IEEE MTT-S International Microwave Symposium digest
Integrating High-Density Microwave Signalling and Packaging With Superconducting Qubits
Saniya Deshpande, Jean-Philip Paquette, Mehrnoosh Vahidpour, Michael Selvanayagam, Rob Lion, Michael Pelstring, Shane Caldwell, Matthew J. Reagor, Damon S. Russell
One of the challenges for building a quantum computer with superconducting qubits is being able to integrate a sufficient density of microwave lines in a cryogenic environment. Traditionally this has been limited by the pitch of SMA cables. When trying to go beyond this approach there are two problems to address. The first is outfitting a dilution refrigerator with enough lines to run from room temperature to the base temperature stage. The second is building a package for a quantum integrated circuit that houses enough microwave lines itself. We propose a solution to both of these problems by using RF flexible circuits to increase signal density in the dilution refrigerator and package while maintaining signal integrity. We also discuss the thermal, mechanical and RF considerations that go into designing such a system. Finally we show a proof-of-concept integration of the entire system including measurements with superconducting qubits.
One of the challenges for building a quantum computer with superconducting qubits is being able to integrate a sufficient density of microwave lines in a cryogenic environment. Traditionally this has been limited by the pitch of SMA cables. When trying to go beyond this approach there are two problems to address. The first is outfitting a dilution refrigerator with enough lines to run from room temperature to the base temperature stage. The second is building a package for a quantum integrated circuit that houses enough microwave lines itself. We propose a solution to both of these problems by using RF flexible circuits to increase signal density in the dilution refrigerator and package while maintaining signal integrity. We also discuss the thermal, mechanical and RF considerations that go into designing such a system. Finally we show a proof-of-concept integration of the entire system including measurements with superconducting qubits.
Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
Abstract Superconducting quantum computers require microwave control lines running from room temperature to the mixing chamber of a dilution refrigerator. Adding more lines without preliminary thermal modeling to make predictions risks overwhelming the cooling power at each thermal stage. In this paper, we investigate the thermal load of SC-086/50-SCN-CN semi-rigid coaxial cable, which is commonly used for the control and readout lines of a superconducting quantum computer, as we increase the number of lines to a quantum processor. We investigate the makeup of the coaxial cables, verify the materials and dimensions, and experimentally measure the total thermal conductivity of a single cable as a function of the temperature from cryogenic to room temperature values. We also measure the cryogenic DC electrical resistance of the inner conductor as a function of temperature, allowing for the calculation of active thermal loads due to Ohmic heating. Fitting this data produces a numerical thermal conductivity function used to calculate the static heat loads due to thermal transfer within the wires resulting from a temperature gradient. The resistivity data is used to calculate active heat loads, and we use these fits in a cryogenic model of a superconducting quantum processor in a typical Bluefors XLD1000-SL dilution refrigerator, investigating how the thermal load increases with processor sizes ranging from 100 to 225 qubits. We conclude that the theoretical upper limit of the described architecture is approximately 200 qubits. However, including an engineering margin in the cooling power and the available space for microwave readout circuitry at the mixing chamber, the practical limit is approximately 140 qubits.
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.
With the demand for scalable cryogenic microwave circuitry continuously rising, recently developed flexible microwave striplines offer the tantalizing perspective of increasing the cabling density by an order of magnitude without thermally overloading the cryostat. We use a superconducting quantum circuit to test the thermalization of input flex cables with integrated 60 dB of attenuation distributed at various temperature stages. From the measured decoherence rate of a superconducting fluxonium qubit, we estimate a residual population of the readout resonator of (2.2±0.9)×10−3 photons and we measure a 0.28 ms thermalization time for the flexible stripline attenuators. Furthermore, we confirm that the qubit reaches an effective temperature of 26.4 mK, close to the base temperature of the cryostat, practically the same as when using a conventional semi-rigid coaxial cable setup.
Scalable quantum computing is limited by the dense network of electrical interconnects linking cryogenic quantum processors to room-temperature control electronics. To overcome this bottleneck, considerable effort has focused on cryogenic CMOS electronics and microwave-to-optical transduction, aiming to reduce wiring complexity and thermal loading. Wireless interconnects have recently emerged as a promising complementary approach, yet their compatibility with superconducting quantum hardware remains largely unexplored. Here, we demonstrate the wireless excitation of a superconducting microwave resonator of the type routinely employed for qubit readout, operating at millikelvin temperatures inside a dilution refrigerator. By directly comparing wired and wireless operation within the same cryogenic environment, we show that wireless coupling preserves the intrinsic resonator response while revealing parasitic electromagnetic pathways arising from stray radiation within the cryostat enclosure. These results establish a framework for the co-design of wireless interconnects, cryogenic packaging and superconducting quantum hardware.
With the demand for scalable cryogenic microwave circuitry continuously rising, recently developed flexible microwave striplines offer the tantalyzing perspective of increasing the cabling density by an order of magnitude without thermally overloading the cryostat. We use a superconducting quantum circuit to test the thermalization of input flex cables with integrated 60dB of attenuation distributed at various temperature stages. From the measured decoherence rate of a superconducting fluxonium qubit, we estimate a residual population of the readout resonator below 3.5⋅10−3 photons and we measure a 0.28ms thermalization time for the flexible stripline attenuators. Furthermore, we confirm that the qubit reaches an effective temperature of 26.4mK, close to the base temperature of the cryostat, practically the same as when using a conventional semi-rigid coaxial cable setup.