by Russell Lake, Senior Scientist, Bluefors, email@example.com
Necessity of mK-cryogenics
During past years, advances in both lithography and millikelvin cryogenics have supported and enabled vast improvement in the sophistication of experimental research on electrical circuits that display uniquely quantum mechanical behavior. It comes as no surprise that dilution refrigerator measurement systems have moved beyond basic physics research contraptions and into central focus in the new era of quantum engineering. Achieving milli-kelvin temperatures remains a prerequisite for many of the leading hardware candidates for quantum computing with solid-state devices. For example, superconducting quantum circuits need temperatures low enough to keep microwave thermal photon populations on the chip negligible. In addition, the amplifiers that are typically required to achieve high fidelity dispersive readout are also based on superconductors and operate at the lowest noise temperatures allowed by physical limits.
Enabling quantum technology
Since 2008, Bluefors in Helsinki, Finland has aimed to support sub-Kelvin measurement applications by providing well-engineered reliable systems that are easy to use. To continue technical leadership, Bluefors pursues several important developments aimed specifically at improving functionality for quantum measurement applications. These include scaled-up wiring infrastructure, coaxial infrared (IR) filtering, and advanced diagnostics for measuring noise temperatures (Figure 1). In immediate applications, high density wiring addresses the problem of sending control and readout tones to the largest quantum processor units currently available. The lines are thermalized at each temperature stage using matched cryogenic microwave attenuators. Signals are conditioned using high density filter banks realized using cryogenically compatible materials, that are installed at the base temperature stage. Bluefors also developed IR filters that are designed to protect superconducting qubits devices from photons with higher energy than typical superconductor pair breaking energies. The IR filter has low insertion loss in the spectral range where signals are sent to the qubit but it utilizes a microwave absorber material to dissipate high frequencies. Diagnostic tools such as the cryogenic variable temperature noise source also offer new ways to measure the quality of readout lines by implementing Y-factor analysis in the cryogenic environment. [1, 2]
In spring 2021, Bluefors published two application notes to provide working examples of quantum measurements from within the manufacturing environment. The first application note (in collaboration with Keysight Technologies) was a concise guide to setting up and running a single-qubit characterization measurement including integration of the room temperature electronics with the cryogenic system. The measurement results demonstrated the signal input-output, hardware-software interface, and a qubit measurement system capable of reaching long qubit energy relaxation times of order 100 µs.  The second application note (in collaboration with Zurich Instruments) reported statistics spanning 100 hours of measurement time of the qubit energy-relaxation times: a crucial system parameter for qubit applications.  In addition, the measurements enabled a comparison between Bluefors measurement systems installed at Chalmers University (Gothenburg, Sweden) and in the Bluefors factory itself. Interlaboratory comparisons are often performed in the context of international metrology but have not been commonly applied to the field of solid-state qubits. Can the cryogenics industry demystify the factors that lead to high performance quantum devices through benchmarking data, measurement examples and clear documentation? Building integrated quantum systems is a multidisciplinary pursuit that needs innovations in inter-
connects, room temperature and cryogenic electronics – and of course the cryogenic environment itself.
Integrating the components required for a turnkey quantum measurement system is a major engineering effort that involves many partners around the world. Improving measurement setups and publishing the results promotes the utilization of cryogenics and will have a broad positive impact on quantum measurement science. Customers and stakeholders benefit from reduced time to bring up new systems, and the cryogenics industry can benefit from improved knowledge of the application of dilution refrigerator systems. http://www.bluefors.com
 S. Simbierowicz, V. Vesterinen, J. Milem, A. Lintunen, M. Oksanen, L. Roschier, L. Grönberg, J. Hassel, D. Gunnarsson and R. E. Lake, “Characterizing cryogenic amplifiers with a matched temperature-variable noise source,” Review of Scientific Instruments, vol. 92, p. 034708, 2021.
 M. Bal, J. Long, R. Zhao, H. Wang, S. Park, C. R. H. McRae, T. Zhao, R. E. Lake, V. Monarkha, S. Simbierowicz, D. Frolov, R. Pilipenko, S. Zorzetti, A. Romanenko, C.-H. Liu, R. McDermott and D. P. Pappas, “Overlap junctions for superconducting quantum electronics and amplifiers,” Applied Physics Letters, vol. 118, p. 112601, 2021.
 R. Lake, S. Simbierowicz, P. Krantz, F. Hassani and J. Fink, “The Bluefors Dilution Refrigerator as an Integrated Quantum Measurement System,” 2021. [Online]. Available: https://bluefors.com/wp-content/uploads/2021/06/Application-Note-Bluefors-Dilution-Refrigerator-as-an-Integrated-Quantum-Measurement-System.pdf.
 S. Simbierowicz, C. Shi, M. Collodo, M. Kirste, F. Hassani, J. Fink, J. Bylander, D. P. Lozano and R. Lake, “Qubit Energy-Relaxation Statistics in the Bluefors Quantum Measurement System,”2021. [Online]. Available: https://bluefors.com/wp-content/uploads/2021/06/application-note-qubit-energy-relaxation-statistics-bluefors-quantum-measurement-system.pdf. ■