The goal of my thesis was to construct a general purpose cryostat, capable of measuring multiple devices at 100mK. This included modifying an existing SHASTA model 106 Adiabatic Demagnetization Refrigerator (ADR) by adding a new physical support structure to allow for more weight to be added to the cryostat’s testing stages, as well as custom pieces of electronics, such as a room temperature programmable current source. This allowed for computer programs to be written so we could automate the data acquisition process. \nWe successfully measured multiple superconducting quantum interference devices (SQUIDs), with the goal of proving that our custom electronics were able to control and record the output of the SQUID. This included measuring the biasing conditions of the SQUID with \nthe automated current sources, as well as measuring the noise temperature of a SQUID down to 300mK. Furthermore, we measured the thermal conductivity of our new sup- port structure for the cryostat and were able to prove that the material used was new \nand novel approach for high strength, low thermal conductivity in cryogenic research. We also investigated a new technique in permanently modifying the operational frequency of a SQUID, and were able to show that this new method of laser ablating the input coil could successfully shift the frequency. While the focus of cryogenic testing was on SQUIDs the cryostat itself was designed in such a way such that many types of low temperature devices could be tested within the system.
The goal of my thesis was to construct a general purpose cryostat, capable of measuring multiple devices at 100mK. This included modifying an existing SHASTA model 106 Adiabatic Demagnetization Refrigerator (ADR) by adding a new physical support structure to allow for more weight to be added to the cryostat’s testing stages, as well as custom pieces of electronics, such as a room temperature programmable current source. This allowed for computer programs to be written so we could automate the data acquisition process. \nWe successfully measured multiple superconducting quantum interference devices (SQUIDs), with the goal of proving that our custom electronics were able to control and record the output of the SQUID. This included measuring the biasing conditions of the SQUID with \nthe automated current sources, as well as measuring the noise temperature of a SQUID down to 300mK. Furthermore, we measured the thermal conductivity of our new sup- port structure for the cryostat and were able to prove that the material used was new \nand novel approach for high strength, low thermal conductivity in cryogenic research. We also investigated a new technique in permanently modifying the operational frequency of a SQUID, and were able to show that this new method of laser ablating the input coil could successfully shift the frequency. While the focus of cryogenic testing was on SQUIDs the cryostat itself was designed in such a way such that many types of low temperature devices could be tested within the system.
Mostafa, Md Asif Installation and Characterization of a Superconducting Quantum Interference Device (SQUID) in an Adiabatic Demagnetization Refrigerator (ADR) Master’s thesis Department of Physics, University of Jyväskylä, 2026. This thesis presents the installation and characterization of a two-stage dc superconducting quantum interference device (SQUID) system in an adiabatic demagnetization refrigerator (ADR). The work combines the physical principles of superconductivity, Josephson junctions, SQUID operation, flux-locked-loop readout, and ADR cooling with practical low-temperature implementation. The SQUID system was integrated into the ADR through a cryogenic signal chain including electrical interface mapping, cryogenic wiring, filtering, thermal anchoring, and room-temperature readout electronics. The experimental characterization included voltage–flux modulation measurements, transfer-function and mutual-inductance calculations, gain calibration, and noise analysis. The measured two-stage combined transfer function was close to the value in the datasheet, confirming that the installed system reached the expected operating regime. The feedback and input-coupling parameters were extracted and compared with datasheet values, while the gain calibration provided the transimpedance needed to convert output-voltage noise into equivalent input- current noise. The initial commissioning-stage measurements showed a white input-current noise level of a few picoamperes per square root hertz and a feedback-resistance- dependent bandwidth. Later optimized measurements demonstrated that the ADR- based SQUID platform could reach a lower noise floor and larger bandwidth after reducing setup-related noise sources. These results show that the installed platform can support low-noise SQUID operation and provides a useful basis for future cryogenic detector and superconducting-device measurements. Keywords: SQUID, Adiabatic Demagnetization Refrigerator (ADR), Josephson junction, flux-locked loop, two-stage SQUID, mutual inductance, cryogenic readout, low-temperature physics.
A cryogen-free cryostat was set up in order to perform measurements at ultra low temperatures. The setup involved the removal of an adiabatic demagnetization refrigerator (ADR) and replacing it with a dilution refrigerator system. The modified cryostat is able to provide continuous cooling power, enabling greater flexibility over the ADR in the type of low temperature experiments that can be performed. Condensation stages, main flow impedance and radiation baffles were designed, constructed and installed into the cryostat. Several general cryostat design considerations are also discussed. Condensation of the incoming 3He - 4He mixture utilizes only the cooling power of the two-stage pulse tube refrigerator. For a circulation rate of 120 μmols/sec, the base temperature of the cryostat is 70 mK with a cooling power of 33 μW of heat at 100 mK. \nWith this cryostat, a measurement system is developed for studying superconducting microwave resonators. Residual losses in these devices continue to impede their performance in applications such as ultra sensitive detectors and quantum computing. A microwave circuit consisting of coaxial cables, attenuators and amplifiers was installed on the cryostat to measure losses in these devices. Data analysis and fitting procedures were written in Python programming language to extract key resonator properties. Characterizing and understanding loss mechanisms for these devices will lead to improved materials and fabrication methods. This will ultimately result in losses that approach the fundamental limit set by the Mattis-Bardeen theory.
