A compact cryogen-free continuous adiabatic demagnetization refrigeration platform for quantum technology applications
P. Schüßler, R. A. Chandavar, S. I. Afroozeh Borjeni, A. Kinast, A. Reinold, F. Galli Geleilate, M. Weisser, Ruben J. J. van Gulik, Sander Dominicus, Jakob Kammhuber, K. Eibensteiner, Jan Spallek, S. Säubert
We present the design and performance of a compact, fully cryogen-free cooling platform as a technology demonstrator for quantum hardware applications. The system leverages a four-stage continuous adiabatic demagnetization refrigerator (cADR) to achieve continuous cooling below 30 mK without the use of helium-3. Integrated mechanical and superconducting heat switches enable efficient thermal cycling, while high-density radio frequency (RF) wiring provides the infrastructure required for control and readout of a five-qubit superconducting quantum processor. Housed in a single rack with a spacious, accessible sample stage, the platform demonstrates that cADR technology can be realized in a compact form factor and provides a basis for future quantum hardware platforms operating under stringent space and infrastructure constraints.
We present the design and performance of a compact, fully cryogen-free cooling platform as a technology demonstrator for quantum hardware applications. The system leverages a four-stage continuous adiabatic demagnetization refrigerator (cADR) to achieve continuous cooling below 30 mK without the use of helium-3. Integrated mechanical and superconducting heat switches enable efficient thermal cycling, while high-density radio frequency (RF) wiring provides the infrastructure required for control and readout of a five-qubit superconducting quantum processor. Housed in a single rack with a spacious, accessible sample stage, the platform demonstrates that cADR technology can be realized in a compact form factor and provides a basis for future quantum hardware platforms operating under stringent space and infrastructure constraints.
Accessibility to the microkelvin temperature regime is important for quantum technology research. Cryogen-free dilution refrigerators opened up the millikelvin temperature range, but the perception is that ultralow temperatures are difficult to reach and require specialist infrastructure. The authors report the design of a microkelvin platform based on a nuclear-demagnetization stage, engineered and optimized for operation on a cryogen-free dilution refrigerator. The study demonstrates temperatures as low as 395 $\ensuremath{\mu}$K, and a protocol that enables experiments operated below 1 mK for 95% of the time, providing an efficient cryogen-free microkelvin environment for a wide range of applications.
Rapid thermal cycling is a central bottleneck in the development of superconducting quantum devices: conventional dilution refrigerators require cooldowns of a day or more and substantial cryogenic infrastructure, which throttles the fabricate-measure-redesign loop. We present an ultracompact dilution refrigerator (3 kg in mass and 100 mm in diameter) that completes a full cooldown-warm-up cycle to a base temperature of 70 mK in 1.2 hours when unloaded, and in 2.1 hours when fully equipped with the microwave wiring required for qubit measurements, while delivering 20 microW of cooling power at 100 mK. We validate the platform through a complete characterization of a two-fluxonium device: we extract the full circuit Hamiltonian by two-tone spectroscopy, measure energy-relaxation and coherence times, and benchmark single-qubit control. Although the relaxation time is limited by the base temperature of the system, we reach a single-qubit gate fidelity of up to 99%, at the coherence limit set by our operating temperature. These results demonstrate that compact, fast-cycling dilution refrigeration can support state-of-the-art quantum-device characterization without sacrificing measurement quality, offering a practical route to high-throughput quantum-hardware development.
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.
