Low-noise SQUID Research Articles

Frequency domain multiplexing readout for large arrays of transition-edge sensors

We report our most recent progress and demonstration of a frequency domain multiplexing (FDM) readout technology for transition-edge sensor (TES) arrays, both of which we have been developing in the framework of the X-IFU instrument on board the future Athena X-ray telescope. Using Ti/Au TES micro-calorimeters, high-Q LC filters and analog/digital electronics developed at SRON and low-noise two-stage SQUID amplifiers from VTT Finland, we demonstrated the feasibility of our FDM readout technology, with the simultaneous readout of 37 pixels with an energy resolution of 2.23 eV at an energy of 6 keV. We finally outline our plans for further scaling up and improving our technology in the future.

Progress in the Development of Frequency-Domain Multiplexing for the X-ray Integral Field Unit on Board the Athena Mission

Frequency domain multiplexing (FDM) is the baseline readout system for the X-ray Integral Field Unit (X-IFU) on board the Athena mission. Under the FDM scheme, TESs are coupled to a passive LC filter and biased with alternating current (AC bias) at MHz frequencies. Using high-quality factor LC filters and room temperature electronics developed at SRON and low-noise two-stage SQUID amplifiers provided by VTT, we have recently demonstrated good performance with the FDM readout of Mo/Au TES calorimeters with Au/Bi absorbers. We have achieved a performance requested for the demonstration model (DM) with the single pixel AC bias ($\Delta E=$1.8 eV) and 9 pixel multiplexing ($\Delta E=$2.6 eV) modes. We have also demonstrated 14-pixel multiplexing with an average energy resolution of 3.3 eV, which is limited by non-fundamental issues related to FDM readout in our lab setup.

Study of Nb and NbN Resonators at 0.1 K for Low-Noise Microwave SQUID Multiplexers

The quality factors and noise performances of superconducting resonators play an important role in microwave SQUID multiplexer (MW-MUX). Since we develop microwave SQUID multiplexer for transition edge sensor arrays, the operation temperature is around 100 mK where the effects of two level system (TLS) can be significant. In this paper, we have investigated the electrode-material or wafer dependence of noise performance of MW-MUX by characterizing four types of resonators. Although the temperature dependence of resonance frequency and the power dependence of ${{\boldsymbol{Q}}_{\boldsymbol{u}}}$ show clear difference among the materials, frequency noises which contribute the readout noise of MW-MUX are similar. Theoretical model predicts that not TLS but other loss dominates ${{\boldsymbol{Q}}_{\boldsymbol{u}}}$ in the power region suitable for the low-noise operation of our MW-MUX. Further, the estimated flux noise calculated from the measured frequency noise and the conversion coefficient from frequency to flux were 1–5 $({{{\mathbf \mu }}{{{\mathbf \Phi }}_0}/\sqrt {{{\mathbf Hz}}} })$ @ 1 Hz, 0.2–0.8 $({{{\mathbf \mu }}{{{\mathbf \Phi }}_0}/\sqrt {{{\mathbf Hz}}} })$ @ 3 kHz. These results indicate the noise of our MW-MUX has small material dependence and the compatible low noise operation to typical dc SQUIDs would be possible. Therefore, the MW-MUX with our materials would not suffer from the low frequency noise due to TLS.

Analysis of a dc SQUID readout scheme with voltage feedback circuit and low-noise preamplifier

