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arXiv:2405.09211 · Applied Physics Letters

Equivalence of flexible stripline and coaxial cables for superconducting qubit control and readout pulses

V. Yu. Monarkha, Slawomir Simbierowicz, Massimo Borrelli, Ruben J. J. van Gulik, Дроботун Николай Борисович, Daan Kuitenbrouwer, Daniël Bouman, Debopam Datta, Patrik Eskelinen, E. T. Mannila, J.-P. Kaikkonen, Visa Vesterinen, Joonas Govenius, Russell E. Lake

Published May 27, 2024• 9 citations
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Abstract

We report a comparative study on microwave control lines for a transmon qubit using (i) flexible stripline transmission lines and (ii) semi-rigid coaxial cables. During each experiment, we performed repeated measurements of the energy relaxation and coherence times of a transmon qubit using one of the wiring configurations. Each measurement run spanned 70–250 h of the measurement time, and four separate cooldowns were performed so that each configuration could be tested twice. From these datasets, we observe that changing the microwave control lines from coaxial cables to flexible stripline transmission lines does not have a measurable effect on coherence compared to thermal cycling the system or random coherence fluctuations. Our results open up the possibility of large-scale integration of qubit control lines with integrated component with planar layouts on flexible substrate.

DOI
10.1063/5.0203101

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We report a comparative study on microwave control lines for a transmon qubit using (i) flexible stripline transmission lines and (ii) semi-rigid coaxial cables. During each experiment, we performed repeated measurements of the energy relaxation and coherence times of a transmon qubit using one of the wiring configurations. Each measurement run spanned 70–250 h of the measurement time, and four separate cooldowns were performed so that each configuration could be tested twice. From these datasets, we observe that changing the microwave control lines from coaxial cables to flexible stripline transmission lines does not have a measurable effect on coherence compared to thermal cycling the system or random coherence fluctuations. Our results open up the possibility of large-scale integration of qubit control lines with integrated component with planar layouts on flexible substrate.
We report a comparative study on transmon qubit control using (i) conventional attenuated coaxial microwave line and (ii) an optical control system using modulated laser light delivered over telecommunications optical fiber to a photodiode located at the 1K stage of a dilution cryostat. During each experiment, we performed repeated measurements of the energy relaxation and coherence times of a transmon qubit using one of the control signal delivery methods. Each measurement run spanned 20 hours of measurement time and from these datasets we observe no measurable effect on coherence of the qubit compared to random coherence fluctuations. Our results open up the possibility of large scale integration of the optical qubit control system.
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.
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 60 60\,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⋅10−33.5\cdot10^{-3}3.5⋅10−3 photons and we measure a 0.28 0.28\,0.28ms thermalization time for the flexible stripline attenuators. Furthermore, we confirm that the qubit reaches an effective temperature of 26.4 26.4\,26.4mK, close to the base temperature of the cryostat, practically the same as when using a conventional semi-rigid coaxial cable setup.
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.
Superconducting qubits are a promising technology for building a scalable quantum computer. An important architecture employed in the field is called Circuit Quantum Electrodynamics (circuit QED), where such qubits are combined with high quality microwave cavities to study the interaction between artificial atoms and single microwave photons. The ultra-strong coupling achieved in these systems allows for control and readout of the quantum state of qubits to perform quantum information processing. The work on circuit QED performed in this thesis consisted of realizing an experimental setup for qubit experiments in a new laboratory, investigating the coherence and decay of higher energy levels of superconducting transmon qubits and finally demonstrating a novel coaxial form of circuit QED. Designing and building a 3D circuit QED setup involved the following main accomplishments: producing high quality 3D cavities; designing and installing the cryogenic microwave setup as well as the room temperature amplification and data acquisition circuitry; successfully developing a recipe for the fabrication of Josephson junctions; controlling and measuring superconducting 3D transmon qubits at 10mK. Several qubits were fully characterised and have shown coherence times of several microseconds and relaxation times up to 25μs. Superconducting qubits in fact possess higher energy levels that can provide significant computational advantages in quantum information applications. In experiments performed at MIT, preparation and control of the five lowest states of a transmon qubit was demonstrated, followed by an investigation of the phase coherence and decay dynamics of these higher energy levels. The decay was found to proceed mainly sequentially with relaxation times in excess of 20μs for all transitions. A direct measurement of the charge dispersion of these levels was performed to explore their characteristics of dephasing. This experiment was also reproduced on a 3D transmon fabricated and measured in Oxford, where due to a higher effective qubit temperature a multi-level decay model including thermal excitations was developed to explain the observed relaxation dynamics. Finally, a coaxial transmon, which we name the coaxmon, is presented and measured with a coaxial LC readout resonator and input/output coupling ports placed inline along the third dimension. This novel coaxial circuit QED architecture holds great promise for developing a scalable planar grid of qubits to build a quantum computer.
Superconducting circuits exhibit remarkable potential for constructing large-scale quantum simulation and computation systems, featuring numerous qubits, extended coherence time, and precise control. Nevertheless, the growing number of signal cables poses a challenge in dilution refrigerators due to space and heat load constraints. To overcome this issue, we experimentally implemented an optically-assisted transmission line as an alternative to coaxial cables. By modulating microwave signals on laser intensities at room temperature and regenerating the signals at a cryogenic plate within the dilution refrigerator, we demonstrated full control of superconducting qubits using photocurrent. We demonstrate and benchmark both single-qubit and two-qubit gates on frequency tunable transmon qubits, achieving fidelities of 99.915% ±\pm± 0.005% and 99.676% ±\pm± 0.041%, respectively, which have reached the requirement of the surface code.