Quantum computing promises to revolutionize our lives, achieving unprecedented computational powers and unlocking new possibilities in drug discovery, chemical simulations and cryptography. The fundamental unit of computation of a quantum computer is the quantum bit (qubit). It can be implemented in a number of technologies, such as superconducting qubits, spin qubits, photonic systems, trapped ions and NV-centers. Solid-state superconducting qubits and spin qubits seem to be the most promising ones, as shown by the recent demonstration of quantum supremacy with a superconducting qubit processor. Such quantum processors, to operate in the quantum regime, require very low temperatures around 10 mK, and they are hence kept inside dilution fridges. The electronics required to read out and control such processors is typically realized by discrete components or bulky scientific instruments at room temperature wired into the dilution fridge. Such approach is only feasible until few (< 100) qubits are handled, but it is not scalable for million-qubit processors required for fault-tolerant operation. In this thesis, readout and control electronics is proposed to be integrated and operated directly at cryogenic temperatures, in close proximity with the qubits and potentially even co-integrated. CMOS technology is advocated for its scalability, compactness and low prototyping cost, to create a comprehensive cryo-CMOS class of fully-integrated transceivers to read out and control solid-state qubits. In particular, the thesis focuses on cryo-CMOS radio-frequency integrated circuits for the readout of silicon quantum computers, looking to integrate the whole readout system, from the quantum layer to the room temperature user interface. Cryogenic CMOS devices are explored first, to lay the foundations of integrated circuit design. The realization of quantum dots in standard CMOS technology is explored as a platform to realize solid-state qubit arrays, demonstrating functionality at 50 mK and good reproducibility. Moreover, the operation of integrated transistors at 50 mK and 4.2 K is explored to understand their basic cryogenic functionality, through characterization and modeling. Then also integrated passives, such as capacitors, inductors and transformers, are studied and modeled at cryogenic temperatures. Then, a fully-integrated readout platform spanning across all the blocks needed for dispersive readout is proposed. A fully-integrated matrix of quantum dots with radio-frequency gate-based readout capabilities is demonstrated along with time- and frequency-multiplexing. This quantum-classical circuit shows all the features for a scalable approach and is the core of the platform following it. A cryogenic CMOS circulator based on a new all-pass filter architecture is demonstrated, to be used as integrated replacement for current bulky counterparts; it allows power reduction, insertion loss and bandwidth enhancement with respect to state-of-the-art integrated circulators. The rest of the front-end is also explored with dedicated test multiplexers, a low noise-amplifier and an oscillator, that are finally merged in a single fully-integrated low-noise receiver with frequency synthesizer operating at 3.5 K for scalable multiplexed readout of silicon qubits. The platform presented in this thesis, across temperature boundaries, carries all the features required for future scalable silicon quantum computing platforms and could therefore form their core.
Recent advances in solid-state qubit technology are paving the way to fault-tolerant quantum computing systems. However, qubit technology is limited by qubit coherence time and the scaling of quantum processors is limited by the complexity of coupling quantum systems with a classical electronic infrastructure. Hence, the proposed approach is to use CMOS at cryogenic temperatures, in order to ensure physical proximity to the quantum bits, thus reducing thermal gradients and increasing compactness. In this thesis, several RF circuits designed using commercial CMOS technology have been shown to be working at cryogenic temperatures. Such circuits could be easily interfaced (or integrated, in the future) with a qubit processor. The performance of the circuits is comparable to the state-of-the-art systems, currently used as control units for qubits. Moreover, these circuits have a much smaller form factor and could be made to work inside a dilution refrigerator where the qubits are operational, hence providing the first demonstration of a scalable solution.
Scaling superconducting quantum processors to hundreds of qubits is essential for demonstrating quantum advantage and implementing quantum error correction codes with larger distances. A key challenge is integrating control and readout wiring without compromising cryogenic or electronic performance. To address this, we performed a systematic analysis of the thermal budget within a dilution refrigerator and developed high-density 0.5 mm SCuNi-CuNi coaxial cable sets optimized for heat load management. This enables the integration of 696 control lines and 40 readout amplification chains into a dilution refrigerator with a cooling power of 1 mW at 100 mK, maintaining a stable base temperature of ∼8 mK for over a year. We validated the platform's performance with two large-scale quantum processors: a 540-qubit processor with an average T1 of 35 μs and a 156-qubit processor with 182 tunable couplers, achieving average fidelities of 99.9% for single-qubit gates and 99.0% for two-qubit gates. This study demonstrates a viable high-density wiring solution for controlling and measuring processors at the 500-qubit scale and provides critical engineering insights for larger-scale quantum computing systems.