We analyzed the dc SQUID with voltage feedback circuit (VFC) and a low-noise room-temperature preamplifier to evaluate the feasibility of a low-noise SQUID direct-coupled readout scheme (DRS), possibly eliminating the need for a two-stage scheme employing a SQUID preamplifier. The passive VFC, connected in parallel to the SQUID, consists of a resistor Rs in series with an inductor Ls. This inductor is coupled to the SQUID by a mutual inductance Ms. The purpose of the VFC is to increase the SQUID’s flux-to-voltage transfer coefficient ∂V/∂Φ, thus reducing the preamplifier noise contribution δΦpreamp. However, at the same time, VFC introduces the thermal noise of Rs, δΦR, which may not be negligible. Generally, the noise of the readout scheme, δΦreadout, may thus include both δΦpreamp and δΦR, i.e., δΦreadout2 = δΦpreamp2 + δΦR2. To characterize the SQUID operation with VFC we introduced two dimensionless parameters, r = Rs/Rd and Δ = (Ms/Mdyn) − (Rs/Rd), where Rd and Mdyn = 1/(∂i/∂Φ) are dynamic properties of the SQUID itself. For assumed intrinsic SQUID parameters, we then numerically analyzed the dependence of δΦreadout noise components on r and Δ to determine their suitable ranges and the minimum of δΦreadout. To verify our analysis, we experimentally characterized, in liquid helium, three niobium SQUIDs with VFC, having suitably chosen r and Δ. The measured SQUID system flux noise was on the order of 1 μΦ0/√Hz, comparable to the intrinsic noise of the SQUID itself. The deduced equivalent voltage noise was comparable to that of a SQUID preamplifier in the two-stage readout. Simple single-stage ultra-low-noise SQUID DRS readout was thus demonstrated.

White noise of Nb-based microwave superconducting quantum interference device multiplexers with NbN coplanar resonators for readout of transition edge sensors

White noise of dissipationless microwave radio frequency superconducting quantum interference device (RF-SQUID) multiplexers has been experimentally studied to evaluate their readout performance for transition edge sensor (TES) photon counters ranging from near infrared to gamma ray. The characterization has been carried out at 4 K, first to avoid the low-frequency fluctuations present at around 0.1 K, and second, for a feasibility study of readout operation at 4 K for extended applications. To increase the resonant Q at 4 K and maintain low noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1 × 104 ≤ Q ≤ 2 × 104 and the square root of spectral density of current noise referred to the SQUID input √SI = 31 pA/√Hz. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Two-directional readout on the complex plane of the transmission component of scattering matrix S21 enables us to distinguish the flux noise from noise originating from other sources, such as the cryogenic high electron mobility transistor (HEMT) amplifier. Systematic noise measurements with various microwave readout powers PMR make it possible to distinguish the contribution of noise sources within the system as follows: (1) The achieved √SI is dominated by the Nyquist noise from a resistor at 4 K in parallel to the SQUID input coil which is present to prevent microwave leakage to the TES. (2) The next dominant source is either the HEMT-amplifier noise (for small values of PMR) or the quantization noise due to the resolution of 300-K electronics (for large values of PMR). By a decrease of these noise levels to a degree that is achievable by current technology, we predict that the microwave RF-SQUID multiplexer can exhibit √SI ≤ 5 pA/√Hz, i.e., close to √SI of state-of-the-art DC-SQUID-based multiplexers.

AC Read-Out Circuits for Single Pixel Characterization of TES Microcalorimeters and Bolometers

SRON is developing Frequency Domain Multiplexing (FDM) for the read-out of transition edge sensor (TES) soft x-ray microcalorimeters for the XMS instrument of the International X-ray Observatory and far-infrared bolometers for the SAFARI instrument on the Japanese mission SPICA. In FDM the TESs are AC voltage biased at frequencies from 0.5 to 6 MHz in a superconducting LC resonant circuit and the signal is read-out by low noise and high dynamic range SQUIDs amplifiers. The TES works as an amplitude modulator. We report on several AC bias experiments performed on different detectors. In particular, we discuss the results on the characterization of Goddard Space Flight Center x-ray pixels and SRON bolometers. The paper focuses on the analysis of different read-out configurations developed to optimize the noise and the impedance matching between the detectors and the SQUID amplifier. A novel feedback network electronics has been developed to keep the SQUID in flux locked loop, when coupled to superconducting high Q circuits, and to optimally tune the resonant bias circuit. The achieved detector performances are discussed in view of the instrument requirement for the two space missions.

