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New Measurements Show the Potential of Flexible Striplines for Qubit Control

In a pre-print paper posted today on arXiv, scientists from Bluefors, Delft Circuits, and VTT Technical Research Centre of Finland report new findings on qubit microwave control. In this work, the performances of flexible striplines and coaxial cables operating as drive lines were compared and no measurable differences in the qubit’s coherence and relaxation time were observed.

These results pave the way to high density scale-up of qubit control lines using flexible transmission lines, and show the technological readiness of Bluefors cryogenic platforms combined with the performance of Delft Circuits’ i/o products.

Need for More Microwave Channels

Quantum computing devices require low noise environments to improve the performance of qubits. Because of this, the microwave components used to send control signals to the qubits must meet strict technical requirements for cryogenic thermalization and noise. Currently, semi-rigid coaxial cables are utilized to provide a suitable means for signal delivery when integrated with microwave attenuators, low-pass filters and infrared filters to further reduce passive and radiative heating.

Current estimates indicate that between 2.5 and 5 lines per physical qubit will be needed to operate fault-tolerant quantum computers. In the near future, quantum computers consisting of a few thousand physical qubits will require multiple thousands of physical microwave channels. This poses serious challenges to semi-rigid coaxial cables, both in terms of mechanical integration and thermal loads.

Miniaturizing and multiplying microwave wiring channels using circuit elements embedded on flexible polymer substrates shows great promise for solving scaling and connectivity challenges in quantum computing. However, there have been questions regarding the effects on qubit coherence, and a scarcity of published data verifying this alternative method of control.

Similar Qubit Performance Observed

In this experiment, Bluefors scientists from the Quantum Applications team performed repeated measurements of energy relaxation and coherence time of a transmon qubit using two very different microwave transmission lines: firstly Cri/oFlex®, the Delft Circuits flexible stripline, and secondly, a semi-rigid coaxial cable, both installed inside the same Bluefors LD250 Dilution Refrigerator.

To make a meaningful comparison, Bluefors scientists used signal conditioning setups that were as nominally identical as possible, and observed no detectable change in the measured qubit properties due to the different wiring type. Only a small change in the mean dephasing rates of the qubits occurred when changing the drive line constructions, but coherence fluctuations appeared not to be caused by the different configurations.

Pathway for Scaling Qubit Control Lines

The results of the study open up the possibility for the large-scale integration of qubit control lines with integrated components in planar layouts on flexible substrate. The study shows flexible striplines to be a suitable solution for qubit control, as coherence times were similar with both wiring configurations. At the same time, the paper showcases the compatibility of Bluefors Dilution Refrigerators with this technology.

This result has a positive impact on the development of the quantum computing field, as it shows a pathway for the future scaling of qubit control lines. The technology offers a promising solution for larger quantum systems, as it provides higher channel density and the potential for lower cost qubit control lines.