Millimetre-Wave Superconducting Quantum Circuits

Project Information

Milli-Q

Grant agreement ID: 101054327

DOI

10.3030/101054327

EC signature date 22 August 2022

Start date 1 October 2022

End date 30 September 2027

Funded under

• European Research Council (ERC)

Total cost

€ 2 736 708,75

EU contribution

€ 2 736 708,75

Coordinated by

KARLSRUHER INSTITUT FUER TECHNOLOGIE
Germany

Periodic Reporting for period 1 - Milli-Q (Millimetre-Wave Superconducting Quantum Circuits)

Reporting period: 2022-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

In the Milli-Q project, we hope to develop a new generation of faster, more resilient, and energy-efficient superconducting qubits. To achieve these goals, the operating frequency of the qubits will be increased from today’s average of 5-6 GHz to 100 GHz. To operate in higher frequencies, the size of quantum circuit components will be reduced. These new quantum processors should be able to operate at significantly higher temperatures than before, thereby reducing the infrastructure and energy costs associated with cooling. During the initial two years of the project, we have focused on establishing the infrastructure and technology in our lab to be able to perform cryogenic experiments with superconducting circuits at mm-wave frequencies. We have constructed, manufactured and assembled fully functioning mm-wave measurement setup in a dilution cryostat operating at ultra-temperatures down to 10 mK. The problem of bringing the mm-wave signals to the low temperatures and then back from to room temperature was solved by combining hollow metallic waveguides and dielectric waveguides. This solution is compatible with scaling up in order to develop high density of cryogenic mm-wave lines for larger scale qubit system. On the qubit fabrication side, we have established a novel fabrication process of Nb/Al-AlOx/Nb Josephson junctions and have been working on reducing the junction size to the sub-micrometer range while keeping the critical current density high as required for mm-wave frequencies. As the first step towards qubit measurements at high frequencies, we successfully detected Josephson plasma resonance at frequencies over 100 GHz using in-house made niobium-based Josephson tunnel junctions and are now able to perform mm-wave spectroscopy and determine the eigenfrequencies of the qubits. Upcoming technical challenges are in avoiding parasitic resonances in the sample housing, chip substrate and interconnects, as well as performing time-domain measurements of qubits at mm-wave frequencies.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The most significant achievements after the first two years of the project to be the following:
1. We solved the problem of thermal conditioning and minimized the heat load of mm-wave waveguides in a dilution cryostat operating at temperatures down to 20 mK. This achievement required a lot of systematic technical work, including fabricating many metallic and dielectric components, development of a vacuum-tight low-loss feed-through for W-band hollow waveguides, making and testing transitions between the hollow and dielectric waveguide segments, minimizing the cross-talk between dielectric waveguides by using flexible mesh shields, developing and testing attenuators for the W-band waveguide thermal posts. We thus achieved the important point of being now able to perform low-noise measurements of mm-wave for quantum circuits at mK-temperatures in order to compare their properties with their performance at the elevated temperatures of up to several Kelvin.
2. We developed a new in-house fabrication process of Nb/Al-AlOx/Nb Josephson junctions with high plasma frequency in the range of 100 GHz. This newly developed technology will allow us to minimize losses at mm-wave frequencies and thus achieve long coherence times for our future qubits made of a superconductor with high energy gap allowing for operation at high temperatures and frequencies.
3. We have established Josephson plasma resonance spectroscopy at 100 GHz frequency range using current-biased phase escape measurement procedure. We have performed mm-wave spectroscopy of current-biased Nb/Al-AlOx/Nb junctions in the regime of thermally activated escape. The determined plasma frequencies of the junctions are in good agreement with expectations. The experimental data are well described by the strong-driving model.
4. We have designed, manufactured and characterized superconducting Fabry-Perot resonators allowing for superconducting qubit experiments in W-band using 3D environment with minimal loss participation of on-chip dielectrics. Here our initial goal was to develop and fabricate reliable high-Q superconducting resonators in order to characterize transmission measurements using a newly constructed mK setup for mm-wave measurements. We have achieved very promising performance of resonators and are considering to employ them for qubit measurements in the future.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The key novelty of the project is in exploring properties of superconducting quantum circuits in the mm-wave frequency range between 75 GHz and 110 GHz. When approaching this goal, in the very beginning of the project we had to develop from scratch new scalable methods and techniques of bringing mm waves down to mK temperatures, to explore appropriate materials and Josephson junction fabrication technology suitable for this frequency range, to construct and verify the performance of new mm-wave circuit holders, resonators and couplers. Looking back to our achievements during the first two years of the project, we believe that we achieved three major breakthroughs beyond the state of the art, which very significantly advance our work:
1. We have constructed, manufactured and assembled fully functioning mm-wave measurement setup in a dilution cryostat operating at ultra-temperatures down to 10 mK. The setup allows for mm-wave transmission and reflection measurement using a W-band network analyzer and is equipped with a cryogenic HEMT amplifier, isolators, and sample housing box. We have verified that the heat load from the W-band waveguides developed and constructed by us is extremely low, and is therefore scalable to a large qubit number. This setup is going to be the main “work horse” in the future mm-wave experiments with superconducting qubits as planned initially in our project.
2. Learning from our already existing experience with microwave-range qubit coherence limits, we have developed and established a novel fabrication process of Nb/Al-AlOx/Nb Josephson junctions with the critical current density over 2 kA/cm2 and zero-bias plasma frequency over 130 GHz. The key feature of this fabrication process, which we believe to be needed for qubits with long coherence times, is in avoiding using dielectric layers surrounding the junction tunnel barrier. This dielectric-free and free of fabrication residuals junction environment should significantly reduce dielectric energy loss at mm-wave frequencies.
3. We successfully detected Josephson plasma resonance at frequencies over 100 GHz using in-house made niobium-based Josephson tunnel junctions. By measuring the statistics of the phase escape events as a function of the bias current, in dependence on the mm-wave irradiation frequency, we are now able to perform mm-wave spectroscopy and determine the eigenfrequencies of the fabricated junctions. In the near future, by decreasing the measurement temperature, we would be able to explore the energy level structure of the Josephson junctions. This should open path towards coherence time measurements in the mm-wave range using the phase qubit regime