Resistively shunted YBa2Cu3O7 grain boundary junctions and low-noise SQUIDs patterned by a focused ion beam down to80 nm linewidth

YBa2Cu3O7 24° (30°) bicrystal grain boundary junctions (GBJs), shunted with 60 nm (20 nm)thick Au, were fabricated by focused ion beam milling with widths80 nm ≤ w ≤ 7.8 µm. At 4.2 K we find criticalcurrent densities jc inthe 105 A cm − 2 range (without aclear dependence on w) and an increase in resistance times junction areaρn with an approximate scaling . For the narrowest GBJs jcρn = IcRn≈100 µV (with critical current Ic and junction resistance Rn), which is promising for the realization of sensitive nanoSQUIDs for thedetection of small spin systems. We demonstrate that our fabrication processallows the realization of sensitive nanoscale dc SQUIDs; for a SQUID withw≈100 nm wide GBJs we find an rms magnetic flux noise spectral density ofSΦ1/2≈4 μΦ0 Hz − 1/2 in the white noise limit. We also derive an expression for the spin sensitivitySμ1/2, whichdepends on SΦ1/2, on the location and orientation of the magnetic moment of a magnetic particle to be detected bythe SQUID, and on the SQUID geometry. For the unoptimized SQUIDs presented here, we estimateSμ1/2 = 390 μB Hz − 1/2, which could be further improved by at least an order of magnitude.

Full noise characterization of a low-noise two-stage SQUID amplifier

From measurements performed on a low-noise two-stage SQUID amplifier coupled to a high-Q electrical resonator we give a complete noise characterization of the SQUID amplifier aroundthe resonator frequency of 11 kHz in terms of additive, back action and cross-correlationnoise spectral densities. The minimum noise temperature evaluated at 135 mK is10 µK and corresponds to an energy resolution of .

Low noise SQUID based NDE with non-magnetic scanning system in unshielded environment

A Non-magnetic scanning Robotic system with special EMC considerations has been designed for SQUID-Based NDE of room temperature stationary samples in unshielded environment. Considerable efforts have been made to cancel out noise of the system which resulted in detection of two major noise sources. Characterizing the noise contribution of the involved parts, a minor noise component was found to be due to the robot and the other due to liquid nitrogen bubbling in some frequencies which could be avoided by choosing proper excitation frequency. Using our NDE system we performed a NDE scan of hidden cracks in aluminum plates with white noise level of 50 μΦ0/Hz-1/2.

Design, Fabrication, and Multiplexing of Magnetic Calorimeter X-ray Detectors with High-Efficiency SQUID Readout

Metallic magnetic calorimeters, where deposited energy is detected by measuring a temperature-dependent magnetization with a low-noise SQUID, remain a promising potential route to X-ray spectrometers with energy resolution approaching 1 eV. In this paper we describe our recent work toward array-compatible, high-resolution MMCs fabricated entirely using thin-film techniques. We describe a meander-style pickup loop designed for good coupling to high-efficiency, low noise SQUIDs, as well as considering various routes to a thin-film paramagnetic sensor. We also briefly overview the most promising technology for multiplexing arrays of non-dissipative metallic magnetic calorimeters.

Fabrication of Metallic Magnetic Calorimeter X-ray Detector Arrays

Microcalorimeters with metallic magnetic sensors show great promise for use in astronomical X-ray spectroscopy. We describe the design and fabrication of a lithographically patterned magnetic microcalorimeter. A paramagnetic AuEr film is sputter-deposited as the sensor, which is coupled to a low noise SQUID via a meander superconducting pickup loop used as an inductor. This inductor also provides the magnetic field bias to the sensor. The AuEr film is deposited over this meander such that the field created by a large current flowing in the loop magnetizes the sensor material. The use of thin film techniques in the fabrication of these magnetic sensors not only allows strong magnetic coupling between the sensor and the inductor, it also is scalable for array fabrication.

