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IEEE Transactions on Applied Superconductivity

Inductive sensor based on nano-scale SQUIDs

L. Hao, J.C. Macfarlane, S. K. H. Lam, C. P. Foley, Patrick Josephs-Franks, John C. Gallop

Published Jun 1, 2005• 31 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2005.849892

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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.
We present a new type of transition-edge sensor for single-photon and macro-molecule detection. In our detector the absorber element is an isolated, passive absorber of extremely low thermal mass, maintained close to, but below, its superconducting–normal transition, and strongly inductively coupled to a SQUID sensor. Incoming particles or photons are sensed in terms of a transient change in the inductive coupling, rather than a change in resistance. The detector's energy sensitivity and response time can be defined by the thermal mass of the absorber and its thermal contact with a substrate, independently of any electrical connections. We have modelled the energy sensitivity of our inductive superconducting transition-edge sensor using a sub-micron SQUID as an inductive read-out device. An ultimate energy resolution of order 10 −25 J Hz −1 is theoretically estimated based on the properties of the read-out SQUID and the dimensions of the absorber. We also report our initial work on fabrication of the Nb nanoscale SQUID where we have used the same material deposited on top of the SQUID as a thin-film absorber.
Nano-SQUIDs which can be the readout for inductive superconducting transition edge detectors (ISTED) were fabricated on monolayer niobium (Nb) film by focus ion beam (FIB) milling. The dimension of the nanobridges which formed the junctions was less than 60 nm and the diameter of the loop was 20 μm, The properties of this nano-SQUID were investigated. The design of ISTED was introduced and the transition temperature (Tc) of the absorbing layer was adjusted by both the thickness and the proportion Si doping of Nb.
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.
Abstract Single-photon detectors based on the superconducting transition-edge sensor are used in a number of visible to near-infrared applications, particularly for photon-number-resolving measurements in quantum information science. To be practical for large-scale spectroscopic imaging or photonic quantum computing applications, the size of visible to near-infrared transition-edge sensor arrays and their associated readouts must be increased from a few pixels to many thousands. In this manuscript, we introduce the kinetic inductance current sensor, a scalable readout technology that exploits the nonlinear kinetic inductance in a superconducting resonator to make sensitive current measurements. Kinetic inductance current sensors can replace superconducting quantum interference devices for many applications because of their ability to measure fast, high slew-rate signals, their compatibility with standard microwave frequency-division multiplexing techniques, and their relatively simple fabrication. Here, we demonstrate the readout of a visible to near-infrared transition-edge sensor using a kinetic inductance current sensor with 3.7 MHz of bandwidth. We measure a readout noise of 1.4 pA/Hz1.4\,{{{\rm{pA}}}}/\sqrt{{{{\rm{Hz}}}}}1.4pA/Hz​ 1.4 pA / Hz , considerably below the detector noise at frequencies of interest, and an energy resolution of (0.137 ± 0.001) eV at 0.8 eV, comparable to resolutions observed with non-multiplexed superconducting quantum interference device readouts.
Abstract Superconducting transition edge sensors (TESs) have made dramatic progress since their invention some 65 years ago (Andrews et al 1949 Phys. Rev. 76 154–155 ; Irwin and Hilton 2005 Topics Appl. Phys. 99 63–149) until now there are major imaging arrays of TESs with as many as 7588 separate sensors. These are extensively used by astronomers for some ground-breaking observations (Hattori et al 2013 Nucl. Instrum. Methods Phys. Res. A 732 299–302 ). The great success of TES systems has tended to overshadow other superconducting sensor developments. However there are other types (Sobolewski et al 2003 IEEE Trans. Appl. Supercond. 13 1151–7 ; Hadfield 2009 Nat. Photonics 3 696–705 ) which are discussed in papers within this special edition of the journal. Here we describe a quite different type of detector, also applicable to single photon detection but possessing possible advantages (higher sensitivity, higher operating temperature) over the conventional TES, at least for single detectors.
Large area superconducting quantum interference devices (SQUIDs) are known to be ultimate sensors of magnetic flux. By diminishing the size of a SQUID loop, the energy sensitivity of the device can be increased to near quantum-limited operation whilst also making it less sensitive to external magnetic field noise and improving its coupling to any nano-scale magnetic particle within the loop. This makes it an ideal nano-sensor for magnetic particles including single molecules. It is, however, important to optimize the coupling between any magnetic nano-particle placed within the SQUID and the SQUID itself in order to achieve maximum sensitivity for the spin detection. We have made some preliminary calculations of the expected SQUID response to a magnetic particle placed at different locations within the SQUID device, and using a magnetic tip in a scanning tunnelling microscope (STM) we are able to produce a spatial map of the SQUID sensitivity. Initial results on a 3 µm diameter loop SQUID are presented to demonstrate this method.