We have designed and tested a superconducting quantum interference device (SQUID) readout circuit, which can be operated at liquid helium temperatures. Although room-temperature SQUID electronics perform well, it is sometimes desirable to keep the wires between SQUID, readout electronics, and feedback coil as short as possible to minimize phase shifts and time delays. Cooling the readout circuit to low temperatures can also decrease its thermal noise. Our readout circuit uses conventional ac-flux modulation, which significantly reduces low frequency excess noise and drift in the preamplifier. In this case, simple complementary metal–oxide–semiconductor circuits with low power dissipation can be used as amplifier, phase-sensitive detector and integrator. The power dissipation of the complete readout is less than 15mW at 5V supply voltage.
Thousands to millions of sensitive signals will need to be conveyed through all the temperature stages of a dilution fridge to operate future large-scale quantum processors made of many quantum bits. The exploding number of heat-conductive coaxial cables will overwhelm the fridge cooling capabilities, detrimental to the quantum core. Moving the control electronics down to cryogenic temperatures allows the use of already-available superconducting cables, alleviating heat conduction between low-temperature stages, and appears as a clear path towards scalability in the number of operating qubits.This Ph.D. work aimed to explore the use of the industrial CMOS 28nm Fully-Depleted Silicon-On-Insulator (FD-SOI) technology at cryogenic temperatures for quantum computing applications. Our first objective is to extend the sparse existing knowledge about the FD-SOI 28nm transistors at cryogenic temperatures for practical aspects of circuit design and later for developments of compact models.To speed-up the characterization of single devices with the inherent hour-long cooling cycles, we designed an integrated circuit multiplexing a thousand of transistors for different geometries and gate-stack flavors for low-frequency measurement of the current-voltage characteristics and pair-matching analysis from 300 down to 0.1K. We discuss and analyze the evolution trends with varying geometry at different temperatures for important quantities in circuit design, such as the transconductance, the conductance, and the transconductance over drain current ratio of individual transistors.Secondly, we explore the low-temperature co-integration and the full on-chip integration of semiconductor quantum devices with classical electronics aimed at specific measurements down to the millikelvin range.We first focus on the sub-nanoampere current measurement of quantum dot devices by designing and characterizing a low-power transimpedance amplifier (TIA). The high-gain amplifier is successfully applied to measure the current across single- and double quantum-dot devices, respectively wire-bonded a few mm away or on-chip integrated a few micrometers-away. To further leverage the integration into the same substrate, we connected GHz-range voltage-controlled oscillators to one of the gates of the double dot in an attempt to observe discrete charge pumping in a fully-integrated device.Lastly, we tackled the measurement of the gate capacitance of single quantum devices by proposing a new measurement scheme making use of the cryogenic electronics capabilities as an alternative to the well-known reflectometry. By integrating a voltage-controlled current excitation and a voltage-sensing amplifier in the 200 MHz range, both in close proximity to the quantum device connected to an LC tank, the read-out circuitry of a variation in the device capacitance becomes a purely lumped-element system with impedance measurement of the resonant circuit without any wave propagation like in reflectometry. This approach increases simplicity and compactness of the measurement set-up. We even replace the bulky passive inductor used in reflectometry by an active inductor made of transistors and capacitors, offering improved scalability with a 3-orders of magnitude lower area for the same inductance. The resulting circuit successfully measured aF-capacitance variations of nanometric transistors at 4.2K revealing oscillatory quantum effects in the gate capacitance as a function of the gate and back-gate voltages.At the end of this dissertation, a picture is given with the challenges laying ahead related to circuit architecture and design for the ultimate goal of entering into the era of large-scale quantum processing.
We have developed a cryogenic probe and cryostat system to test both active and passive superconducting devices and circuits up to microwave frequencies at variable temperatures. Our system consists of two basic parts: the cryostat and the matching probe. The cryostat is a unique, very efficient, variable temperature, flow-type cryostat, where we control temperature without electrical heaters. It is magnetically shielded and allows rapid testing between 4.2 K and room temperature. Probes developed for this cryostat have a larger number of shorter cables than standard dip-type probes. They are designed to test chips or chip assemblies without additional packaging. Chips or chip assemblies can be quickly mounted and dismounted from a fixed spring-contact assembly. Each probe has 56 wide-bandwidth signal cables. We have repeatedly used these probes for testing both active and passive superconducting integrated circuits up to 20 GHz. The probe and cryostat combination provides a testing capability that is simultaneously high frequency, dc magnetic shielded, has variable cryogenic temperature, and quick turnaround.
At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interference Device (SQUID) acts as a macroscopic quantum variable whose dynamic is described by a double well potential. A rf SQUID consists of a superconducting loop interrupted by a thin insulating barrier, known as a Josephson junction. The modified rf SQUID design discussed in this thesis has independent, in situ, controls that allow varying the barrier height between the potential wells and their relative tilt. This high level of control of the potential together with the capability to implement an experimental setup optimized to reduce coupling to external sources of noise, made this system ideal not only for tests of quantum transition at a macroscopic level, but also as a scalable solid state device for quantum computation. A variety of measurements of quantum phenomena are reported in this thesis, including microwave level spectroscopy, coherent oscillations, and macroscopic resonant tunneling (MRT). These measurements are compared with theoretical models to study the possible sources of decoherence in a rf SQUID qubit. The analysis confirmed that the main source of decoherence is not due to coupling to the external environment, but it can be linked to intrinsic low frequency flux noise. Work from many groups over the last several years has shown that the performance of a superconductive qubit is limited by coupling to spurious defects in the materials used for the device fabrication. In this work we analyze the possibility of using the MRT as a tool for future studies of effects of materials and fabrication process in the realization of improved qubits, and we also explore the feasibility for measurements of quantum phenomena that involve fast pulsing of the potential barrier height.