INVERSE-SQUARE LAW EXPERIMENT IN SPACE

The objective of ISLES (Inverse-Square Law Experiment in Space) is to perform a null test of Newton's law in space with a resolution of 10 ppm or better at a 100 μm distance. ISLES will be sensitive enough to detect the axion, a dark matter candidate, with the strongest allowed coupling and probe large extra dimensions of string theory down to a few micrometers. The experiment will be cooled to < 2 K , which permits superconducting magnetic levitation of the test masses. This soft, low-loss suspension, combined with a low-noise SQUID, leads to extremely low intrinsic noise in the detector. To minimize Newtonian errors, ISLES employs a near-null source, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a nominal distance of 100 μm. The signal is detected by a superconducting differential accelerometer.

A low noise SQUID magnetometer array probe designed for mouse and rat MCG measurements

We designed and fabricated a 9-channel magnetometer array probe with low noise and high spatial resolution for magnetocardiograph (MCG) measurements of mice and rats. The magnetometer based on a low-Tc superconducting interference device (SQUID) has a circular pick-up loop with the diameter of 2.5 mm and is arrayed in a 3 × 3 matrix with the spatial interval of 2.75 mm on a 10 mm × 10 mm silicon chip. The chip was mounted on a glass epoxy substrate with the diameter of 19 mm using flip-chip connections so that the cross talk due to wiring is reduced as small as possible. To protect the array during repetitive thermal cycles, the chip is covered with an epoxy resin. Typical field resolution of the array was 8 fT/√Hz at 1 kHz and the maximum cross talk in the array was measured to be less than 2%. Setting up this probe in a small cryostat with the warm-to-cool distance of about 2 mm, we demonstrated a real-time monitoring of MCG signals from a mouse and a rat in a magnetically shielding.

Short-Range Inverse-Square Law Experiment in Space

The objective of ISLES (Inverse-Square Law Experiment in Space) is to perform a null test of Newton's law in space with a resolution of one part in 105 or better at 100 μm. ISLES will be sensitive enough to detect axions with the strongest allowed coupling and probe large extra dimensions of string theory down to a few μm. The experiment will be cooled to ≤2 K, which permits superconducting magnetic levitation of the test masses. This soft, low-loss suspension, combined with a low-noise SQUID, leads to extremely low intrinsic noise in the detector. To minimize Newtonian errors, ISLES employs a near null source, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a nominal distance of 100 μm. The signal is detected by a superconducting differential accelerometer.

120 ℏ SQUID amplifiers with a high-Q resonating input load

We report on the behaviour of a low-noise two-stage SQUID amplifier, which is strongly coupled to high-Q electrical resonators operating at audio frequencies. The behaviour of the system has been made stable, without adding noise, by means of a damping network. Thanks to the very high quality factor of the resonator, it is possible to evaluate the back action noise of the SQUID amplifier. The additive noise is evaluated as usual by measuring the open input noise. From measurements of back action and additive noises, performed in the temperature range 1.33 K–4.17 K and at 1.67 kHz and 11 kHz, the approximate noise temperature T′n of the SQUID amplifier is evaluated and its minimum value corresponds to 120 quanta. Besides providing a full noise characterization of the SQUID amplifier, this system constitutes a simulator of a cryogenic resonant gravitational wave detector as regards operation frequency, quality factors and stability problem. On the basis of these results the operation of a resonant gravitational wave detector with an amplifier with energy sensitivity better than 100 ℏ seems achievable.

マイクロ波共振に基く超伝導転移端検出器の周波数多重読出回路

Dissipationless microwave radio frequency superconducting interference device (RF-SQUID) multiplexers are attractive for readouts of large format arrays of transition edge sensors (TES). This multiplexer has the potential for readout bandwidth of several GHz and power consumption of ≈0.1 pW per pixel, which are ≈103 and ≈10-4 times as large as those of DC-SQUID-based conventional multiplexers. To evaluate their readout performance for TES photon counters ranging from near infrared to gamma ray, white noise of this multiplexer was experimentally studied at 4 K. The reason for this was, firstly, to avoid the low-frequency fluctuation obvious at around 0.1 K but trivial for the energy resolution of photon counters, and secondly for a feasibility study of readout operation at 4 K for extended applications. To increase resonant Q at 4 K and maintain low-noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1×104≤QI≤2×104 and the square root of spectral density of current noise referred to the SQUID input √SI =31 pA/√Hz, where QI is the internal Q of resonators. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Systematic noise measurement with various microwave readout power PMR and two-directional readout on the complex plane of transmission component of scattering matrix S21 makes the contribution of each noise source obvious. By decreasing these noises to the degree achievable by current technology, we predicted the microwave RF-SQUID multiplexer would exhibit √SI ≤5 pA/√Hz; i.e., close to √SI of state-of-the-art DC-SQUID-based multiplexers.

