The microwave cavity experiment for the dark matter axion search is based on the theoretical prediction that axions could convert into microwave photons inside a High-Q cavity immersed in a strong magnetic field. In such experiments a weak, of order 10-22 W, quasi-monochromatic microwave signal should be detected by scanning it in a very wide frequency range. The best cryogenic semiconductor amplifiers have the lowest noise temperature plateau of about 1.0 K even at significantly lower ambient temperature. Superconducting quantum interference devices, or SQUIDs, can work as microwave amplifiers with temperature noise close to the standard quantum limit that is about 50 mK at 1 GHz [1]. Previously designed SQUID-based high frequency amplifiers have narrow bandwidth due to a microstrip resonant input coil. It requires serial replacements of SQUID preamplifiers in order to scan wide frequency range. This procedure is complex and time consuming because of a large mass of hardware should be cooled down below 100 mK. For suitable amplification at high frequency a SQUID should be designed with the smallest possible Josephson junction capacitance, reasonably low SQUID loop inductance, and maximal steepness of the transfer function. Sub-micron size cross-type Josephson junctions have a very low capacitance of about 0.04 pF that is one order smaller than a capacitance of Josephson junctions fabricated using conventional window-type technology. Such junctions initially were designed and used at IPHT for low-frequency SQUID current sensors [2]. Slightly modified current sensors were tested as amplifiers at frequency range 0.5 – 5 GHz. They showed high gain and both wideband and resonant performance. In this paper we describe wideband SQUID microwave amplifiers fabricated at IPHT using sub-micron size Josephson junctions with very low capacitance.
The microwave cavity experiment for the dark matter axion search is based on the theoretical prediction that axions could convert into microwave photons inside a High-Q cavity immersed in a strong magnetic field. In such experiments a weak, of order 10-22 W, quasi-monochromatic microwave signal should be detected by scanning it in a very wide frequency range. The best cryogenic semiconductor amplifiers have the lowest noise temperature plateau of about 1.0 K even at significantly lower ambient temperature. Superconducting quantum interference devices, or SQUIDs, can work as microwave amplifiers with temperature noise close to the standard quantum limit that is about 50 mK at 1 GHz [1]. Previously designed SQUID-based high frequency amplifiers have narrow bandwidth due to a microstrip resonant input coil. It requires serial replacements of SQUID preamplifiers in order to scan wide frequency range. This procedure is complex and time consuming because of a large mass of hardware should be cooled down below 100 mK. For suitable amplification at high frequency a SQUID should be designed with the smallest possible Josephson junction capacitance, reasonably low SQUID loop inductance, and maximal steepness of the transfer function. Sub-micron size cross-type Josephson junctions have a very low capacitance of about 0.04 pF that is one order smaller than a capacitance of Josephson junctions fabricated using conventional window-type technology. Such junctions initially were designed and used at IPHT for low-frequency SQUID current sensors [2]. Slightly modified current sensors were tested as amplifiers at frequency range 0.5 – 5 GHz. They showed high gain and both wideband and resonant performance. In this paper we describe wideband SQUID microwave amplifiers fabricated at IPHT using sub-micron size Josephson junctions with very low capacitance.
Radio-frequency (RF) amplifiers based on direct current (DC) superconducting quantum interference device (SQUID) have low-noise performance for precision physics experiments. Gain curves of SQUID RF amplifiers depend on several parameters of the SQUID and operation conditions. We are developing SQUID RF amplifiers for application to measure very weak RF signals from ultra-low-temperature high-magnetic-field microwave cavity in axion search experiments. In this study, we designed, fabricated and characterized SQUID RF amplifiers with different SQUID parameters, such as number of input coil turn, shunt resistance value of the junction and coupling capacitance in the input coil, and compared the results.
We report results of our development of two types of microwave amplifiers based on Superconducting quantum interference devices (SQUIDs) for CAPP (Center for Axion and Precision Physics Research) axion search experiments. The first amplifier, Microstrip SQUID Amplifier (MSA) has a wide bandwidth. Power dissipation in resistive shunts results in a device overheating and limits the MSA performances. A thermal models of shunt cooling processes is developed and recommendations for a future design is provided. The second amplifier, Josephson Parametric Amplifier (JPA), at limited bandwidth showed a low noise near the standard quantum limit (SQL) of 196 mK. Implementation of a JPA in CULTASK (CAPP's Ultra-Low Temperature Axion Search in Korea) experiment is planned in the end of 2019.
