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Calibration of a Multi-Qubit Microwave Package

The packaging around a quantum device has a substantial role in creating a good operating environment for qubits. A qubit sample holder needs to enable high-fidelity qubit operations by increasing circuit coherence, decreasing losses, ensuring good thermalization, and isolating the device from external electromagnetic fields. The field of quantum technology faces outstanding challenges in the testing of quantum device packaging due to a general lack of cryogenically compatible microwave calibration standards for scattering parameter (S-parameter) measurements in the microwave regime.

Our new article demonstrates a method for characterization of quantum device sample holders. The development of the method and the measurements were performed in the quantum applications labs at Bluefors using a QDevil QCage.24 sample holder developed by Quantum Machines. The measurement and analysis method enables the detection of insertion loss and return loss in the frequency band utilized for quantum experiments, < 20 GHz. Furthermore, analyzing the data with a time-domain signal propagation model reveals the control pulse distortions arising from reflections in the millikelvin-stage wiring that immediately precedes the input port of a quantum device. The results highlight the utility of calibrated cryogenic S-parameter measurements for validation of qubit packaging and the wiring in its immediate vicinity. The method is applicable to a variety of microwave wiring cascades that operate in the cryogenic environment.

2-port SOLT Calibration

Our previous work demonstrated the accurate determination of reflections from qubit drive line components, namely cryogenic attenuators and coaxial cables, using databased short-open-load (SOL) calibration.  The present study improves the technique by adding an unknown “through” (T) at cryogenic temperatures to extend the technique to a full 2-port cryogenic databased SOLT calibration. Calibration of a network analyzer for S-parameter analysis is a standard tool for room temperature test and measurement, but the application of calibrated S-parameter measurements to the cryogenic hardware of quantum computers may reveal new insight into the specifications and requirements for quantum computing wiring.

Calibrated Transmission and Reflection Measurements

The method was demonstrated with calibrated transmission and reflection measurements of a quantum device holder, done at millikelvin temperatures through a printed circuit board installed into the device holder. The results were published in the Review of Scientific Instruments.

The measurement and analysis method described here enables accurate characterization of quantum device holders and will be used to probe the limits of wiring density in the vicinity of a quantum device in the future.

Magnitude of the two-port S-parameters for the QCage.24 sample holder at 300 K and 40 mK. (a) Reflection at the input of the sample holder at the coaxial-to-CPW interface. (b) Transmission of the full sample holder assembly including coaxial cables.

Simulation of Qubit Drive Signal Distortion

How do we interpret the measured reflection data? Are the wiring and packaging components good enough to control qubits?  To address these questions, we also demonstrate a method of simulating deviations that arise in single qubit gate fidelity due to amplitude and phase distortions in the qubit drive pulse that arise because of interference of the drive pulse with the reflections. By introducing the measured return loss for the QCage.24 sample holder into the simulation, it is shown that the deviation in fidelity introduced by the sample holder should be four orders of magnitude lower than the exaggerated case of 15 dB return loss. Thus, the sample holder should not limit fidelity of single qubit gate operations. Looking forward, augmenting simulations of quantum dynamics with experimentally determined input parameters will improve the accuracy of specifications for quantum computer wiring requirements.

Frequency-dependent fidelity deviation for the last attenuator and coaxial cable terminating at the sample holder under test.

Frequency-dependent fidelity deviation for the last attenuator and coaxial cable terminating at the sample holder under test.

Learn more about our research from our publications page.