Lithographically patterned magnetic calorimeter X-ray detectors with integrated SQUID readout

We describe the design, fabrication and performance of a fully lithographically patterned magnetic microcalorimeter X-ray detector. The detector is fabricated on the same chip as a low-noise SQUID that measures the change in the magnetic sensor film's magnetization as the film is heated by absorbed X-rays. Our proof-of-principle detectors use a 100μm×100μm–2μm paramagnetic Au:Er film coupled to a low-noise on-chip SQUID via a meandering superconducting pickup loop that also provides the magnetic field bias to the film. Absorption of 6keV X-rays in the film causes heating on the order of 1mK with a decay time of 1ms or less, the fastest reported using a magnetic calorimeter. However, the resolution is currently poor due to poor Au:Er film properties and non-optimized coupling to the SQUID. We describe the design and fabrication of this device and present measurements of the heat capacity, decay time constant and effective thermal conductance of the microcalorimeter as a function of temperature. Because the SQUID and calorimeter are lithographically patterned on the same substrate, this technology can be readily applied to the fabrication of arrays of multiplexed magnetic microcalorimeter detectors.

Short-range inverse-square law experiment in space

The objective of ISLES (inverse-square law experiment in space) is to perform a null test of Newton’s law on the ISS with a resolution of one part in 105 at ranges from 100 μm to 1 mm. ISLES will be sensitive enough to detect axions with the strongest allowed coupling and to test the string-theory prediction with R⩾5 μm. To accomplish these goals on the rather noisy International Space Station, the experiment is set up to provide immunity from the vibrations and other common-mode accelerations. The measures to be applied for reducing the effects of disturbances will be described in this presentation. As designed, the experiment will be cooled to less than 2 K in NASA’s low temperature facility the LTMPF, allowing superconducting magnetic levitation in microgravity to obtain very soft, low-loss suspension of the test masses. The low-damping magnetic levitation, combined with a low-noise SQUID, leads to extremely low intrinsic noise in the detector. To minimize Newtonian errors, ISLES employs a near-null source of gravity, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a distance of 100 μm to 1 mm. The signal is detected by a supercon-ducting differential accelerometer, making a highly sensitive sensor of the gravity force generated by the source mass.

1:30 000 cryogenic current comparator with optimum squid readout

We have developed a 1:30000 winding ratio cryogenic current comparator (CCC) for measurement of small currents, e.g., generated by a single electron transport device. A dedicated low-noise SQUID is used for the readout, which allows reaching a sensitivity close to ideal, that is without loss of sensitivity in the flux transformer coupling the CCC and the SQUID. The CCC-SQUID equivalent current input noise using a 1:30 000 winding ratio is 2.1 fA/Hz/sup 1/2/, measured down to 0.1 Hz. Room temperature impedance measurements on CCCs appear to be a useful method for the determination of the screening by shields surrounding the CCC as well as for the detection of internal CCC resonances.

Effects of microwave radiation on the noise in high- Tc YBa 2Cu 3O 7 dc SQUID magnetometer

With a properly fabricated low-noise SQUID, we have studied the effects of microwave radiation on the noise behavior. Under an ac bias, with increasing microwave power, the critical current and the voltage modulation depth Δ V= Φ 0 V Φ /π decrease, but the SQUID resistance R s increases, suggesting a dynamical interaction between the charge carriers and the electromagnetic wave. Furthermore, the microwave radiation modifies drastically the functional dependence of S Φ and Δ V on ac bias current.