Abstract IBS/CAPP in South Korea is home to multiple active axion search experiments using cavity haloscopes that operate within the frequency range of 1-6 GHz. The haloscopes convert axions to photons, resulting in an output power of about 10-24 to 10-22 W. To detect such a small signal amidst noise, quantum-limited noise amplifiers and ultra-low-temperature environment (∼10 mK) are required for all critical read-out components to minimize noise from all of active and passive lossy components. Additionally, operating at low dissipation levels is necessary to maintain the haloscope and amplifier at a temperature of 25-40 mK. Our primary objective is to achieve the highest possible scanning-frequency speed, which includes the time for maintenance and system calibration. For the CAPP-MAX flagship experiment, we achieved a record speed of 3 MHz/day for the DFSZ-level axion sensitivity during uninterrupting measurements. This paper presents the development and operation of low-noise amplifiers for five haloscope experiments targeting different frequency ranges and provides design, operational, and performance details of the amplifiers.
We have designed and characterized a micro-SQUID with dispersive readout for use in low temperature scanning probe microscopy systems. The design features a capacitively shunted RF SQUID with a tunable resonance frequency from 5 to 12 GHz, micrometer spatial resolution, and integrated superconducting coils for local application of magnetic fields. The SQUID is operated as a nonlinear oscillator with a flux- and power-dependent resonance frequency. Measurements for device characterization and noise benchmarking were carried out at 4 K. The measured flux noise above 10 kHz at 4 K is 80 nΦ0 Hz−1∕2 at a bandwidth of 200 MHz. Estimations suggest that one can benefit from parametric gain based on inherent nonlinearity of the Josephson junction and reduce the flux noise to 30 nΦ0Hz–1∕2 at 100 mK, which corresponds to 10.6 μBHz–1∕2 for a magnetic moment located at the center of the pickup loop.Measuring the magnetic response of mesoscopic samples or mapping it vs. position for extended systems is effective methods for revealing the fundamental quantum properties of condensed matter systems. Over the past few decades, many advanced magnetic imaging schemes have been developed, including magnetic force microscopy,1 scanning Hall probe microscopy,2 superconducting interference devices (SQUIDs),3,4 and NV center based magnetometry.5 The high sensitivity, low back action, and low power dissipation of SQUIDs make them attractive for many types of low temperature experiments. One classic example is the central role of scanning SQUID microscopy in tests of pairing symmetry of high-Tc cuprate superconductors.6 Other prominent applications are the thermodynamic characteristics of persistent currents in normal metal rings7 and proof of edge states in topological insulators in the quantum spin Hall regime.8A key performance for a SQUID is its flux sensitivity. Parametric amplification has been harnessed in nano-SQUIDs based on aluminum junctions9,10 in order to reduce the flux noise down to 30 nΦ0Hz–1∕2 with a bandwidth exceeding 60 MHz.10 Recent studies on parametric amplifiers and dispersive readout of superconducting qubits also harness the nonlinearity of the Josephson junction to boost sensitivity.11–13In scanning SQUID microscopy, signal sources often behave dipole-like, such as superconducting vortices in mesoscopic dots or currents in mesoscopic rings. Smaller SQUIDs provide better coupling to smaller samples which leads to higher spin sensitivity3,14 as the magnetic field of a dipole decreases with 1/r3. Various approaches have been pursued to reduce the size. The smallest nano-SQUID to date has been fabricated by evaporating Nb or Pb15 onto the apex of a sharp quartz tip reaching a loop diameter of below nm and a spin sensitivity of 0.38 μBHz−1/2 at tens of kHz bandwidth. Compared to such size-optimized devices, micro-SQUIDs fabricated using a standard Nb technique have the advantage of on-chip field coils and modulation coils,14 and can be operated at 4 K allowing for a wide range of applications. However, these come at the cost of larger pick-up loops of a few microns and a resulting poorer spin sensitivity of 200 μBHz−1/2 for a magnetic moment located at the center of the pickup loop.The limitation of large pickup loops was previously only overcome by devices with pickup loops defined by the focused-ion-beam,16 but has recently also been achieved by improvements in lithography and shown to offer better spatial resolution as well.17 Here, we present a scanning micro-SQUID which is based on the same fabrication technology incorporating another major advancement. Our design exploits the parametric amplification based on nonlinearities of the SQUID to boost sensitivity. The combination of a smaller pickup loop and parametric amplification of the signal leads to better spin sensitivity (10.6 μBHz−1/2) and higher bandwidth (200 MHz) compared to traditional DC micro-SQUIDs in which the flux sensitivity is mainly limited by internal dissipation.Our devices were fabricated at Hypres Inc. using planarized Nb/AlOx/Nb trilayer Josephson junction technology, including two planarized Nb layers with