnology formulates the fabrication of SQUIDs loop less than 100 nm; its obtaining an adequate sensitivity to detect the magnetic nano-objects. These nano-SQUIDs can be employed to measure small clusters of atomic magnetic moment, to obtain an inductive readout sensor in single-photon detectors and they have been proposed as a magnetic qubit for quantum computing. The research activities at CNR-ICIB has been devoted to the design, fabrication and characterization of nano-SQUIDs based on niobium Dayem Bridge and its magnetization measurements of nanoparticles for advance study of nanomagnetism. Here, I have reported the designs, the fabrication processes and the experimental performances (voltage flux characteristics, spin noise performance) of these nano-devices and also the groundwork measurement of nanoparticles magnetization using switching current measurement method operating in CNR-ICIB laboratory. Design and Fabrication: Many devices are fabrication using advanced electron beam lithography techniques with conventional photolithography, lift off and reactive ion etching techniques. The fabricated devices can be divided into two categories, first bilayer device or non hysteretic device and second single layer device or hysteretic device. a)Non-Hysteretic Device: The bilayer device consists of a niobium loop with a hole of 200nm interrupted by two bridges having length and width dimensions of 100nm X 80nm, 80nm X 60nm etc. The superconducting loop has a washer shape in order to enhance the heat dissipation during the working operations when the sensor is current biased in resistive mode. The device includes a micrometric integrated niobium coil located very close to the nano-sensor in order to modulate, tune and operate the SQUID in FLL mode. b)Hysteretic Device: The single layer devices having two Dayem Bridges of 90nm x 250nm and loop sizes of 2, 1 and 0.75μm, consist of a single niobium layer 20nm thick pattern by electron beam lithography and shaped by lift-off. The SQUIDs were designed to have a hysteretic current-voltage characteristic in order to work as a magnetic flux-current transducer. Performances: a)Non-Hysteretic Device: An intrinsic voltage swing and a maximum responsivity of 75 μV and 1.5 mV/Φ0 can be estimated from the experimental data. Our interest is focused on the characterization in small signal mode, because the magnetic flux coupled into device is expected to be much smaller than Ф0. In fact, such devices are designed to measure very small magnetic fields arising from a local nano-object. The sensor exhibits a magnetic flux noise level of 1.5μΦ0/Hz1/2 in the white region corresponding to a spin noise, in unit of Bohr magneton, of Sn1/2=2aSΦ1/2/(μBμ0)~60 spin/Hz1/2 where a is the radius of the SQUID loop, μ0 is the magnetic vacuum permeability and μB = 9.27 × 10−24 J/T. This value is comparable with the best value reported in the literature. It is worth noting that these sensors show a wide linear region in the V- Φ characteristics; the non-linearity effects are not evident up to 0.02 Φ0. b)Hysteretic Device: The second single layer device shows hysteretic I-V characteristics which are used as magnetic flux-current transducer. The magnetic pattern determines the responsivity of the SQUID, defined as the variation of the critical current as function of the external magnetic flux variation dIc/dΦ; in the present work responsivity up to 30μA/Φ0 have been obtained. The maximum magnetic flux coupled to the 2 and 1 micron loop size is about 2 and 0.5 Φ0 respectively. A critical current modulation is about 20%. An intrinsic sensor magnetic flux resolution less than 1 mΦ0, can be estimated from the intrinsic current fluctuation. The main interest of such devices is magnetization measurement of small magnetic particles by switching current techniques. Simulation Results: We have computed performance of nano-superconducting-quantum-interference devices (SQUIDs) in view of their employment in the detection of small spin populations. The analysis has been focused on nano-SQUID sensors having a square loop with a side length of 200 nm. We have calculated the spin sensitivity and the magnetic response relative to the single Bohr magneton (single spin), as a function of its position within the SQUID hole. The results predict that the SQUID response depends strongly on the spin position. The projected information’s are very useful to optimize the sensor performance in view of the most nanomagnetism applications. Preliminary Measurements of Nanoparticle Magnetization: We have effectively employed non-hysteretic sensor for nanoparticle magnetization investigation in view of nano-magnetism applications. We have performed preliminary measurements with and without iron oxide nanoparticles on the SQUID loop showing that the presence of magnetic nanoparticles can be easily detected and the magnetic relaxation curve measured. To test the capability of our devices in order to detect the magnetization of nanoparticles, we deposited iron oxide nanoparticles, having an average diameter of about 16nm, on chips containing characterized sensors. Here we report the measurement performed on the device with 1μm loop. The presence of iron nanoparticles did not change appreciably the magnetic pattern obtained using the integrated coil (magnetic field normal to the SQUID plane), however the nanoparticle presence was evident by the critical current measurement performed with magnetic field generated using the external solenoid (magnetic field parallel to the SQUID plane). Using the measured responsivity for this particular device (27μA/Φ0) the critical current variation was converted to magnetic flux variation. It is worth to note that with the present electronic at 100Hz to acquire 104 measurements it takes about 100s. The above results clearly indicating that the sensor can be effectively used in nano-magnetism applications.
SQUIDs (superconducting quantum interference devices) have been in use for more than half a century and constitute one of the first macroscopic quantum devices. SQUIDs operating at millikelvin temperatures can act as qubits for quantum computers and the recent reports of more than 50 qubit circuits indicate how far the technology has developed. In this talk, the author will describe two other applications of SQUIDs as quantum detectors, focusing on single spin and single photon energy resolving detection. Most superconducting devices rely on tri-layer Josephson tunnel junctions which are not easily scalable to the nanoscale. We have developed a Josephson junction fabrication method, based on electron beam lithography or focused ion beam milling of a single thin film of superconductor (Nb generally) which can provide sizes down to 50 nm. These devices are particularly relevant for two main applications in quantum technology and metrology. First, by shrinking the size of the SQUID loop and the junctions to around 200 nm, the sensitivity of the SQUID for magnetization measurements is improved to the level where a single electron spin flip may be detected. This is possible at the relatively elevated temperature of 4K.
We are working with Surrey University to implant single magnetic ions within the SQUID loop to provide a platform to test this combination as the basis for a new form of qubit operating at higher temperatures than the conventional Transmon superconducting devices. A second SQUID based detector which we are developing is an inductive transition edge sensor device (ISTED) for energy resolving measurements of single photons. This is based on the development of conventional transition edge sensors where we detect the change of the penetration depth of a small thin film of superconductor when it absorbs a photon. In this way, a major source of noise in conventional TES may be avoided since the absorber remains in the superconducting state at all times. In this way we have demonstrated single photon detection at 633 nm with 0.1eV resolution at operating temperature of 7.5K.