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.
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.
This article reviews recent progress in superconducting nanodevices, with particular emphasis on fabrication methods developed for superconducting nanowires and nanoscale Josephson junctions based on different barrier materials. It evaluates the future potential of superconducting nanodevices, including nano-superconducting quantum interference devices (nanoSQUIDs), in light of improvements in nanoscale fabrication and manipulation techniques, along with their likely impacts on future quantum technology and measurement. The article first considers efforts to realize devices at the physical scale of 100 nm and below before discussing different types of Josephson junction such as trilayer junctions. It also describes the use of focused ion beam milling and electron beam lithography techniques for junction fabrication at the nanoscale and the improved energy sensitivity detectable with a nanoSQUID. Finally, it looks at a range of applications for nanoSQUIDs, superconducting single photon detectors, and other superconducting nanodevices.
It has been nearly 110 years since the discovery of superconductors, and more than 30 years since the discovery of high temperature superconductors (HTS). Great progress has been made in the application of superconducting electronics in the last two decades. HTS microwave devices have shown much higher perfomance than the traditional ones and have found their ways to the industry applications in mobile communication, radar, and special communication applications. Owing to the ultrahigh sensitivity to magnetic fields and currents, superconducting quantum interference devices (SQUIDs) have been used as the irresplacible sensors in geological surveying, magnetic resonanc imaging, biomagnetic imaging, and other areas. The sensitivity of superconducting radiation detectors such as superconducting SIS mixer, superconducting hot electron bolometer, superconducting transition edge sensor, superconducting nanowire single photon detector, and superconducting microwave kinetic inductance detector are near the quantum limitation. They are now key technology in geophysics, astrophysics, quantum information science, biomedicine, and so on. Superconducting Josephson parametric amplifier has become a key element for superconducting quantum computing. Superconducting integrated circuit has been included in the international roadmap for devices and systems, and shows that having the potential to become one of the mainstreams for post-Moore information processing technology. In metrology, superconducting Josephson effect and Josephson junction array devices have been widely used in the redefinition of quantum voltage reference and basic units of the International system of Units. Superconducting electronics plays an important role in the current quantum information technology boom, which in turn promotes the development of superconducting electronics. This review will brief introduce the research and application of superconducting electronics in China in recent years.
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.
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.
This Thesis studies opportunities to create a quantum standard for the electric current with the help of tunnel junctions. We use two types of tunnel junctions: superconducting Josephson junctions, and NIS junctions where one of the electrodes is normal (N) and the other one superconducting (S). In both cases, tunnel junctions are employed in a single-electron transistor (SET) structure, which is used to transfer a controlled number k of electrons (e) with the repetition frequency f. The magnitude of the resulting current is thus I = kef. First, we study the Cooper pair sluice, where Josephson junctions are connected as two superconducting quantum interference devices (SQUID). A gate electrode is used to control the charge state of the superconducting island formed between the SQUIDs. The sluice is based on tuning the tunneling rates through the SQUIDs with local magnetic fluxes, which allows to control the direction of the charge transfer. Weak traces of current quantization can be observed up to above 1 nA, which is large enough current for many metrological purposes. However, the accuracy of the current is still far from what is required in metrology. We study also a new type of SQUID structure, the balanced SQUID, which could be used to improve the accuracy of the sluice or in, e.g., some quantum computing applications. Second, we employ NIS junctions in the hybrid (SINIS) single-electron transistor with superconducting leads and a normal-metal island. This structure can be used as the SINIS turnstile which lets electrons to flow one by one in the direction determined by the bias voltage. We report the first experimental results on the SINIS turnstile and two methods to improve the accuracy of the current: increasing the charging energy of the island or connecting the turnstile in a resistive environment. We also show that the SINIS turnstile can be used as an electronic radio-frequency refrigerator where tunneling processes cool the electron temperature of the normal-metal island.
Nanoscale Superconducting QUantum Interference Devices (SQUIDs) have spin sensitivities approaching that required to detect the flip of a single spin in close proximity. There is considerable interest in developing them for measuring the properties of small spin populations in magnetic systems. It is desirable that such measurements can be realised at sub-kelvin temperatures thereby allowing the study of magnetic systems that undergo phase changes at such temperatures. However, most nanoscale SQUIDs use nanobridges as the Josephson elements which limits the operating range to temperatures close to the transition temperature of the device. Well below this, the current-phase relation can become non-sinusoidal, and hot spots arising from the large critical current lead to hysteretic I-V characteristics. To extend the temperature range downwards, we have developed a range of nanoSQUIDs fabricated from alternative materials with lower transition temperatures including Ti/Au and Al/Ag bilayers patterned using lift-off and e-beam lithography (EBL). We report on their I-V characteristics, noise performance and behaviour in applied magnetic fields at temperatures down to 60 mK. We discuss theoretical analysis and computer modelling of the heat flow in the nanobridge structure, and consider the effects of bank geometry and kinetic inductance on the overall device performance. Finally, we present measurements of several magnetic systems: control lines, superconducting islands and ground planes and discuss the feasibility of magnetic measurements of novel materials of interest, including the heterointerface between lanthanium aluminate and strontium titanate.