approximately 0.8 μm minimum feature size. The SQUID consists of a superconducting ring shunted by one Josephson junction with a designed critical current of 20 μA [Fig. 1(a)]. With this geometry, the SQUID is gradiometric, i.e., the effect of any homogeneous background magnetic field approximately cancels out. The smallest pickup loops implemented have an inner (outer) diameter of 1 μm (2 μm). The superconducting loop is shunted by an on-chip parallel-plate capacitor with amorphous silicon dioxide as dielectric which was designed to have a capacitance of 80 pF. The superconducting loop and the capacitor form a resonator with a flux dependent resonance frequency from 5 to 12 GHz. The pickup loops are placed on opposite corners of the parallel-plate capacitor at a distance of about 450 μm so that one loop can be located in close proximity to the sample while the other is kept far away from it. External fields can be applied to the sample by local single-turn field coils situated around each pickup loop. The field coils are fully integrated into the chip layout and can also be used to bias the SQUID at its most sensitive point. Compared to using additional modulation coils,14 this approach has the advantage of reducing the total device inductance, which leads to a higher sensitivity.18 In a scanning microscope, the field coil at the rear end (away from the sample) can be used to bias the SQUID or operate it in flux feedback such that resulting stray fields will not act on the sample, apart from a weak screening current through the pickup loop. The effective diameter of each coil is 7.5 μm, resulting in a mutual inductance of about 0.34 Φ0/mA (for the smallest pickup loop). The SQUID resonator is connected by an on-chip Z0 = 50 Ω coplanar waveguide (CPW) transmission line to the bonding pads. The two field coils are connected by two smaller CPWs to the bottom edge bonding pads [Fig. 1(c)], ending in two parallel-stripline waveguides running alongside the edges of the capacitor towards the respective field coils. Based on estimations, it should thus be possible to apply high frequency signals of at least several GHz to SQUID and sample.
The axion, a pseudoscalar particle originally introduced by Peccei, Quinn, Weinberg, and Wilczek to solve the "strong CP problem", is a well motivated dark-matter candidate with a mass lying in a broad range from peV to few meV. The last decade witnessed an increasing interest in axions and axion-like particles with many theoretical works published and many new experimental proposals. A major challenge for cosmological-axion discovery is the detection of the faint signal expected in detectors with power as low as a fraction of yoctowatt corresponding to a single microwave photon per minute. Early experiments used GaAs field-effect-transistors as well as pioneering technology such as Rydberg-atom single-photon detection. In the attempt of reducing the noise temperature superconductive devices were soon introduced. Microstrip SQUID Amplifiers allowed SQUIDs to operate at frequencies of a few GHz with a noise temperature reaching the standard quantum limit. Josephson Parametric Amplifiers have been recently employed extending the search to higher frequency while boradband Traveling Wave Parametric Amplifiers are now under study. The ultimate sensitivity, beyond the quantum limit, is however expected from single microwave-photon detectors. Solutions for quantum sensing include quantum non-demolition measurements and switching detectors based on superconducting qubits and hot-electron detectors. A further improvement in signal sensitivity and noise reduction is expected exploiting arrays of superconducting qubits as proposed by the SUPERGALAX project.
The dynamics and noise of a dc SQUID (the Superconducting Quantum Interference Device), with the McCumber parameter /spl beta//sub c/=2/spl pi/R/sup 2/l/sub c/C//spl Phi//sub 0/ close to the unity (where I/sub c/, R, C is the critical current, the shunt resistance and the capacitance of the Josephson junctions comprising the SQUID, respectively, /spl Phi//sub 0/=2.07 10/sup -15/ Wb is the magnetic flux quantum) integrated with a planar spiral input coil, have been experimentally studied. The length of the spiral input coil was chosen so as to match its /spl lambda//4 microwave resonance frequency to the plasma resonance frequency of the SQUID. The input coil resonance is found to enhance the overall quality factor Q of the Josephson oscillations in the SQUID and, as a result, to increase the dynamic resistance R/sub d/ and the gradient of the flux-to-voltage characteristics, /spl delta/V//spl delta//spl Phi/, without hysteresis. A dc SQUID with a loop inductance L=30.5 pH, /spl beta//sub c/=0.72, and a 6 turn input coil demonstrated a non-distorted quasi-sinusoidal flux-to-voltage transfer function with an exceptionally large modulation depth of 140 /spl mu/V peak-to-peak. The spectral density of the magnetic flux noise was as low as 3.5/spl times/10/sup -7/ /spl Phi//sub 0//Hz/sup 1/2/ in the double stage configuration, measured at a temperature 4.2 K using direct read out electronics. In combination with an intermediary transformer, the current resolution of the SQUID is as low as 1.25 pA/Hz/sup 1/2/ with an input coil inductance of 100 nH.