Acquisition of two dilution refrigerators

The Niels Bohr Institute is part of the Faculty of SCIENCE at the University of Copenhagen. The Niels Bohr

Institute has a 100 year long tradition of excellence within science, in particular within the area of quantum

physics. The institute was inaugurated on March 3, 1921, by Professor Niels Bohr. We are approx. 430

researchers, 70 administrative and technical staff, 100 PhD students and 400 students. The European

Physical Society has declared the Niels Bohr Institute a Historic Site of great international importance for the

development of physics and research.

The experimental tasks in my upcoming InnovationFund and Novo Nordisk Foundation Quantum Computing Programme project requires me to operate and develop sensitive superconducting qubit experiments at ~10 mK. Establishing and controlling such conditions, as well as sharing techniques and samples between multiple experimental setups involved in this work are a prerequisite for experimental success in establishing infrastructure for realizing a logical qubit with superconducting qubits. Over the last few years, we have developed experience with Bluefors XLD dilution refrigerators that have been shown to be exceedingly compatible with exactly the type of research to be conducted.

Two cryostat systems are required (of the same general design), one with 400µW and one with 1000µW of cooling power at 100mK (corresponding to 16µW and 34µW at 20mK). The systems should be compatible with each other (wiring, loading, shielding etc), and be compatible with very high coherence superconducting qubit measurements and very high fidelity operations. In particular we will need cryostats that have already demonstrated compatibility with >1 ms T1 times of fluxonium and >99.9% fidelity two-qubit gate operations of transmon/fluxonium qubits.

In particular, the research to be conducted involves developing and benchmarking very high coherence superconducting transmons , fluxonium, and other related qubit architectures, as well as operating them to high fidelity. A cornerstone of the planned research with both System 1 and System 2 will be to benchmark superconducting qubit devices produced in a novel way that is expected to dramatically increase the device cleanliness leading to very long coherence times. We expect to fabricate devices with T1 coherence times exceeding 1ms, ultimately compatible with up to ~400 qubits on a experimental cooldown, necessitating a combination of very high cooling powers, significant sample space at the mixing chamber and cryostats that have previously demonstrated compatiblity with >1ms T1 times and high fidelity gates. To the best of our knowledge BlueFors systems have demonstrated compatibility with T1 >1ms in fluxonium superconducting qubits and is the only commercially available cryostat to have done so.

Moreover, there is a laboratory constraint that necessitates the use of BlueFors XLDs: System 1 and System 2 will replace two previous Bluefors cryostats that were installed in the intended lab where the research is to be executed. To the best of our knowledge, Bluefors is the only manufacturer that can readily make a cryostat with identical foot print, mechanical and infrastructure requirements (pulse tube cooling, ceiling hangers, electricity requirements), which are already directly compatible with all the infrastructure that was set up for the previous BlueFors XLDs.

Finally, this resesarch will be conducted in partnership with other research labs (external to U of CPH) currently using BlueFors cryostats of similar make to System 1 and System 2. Interchanging samples to compare and contrast performance will be a central component of the research to be conducted, thus necessitating that the systems to be used for the research is of similar make and model.

Because the fridge will be used for extensive calibration, prolonged times at base temperature is essential, here Bluefors has a patent (EP 3167235B) for long-lived cold traps that enable XLD base operation for up to 3 years, without warming up.

Due to space constraints in our laboratory, these two cryostats will physically replace two previously existing Bluefors XLD cryostats which does not belong to our research activities and were removed for other purposes.